ORIGINAL RESEARCH
published: 12 February 2021
doi: 10.3389/feart.2020.594355
Tectonic Fabric, Geochemistry, and
Zircon-Monazite Geochronology as
Proxies to Date an Orogeny: Example
of South Delhi Orogeny, NW India, and
Implications for East Gondwana
Tectonics
Subhash Singh 1, Bert De Waele 2, Anjali Shukla 1, B. H. Umasankar 1 and
Tapas Kumar Biswal 1*
1
Department of Earth Sciences, Indian Institute of Technology Bombay, Mumbai, India, 2SRK Consulting (Australasia) Pvt Ltd.,
West Perth, WA, Australia
Edited by:
Guillermo Booth-Rea,
University of Granada, Spain
Reviewed by:
Junpeng Wang,
China University of Geosciences
Wuhan, China
Christoph Von Hagke,
RWTH Aachen University, Germany
*Correspondence:
Tapas Kumar Biswal
tkbiswal@iitb.ac.in
Specialty section:
This article was submitted to
Structural Geology and Tectonics,
a section of the journal
Frontiers in Earth Science
Received: 13 August 2020
Accepted: 17 December 2020
Published: 12 February 2021
Citation:
Singh S, De Waele B, Shukla A,
Umasankar BH and Biswal TK (2021)
Tectonic Fabric, Geochemistry, and
Zircon-Monazite Geochronology as
Proxies to Date an Orogeny: Example
of South Delhi Orogeny, NW India, and
Implications for East
Gondwana Tectonics.
Front. Earth Sci. 8:594355.
doi: 10.3389/feart.2020.594355
We have dated the South Delhi orogeny, Aravalli-Delhi Mobile Belt (ADMB), NW India, using
the tectonic fabric, geochemistry, and zircon-monazite geochronology as the proxies. The
South Delhi Terrane (SDT), a passive margin domain in the ADMB, consists of multiply
deformed (D1–D4) greenschist facies rocks and several granite plutons. The D1
deformation is characterized by pervasive isoclinal recumbent F1 fold and axial planar
tectonometamorphic fabric, S1, developed in all rock types. The S1 minerals belong to
peak greenschist facies metamorphism, M1, suggesting syntectonic nature of M1 with D1.
The age of the D1-M1 is constrained by the syncollisional peralkaline S type Sewariya
granite which is characterized by magmatic/submagmatic fabric (Sm) coplanar with the S1.
The margin of the pluton is turned into quartzofeldspathic gneiss carrying the evidence of
high temperature deformation. The age of Sewariya granite is estimated at ca. 878 Ma by
zircon geochronology. The D1-M1 is further constrained by monazite geochronology of the
mica schist at ca. 865–846 Ma. The other granite plutons and metarhyolite are pre-D1 and
emplaced at ca. 992–946 Ma. The D2 deformation produced NE-SW trending open
upright F2 folds coaxial with the F1, and northwesterly vergent F2–axial planar thrusts.
Monazite geochronology constrains the D2 at ca. 811–680 Ma. The D3 is characterized by
small to large scale NW-SE folds, and the D4 by faults and fractures marking the brittle
deformation in the rocks. The D4 is constrained by monazite geochronology at ca.
588–564 Ma. There are upper amphibolitic tectonic slivers along the D2-Phulad thrust,
belonging to the pre-Delhi rocks, which show ca. 1,638 Ma metamorphism age. From the
above study, it is suggested that the South Delhi orogeny belongs to ca. 878–680 Ma
marking the final amalgamation of Marwar Craton with the rest of India. This overlaps the
early phase of the Pan-African orogeny (900–630 Ma). The brittle deformation, D4,
coincides with Kuunga orogeny (650–500 Ma). Our study implies that India, like other
continents in the East Gondwana, underwent amalgamation of internal blocks until the late
part of the Neoproterozoic.
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Singh et al.
Age of South Delhi Orogeny
Keywords: blastesis-deformation relationships, granite, geochemistry, zircon-monazite geochronology, South
Delhi orogeny, ca. 878–680 Ma, East Gondwana
in the metasediments and granite plutons to know chronological
sequence of granite intrusion with deformation. Further,
geochemistry of granites was studied to decipher the tectonic
setting of their intrusion. Furthermore, zircon and monazite
geochronology were conducted to date the granite plutons and
tectonometamorphic fabric in the metasediments. Based on these
studies, the South Delhi orogeny was found not directly related to
Grenvillian or Kuunga orogeny (late part of Pan-African) rather
to the early part of Pan-African orogeny. Therefore, we suggest
that some of the orogenies observed in different continents may
not have direct match with global scale orogenies which was
responsible for building supercontinental assembly.
INTRODUCTION
Amalgamation of continents took place through accretion and
continental collision giving rise to supercontinental assembly.
Orogens were created along the zone of amalgamation, which are
marked by ductile as well as brittle deformation of rocks, different
types of metamorphism, and intrusion of variety of magmatic
rocks (Twiss and Moores, 1992; Kearey et al., 2009). A correlative
study between tectonometamorphic fabric with geochronology of
magmatic rocks and metamorphic mineral could estimate the life
span of a supercontinental cycle (Hawkesworth et al., 2017). The
earth underwent several orogenic cycles in the past since
Neoarchean to Tertiary period. The Nuna orogeny was
responsible for the formation of the Columbia Supercontinent
(ca. 2.1 to 1.8 Ga, Rogers and Santosh 2002; Zhao et al., 2002), the
Grenvillian orogeny was responsible for the Rodinia
Supercontinent (ca. 1.3 to 1.0 Ga; Valentine and Moores, 1970;
McMenamin and McMenamin, 1990; Meert and Torsvik, 2003;
Cawood, 2005; Li et al., 2008), and the Pan-African orogeny
created the Gondwana Supercontinent (0.9–0.5 Ga, Stern, 1994;
Kroner and Stern, 2005; Fritz et al., 2013; Oriolo et al., 2017). Each
orogeny comprises several phases of subduction and arc accretion
that vary temporally and spatially at different parts of the
Supercontinent. For example the Pan-African orogeny consists
of an earlier phase of subduction and arc accretion (0.9–0.63 Ga)
as observed in the Arabian-Nubian shield (Kroner and Stern,
2005) and a late phase of collision (Kuunga orogeny, ca.
0.65–0.50 Ga) in Madagascar, Tanzania, and other parts of
Africa (Meert and Lieberman, 2008; Lehmann et al., 2016).
In this paper, we studied the South Delhi orogeny of the
Archean-Neoproterozoic Aravalli-Delhi Mobile Belt (ADMB) in
NW India that consists of several terranes juxtaposed along shear
zones (Figures 1A,B). The terranes are Hindoli-Jahazpur,
Mangalwar, Sandmata, Aravalli, North Delhi, South Delhi, and
Sirohi terranes. The ADMB bears the imprint of the Nuna,
Grenvillian, and Pan-African orogenies. The age of the South
Delhi orogeny is still debated, if it is equivalent to Grenvillian or
Pan-African, because most of the studies are based on single
proxy. For instance, with very little analysis of magmatic vs. solid
state fabric of the diorite (1.0 Ga, Volpe and Macdaugall, 1990),
the Godhra granite (0.95 Ga, Gopalan et al., 1979), and the Chang
granite (Table 1, 0.97 Ga, Tiwana et al., 2019), a Grenvillian age
for the South Delhi orogeny was advocated (Roy, 2001). Similarly,
based on monazite ages of Pilwa-Chinwali area without much
consideration to deformation fabric Grenvillian age has been
considered (Bhowmik et al., 2018). Contrarily, Singh et al. (2010)
used zircon geochronology of granite and deformation history of
host rocks and deduced ca. 0.87–0.65 Ga age, suggesting PanAfrican age. Similar ages (ca. 0.87–078 Ga) were obtained by
Tiwari and Biswal (2019a) who applied a combined study of
monazite geochronology and deformation fabric to Ambaji
granulite. We adopted a new approach wherein multiple
proxies have been applied that include deformational structure
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REGIONAL GEOLOGICAL SETTING AND
OROGENIC CYCLES IN THE
ARAVALLI-DELHI MOBILE BELT
The ADMB underwent three orogenic cycles, namely, Bhilwara,
Aravalli, and South Delhi orogenic cycles (Figure 1;
Synchanthavong and Desai, 1977; Sinha-Roy, 1988; Gupta
et al., 1997; Biswal et al., 1998a,b; Bhowmik and Dasgupta,
2012). The Bhilwara orogenic cycle belongs to Archean age
and is represented by multiply deformed, metamorphosed,
migmatized
greenstone-tonalite-trondhjemite-granodiorite
gneisses of the Mewar gneiss, Mangalwar, Sandmata, HindoliJahazpur terranes, and Beawar and Anasagar gneisses
(Figures 1B–D) (ca. 3.3 Ga to 2.8 Ga age, Kaur et al., 2020
and reference therein). All these terranes were part of
Marwar-Bundelkhanda Craton (Figure 1C). The Bhilwara
orogeny terminated with westerly subduction and intrusion of
arc setting Berach, Gingla, and Jhiri granites at 2.6 Ga
(Figure 1D). The Aravalli orogenic cycle initiated during
Paleoproterozoic period and was manifested in the Aravalli
and North Delhi terranes (Figure 1B). The cycle was marked
by rifting and creation of expansive continental margin (ca.
2.0 Ga, Figure 1E) where thick sequence of sandstone-shalecarbonate rocks with interbedded rift-generated bimodal
volcanics were deposited with conglomerate and palaeosol at
the base (Pandit et al., 2008; De Wall et al., 2012; Mehdi et al.,
2015; Wang et al., 2014). The N-S aligned inverted V-shaped
Aravalli Terrane comprises shallow water stromatolites bearing
rocks in the east and low metamorphic grade carbonate and
pelitic rocks in the west. The Aravalli orogeny is marked by
subduction along Rakhabdev shear zone (Figure 1F),
emplacement of ophiolites at 1.8 Ga, intrusion of granulite and
charnockite plutons in the Sandmata Terrane at ca. 1.7 Ga,
granulite facies metamorphism in the Pilwa-Chinwali area at
1.7 Ga (Fareeduddin et al., 1994; Bhowmik et al., 2018), and arc
setting rhyolite and volcanic tuffs volcanism in the HindoliJahazpur Terrane at 1.8 Ga (Deb et al., 1989; Verma and
Greiling, 1995; Deb and Thorpe, 2001; Raza and Siddiqui,
2012). The Mangalwar Terrane was completely metacratonized
2
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Singh et al.
Age of South Delhi Orogeny
FIGURE 1 | (A) Peninsular India with location of various cratons and mobile belts. Abbreviations: ADMB: Aravalli-Delhi Mobile Belt, BC: Bundelkhanda Craton, DC:
Dharwar Craton, EGMB: Eastern Ghats Mobile Belt, MC: Marwar Craton, PMB: Proterozoic mobile belt, SGT: Southern Granulite terrane. (B) Simplified terrane map of
the ADMB (Singh et al., 2010). Abbreviations: AT: Aravalli Terrane, BC: Bundelkhanda Craton, GBT: Great Boundary Thrust, HJT: Hindoli-Jahazpur Terrane, KSZ:
Kaliguman Shear Zone, MT: Mangalwar Terrane, MC: Marwar Craton, NDT: North Delhi Terrane, PSZ: Phulad Shear Zone, RSZ: Rakhabdev shear zone, SDT:
South Delhi Terrane, ShT: Sirohi Terrane. (C) Schematic diagram of the Bhilwara, Aravalli, and South Delhi orogenic cycles: The Archean crust was 2.8–3.5 Ga old, the
relicts are left out as Mewar gneiss, Untala granite, Beawar gneiss, and Sandmata-Mangalwar gneisses. Hindoli group was probably a greenstone belt; all these were
(Continued )
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February 2021 | Volume 8 | Article 594355
Singh et al.
Age of South Delhi Orogeny
FIGURE 1 | part of Marwar-Bundelkhanda Craton. (D) Subduction/collision at 2.6 Ga, between Bundelkhanda Craton and Marwar Craton, marking the Bhilwara
orogeny. Berach granite, Gingla granite, and Jhiri granite produced from the remelting of the subducting slab. (E) Aravalli orogenic cycle initiated with opening of Aravalli
basin and North Delhi basin on the Archean-Paleoproterozoic Craton, at ca. 2.0 Ga. Ahar river granite served as basement of Aravalli sediments. (F) Aravalli orogeny
occurred at 1.7 Ga, subduction along Rakhabdev suture zone, ophiolites were obducted, Sandmata granulites intruded to gneissic basement, and Amet granite
intruded. (G) South Delhi orogenic cycles initiated with opening of South Delhi basin at 1.0 Ga; the Sirohi basin may be at the western flank of the South Delhi basin;
during rifting Sendra granite, Ranakpur diorite, Bilara granite intruded. (H) South Delhi orogeny at 0.88 Ga, subduction/collision, arc magmatism in form of Ambaji granite,
Erinpura granite and Sewariya granite, Marwar Craton was metacratonized, postorogenic extension created normal faulting, opened Punagarh basin.
during the Aravalli orogeny. The North Delhi Terrane, of nearly
equidimensional shape (Heron, 1953), consists of shallow water
sedimentary and volcanic sequences deposited in several grabens
(age ca. 2.5–1.7 Ga) (Kaur et al., 2006; Mehdi et al., 2015). The
sediments were folded along NE-SW axis (Ray, 1974; Gupta et al.,
1998), metamorphosed in greenschist facies (0.95 Ga, Pant et al.,
2008), and intruded by several granite plutons ranging in age
from ca. 2.5 Ga to 0.8 Ga (Kaur et al., 2006, 2013; Misra et al.,
2020). The Aravalli orogeny could be synchronous with
Columbia Supercontinent amalgamation event (monazite ages
of Mangalwar Terrane, 1.8–1.7 Ga, Bhowmik and Dasgupta,
2012; Columbia assembly ca. 2.1 to 1.7 Ga, Meert and Santosh,
2017). Ca. 1.0 Ga Grenvillian orogeny overprinted the BhilwaraAravalli rocks in form of granulite facies metamorphism in the
Sandmata granulite (ca. 0.94 Ga, Bhowmik et al., 2010),
Mangalwar Terrane (ca. 0.99 Ga, Ozha et al., 2016), and
northern part of Bhilwara belt (Kumar et al., 2019). The South
Delhi orogenic cycle initiated during Meso-Neoproterozoic
period following Grenvillian orogeny, when the Marwar
Craton underwent rifting to form the South Delhi and Sirohi
basins (Figure 1G). The NE-SW trending linear South Delhi
Terrane (SDT, Figure 1B) shows variation in lithological
association. The northern part consists of low grade pelitequartzite-carbonate sequence, the central part is dominated by
metavolcanics known as Phulad ophiolite, and the southern part
exposes amphibolite-granulite facies rocks (Biswal et al., 1998a, b;
Khan et al., 2005). Distinct erosional unconformity is preserved in
the northern part where the SDT-metasediments unconformably
overlie the gneissic-granulitic rocks of Mangalwar and Sandmata
terranes (Heron, 1953). Deposition of the SDT-metasediments
was constrained between ca.1.2–0.87 Ga by Singh et al. (2010)
and ca.1.74–1.06 Ga by Wang et al. (2014), and the granulite
facies metamorphism was constrained at ca. 0.87–0.85 Ga (Tiwari
and Biswal, 2019a). Bimodal volcanics and synrift extensional
granitoids (Sendra granite, Ranakpura diorite, Bilara granite,
ca.1.0 Ga) occur in the entire stretch of the SDT (Table 1,
Biswal et al., 1998a, b; Bhattacharjee et al., 1988; Pandit et al.,
2003; Singh et al., 2010). The South Delhi orogeny is marked by
westerly subduction and formation of island arc system with
intrusion of Ambaji, Erinpura, and Sewariya granites at
0.88–0.86 Ga (Figure 1H). The terrane is marked by multiple
stages of folding and many longitudinal thrusts and faults (Naha
et al., 1984, 1987; Biswal, 1988; Mukhopadhyay, 1989;
Mukhopadhyay and Martin, 1991; Ghosh et al., 1999, 2003;
Dasgupta et al., 2012). The basement rocks were thrusted up
and occur as tectonic slices, e.g., the granite gneisses at Beawar
and Ajmer (Tobisch et al., 1994; Chattopadhyay et al., 2012) and
granulitic outcrop at Pilwa-Chinwali (1.7–1.5 Ga to 1.0 Ga age,
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Fareeduddin et al., 1994; Bhowmik et al., 2018; Singh et al., 2020).
The Sirohi Terrane occurs to the west of the SDT in the form of
isolated remnants of low grade metasediments in the western
trans-Aravalli plain (probable age of sediments ca. 1.0 Ga, Roy
and Sharma, 1999; Purohit et al., 2012) within expansive ca.
0.86–0.76 Ga old granites (Erinpura granite: Just et al., 2011;
Malani Igneous Suite: Dharma Rao et al., 2013). Deformation and
metamorphic events were dated ca. 0.9–0.8 Ga (Arora et al., 2017).
Postorogenic extensional volcano-sedimentary sequences of the
Sindreth and Punagarh basins overlie the Sirohi Terrane
(Figure 1H) (Sharma, 2005; De Wall et al., 2014; Schobel
et al., 2017; Bhardwaj and Biswal, 2019; Tiwari et al., 2020)
and Malani Igneous Suite intruded along extensional fractures
(Sharma, 2005).
We studied a part of the SDT near Beawar-Rupnagar-Babra
in Rajasthan (Figure 2A). The SDT, in this part, is flanked by
the pre-Delhi rocks in the east and west (Heron 1953). Later
studies interpreted the pre-Delhi rocks in the western flank
were completely metacratonized by the intrusion of ca.
≤0.87 Ga old granites (Gupta et al., 1997; Singh et al., 2020).
Further, an inlier of pre-Delhi rocks, called Beawar gneiss,
divides the SDT into NW and SE synclines (Heron, 1953). The
Beawar gneiss is as old as ca. 2.8 Ga (Tobisch et al., 1994;
detrital zircon ages range from 2.5 to 1.6 Ga; Kaur et al., 2019)
and was overprinted by later thermal events at 0.96 and 0.89 Ga
(Kaur et al., 2020). Our study belongs to the NW syncline that
Heron (1953) further subdivided into Barotiya and Sendra
sequences (Figure 2A). While Barotiya sequence is
dominated by conglomerate, mica schist, and amphibolite,
the Sendra sequence consists of calc schist and mica schist.
Both have undergone NE-SW folding and greenschist facies
metamorphism. Mylonite and cataclasite are developed along
ductile shear zones and faults. Several granite plutons, namely,
Sewariya, Pratapgarh, Sumel, Chang, and Sendra plutons,
intrude the belt (Figure 2A) (Sendra granite, ca. 0.98 Ga,
Gangopadhyay and Mukhopadhyay, 1984; Tobisch et al.,
1994; Pandit et al., 2003; Sewariya granite, ca. 0.86 Ga,
Sivasubramaniam et al., 2019).
GEOLOGICAL SETTING OF THE STUDY
AREA-PRESENT STUDY
Field Setting
Mapping of litho-units and meso-microfabric study of rocks were
carried out and the structural map of the area (Singh et al., 2020)
was updated with addition of a greater amount of structural data
and stereoplots for areas like large scale fold hinges and shear
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Age of South Delhi Orogeny
TABLE 1 | Granite ages, from the SDT.
Name of the granite
Granites, Bilara near Jodhpur
Plagiogranite Sirohi Terrane
Diorites, Ranakpur
Xenocryst granite gneisses of Sirohi
Rhyolite, Deri
Foliated granite Sirohi Terrane
Rhyolite flows in Ambaji granite
Sendra granite
Sendra granite
Sendra granite
Chang pluton
Godhra pluton
Erinpura granites
Granulite metamorphism, anatexis Ambaji
Granite gneisses, Ambaji
Rapakivi granite intrusion, Ambaji granulite
Foliated granite, Siyawa
Granites, Sirohi
Granite, Tosam
Porphyritic granite Sirohi Terrane
Rhyolite, Tosam
Malani Igneous Suite
Sindreth volcanic rocks
Microgranite, Ambaji
Sindreth rhyolite Sirohi Terrane
Malani Igneous Suite
Balda granites
Microgranite, Ambaji granulite
Charnockite, Ambaji granulites
Mt Abu granite
Gabbar Hill granite near Ambaji
Mt Abu, fabric formation
Present study
Sewariya granite (TKB-1)
Xenocryst from Sewariya granite (TKB-1)
Rupnagar metarhyolite (TKB-2)
Pratapgarh granite (TKB-3)
Sumel granite (TKB-4)
Sumel granite (TKB-4)
Pre-Delhi metamorphism
D1 deformation metamorphism
D2 deformation and shearing
Brittle shearing
Age
Method
1101 ± 13 Ma
1015 Ma
1012 ± 78 Ma
992 ± 1.3 Ma
987 ± 6 Ma
966 Ma
960 Ma
840 Ma
967.8 ± 1.2 Ma
966 Ma
970 Ma
955 Ma
863 ± 23
860 Ma
850 Ma
840 Ma
836+7/−5 Ma
822.8 ± 0.8 Ma
818 ± 3.6 Ma
808 Ma
793 ± 18 Ma
771 Ma
765.9 ± 1.6 Ma
765 Ma
765 Ma
765 Ma
763 ± 22 Ma
759 Ma
757.8 ± 0.9 Ma
735 ± 15 Ma
535 ± 15 Ma
509 ± 2 Ma
Zircon 207Pb/206Pb
SHRIMP Zircon dating
Sm-Nd Isochron age
Pb-Pb Zircon Xenocryst
U-Pb zircon age
SHRIMP Zircon dating
SHRIMP Zircon dating
Rb-Sr age
Pb/Pb ages
Nd-Sm
U/Pb age
Rb/Sr age
Monazite dating
SHRIMP Zircon dating
Rb-Sr age
SHRIMP Zircon dating
U-Pb Zircon age
Pb-Pb Zircon age
Ar-Ar ages
SHRIMP dating
Ar-Ar ages
U-Pb and 40Ar/39Ar
U-Pb Zircon age
Rb-Sr age
SHRIMP Zircon dating
Zircon age
Whole Rock Rb-Sr age
SHRIMP Zircon age
Single zircon age
Rb-Sr age
Biotite mineral isochron
Ar-Ar, Hb crystallization
Meert et al. (2013)
Dharma Rao et al. (2013)
Volpe and Macdougall (1990)
Purohit et al. (2012)
Deb et al. (2001)
Dharma Rao et al. (2013)
Singh et al. (2010)
Choudhary et al. (1984)
Pandit et al. (2003)
Tobisch et al. (1994)
Tiwana et al. (2019)
Gopalan et al. (1979)
Just et al. (2011)
Singh et al. (2010)
Choudhary et al. (1984)
Singh et al. (2010)
Deb et al. (2001)
Purohit et al. (2012)
Murao et al. (2000)
Dharma Rao et al. (2013)
Murao et at. (2000)
Meert et al. (2013)
Van Lente et al. (2009)
Choudhary et al. (1984)
Dharma Rao et al. (2013)
Dharma Rao et al. (2012)
Sarkar et al. (1992)
Singh et al. (2010)
Roy et al. (2005)
Crawford (1975)
Crawford (1975)
Ashwal et al. (2013)
878 ± 9 Ma
1634 ± 22 Ma
982 ± 3 Ma
992 ± 12 Ma,
946 ± 18 Ma
270 ± 12 Ma
1611 Ma
865 − 846 Ma
81a − 680 Ma
588 Ma
Zircon, SHRIMP
Zircon, SHRIMP
Zircon, SHRIMP
Zircon, SHRIMP
Zircon, SHRIMP
Zircon, SHRIMP
Monazite dating
Monazite dating
Monazite dating
Monazite dating
Intrusion, Age of D1
Basement metamorphism
Intrusion
Intrusion
Intrusion
Pb-loss
zones. Large scale structures were interpreted with the help of
structural cross sections. Mica schist and calcareous schist are the
major metasedimentary units in the area (Figures 2B,C). The
mica schist is marked by close spaced schistosity (S1) (Figures
3A,B) and calcareous schist contains rib structure produced by
differential weathering between carbonate layer and silicate layers
(Figures 3C–H). The calcareous schist grades into impure marble
and calcareous quartzite. The impure marble carries bedding
parallel epidote-garnet skarnoid bands as at Rupnagar
(Figure 3D). Metaconglomerate formation having width
ranging from m to 10 m occurs within mica schist, in the west
of Babra (Figure 2B). The metaconglomerate consists of alternate
mica schist and quartzite layers with several ellipsoidal quartzite
and granite pebbles (Figures 3I,J). The Phulad thrust passes
along the metaconglomerate formation and converted the rock
into a micaceous mylonite. The pebbles and quartzite layers
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References
within the thrust are flattened and stretched down-dip,
forming boudin, mullion, and rodding structures. Further,
meter scale slivers of basement pelitic-gneissic rocks
(Figure 3K) occur within the thrust zone; they carry garnet,
tourmaline, quartzofeldspathic pockets, and sillimanite needles
visible on the schistosity surface.
Concordant bands of metarhyolite and amphibolite occur
within mica schist and calcareous schist (thickness varying
from few cm to m) near Rupnagar and Babra (Figures 2B,C,
3L,M). These rocks are marked by flow layers (S0) and filled-in
vesicles (Figure 3M).
Several deformed granite plutons occur in the area, namely,
Sendra, Chang, Pratapgarh, Sumel, and Sewariya plutons
(Figures 2A–C). Heron (1953) included all of them within
Erinpura granite while later workers (Bhattacharjee et al.,
1993; Gupta et al., 1997; Pandit et al., 2003) classified them
5
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Singh et al.
Age of South Delhi Orogeny
FIGURE 2 | (A) Geological map of the SDT around Beawar-Rupnagar-Babra; pre-Delhi gneiss outcrop exposed around Beawar, called Beawar gneiss, divides the
SDT into NW and SE syncline. The study areas (marked by polygons) lie in the NW syncline around Babra and Rupnagar. (B) Geological map of the area west of Babra.
(C) Geological map of the area west of Rupnagar. (D–G) Stereoplots of the S1 fabric, F1-F2 axis, stretching lineation. (H–I) Geological cross sections. Sample locations
are marked in the geological map, PT: Pratapgarh thrust, RT: Rupnagar thrust Geological Setting of the Study Area-Present Study.
plagioclase, and K-feldspar minerals (Figure 3R). It is
coplanar with S1 fabric albeit the S1 fabric anastomoses
around the coarse grains of feldspar and quartz. The pluton
exhibits an increase in intensity of deformation from core to the
margin. The granite converts to a quartzofeldspathic gneiss near
the host rock contact (Figure 3R). The pluton and host rock
fabric are coupled as the Sm parallel S1 fabric in the granite is
coplanar with the S1 fabric in the host rock and together
participated in subsequent folding. This evidences the
syntectonic emplacement of granite with D1 deformation (cf.
Vernon et al., 1989; Bouchez et al., 1990; Miller and Paterson,
1994; Paterson et al., 1998; Buttner, 1999; Biswal et al., 2007).
Such gneisses occur at several places, e.g., Kalakot, Bar, and Babra,
which were earlier identified as pre-Delhi gneiss (Figure 2A,
Heron, 1953). At Bar (near Makarwali village, Figure 2A), there
is a spectacular gradation of mica schist into granite gneiss.
The mica schist contains quartzofeldspathic layers derived
from lit-par-lit injection of Sewariya granite pluton; the
into Sendra granite (Sendra, Chang, Pratapgarh, and Sumel
pluton) and Erinpura granite (Sewariya pluton). Field relation
suggests that the Sewariya granite is the youngest as its apophyses
crosscut the metasediments as well as other granite plutons
(Figures 3F,N). The Pratapgarh and Sumel plutons are coarse
grained with prominent NE-SW trending S1 fabric; it carries
several metasedimentary and metavolcanic roof pendants and
xenoliths which are folded and metamorphosed along with the
pluton (Figure 3N). Sewariya pluton occurs as a linear body in
the western part of the area (Figures 2A,b) containing magmatic/
semimagmatic (Sm) and solid state deformation fabric, S1
(Figures 3O,Q). Tourmalines are segregated into ellipsoidal to
ribbon shape pockets (Figure 3R-inset 1) and amphibolite-mica
schist xenoliths are more common in the central part of the
Sewariya granite (Figure 3R-inset 2). The pluton shows distinct
textural variation from core to the margin (Figure 3R). The core
part of the pluton is characterized by magmatic/submagmatic
fabric (Sm) due to shape preferred alignment of quartz,
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FIGURE 3 | (A) Mica schist, showing hinge zone of a large scale F1 fold, S1 fabric (scale parallel) is developed parallel to the axial plane of the fold, S1 fabric crosscut the S0
(bedding, color bandings), subhorizontal view, Rupnagar. (B) Mica schist showing crenulation cleavage (scale parallel) due to folding of S1 by upright F2, Rupnagar. (C) Calc schist
showing recumbent F1 fold (coins at the F1 hinge), vertical view, Pratapgarh. (D) Epidote (Ep) garnet (Grt) skarnoid near Rupnagar. (E) Calc schist/impure marble showing upright F2 fold,
having axial plane parallel shear (NW vergence thrust, scale parallel), vertical view, Rupnagar. (F) Calc schist with Sewariya granite vein, folded by NNW-SSE trending F3 fold (scale
parallel), horizontal view, Pratapgarh. (G) Dome structure (type 1) due to interference between NE trending F2 and NW trending F3 folds, in calc schist, vertical view near Pratapgarh. (H)
Crude type 2 int pattern due to F3 (scale parallel) superimposed on F1 near Rupnagar. (I) Metaconglomerate formation, sheared at the center, S-C fabric has small internal angle (15°), NW
vergence thrusting, vertical view, Babra. (J) Mylonite surface, subvertical stretching lineation (hammer parallel) and low plunging intersection lineation (scale parallel), vertical view, Babra.
(K) Garnet, staurolite, sillimanite bearing pockets within metaconglomerate, horizontal view, Babra. (L) Alternate metarhyolite (pink colored) and amphibolite layers (green colored), vertical
view, Rupnagar. (M) Amphibolite carrying vesicles (white pockets), vertical view, Rupnagar. (N) Pratapgarh granite course grained (PG), intruded by Sewariya granite vein (SG), horizontal
view, near Pratapgarh. (O) Sewariya granite from the core of the pluton showing magmatic fabric Sm, pen parallel, west of Babra. (P) Faulted rock along Chang fault, north of Rupnagar,
brittle fractures (Fr) are present along which thin pegmatite veins are present (PV). (Q) Sewariya granite with solid state deformation fabric, S1, close to the host rock contact. (R)
Landscape view of Sewariya granite near Babra, on the NW side, in the core of the pluton, magmatic/submagmatic fabric, Sm, is more prominent and in the SE side near mica schist
contact, solid state S1 fabric is more prominent (scale parallel) (width of the photograph 2 km).
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Age of South Delhi Orogeny
dimension of the euhedral plagioclase and K-feldspar crystals
(Figure 4L). Some grains are oriented at an angle probably due to
lack of space for rotation during flow of magma (cf. Bouchez et al.,
1990). Under microscope the twin planes of these minerals are
oriented parallel to the Sm. The K-feldspar and plagioclase
phenocrysts contain mica inclusions in mutually perpendicular
orientation, one parallel to cleavage and the other perpendicular
to it (Figure 4M). Surrounding the feldspar phenocrysts, smaller
quartz grains are dynamically recrystallized, which are
characterized by chess board twinning and grain boundary
migration. Plagioclase and K-feldspar occasionally show
dynamic recrystallization into smaller grains (Figure 4N).
Toward the margin of the pluton, the rock exhibits increase in
degree of dynamic recrystallization and segregation into alternate
quartzofeldspathic and mica rich bands (Figures 4O,P). Smaller
grains of plagioclase, K-feldspar, and quartz are developed out of
such dynamic recrystallization; quartz shows chess board
twinning; all these features indicate higher temperature of
deformation at about 650–700 °C (cf. Stipp et al., 2002;
Passchier and Trouw, 2005).
quartzofeldspathic layers increase in number and thickness and
grade into granite gneiss.
Textural Study
Mica schist contains alternate thin quartzofeldspathic and
micaceous bands, and the muscovite and biotite show shape
preferred orientation defining the S1 fabric in the rock
(Figure 4A). The calcareous schist carries layers of tremoliteactinolite with shape preferred orientation parallel to S1
(Figure 4B). Occasionally, plagioclase and epidote
porphyroblasts with inclusions of tremolite and actinolite occur
in the rock. The tectonic slivers within metaconglomerate contain
garnet, staurolite, tourmaline, quartz, feldspar, and ± sillimanite
assemblage (Figure 4C). Along the thrusts, micaceous mylonite is
developed that contains quartz ribbons which are recrystallized
into smaller grains (Figure 4D). Further, the tectonic slivers show
tailed garnet and staurolite grains (Figures 4E,F) and the rock
shows retrogression of sillimanite to muscovite and staurolite to
biotite (Figure 4E).
The metarhyolite comprises quartz (40–50%), plagioclase
(30–40%), hornblende, and epidote; the grain size varies
between 100 and 120 µm (Figure 4H). Hornblende is euhedral
and shows shape preferred orientation parallel to S1 fabric.
Epidote is produced from retrogression of hornblende during
shearing (Figure 4G). Plagioclase retains its euhedral shape
characteristic of magmatic origin. Quartz is subrounded and
show bulging grain margin indicating low temperature
dynamic recrystallization. Amphibolite shows granoblastic to
well-foliated character (with S1 fabric) and contains
hornblende-epidote-biotite-quartz-plagioclase
minerals
(Figure 4I).
The Pratapgarh pluton is coarse grained containing
microcline, quartz, and plagioclase that show an equigranular
mosaic (grain size 450–500 µm) (Figure 4J). Biotite, muscovite,
and rare garnet, sillimanite, and tourmaline are present in the
rock; garnet and sillimanite represent the undigested phases.
Microcline and plagioclase maintain euhedral shape, and the
polysynthetic lamella in plagioclase is in random orientation to
suggest the absence of any magmatic fabric in the rock. However,
the rock shows low temperature deformation fabric indicated by
bulging recrystallization of quartz and shape preferred
orientation of mica minerals parallel to S1 fabric. The Sumel
pluton is inequigranular, coarse grained, and porphyritic
containing plagioclase phenocrysts in the groundmass of
K-feldspar, quartz, tourmaline, and biotite. Plagioclase still
preserves euhedral shape indicating magmatic origin
(Figure 4K). Biotite shows shape preferred orientation
defining deformation fabric (S1) in the rock. The pluton is
marked by N-S trending pegmatite veins intruded along
fractures and ductile shear zones. Absence of magmatic fabric
and development of greenschist facies deformation fabric (S1)
similar to the metasediments point toward pre-D1 intrusion of
these plutons.
The Sewariya granite is coarse grained and consists of quartz,
K-feldspar, plagioclase, muscovite, biotite, and tourmaline. The
magmatic/submagmatic fabric Sm is best visible on the polished
surface of the sample, having been defined by alignment of longer
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Structures
Three stages of ductile deformation (D1-3) and one stage of brittle
deformation (D4) affected the rocks. The D1 deformation
produced cm to m scale NE-SW axial oriented tight to
isoclinal recumbent/reclined to upright F1 folds (Figures
3A,C). These are developed not only in the metasediments but
in the amphibolite, metarhyolite, and granite also. Due to syn-D1
greenschist facies metamorphism (M1), pervasive F1 axial planar
schistosity (S1, Figures 3A,B) and intersection lineation between
bedding and S1 fabric (Figure 3A) are developed in all the rock
types. The D2 deformation developed cm to 10 m scale NE-SW
trending upright F2 folds (Figures 3B,E) coaxially with the F1
(type 3 interference pattern, Ramsay, 1967). Crenulation cleavage
and pucker axes associate with the F2 fold (Figures 3B and 4B)
and axial planar thrusts were developed (Figure 3D). The F2 fold
bears testimony of buckling (parallel folding, disharmonic
folding, etc., Figure 3B) and was ascribed to a NW-SE
horizontal compression during D2. The D3 developed m to
10 m scale NW-SE to N-S trending F3 folds, attributed to NESW shortening of the orogen (Figure 3F). Type 1 (Figure 3G)
and type 2 interference patterns (Figure 3H) were produced
due to superposition of F 3 on the F2 and F1 folds, respectively.
Ductile deformation was followed by brittle deformation (D4 )
which is represented by N-S and NNE-SSW brittle fractures;
late-stage pegmatite veins intrude the fractures at places
(Figure 3P).
Several NE-SW trending F1 folds are mapped in the
metarhyolite-amphibolite-calc schist sequence to the north of
Rupnagar (Figure 2C and profile section Figure 2I); the S1 fabric
crosscut the litho-units at the F1 hinge zone. The F1 axial planes
are refolded by NW-SE trending F3 fold to produce type 2
interference pattern in map scale. Further, to the west of
Rupnagar, the F1 folds in marble-metarhyolite-amphibolite
sequence are refolded by an F2 fold producing type 3
interference pattern which is indicated by girdle distribution of
S1 fabric in stereoplot; the β axis lies in ESE direction (Figure 2G).
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Singh et al.
Age of South Delhi Orogeny
FIGURE 4 | Photomicrographs: (A) Mica schist with alternate muscovite + biotite and quartz rich layers, parallel to S1. (B) Calc schist with tremolite, arranged
parallel to S1, the S1 is folded by F2 fold. (C) Garnet (Grt), staurolite (St), sillimanite (Sil), and tourmaline (Tur) assemblage in pelitic gneiss-tectonic slivers within
metaconglomerate formation. Due to shearing along Phulad thrust, garnets show rotation in top-to-NW reverse sense. (D) Mica mylonite from Pratapgarh thrust, the
quartz (Qtz) shows dynamic recrystallization, S fabric is indicated by long axis of the quartz, top-to-NW reverse sense of shearing, S^C is nearly 60° suggesting
volume gain. (E) Staurolite changing to biotite (Bt) during shearing, Biotite is grown in the strain shadow of the staurolite porphyroclast, top-to-NW reverse sense of
shearing. (F) Garnet porphyroclasts in the Phulad thrust show top-to-NW reverse slip, biotite develops in the tail of the porphyroclast, relict sillimanite (Sil) is present. (G)
(Continued )
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FIGURE 4 | Epidote-biotite mylonite developed on metarhyolite, along Rupnagar thrust, top-to-NW shearing, inset shows detailed view of the biotite fish that indicates
NW vergence shearing. (H) Rupnagar metarhyolite, medium grained, quartz shows bulging, magmatic hornblende (Hbl), plagioclase (Pl) are preserved. (I) Amphibolite
with hornblende porphyroblast (Hbl) and biotite. (J) Pratapgarh granite coarse grained preserves magmatic microcline (Mc) and plagioclase (Pl). (K) Sumel pluton shows
coarse grained, plagioclase (Pl) shows magmatic habit, quartz shows undulose extinction. (L) Sewariya granite, polished sample, Feldspar (Fsp) grains show magmatic
fabric (Sm). (M) Thin section prepared from sample l, coarse plagioclase contains biotite inclusions, surrounding quartz grains show chess board extinction. (N) Thin
section of Sewariya granite shows dynamic recrystallization of quartz, feldspar, and plagioclase surrounding coarse plagioclase grain. (O) Polished hand specimen of
gneissic part of Sewariya granite close to country rock, solid state fabric S1. (P) Thin section of the sample o, solid state fabric S1 is prominent, mica (Mc) recrystallized
parallel to it. Quartz (Qtz), K-feldspar (Fsp), and plagioclase (Pl) are dynamically recrystallized. Quartz shows chess board extinction.
as symmetrical-drawn boudins to asymmetrical-shear boudins
(cf. Goscombe et al., 2004) suggesting effect of vertical shearing in
the formation of boudin structures. As the boudins are detached
both horizontally and vertically, they can be termed as chocolate
boudins (Ghosh, 1988). Vertical shearing with large component
of pure shear component during D2-thrusting has flattened the
boudins and pebbles and made them near parallel to the
mylonitic foliation (Figure 3I, Dasgupta et al., 2012). A low
plunging intersection lineation between S and C fabric is
developed on C-surface in a few instances (Figure 3J). The
lensoidal quartz aggregates show dynamic recrystallization of
quartz grains by subgrain rotation; the quartz grains are
oriented at an angle to the mylonitic foliation (C).
Temperature of deformation is around 450°C (e.g., Stipp et al.,
2002). Lensoidal quartz aggregates, mica fishes, and S-C fabric on
the XZ section of the mylonites indicate top-to-NW sense of
thrust-slip kinematics (Figure 3I). The upper amphibolite grade
tectonic slivers contain several rounded garnet and tourmaline
grains. Garnet contains quartz-biotite inclusion trails
(Figure 4C). The orientation of such inclusion trails varies
from grain to grain suggesting variable amount of rotation of
host garnet grains during D2 shearing. Sillimanite and staurolite
grains are retrogressed during shearing to muscovite and biotite
to form porphyroclastic tail in the strain shadow zone of the
garnet (Figures 4C,E,F). The sigma type garnet porphyroclasts
indicate top-to-NW vergence thrusting (Figures 4C,E–G). In
addition, a pair of thrusts occurs on western and eastern edge of
the Pratapgarh granite pluton, both jointly named as Pratapgarh
thrust. The western thrust passes through the mica schist. The
lensoidal aggregate contains dynamically recrystallized quartz
grains (Figure 4D). These grains show top-to-NW thrust
sense of shear. The S fabric is at high angle (60°) to C fabric,
which suggests there is volume gain during thrusting due to fluid
injection (e.g., Ramsay and Huber, 1987). Rupnagar thrust occurs
on the eastern edge of the metarhyolite formation (Figure 2A).
The metarhyolite is sheared and produced muscovite-biotitequartz-epidote assemblage. The mylonite contains SE plunging
stretching lineations, and the S-C fabric indicates top-to-NW
thrusting (Figure 4G). In addition to these ductile shear zone,
brittle fault, namely, Chang fault, occurs at the contact between
the pre-Delhi granite gneiss and metasedimentary sequence east
of Rupnagar and Babra fault, west of Babra. The fault zone is
marked by crushed rock of the amphibolite and marble
(Figure 3P).
The F1-F2 axes are rotated from NE direction toward ESE
direction by the F3 fold. Further, F2 folds are present to the
NE of Ratariya and Pratapgarh in the calc and mica schist units;
the core of the fold is occupied by the Pratapgarh granite pluton
(profile section Figure 2I). The F1 fold and S1 fabric in
metasediments as well as in the granite are refolded around
the hinge zone of the F2 fold. The axis of the fold is oriented
toward SSW as indicated by the β axis in stereoplot (Figures
2E,F). The lineation plots do not coincide with the β axis
suggesting that the lineations are rotated to a large extent by
the F3 fold. The entire structure surrounding the Pratapgarh
pluton represents dome and basin structure in large scale. In
addition, the mica schist amphibolite sequence describes large
scale F2 fold with NNE-SSW trending β axis to the west of Babra
(Figure 2D, structural profile Figure 2H). The stereoplot shows
an incomplete girdle suggesting either F2 hinges are too tight or
they are obliterated by thrusting. The lineations in the stereoplot
show concentration in NE as well as SW quadrant indicating
plunge reversal due to the F3 folding. In this domain, the Sewariya
pluton is marked by F1 fold and S1 fabric. The S1 fabric remains
parallel with the litho-contact between the pluton and mica schist
as well as the S1 within mica schist. In both the mapped areas,
west of Babra as well as around Rupnagar (Figures 2B,C), the
litho-units are marked by multiple phases of folding; coaxial
folding is predominant. Therefore, the S1 fabric shows girdle
distribution in stereoplots (Figures 2D–G). But the profile
sections (Figures 2H,I) show uniform dip toward SE. This is
interpreted as due to the effect of NW verging thrusts on large
scale structure of the area.
Further, the map depicts large scale D2 thrusts at several
localities, in form a retro-wedge thrust belt (Sections A-B and
C-D, Figures 2H,I, cf. Naylor and Sinclair, 2008). The most
prominent amongst them is Phulad thrust passing along the
metaconglomerate-mica schist formations west of Babra
(Figure 2A). The thrust is marked by quartz-biotite mylonite
with crudely defined S-C fabric (Figure 3I). The mylonitic
foliation dips to SE and the stretching lineation is nearly
down-dip (Figure 2D, 3J). The stretching lineation is defined
by muscovite and biotite minerals. The conglomerate pebbles are
stretched parallel to stretching lineation. Apart from that, large
number of boudins (Figure 3I), mullions, and rodding of
quartzite, granite, and gneisses associate with stretched
pebbles. These boudins are long drawn out on both vertical
and horizontal section; vertical stretched axis is 5–10 times
longer than the horizontal stretched axis. Though the boudin
long axes are parallel with the mylonitic foliation, these are tilted
with a smaller angle with mylonitic foliation (∼15°) nearly in
more than 50% of cases (Figure 3I). The boudins can be classified
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Metamorphism
The rock types show greenschist facies metamorphism. Mica
schist and calcareous schist contain muscovite, biotite,
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Age of South Delhi Orogeny
tremolite-actinolite, and epidote minerals characteristic of
greenschist facies assemblage (Figures 4A,B). Interbedded
metarhyolite contains quartz grains which show bulging;
plagioclase and K-feldspar still preserve magmatic habit
(Figure 4H); the amphibolite band consists of hornblende,
albite, and epidote (Figure 4I). Pratapgarh-Sumel granite
shows growth of mica and grain boundary migration in
quartz; feldspar retains magmatic habit (Figure 4J). All these
corroborate to greenschist facies of metamorphism that
represents the peak metamorphism in the area (M1)
syntectonic with the D1 deformation. The Sewariya granite
shows dynamic recrystallization of plagioclase and K-feldspar
(Figures
4M,N,P)
indicating
higher
temperature
of
metamorphism (M1), nearly 700°C (Pryer, 1993; Kruhl, 1996;
Rosenberg and Stuniz, 2003), but under similar pressure condition
as greenschist facies. The D2-D3 indicates greenschist facies
metamorphic condition. This is indicated by the crystallization of
mica along the axial plane of the folds. The pelitic gneiss slivers within
metaconglomerate show upper amphibolite facies metamorphism as
they contain garnet-staurolite-tourmaline-quartz-feldspar-sillimanite
assemblage (Figures 4C,E,F). During D2 shearing, fluid activity along
the thrust retrograded sillimanite to muscovite, staurolite to biotite,
and hornblende to epidote (Figures 4E–G).
et al., 2006). As the closure temperature of monazite for Th-Utotal Pb is ca. 800 °C, each domain retains its age of formation and
can be used as geochronometer (Cherniak et al., 2004; Cherniak
and Pyle, 2008).
Geochemistry
Geochemistry of granite and metarhyolite was carried out with 15
samples collected at 100 m interval across the strike of the pluton
(Figures 2B,C). Major elements were analyzed for all samples;
however, trace element for seven samples was carried out in an
ICP-AES (ARCOS, Simultaneous ICP Spectrometer) at the
Sophisticated Analytical Instrument Facility, Indian Institute of
Technology, Bombay, adopting the following procedure, and
discriminatory diagrams were plotted (Table 2, Figure 5). A
0.25 g of powdered sample was fused with 0.75 g lithium
metaborate and 0.50 g lithium tetraborate in platinum
crucibles at a temperature of 1,050°C in a muffle furnace.
After cooling, the crucible was immersed in 80 ml of 1 M
HCl contained in a 150 ml glass beaker and then magnetically
stirred until the fusion bead dissolved completely. Then the
sample volume was made up to 100 ml in a standard
volumetric flask. The same procedure was adopted to make
standard solutions and blank sample. The standards used for
the analyses include BIR-1 (Basalt), BHVO-2 (Basalt), BCR-2
(Basalt), AGV-2 (Andesite), and GSP-2 (Granodiorite) (USGS
Reference standards, 2005). Major and trace elements and LOI
were estimated from the above solution by ICP-emission
spectrometry.
MATERIALS AND METHOD FOR
GEOCHEMISTRY AND GEOCHRONOLOGY
Principle
Zircon U-Pb Geochronology
Geochronology, geochemistry, and tectonic fabric of a granite
serve as important proxies for dating an orogeny. For example,
rifting of continent creates synsedimentary bimodal volcanism
and granite intrusion in the basin (e.g., New England Orogen,
Allen et al., 1998; Liu and Han, 2019). Further, when the basin
closes through subduction/collision the rocks undergo high grade
metamorphism and melting at depth producing synorogenic
granites (Finnacca et al., 2019). Postorogeny granite is
emplaced in extensional setting (Pearce, 1996; Zhang et al.,
2018). Granites intruding at different period of orogenesis are
distinguished by their geochemical signature and tectonic fabric.
For instance, the preorogenic granite reflects extensional or rift
setting geochemistry and it undergoes ductile deformation
recording multiple solid state deformation fabric during
orogenesis. Synorogenic granite shows S type geochemistry
and contains magmatic/semimagmatic fabric coupled with
solid state deformation fabric (Bouchez et al., 1990; Chappell
and White, 1992; Paterson et al., 1998). Postorogenic granite lacks
solid state deformation fabric. The granites are dated by zircon
geochronology because zircon has the higher closure temperature
to U-Th -Pb system (>900°C.; Dahl, 1997; Cherniak and Watson,
2001). However, zircon cannot reset the age in response to low
temperature shearing. For dating fluid induced low temperature
deformation, monazite geochronology is most suitable. Monazite
crystallizes in response to fluid activity (alkali rich) at different
period of deformation and develops compositional domains
corresponding to each phase of growth (Williams and
Jercinovic, 2002; Pyle et al., 2003; Foster et al., 2004; Mahan
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Zircon geochronology is performed on four granite and
metarhyolite samples, collected one from each pluton
(Sewariya pluton, near Bar, Figure 2A, Sample No TKB-1;
Rupnagar metarhyolite, Sample No TKB-2; Pratapgarh pluton,
Sample No TKB-3; Sumel pluton, Sample No TKB-4, Figures
2B,C). Textural study of zircon was carried out on CL images
(Figure 6). Magmatic zircon was distinguished by its euhedral
shape with rings, from xenocrystic zircon that has irregular
geometry (Biswal et al., 2007). A U-Pb isotope of the zircon
was estimated on the SHRIMP ion microprobe at the John de
Laeter Center for Mass Spectrometry at Curtin University in
Perth, Australia. Zircon of all shapes and sizes was handpicked
and mounted in epoxy resin together with natural zircon
standard BR266 (Stern, 2001), TEMORA-2 (Black et al., 2003,
2004), and CZ3 (Pidgeon et al., 1994). The samples were loaded in
the SHRIMP sample lock 24 h prior to analysis and pumped to ∼5
× 10–7 Torr to allow degassing. Analytical procedure of the
SHRIMP follows methods as described in detail by Claoue-Long
(1994). Working conditions for both sessions included a primary
beam current of 2–3 nA, slightly elliptical spot size of ∼25–30 μm,
the sensitivity of >20 counts per ppm Pb and per nA primary
beam current, and a mass resolution of >4,500. Measurements
were conducted on Zr2O+, 204Pb+, background, 206Pb+,
207Pb+, 208Pb+, 238U+, 232ThO+, and 238UO2 + in sets of
six scans, with a total analysis time of about 15 min per sample
spot. Analyses of unknown and BR266 standard zircon were
interspersed at a ratio 3:1, allowing calibration of 238U/206Pb
11
February 2021 | Volume 8 | Article 594355
TKB12
LOCATION
12
TKB14
TKB16
TKB17
1G2
Rupnagar metarhyolite
2G2
3G2
4G2
5G2
SP1
SP3
Pratapgarh granite
SP4
SP5
SP6
Sewariya granite
78.126
0.126
10.688
0.91
0.017
0.244
0.479
4.533
3.868
0.008
0.43
99.429
70.652
0.27
15.832
0.855
0.017
0.304
0.338
4.328
6.394
0.008
0.61
99.608
70.527
0.378
15.126
0.408
0.008
0.286
1.279
5.595
5.376
0.017
0.51
99.51
76.597
0.037
11.559
0.989
0.018
0.167
0.056
5.891
3.677
0.009
0.16
99.16
74.551
0.018
12.621
0.731
0.009
0.157
0.129
5.599
5.174
0.009
0.11
99.108
75.512
0.038
12.835
0.674
0.052
0.058
0.792
3.751
4.928
0.012
0.26
98.912
75.683
0.044
12.266
0.921
0.057
0.055
0.772
3.812
4.537
0.011
0.14
98.298
76.523
0.033
12.48
0.585
0.043
0.051
0.797
3.928
4.377
0.011
0.36
99.188
76.318
0.032
12.589
0.519
0.047
0.05
0.806
3.834
4.756
0.01
0.11
99.071
75.316
0.034
12.872
0.759
0.053
0.044
0.758
3.853
4.779
0.013
0.28
98.761
75.476
0.09
12.786
0.813
0.023
0.196
0.533
2.762
5.543
0.151
0.53
98.903
73.318
0.249
13.733
1.038
0.033
0.571
1.612
3.178
4.022
0.17
0.54
98.464
75.827
0.139
12.452
0.74
0.024
0.219
0.822
2.847
5.274
0.173
0.47
98.987
76.296
0.14
12.247
0.975
0.016
0.215
0.466
2.165
5.406
0.121
0.99
99.037
78.683
0.176
10.814
0.921
0.03
0.284
0.686
2.497
3.922
0.144
0.47
98.627
37.88752
0
0.443306
2.632737
1.289501
0.263727
0
33.43823
22.87025
0
0.023169
0.042783
0
0
0
0
98.89123
0.917054
0.853185
1.171923
10.99853
0
0
1.698851
0.591337
0.607271
0.473099
34.71066
49.58176
0
0.023169
0.042783
0
0
0.272796
0
99.00026
#DIV/0!
#DIV/0!
0.676885
15.53595
0.256329
0
0
1.558147
0.904838
0
47.38544
31.79379
1.117903
0.046339
0.021391
0
0
0.41
0
99.03013
1.005838
0.870921
1.040737
33.06094
0
1.772851
2.864186
0.133484
0.042878
0.361549
38.97099
21.74742
0
0.023169
0.042783
0
0
0
0
99.02026
0.844858
0.838606
1.602121
27.45183
0
2.062275
2.111976
0.427471
0.021439
0.200356
36.12746
30.55276
0
0.023169
0.021391
0
0
0
0
99.00013
0.851298
0.838016
1.082141
33.12
0
0
0
0
0
0.07
35.37
29.13
0
0.02
0.08
0
0
0.64
0.05
98.48
1.114304
0.99028
0.761161
34.6
0
0
0
0.32
0
0
35.21
26.83
0.16
0.02
0.08
0
0
0.86
0.08
98.16
1.095718
0.973444
0.840203
35.23
0
0
0
0.27
0
0
36.73
25.88
0.03
0.02
0.06
0
0
0.56
0.04
98.82
1.113122
0.985519
0.897418
34.23
0
0
0
0.27
0
0
35.51
28.13
0.21
0.02
0.06
0
0
0.47
0.08
98.98
1.09772
0.973147
0.80614
32.97
0
0
0
0
0
0.1
36.28
28.25
0
0.02
0.06
0
0
0.71
0.08
98.47
1.116922
0.99743
0.806236
37.21
0
0
0
0
0
0.5
25
32.74
0
0.35
0.04
1.65
0.07
0.81
0
98.37
1.210947
1.108981
0.498286
35.62
0
0
0
0
0
1.42
33.78
23.76
0
0.39
0.06
1.63
0.22
1.04
0
97.92
1.431621
1.096118
0.790154
37.48
0
0
0
0
0
0.55
27.07
31.14
0
0.39
0.04
0.97
0.12
0.74
0
98.5
1.196545
1.046049
0.539818
41.98
0
0
0
0
0
0.55
19.91
31.97
0
0.28
0.04
2.26
0.12
0.98
0
98.09
1.299022
1.191731
0.400481
47.64
0
0
0
0
0
0.7
23.66
23.17
0
0.32
0.06
1.53
0.15
0.92
0
98.15
1.29296
1.124905
0.636665
7.384
285.071
0
182.216
126.639
96.495
74.883
70.464
386.648
13.125
574.892
24.9
46.218
9.284
332.427
162.717
112.515
138.276
205.082
162.463
74.695
638.98
12.021
1,498.938
30.258
34.162
8.776
188.665
0
199.345
0
122.232
170.489
50.537
670.185
11.862
1,115.323
14.419
12.442
10.555
259.347
0
0
0
103.374
4.963
69.944
100.038
12.069
453.445
5.881
9.52
5.775
166.635
55.31
0
0
142.536
5.551
37.433
76.231
11.659
599.166
5.722
2.782
3.221
ND
ND
ND
54.326
119.429
23.97
91.352
48.759
12.365
475.123
1.54
13.498
2.665
ND
ND
ND
41.451
122.329
16.476
82.352
51.271
12.621
475.123
1.53
12.319
3.618
ND
ND
ND
38.787
ND
14.568
ND
36.238
ND
ND
ND
ND
3.599
ND
ND
ND
37.017
ND
17.496
ND
43.722
ND
ND
ND
ND
3.471
ND
ND
ND
40.96
ND
16.526
ND
43.943
ND
ND
ND
ND
2.445
ND
ND
ND
46.628
ND
11.374
ND
29.725
ND
ND
ND
ND
4.73
ND
ND
ND
57.674
ND
48.241
ND
130.46
ND
ND
ND
ND
2.823
2.793
3.461
ND
ND
ND
ND
ND
ND
ND
ND
ND
56.232
58.329
65.825
ND
ND
ND
3.038
13.344
0.933
ND
ND
ND
59.312
69.237
92.05
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
(Continued on following page)
Age of South Delhi Orogeny
February 2021 | Volume 8 | Article 594355
SiO2
TiO2
Al2O3
Fe2O3(t)
MnO
MgO
CaO
Na2O
K2O
P2O5
LOI
SUM
CIPW
Quartz
Anorthite
Na2SiO3
Acmite
Diopside
Sphene
Hypersthene
Albite
Orthoclase
Wollastonite
Apatite
Ilmenite
Corundum
Rutile
Hematite
Magnetite
SUM
A/NK
A/CNK
Na2O/K2O
Trace elements
Sc
Cr
Ni
Cu
Zn
Rb
Sr
Y
Zr
Nb
Ba
Th
La
TKB13
Singh et al.
Frontiers in Earth Science | www.frontiersin.org
TABLE 2 | Major element geochemistry of the granites. Few samples have been analyzed for trace elements. LOI is negligible (<0.001%).
ratios and U content using an age of 559 Ma and U content of
909 ppm (Stern, 2001). TEMORA-2 and CZ3 were used as
control standards and yielded 206Pb/238U ages within the
error of those reported for them (Pidgeon et al., 1994; Black
et al., 2003, 2004). Common Pb correction is based on measure
nonradiogenic 204Pb isotope, and a common Pb composition
applied following the Pb-evolution model of Stacey and Kramers
(1975). Because analyses that recorded high counts on 204Pb
during the first scan were aborted, corrections are small and
insensitive to the choice of common Pb composition.
Nevertheless, some analyses were characterized by very low
contents of U, which, combined with the relatively young age,
lead to low amounts of radiogenic Pb. In these cases, proportions
of common Pb can become relatively high, even though counts on
204Pb were barely above background. Because of the low signalto-noise ratio of the 204Pb signal, 204-correction suffers from
imprecision particularly in these cases, and we, therefore, report
uncorrected ratios in the table (Table 3) and in some cases used
these uncorrected values to regress the data to common Pb and
constrain an intercept age. Standard calibration errors are
reported in Table 3 but were not included in single spot ages
and pooled age calculations. Single spot ages are reported at the
1σ confidence level, while pooled ages are reported at 95%
confidence (Figure 7).
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
Sewariya granite
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
SP5
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
Monazite geochronology was carried out on monazite grains in
the mica schist samples (sample location in Figures 2B,C; BSE
and X-ray images in Figures 8, 9, analytical data in Table 4).
Monazite developed during metamorphism of the mica schist.
Either a detrital monazite was recrystallized or elements like REE,
Th, U, and P present in different minerals reacted in Ca poor
condition producing monazite grains (Wawrzenitz et al., 2012).
Crystallization of monazite occurred along with other minerals
like quartz, feldspar, and biotite. The monazite grains have equant
shape and are aligned parallel to S1 fabric that results from
dislocation creep, Sample No. B5 and R4 (cf. Passchier and
Trouw, 2005). Subsequently, fluid action along the shear zone
reprecipitates the monazite by dissolution precipitation creep
(e.g., Wawrzenitz et al., 2012). Those monazites are elliptical in
shape and aligned parallel to the mylonitic fabric, Sample No.
CG1 and B4. Further, monazites undergo reprecipitation during
brittle deformation due to fluid action and those monazites
contain fractures. All these microfabrics of monazite in
relation to host rock fabric were studied under BSE images
and the events of crystallization/precipitation of monazite were
identified.
Nine samples were analyzed, and 194 spot analyses were
performed. Finally, we presented four representative samples
(CG1, B4, B5, and R4) where the monazite shape could be
unequivocally correlated with a particular deformation event.
We used the Cameca SX-FIVE Electron Probe Micro-Analyzer
(with five WDS spectrometers including LLIF and LPET crystals)
for monazite geochronology, in the Department of Earth
Sciences, IIT, Bombay. The single point/average method was
used for finding the dates of the monazite grains (Montel
et al., 1996). The age analyses were conducted at an
46.429
31.189
9.21
7.264
10.121
0.659
9.488
2.789
13.697
3.322
10.855
1.747
10.332
170.6
0.134614
0.530299
41.874
30.256
7.64
6.12
10.491
0.981
10.676
1.984
12.654
3.189
11.298
1.894
11.255
162.631
0.11278
0.622572
Pratapgarh granite
Th-U-Total Pb Monazite Geochronology
11.907
1.8
6.384
2.663
0.802
2.952
0.951
14.104
3.272
9.087
1.726
9.609
1.591
69.63
0.180173
0.067356
31.721
5.156
22.122
8.384
0.894
6.874
1.99
4.034
1.13
2.806
0.832
3.897
0.89
100.25
1.102175
0.027006
37.524
3.732
13.274
3.679
1.382
3.975
1.243
10.228
2.435
7.342
1.311
6.772
1.211
106.55
1.058644
0.08498
106.703
9.198
34.826
73.627
2.074
8.284
2.178
4.551
1.203
2.869
0.737
2.33
0.57
283.312
6.175502
0.010563
104.05
11.828
44.072
9.675
1.581
10.259
2.35
4.223
1.187
2.73
0.772
2.403
0.608
241.956
7.832698
0.037274
Rupnagar metarhyolite
LOCATION
Ce
Pr
Nd
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
Σ REE
(La/Lu)N
Eu/Eu*
SP4
SP3
SP1
5G2
4G2
3G2
2G2
1G2
TKB17
TKB16
TKB14
TKB13
TKB12
TABLE 2 | (Continued) Major element geochemistry of the granites. Few samples have been analyzed for trace elements. LOI is negligible (<0.001%).
Age of South Delhi Orogeny
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
SP6
Singh et al.
Frontiers in Earth Science | www.frontiersin.org
13
February 2021 | Volume 8 | Article 594355
Singh et al.
Age of South Delhi Orogeny
FIGURE 5 | (A) A/NK vs. A/CNK plot, (B) Na2O vs. K2O, (C) TiO2 vs. Zr plot, peraluminous, meta-aluminous, peralkaline, I type, S type granite fields are after
Chappell and White (1992), (D) Na2O + K2O-CaO vs. SiO2 plot, alkalic-alkali-calcic, calc-alkalic, and calcic fields are after Frost et al. (2001), (E) Rb vs. Y + Nb plot within
plate, syncollisional, volcanic arc, oceanic ridge granite fields are after Pearce (1996), (F) Na2O + K2O–(CaO + MgO)*5 Fe2O3 (t) * 5 ternary plot, fields of A1, A2, rift, island
arc granites are after Grebennikov (2014).The fields of Chang and Sewariya plutons in Fig. 5D and 5E are after Tiwana et al., 2019 and Ray et al. (2015),
respectively.
accelerating voltage of 20 keV and a 200 nA prob current with
1μm beam diameter (Wawrzenitz et al., 2012). X-ray element
mapping for Ce, La, Y, Pb, Th, and U in monazite was acquired
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with an accelerating voltage of 20 keV, beam current of 100 nA,
and spatial resolution of 1–3 μm/pixel dwell times varying
between 50 and 80 ms/pixel. Both natural and synthetic glass
14
February 2021 | Volume 8 | Article 594355
Singh et al.
Age of South Delhi Orogeny
FIGURE 6 | CL images of zircon, sample numbers TKB-1, TKB-2, TKB-3, and TKB-4.
standards were used in calibrating major and trace elements in
monazite. PbMa, ThMa, and UMb spectral lines were calibrated
with crocoite (PbCrO4), Th glass (ThO2-5 wt.%), and U glass
(UO2-5 wt.%) standards and simultaneously analyzed in two
spectrometers for 240, 160, and 160 s, respectively, using
subcounting methodology (cf. Spear and Wark, 2009;
Prabhakar, 2013). The total counts of PbMa were acquired in
the exponential mode to better define the distantly located
background positions (Jercinovic and Williams, 2005;
Jercinovic et al., 2008; Spear and Wark, 2009; Gonçalves et al.,
2016). Background values for Th, U, Pb, and K are calculated
from a nonlinear regression of high-precision wavelength
dispersive scans (Jercinovic et al., 2008; Williams et al., 2006).
Background values for the rest of the elements are based on linear
interpolation of intensities between paired off-peak wavelength
positions. The matrix effects (ZAF) were reduced with X-PHI
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method (Merlet, 1992). The significant peak interference of
ThM2-O4, ThMζ1, ThMζ2, YLC2, YLC3, and LaLa on PbMa
and ThMc and ThM3-N4 on UMb was corrected during
quantification following the values given in Supplementary
Table S1 (Tiwari and Biswal, 2019a). More details on
monazite dating protocol and interference corrections were
outlined by Pant et al. (2009), Prabhakar (2013), Deshmukh
et al. (2017), Chatterjee et al. (2017), Pandey et al. (2013), and
Tiwari and Biswal, (2019a). By applying these conditions,
detection limit was achieved as 100 ppm for Th, 110 ppm for
U, and 80 ppm for Pb.
BSE images and X-ray mapping of all monazite grains in
individual samples were documented. Individual domains were
identified from such images, and two to three points in each
domain were chemically analyzed (Table 4). PbO vs. ThO2*
diagram plotted (ThO2* measured ThO2 plus ThO2 equivalent
15
February 2021 | Volume 8 | Article 594355
Spot
16
ppm (U)
Ppm (Th)
232Th/
238U
±%
206Pb/
238U (Age)
207Pb/
206Pb (Age)
208Pb/
232Th (Age)
0.06
0.09
0.45
0.06
2.02
1.49
0.15
0.13
0.37
0.00
0.09
0.09
0.33
0.15
0.28
0.10
0.08
296
190
170
151
250
287
292
132
300
127
184
434
213
118
187
162
193
86
56
101
55
94
470
137
72
70
101
61
209
117
60
88
83
78
0.30
0.30
0.61
0.38
0.39
1.69
0.49
0.56
0.24
0.82
0.34
0.50
0.57
0.53
0.49
0.53
0.42
0.88
1.11
0.34
0.42
7.65
2.85
0.82
0.39
1.13
4.14
0.39
1.39
6.25
0.47
1.06
0.36
0.33
895
881
887
882
832
744
885
873
857
885
879
1,619
849
874
884
884
879
±18
±18
±18
±18
±21
±16
±17
±23
±20
±22
±18
±32
±17
±18
±21
±18
±18
897
874
793
888
954
929
879
793
880
874
910
1,634
873
902
844
899
887
±23
±30
±49
±32
±180
±69
±27
±40
±36
±33
±30
±11
±38
±42
±54
±32
±28
892
920
841
889
664
501
868
846
872
913
886
1,497
308
883
872
887
864
0.14
0.41
0.04
0.35
0.13
0.16
0.03
0.18
0.13
0.01
0.26
0.03
–
0.06
9.51
0.29
0.36
0.12
0.03
0.81
-0.05
0.06
0.06
157
115
197
151
207
330
206
195
355
483
283
230
443
198
158
149
307
220
193
373787
292
137
230
386
147
101
185
134
346
355
212
227
472
681
367
207
512
194
139
113
239
226
183
2E+06
301
107
223
427
0.97
0.9
0.97
0.92
1.73
1.11
1.06
1.2
1.38
1.46
1.34
0.93
1.19
1.01
0.91
0.79
0.8
1.06
0.98
5.23
1.06
0.81
1
1.14
0.3
1.22
0.26
0.3
2.31
0.2
0.25
0.25
0.58
0.45
3.35
0.25
0.28
0.7
0.3
0.59
0.2
0.24
0.26
0.5
0.49
0.72
0.27
0.18
958
981
983
975
956
990
985
971
997
981
948
1,019
986
970
1,070
982
830
982
991
1,357
986
944
967
978
±19
±20
±21
±20
±19
±19
±24
±19
±19
±19
±19
±20
±19
±19
±27
±20
±18
±19
±20
±37
±19
±19
±19
±19
1,015
891
975
1,008
988
957
988
979
951
1,000
1,010
957
1,001
995
838
941
924
959
1,003
846
1,013
1,047
973
1,008
±31
±49
±23
±38
±26
±22
±22
±29
±20
±14
±27
±22
±14
±23
±221
±39
±28
±25
±23
±89
±18
±27
±23
±16
938
955
1,005
954
587
982
993
857
989
972
846
1,033
973
965
2095
965
161
973
979
1,099
971
950
950
964
% Discordant
207Pb*/
206Pb*
±%
207Pb*/
235U
±%
206Pb*/
238U
±%
±26
±31
±27
±35
±135
±20
±31
±30
±44
±76
±27
±40
±25
±29
±30
±25
±24
+0
−1
−13
+1
+14
+21
−1
−11
+3
−1
+4
+1
+3
+3
−5
+2
+1
0.0690
0.0682
0.0656
0.0686
0.0709
0.0700
0.0683
0.0656
0.0684
0.0682
0.0694
0.1006
0.0681
0.0691
0.0672
0.0690
0.0686
1.1
1.5
2.3
1.5
8.8
3.4
1.3
1.9
1.8
1.6
1.4
0.6
1.9
2.0
2.6
1.5
1.3
1.42
1.38
1.33
1.39
1.35
1.18
1.39
1.31
1.34
1.38
1.40
3.96
1.32
1.38
1.36
1.40
1.38
2.4
2.6
3.2
2.7
9.2
4.1
2.5
3.4
3.0
3.1
2.6
2.3
2.8
3.0
3.6
2.7
2.5
0.149
0.146
0.148
0.147
0.138
0.122
0.147
0.145
0.142
0.147
0.146
0.285
0.141
0.145
0.147
0.147
0.146
2.1
2.2
2.2
2.2
2.7
2.3
2.1
2.8
2.5
2.7
2.2
2.2
2.1
2.3
2.5
2.2
2.1
±24
±29
±25
±25
±19
±22
±27
±20
±22
±22
±35
±24
±21
±23
±88
±27
±8
±22
±23
±35
±22
±25
±22
±21
6
−11
−1
3
3
−4
0
1
−5
2
7
−7
2
3
−30
−5
11
−3
1
−67
3
11
1
3
0.073
0.0687
0.0716
0.0728
0.0721
0.071
0.0721
0.0717
0.0708
0.0725
0.0729
0.071
0.0725
0.0723
0.067
0.0704
0.0698
0.071
0.0726
0.0673
0.073
0.0742
0.0715
0.0728
1.5
2.4
1.1
1.8
1.3
1.1
1.1
1.4
1
0.7
1.3
1.1
0.7
1.1
10.6
1.9
1.3
1.2
1.1
4.3
0.9
1.4
1.1
0.8
1.61
1.56
1.63
1.64
1.59
1.63
1.64
1.61
1.63
1.64
1.59
1.68
1.65
1.62
1.67
1.6
1.32
1.61
1.66
2.17
1.66
1.61
1.6
1.64
2.7
3.3
2.6
2.9
2.5
2.3
2.8
2.6
2.3
2.2
2.5
2.4
2.2
2.4
11
2.9
2.7
2.4
2.4
5.2
2.3
2.6
2.4
2.2
(Continued on
0.16
2.2
0.164
2.2
0.165
2.3
0.163
2.2
0.16
2.1
0.166
2.1
0.165
2.6
0.163
2.1
0.167
2.1
0.164
2.1
0.158
2.1
0.171
2.1
0.165
2.1
0.162
2.1
0.18
2.8
0.165
2.2
0.137
2.3
0.165
2.1
0.166
2.1
0.234
3
0.165
2.1
0.158
2.2
0.162
2.1
0.164
2.1
following page)
Age of South Delhi Orogeny
February 2021 | Volume 8 | Article 594355
TKB1
TKB1.30
TKB1.29
TKB1.20
TKB1.24
TKB1.25
TKB1.26
TKB1.27
TKB1.19
TKB1.21
TKB1.22
TKB1.23
TKB1.14
TKB1.15
TKB1.16
TKB1.17
TKB1.18
TKB1.9
TKB2
TKB2.1
TKB2.2
TKB2.3
TKB2.4
TKB2.5
TKB2.6
TKB2.7
TKB2.8
TKB2.9R
TKB2.10R
TKB2.11C
TKB2.12
TKB2.14
TKB2.13C
TKB2.15
TKB2.16
TKB2.17
TKB2.18
TKB2.19
TKB2.20
TKB2.21
TKB2.22
TKB2.23
TKB2.24
% 206Pbc
Singh et al.
Frontiers in Earth Science | www.frontiersin.org
TABLE 3 | Zircon U-Pb SHRIMP data (2σ error of the mean of 0.94%) samples TKB-1 (26,015′50’’/74,013′06″), TKB-2 (26,010′36’’/74,017′30″), TKB-3 (26,012′24’’/74,018′15″), and TKB-4 (26,013′36’’/74,019′48″).
Spot
17
ppm (U)
Ppm (Th)
232Th/
238U
±%
206Pb/
238U (Age)
207Pb/
206Pb (Age)
208Pb/
232Th (Age)
% Discordant
207Pb*/
206Pb*
±%
207Pb*/
235U
±%
206Pb*/
238U
±%
-0.1
0.12
0.05
0.26
0.13
249
256
439
421
181
256
241
262
546
469
170
252
1
1.06
1.28
1.15
0.97
1.02
0.76
0.22
0.41
0.17
0.27
0.22
985
984
997
976
970
987
±19
±19
±19
±19
±19
±19
978
958
967
1,027
948
968
±18
±22
±17
±15
±33
±23
977
964
959
967
944
973
±23
±22
±21
±21
±23
±22
−1
−3
−3
5
−2
−2
0.0717
0.071
0.0713
0.0735
0.0707
0.0714
0.9
1.1
0.8
0.8
1.6
1.1
1.63
1.61
1.65
1.66
1.58
1.63
2.3
2.4
2.2
2.2
2.7
2.4
0.165
0.165
0.167
0.163
0.162
0.165
2.1
2.1
2.1
2.1
2.2
2.1
0.54
0.06
6.15
0.03
0.06
9.99
3.77
7.73
-0.03
0.29
0.05
0.02
21.71
0.39
0.06
–
1.12
–
2.95
1,248
1,200
524
248
120
500
1,383
1802
473
198
100
692
320
11,915
169
495
305
852
112
565
29
247
978
386
86
128
1,011
248
561
282
134
53
334
21,102
134
131
275
646
57
1,071
0.02
0.21
1.93
1.61
0.74
0.27
0.76
0.14
1.23
1.47
1.38
0.08
1.08
1.83
0.82
0.27
0.93
0.78
0.53
1.96
3.42
14
0.97
0.21
0.36
0.89
1.05
2.29
0.6
0.24
17
2.66
0.32
6.54
0.53
0.39
0.7
2.33
0.42
1.33
860
895
565
971
2,697
177
990
404
976
968
963
1,009
996
532
431
1,389
274
1,033
2,358
756
±16
±35
±18
±19
±49
±4
±19
±11
±19
±19
±22
±36
±19
±39
±9
±26
±6
±20
±45
±21
910
935
906
1,005
2,765
1,431
935
980
1,033
1,024
963
999
988
1,686
413
1,407
248
914
2,591
1,035
±26
±10
±102
±20
±9
±171
±51
±67
±14
±22
±50
±104
±18
±874
±79
±10
±44
±42
±11
±84
3,806
921
648
932
2,665
1,573
1,574
2,724
963
945
929
942
973
306
432
1,353
271
871
2,478
807
±484
±186
±24
±21
±61
±60
±55
±105
±21
±21
±225
±50
±21
±89
±14
±31
±8
±33
±61
±28
6
5
39
4
3
89
−6
61
6
6
0
−1
−1
71
−5
1
−11
−14
11
29
0.0694
0.0702
0.0692
0.0727
0.1927
0.0903
0.0702
0.0718
0.0737
0.0733
0.0712
0.0724
0.0721
0.1034
0.055
0.0892
0.0512
0.0695
0.1734
0.0738
1.2
0.5
5
1
0.5
9
2.5
3.3
0.7
1.1
2.5
5.1
0.9
47.3
3.5
0.5
1.9
2
0.6
4.1
1.37
1.44
0.87
1.63
13.8
0.35
1.61
0.64
1.66
1.64
1.58
1.69
1.66
1.23
0.53
2.96
0.31
1.67
10.56
1.27
2.4
4.2
6
2.3
2.3
9.3
3.2
4.3
2.2
2.4
3.5
6.4
2.3
47.9
4.1
2.1
3
3
2.3
5.1
0.143
0.149
0.092
0.163
0.519
0.028
0.166
0.065
0.163
0.162
0.161
0.169
0.167
0.086
0.069
0.24
0.043
0.174
0.442
0.124
2
4.2
3.3
2.1
2.2
2.3
2
2.7
2.1
2.1
2.5
3.9
2.1
7.6
2.2
2.1
2.3
2.1
2.3
2.9
0.14
2.65
0.27
0.07
0.45
23.43
0.12
30.89
36.8
35.62
11.61
37.03
3.43
0.48
33.63
2.67
167
2,576
30
776
1,468
1885
386
1,698
4,253
4,421
3,760
4,903
2,728
371
4,041
2,936
121
14
63
648
1,232
23
119
11
137
71
25
253
15
280
88
19
0.75
0.01
2.14
0.86
0.87
0.01
0.32
0.01
0.03
0.02
0.01
0.05
0.01
0.78
0.02
0.01
0.67
0.66
0.61
3.66
11
2.06
0.27
2.8
1.86
1.7
1.39
1.68
1.8
0.22
7.12
2.03
951
800
2,841
919
773
1,050
625
1,050
835
703
1,032
751
755
270
1,245
774
±19
±19
±67
±17
±35
±36
±12
±30
±20
±21
±35
±23
±31
±6
±33
±15
974
804
2,981
954
994
954
636
896
1,022
1,096
895
1,072
749
210
983
789
±32
±162
±17
±14
±33
±152
±29
±224
±95
±85
±112
±77
±298
±85
±288
±52
970
39.2
2,646
970
591
7,097
593
11,891
2,717
4,614
5,940
1780
-2,756
260
8,141
673
±26
±4,054
±93
±65
±87
±2,442
±18
±5,058
±587
±885
±2,862
±321
-9,261
±9
±2,535
±1,364
3
1
6
4
24
−11
2
−19
19
38
−17
32
−1
−30
−29
2
0.0716
0.0659
0.22
0.0709
0.0723
0.0709
0.0609
0.0689
0.0733
0.076
0.0689
0.0751
0.0642
0.0503
0.0719
0.0655
1.5
7.7
1.1
0.7
1.6
7.4
1.4
10.9
4.7
4.2
5.4
3.8
14.1
3.7
14.1
2.5
1.57
1.2
16.8
1.5
1.27
1.73
0.85
1.68
1.4
1.21
1.65
1.28
1.1
0.3
2.11
1.15
2.7
8.2
3.1
2.1
5.1
8.3
2.5
11.3
5.4
5.3
6.5
5
14.8
4.3
14.4
3.2
(Continued on
0.159
2.2
0.132
2.6
0.554
2.9
0.153
2
0.127
4.9
0.177
3.7
0.102
2.1
0.177
3
0.138
2.6
0.115
3.2
0.174
3.7
0.124
3.2
0.124
4.3
0.043
2.3
0.213
2.9
0.128
2
following page)
Age of South Delhi Orogeny
February 2021 | Volume 8 | Article 594355
TKB2.25
TKB2.26
TKB2.27
TKB2.28
TKB2.29
TKB2.30
TKB3
TKB3.1
TKB3.2
TKB3.5
TKB3.6
TKB3.7
TKB3.8
TKB3.23
TKB3.9
TKB3.10
TKB3.11
TKB3.12
TKB3.13
TKB3.14
TKB3.16
TKB3.24
TKB3.25
TKB3.20
TKB3.21
TKB3.18
TKB3.4
TKB4
TKB4.1
TKB4.2
TKB4.3
TKB4.4
TKB4.6
TKB4.5
TKB4.7
TKB4.17
TKB4.16
TKB4.15
TKB4.14
TKB4.13
TKB4.12
TKB4.18
TKB4.19
TKB4.20
% 206Pbc
Singh et al.
Frontiers in Earth Science | www.frontiersin.org
TABLE 3 | (Continued) Zircon U-Pb SHRIMP data (2σ error of the mean of 0.94%) samples TKB-1 (26,015′50’’/74,013′06″), TKB-2 (26,010′36’’/74,017′30″), TKB-3 (26,012′24’’/74,018′15″), and TKB-4 (26,013′36’’/
74,019′48″).
6.1
4.6
5.7
10.5
10.1
6
6.2
0.9
3.5
8.4
0.073
0.0716
0.0696
0.0762
0.0633
0.0678
0.0681
0.0797
0.0688
0.0696
1.78
1.41
2.11
1.55
1.47
1.03
1.06
2
1.22
1.31
7.3
5.2
6.3
11.4
10.5
6.4
6.8
2.5
4.2
8.7
0.177
0.142
0.22
0.148
0.168
0.11
0.113
0.182
0.129
0.136
4
2.5
2.6
4.4
2.7
2.2
2.6
2.4
2.3
2.1
of measured UO2). Data points are arrayed linearly to provide an
isochron; the slope of the line (m) is used to calculate the age of
the event (Suzuki and Dunkley, 2014).
Further, from the analytical data the spot age is computed
using the formulation of Montel et al. (1996). From the
individual age and associated error data, the inversevariance weighted mean and 2σ error were calculated using
Gaussian distribution and Isoplot logarithm tool (Sambridge
and Compston, 1994; Ludwig, 2012). The entire population of
unmixed ages of monazite of an individual sample was used to
construct a probability density diagram by Isoplot. Dates
constrained by weighted means of monazite dates in
Isoplot are interpreted to constrain the age of monazite
growth in single or multiple events depending upon
number of peaks.
RESULT OF GEOCHEMISTRY AND
GEOCHRONOLOGY
Geochemistry
−4
13
−44
21
−43
23
22
10
13
11
±%
3.67
11
2.11
0.62
7.05
0.85
13
1.67
0.58
5.35
0
0.06
0.01
0.01
0.01
0
0.01
0.38
0.01
0
1,050
858
1,284
888
1,003
674
688
1,077
780
822
±39
±20
±31
±37
±26
±14
±17
±23
±17
±16
1,014
976
916
1,101
718
862
872
1,190
893
916
±125
±94
±117
±211
±215
±124
±129
±17
±73
±173
21,350
1,371
4,094
10,152
2,880
3,283
2,110
1,173
3,657
5,697
±1924
±382
±3,981
±1913
±4,257
±3,244
±1,095
±59
±1,643
±7,863
Metarhyolite
A higher % of SiO2 (70.52–78.12%), moderate to high Fe2O3
(0.4–0.98), moderate to high Al2O3 (10.68–15.83), and higher
Na2O compared to K2O characterize the metarhyolite (Table 2).
The A/NK and A/CNK values are <1.0 and CIPW norm shows
presence of normative quartz, diopside, albite, and ilmenite. The
samples lie in the peralkaline field in the A/NK vs. A/CNK plot
(Figure 5A), I type field in the Na2O vs. K2O and TiO2 vs Zr plots
(Figures 5B,C) (cf. Chappell and White, 1992), and alkali-calcic
to alkalic field in Na2O + K2O-CaO vs. SiO2 plot (Figure 5D) (cf.
Frost et al., 2001). (La/Lu) N is extremely variable ranging from
0.11 to 7.83 suggesting low to moderate REE differentiation and
Eu/Eu* value ranges from 0.01 to 0.62 suggesting -ve Eu anomaly
(Table 2). Rb vs. Y + Nb plot indicates “within plate magmatism”
(Figure 5E) (Pearce, 1996) and Na2O + K2O–(CaO + MgO)*5
Fe2O3 (t) * 5 ternary plot (Figure 5F) shows the samples lie in
“intracontinental-continental margin and intracontinental rift
and continental hot spots setting” (Grebennikov, 2014). The
Zr values range in 48.75–670.18 ppm indicating Zr saturation
temperature is around 750 to 800°C (cf. Watson and Harrison,
1983).
Pratapgarh Pluton
Higher % of SiO2 (75.51–76.52), moderate to high Fe2O3
(0.51–0.92), moderate to high Al2O3 (12.26–12.87), and lower
Na2O compared to K2O characterize the Pratapgarh granite. It
shows A/NK > 1.0 and the A/CNK values <1.0 (Table 2). The
CIPW norm shows these are quartz, diopside, albite, and ilmenite
normative. The samples lie in the meta-aluminous field of the
A/NK vs. A/CNK plot (Figures 5A, I type field of the Na2O vs.
K2O and TiO2 vs. Zr plot (Figures 5B,C) (Chappell and White,
1992) and calc-alkaline field in Na2O + K2O-CaO vs. SiO2 plot
(Figures 5D) (cf. Frost et al., 2001). The (La/Lu) N is 0.11–0.13
suggesting low degree of REE differentiation and Eu/Eu* value is
0.53 and 0.62 suggesting -ve Eu anomaly (Table 2). Rb vs. Y + Nb
plot indicates “within plate magmatism” (Figure 5E) (Pearce,
TKB4.21
TKB4.22
TKB4.23
TKB4.24
TKB4.25
TKB4.26
TKB4.27
TKB4.28
TKB4.8
TKB4.10
36.39
29.78
4.91
26.09
27.17
3.45
11.75
–
15.46
3.38
3,691
3,045
3,647
2,334
2,506
2,210
3,309
324
2,441
1,281
17
181
23
18
24
10
39
118
17
4
±%
Spot
% 206Pbc
ppm (U)
Ppm (Th)
232Th/
238U
206Pb/
238U (Age)
207Pb/
206Pb (Age)
208Pb/
232Th (Age)
% Discordant
207Pb*/
206Pb*
207Pb*/
235U
±%
206Pb*/
238U
±%
Age of South Delhi Orogeny
TABLE 3 | (Continued) Zircon U-Pb SHRIMP data (2σ error of the mean of 0.94%) samples TKB-1 (26,015′50’’/74,013′06″), TKB-2 (26,010′36’’/74,017′30″), TKB-3 (26,012′24’’/74,018′15″), and TKB-4 (26,013′36’’/
74,019′48″).
Singh et al.
Frontiers in Earth Science | www.frontiersin.org
18
February 2021 | Volume 8 | Article 594355
Singh et al.
Age of South Delhi Orogeny
FIGURE 7 | Zircon U-Pb data for samples TKB-1, TKB-2, TKB-3, and TKB-4 locations are in Figure 2. Errors are at 1σ confidence level.
1996) and Na2O + K2O–(CaO + MgO)*5 Fe2O3 (t) * 5 ternary
plot (Figure 5F) shows the sample lies in “intracontinentalcontinental margin and intracontinental rift and continental
Frontiers in Earth Science | www.frontiersin.org
hot spots setting” (Grebennikov, 2014). Zr values range in
47–670 ppm indicating Zr saturation temperature is around
750 to 800 °C (cf. Watson and Harrison, 1983).
19
February 2021 | Volume 8 | Article 594355
Singh et al.
Age of South Delhi Orogeny
FIGURE 8 | Sample No. CG-1, (A) BSE image of mica schist mylonite with elliptical monazite grains, from Phulad thrust, from the inclination of the monazite grain, a
top-to-NW sense of shear is indicated, (B) 813 and 716 ages are obtained from the high Th and low Y domain, 590 Ma age from fractured part of the grain, which is low in
Th and high in Y (X-ray maps in (C and D)). (E) Isochron shows three ages 819, 680, and 588 Ma. (F) Probability curve shows three peaks as 811, 700, and 569 Ma. First
two ages from ‘e and f’ correspond to D2 and last age corresponds to brittle deformation. Sample No. B-4 (G and I) BSE images of the upper amphibolite slivers
containing elliptical and rounded monazite grains, within Phulad thrust. (H) Elliptical grain is fractured in the top end. The grain yields 706 Ma and 550 Ma indicating age of
D2 and brittle deformation. (J) Rounded grain does not show chemical variation and yields higher ages ca. 1,638 Ma that indicates pre-Delhi metamorphism. (K)
Isochron and (L) probability graph display two age clusters, 1,611–704 Ma and 1,609–686 Ma, related to pre-Delhi metamorphism and D2 ages.
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20
February 2021 | Volume 8 | Article 594355
Singh et al.
Age of South Delhi Orogeny
FIGURE 9 | Sample B-5, collected from mica schist: (A) Monazite under BSE image shows more or less equant shape and compositional domains. (B) The lighter
domain that occurs outside is Y poor and Th rich (X-ray images, (C), (D)) yielding higher ages as 886 Ma, indicating the age of the D1. The darker domains, at the center,
are Y rich and Th poor and yield 783 and 597 Ma ages, indicating D2 and brittle deformation ages, respectively; there is minor fracture at the edge (Fr); fluid migrated
through the fracture and rejuvenated the age. (E and F) Isochron and probability curve yield three clusters as 864, 718, 588 Ma, 846, 697, 564 Ma corresponding
to D1, D2 shearing and brittle shearing ages, respectively. Sample R-4. Mica schist near Pratapgarh, Monazites are nearly rounded to equant shape and have domains (G
to J). (K and L) Isochron and probability curve yield two distinct ages 865–720 Ma and 852–743 Ma corresponding to D1–D2 ages, respectively.
Frontiers in Earth Science | www.frontiersin.org
21
February 2021 | Volume 8 | Article 594355
CG1
Point
22
SiO2
P2O 5
CaO
FeO
Y2 O 3
La2O3
Ce2O3
Pr2O3
Nd2O3
Sm2O3
Eu2O3
Gd2O3
Tb2O3
Dy2O3
Tm2O3
PbO
ThO2
UO2
Total
Age
(Ma)
Age
err
0
0.001
0.022
0.018
0.014
0.025
0.066
0.006
0.82
0.674
0.034
0.106
0.06
1.108
0.051
0.003
0
0.072
0.041
0.081
0.148
0.388
0
0.003
0.011
0.038
0.199
0.084
0.028
0.039
0.1
0.04
0.024
7.47
0.123
0.006
0.265
0
0.009
0.035
0.071
0.189
0.264
0.224
0.331
0.795
1.064
0.77
0.791
0.338
2.464
0.479
0.534
0.424
0.37
0.378
0.29
1.084
0.75
1.224
0.599
1.08
0.405
0.783
0.35
0.24
0.218
0.421
0.706
0.659
0.178
0.156
0.304
0.76
0.676
79.071
1.918
0.19
0.183
0.29
0.255
0.201
1.312
0.313
30.409
30.521
30.39
29.168
29.061
30.016
30.954
31.537
26.47
30.105
28.723
30.062
28.809
29.892
28.749
28.37
28.833
28.442
29.386
27.888
28.069
27.717
29.954
30.135
30.144
29.646
31.316
26.397
29.877
27.823
27.642
28.625
28.489
0.451
36.814
29.417
29.193
29.44
30.266
30.594
30.372
31.427
0.999
0.653
0.66
0.532
0.724
0.513
1.214
0.68
0.952
1.137
1.355
0.939
0.665
0.704
0.574
1.957
1.63
2.213
1.52
1.969
1.452
1.604
1.461
0.588
0.576
1.298
0.715
0.624
0.721
0.576
0.761
1.596
1.522
0.133
7.744
0.445
0.567
0.457
0.754
0.801
0.697
0.746
0
0
0
0
0
0
0
0
0.008
0
0
0
0
0
0
0
0
0
0
1.984
0.776
0.804
0.023
0
0
0.457
0
0
0
0
0
0
0
5.17
0
0
0
0
0
0
0
0
1.344
1.365
1.095
0.455
0.487
0.375
1.416
1.438
1.516
1.476
1.535
1.285
1.299
1.223
1.356
1.241
1.485
1.552
1.692
1.42
1.456
1.542
1.499
1.463
1.718
1.397
0.633
0.681
1.516
1.645
1.561
1.574
1.574
0
1.321
0.436
0.536
0.444
1.602
1.518
1.399
1.05
15.06
16.024
16.13
16.525
16.045
16.854
14.189
15.419
14.435
14.302
13.691
14.808
15.42
15.574
15.744
12.346
12.754
11.918
13.338
12.352
13.742
12.977
14.264
15.837
15.879
14.817
16.422
15.756
15.748
15.415
15.046
13.245
13.33
0.316
15.024
12.82
11.592
16.927
15.456
15.805
16.345
16.055
29.664
29.913
30.282
30.304
29.178
30.729
27.668
30.033
28.285
27.81
26.481
28.548
29.344
29.883
29.792
22.619
24.716
23.254
26.485
24.176
26.783
25.589
27.389
30.094
29.496
29.692
29.64
29.459
29.968
29.976
28.516
25.929
26.176
2.367
29.11
30.205
30.067
30.605
29.837
29.937
30.252
30.507
3.263
3.349
3.269
3.283
3.131
3.228
3.106
3.174
3.051
3.105
3.037
3.156
3.169
3.167
3.283
2.771
2.946
2.665
2.991
2.684
2.984
2.933
2.912
3.271
3.232
3.033
3.2
3.242
3.296
3.368
3.195
2.942
2.939
0.106
2.377
3.733
3.757
3.348
3.185
3.208
3.254
3.178
11.397
11.454
11.701
11.635
11.04
11.649
10.891
11.321
11.01
10.625
10.538
11.17
11.132
11.378
11.247
10.275
10.379
9.607
10.569
9.602
10.569
10.443
10.659
11.29
11.045
11.04
11.492
11.61
11.259
11.78
11.316
10.572
10.668
0.183
11.6
14.41
14.999
11.656
11.444
11.199
11.401
10.743
2.222
2.244
2.147
1.816
1.735
1.776
2.178
2.335
2.281
2.2
2.183
2.279
2.247
2.321
2.326
2.239
2.243
2.081
2.259
2.084
2.194
2.163
2.226
2.215
2.252
2.275
1.888
1.955
2.199
2.294
2.226
2.204
2.167
0.073
2.349
2.875
3.095
1.988
2.373
2.378
2.373
2.24
0.248
0.251
0.234
0
0.009
0.093
0.305
0.349
0.329
0.335
0.262
0.316
0.296
0.356
0.277
0.294
0.28
0.276
0.34
0.29
0.272
0.342
0.308
0.289
0.234
0.315
0
0.046
0.281
0.309
0.294
0.235
0.301
0.005
0.341
0.586
0.705
0.07
0.336
0.381
0.309
0.312
1.05
1.033
1.077
0.654
0.708
0.663
1.173
1.191
1.139
1.368
1.277
1.235
1.188
1.222
1.162
1.724
1.56
1.569
1.416
1.361
1.191
1.248
1.308
1.132
1.079
1.149
0.833
0.836
1.097
1.052
1.153
1.296
1.316
0
1.328
0.876
1.02
0.808
1.425
1.335
1.117
1.121
0.053
0.039
0.006
0
0
0
0.046
0.048
0.018
0.081
0.034
0.003
0.045
0.036
0.009
0.055
0.03
0.069
0.05
0.062
0.087
0.044
0.095
0.003
0.004
0
0
0.021
0
0.045
0.026
0.077
0.058
0
0.059
0
0
0
0
0.059
0.071
0.022
0.316
0.304
0.306
0.075
0.07
0.093
0.406
0.379
0.328
0.485
0.413
0.337
0.334
0.383
0.359
0.427
0.419
0.521
0.507
0.443
0.347
0.339
0.409
0.37
0.437
0.33
0.117
0.19
0.305
0.339
0.328
0.372
0.387
0.27
0.487
0
0.025
0.059
0.423
0.445
0.324
0.255
0.083
0.157
0.124
0.063
0.04
0.015
0.119
0.12
0.15
0.088
0.116
0.095
0.095
0.148
0.104
0.133
0.153
0
0.117
0.118
0
0
0
0.084
0.03
0.078
0.061
0.056
0.125
0.162
0.121
0.118
0.197
0.036
0.174
0.192
0.183
0.149
0.114
0.204
0.12
0.138
0.143
0.094
0.092
0.16
0.237
0.149
0.208
0.117
0.143
0.224
0.27
0.166
0.118
0.097
0.079
0.436
0.327
0.495
0.277
0.397
0.246
0.199
0.265
0.096
0.163
0.137
0.148
0.14
0.05
0.036
0.091
0.311
0.298
0.031
0.093
0.022
0.016
0.032
0.099
0.072
0.051
0.099
3.798
3.084
3.407
5.053
6.229
4.519
5.876
2.821
3.571
4.886
7.286
4.629
2.869
2.614
2.073
12.449
8.404
12.361
6.664
11.001
6.159
6.089
6.533
2.643
1.792
2.389
4.201
4.746
2.158
1.64
3.342
8.404
7.445
5.56
2.635
1.403
1.076
1.489
3.15
2.291
1.677
2.749
0.252
0.067
0.074
0.126
0.17
0.106
0.399
0.426
0.55
0.762
0.561
0.42
0.463
0.406
0.468
0.009
0.281
0.325
0.697
0.402
0.618
0.513
0.635
0.566
1.815
0.487
0.204
0.128
0.098
0.113
0.138
0.411
0.365
0.734
0.817
0
0
0
0.265
0.207
0.179
0.146
100.566
100.776
101.345
100.662
99.945
101.573
101.005
101.732
97.519
100.141
98.33
99.976
97.922
100.89
97.943
98.431
97.189
98.643
98.947
99.394
97.499
95.716
100.293
100.319
100.125
99
101.774
96.63
98.905
96.769
96.16
98.711
97.933
101.975
114.313
97.614
97.28
97.764
100.993
100.67
101.324
101.29
720
663
590
686
813
716
677
649
621
705
692
647
627
577
515
815
817
858
722
752
702
601
718
502
496
796
710
635
475
419
564
743
803
93
414
370
349
504
576
565
526
719
46
52
47
40
36
44
32
47
39
33
27
36
45
46
46
22
28
22
28
23
30
27
29
39
27
53
43
39
52
53
43
26
29
21
33
67
74
70
44
53
60
58
0
0
0.001
0
0.302
0.325
0.37
0.281
30.646
30.743
30.623
30.514
1.808
1.797
1.823
1.537
0
0
0
0
1.191
1.172
1.139
1.496
13.522
13.755
13.86
15.219
26.641
26.86
26.838
27.433
3.09
3.089
3.041
2.905
10.229
10.176
10.113
9.677
2.101
2.04
1.971
1.813
0.059
0.087
0.113
0.009
1.089
1.123
1.014
0.887
0
0
0
0
0.309
0.286
0.21
0.299
0.109
0.147
0.124
0.069
0.71
0.738
0.753
0.663
7.227
7.056
7.118
5.92
0.838
99.871
1,585
37
0.902 100.297 1,632
37
0.933 100.044 1,638
36
0.994
99.715
1,595
38
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Age of South Delhi Orogeny
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Al2O3
Singh et al.
Frontiers in Earth Science | www.frontiersin.org
TABLE 4 | EPMA analytical result of monazite for the samples CG-1 (26,014′12’’/,74,013′25″), B-4 (26,013′44’’/,74,013′12″), B-5 (26,013′27’’/,74,013′12″), and R-4 (26,012′56’’/,74,018′30″).
CG1
Point
23
SiO2
P2O 5
CaO
FeO
Y2 O 3
La2O3
Ce2O3
Pr2O3
Nd2O3
Sm2O3
Eu2O3
Gd2O3
Tb2O3
Dy2O3
Tm2O3
PbO
ThO2
UO2
Total
Age
(Ma)
Age
err
0.006
0
0.014
0.016
0.016
0.01
0
0.034
0
0.013
0
0.007
0
0.02
0
0.027
0.004
0.002
0.307
0.274
0.25
0.308
0.195
0.297
0.152
0.191
0.338
0.324
0.265
0.266
0.302
0.43
0.368
0.166
0.226
0.213
30.645
30.53
29.996
29.902
30.181
29.769
30.164
29.689
30.156
30.85
30.64
30.642
30.702
30.635
30.14
30.625
31.387
30.896
1.54
1.492
0.63
0.928
1.128
0.877
1.081
1.287
0.669
0.898
0.874
1.297
0.937
1.183
0.956
0.924
0.289
0.484
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
1.446
1.428
1.242
1.25
1.366
1.291
1.609
1.458
0.961
0.979
0.901
1.459
1.144
1.122
1.071
1.386
0.892
1.062
15.183
15.85
14.741
14.604
14.711
14.639
13.985
14.184
14.989
15.462
15.043
14.23
14.891
14.753
15.154
13.893
11.546
12.785
27.451
27.684
29.557
28.792
28.234
28.825
27.78
27.827
29.132
29.602
29.645
28.215
29.156
28.538
29.576
28.836
29.566
29.858
2.97
2.982
3.186
3.136
3.049
3.117
3.012
3.149
3.211
3.113
3.311
3.084
3.227
3.108
3.128
3.337
3.781
3.728
9.704
9.189
11.252
10.99
10.201
10.773
10.735
10.591
10.936
10.931
11.302
10.668
10.833
10.664
10.915
11.972
15.674
13.432
1.807
1.601
2.359
2.245
2.15
2.299
2.182
2.145
2.237
2.196
2.284
2.086
2.197
2.155
2.257
2.418
3.165
2.564
0
0
0.298
0.302
0.278
0.236
0.436
0.366
0.257
0.157
0.299
0.279
0.182
0.151
0.25
0.414
0.597
0.51
0.927
0.843
1.456
1.481
1.435
1.388
1.27
1.254
1.359
1.42
1.331
1.282
1.325
1.265
1.313
1.327
1.392
1.285
0.033
0.058
0.067
0.11
0.055
0.062
0.05
0.054
0.061
0.032
0.066
0.076
0.11
0.12
0.085
0.042
0
0.001
0.371
0.355
0.466
0.476
0.419
0.439
0.435
0.397
0.383
0.396
0.346
0.484
0.401
0.354
0.403
0.426
0.351
0.451
0.11
0.151
0.115
0.141
0.147
0.142
0.139
0.147
0.123
0.094
0.089
0.143
0.087
0
0.079
0.167
0.125
0.095
0.668
0.692
0.099
0.188
0.204
0.145
0.214
0.202
0.102
0.15
0.147
0.221
0.142
0.183
0.139
0.17
0.071
0.109
6.066
5.772
3.055
5.289
4.089
4.155
3.855
3.945
2.822
4.25
3.766
4.716
3.917
5.395
4.167
2.9
1.313
2.239
0.982
1.218
0.362
0.431
0.8
0.416
1.199
0.982
0.197
0.228
0.518
0.924
0.345
0.366
0.32
1.1
0.438
1.216
100.217
100.122
99.145
100.588
98.656
98.88
98.299
97.9
97.932
101.094
100.825
100.079
99.899
100.443
100.322
100.134
100.816
100.93
1,590
1,560
550
656
706
615
640
656
684
699
629
665
658
648
625
611
605
414
38
36
44
34
35
38
30
33
56
43
40
31
42
34
40
34
67
32
0
0.015
0.001
0
0.012
0.005
0.037
0.001
0
0.013
0.012
0.04
0.01
0
0
0.001
0.001
0
0.011
0.009
0.013
0.003
0.005
0.015
0.012
0.003
0.027
0
0.238
0.231
0.454
0.449
0.693
0.682
0.239
0.267
0.268
0.245
0.217
0.52
0.199
0.21
0.562
0.308
0.229
0.185
0.141
0.273
0.145
0.16
0.251
0.268
0.234
0.417
0.414
0.195
30.462
30.216
30.122
29.771
29.605
29.721
30.113
29.943
30.391
29.724
29.998
29.624
29.382
29.503
29.764
29.433
29.762
29.631
30.028
29.455
29.898
30.132
29.738
29.615
29.226
29.382
29.135
29.725
1.875
1.857
0.962
1.257
2.31
1.67
0.901
0.932
1.643
0.483
0.632
1.312
1.61
1.445
1.386
1.421
0.373
0.305
1.5
1.188
0.38
1.155
1.907
1.74
0.753
1.226
1.229
0.553
0
0
0
0
0.371
0.045
0.394
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0.511
0.574
0.183
0.373
0.13
0.152
0.4
0.46
0.7
0.215
0.258
0.204
0.391
0.408
0.353
0.365
0.387
0.258
1.055
0.357
0.34
0.836
0.818
0.431
0.101
0.849
0.39
0.461
14.152
13.911
15.558
14.939
13.39
15.462
16.538
11.178
12.089
11.975
15.579
13.971
14.61
14.715
6.292
14.731
11.945
14.709
14.02
5.987
10.432
14.28
13.93
14.521
18.321
14.987
15.528
18.251
27.311
27.294
29.05
29.215
25.292
27.579
30.486
28.877
25.091
31.327
31.393
25.433
28.064
28.447
23.07
28.287
31.411
33.041
27.623
23.398
30.642
29.978
26.924
27.633
30.611
27.909
28.667
30.346
2.959
2.875
3.068
2.955
2.766
2.797
3.037
3.529
3.032
3.727
3.218
2.923
2.998
2.918
3.657
2.935
3.737
3.58
2.995
3.772
4.009
3.015
2.79
2.87
2.975
2.834
2.869
2.921
10.051
10.126
10.45
10.266
9.454
9.584
10.086
13.474
11.691
13.73
11.226
9.216
10.142
10.356
16.195
10.189
13.632
12.213
10.187
17.472
15.437
10.669
9.418
9.633
9.269
9.607
9.648
9.294
2.11
2.181
2.277
2.118
2.079
2.065
1.998
2.948
3.387
2.763
2.197
1.916
2.083
2.149
4.95
2.192
2.87
2.205
2.172
5.205
3.271
2.209
1.975
2.011
1.735
1.984
2.103
1.822
0.31
0.31
0.217
0.288
0.256
0.213
0.186
0.675
0.774
0.709
0.331
0.22
0.235
0.359
1.268
0.31
0.641
0.326
0.346
1.465
0.827
0.396
0.278
0.278
0.108
0.238
0.307
0.119
1.274
1.318
0.898
1.073
0.937
0.862
0.645
1.657
1.842
1.134
0.769
0.691
1.079
1.224
2.81
1.366
1.383
0.783
1.271
3.084
1.378
1.215
1.191
0.988
0.667
1.24
1.266
0.954
0.026
0.074
0
0.004
0
0
0
0.039
0
0
0
0
0.057
0.01
0
0.048
0.012
0
0.028
0
0
0.026
0.017
0
0
0.028
0.001
0.003
0.193
0.17
0.049
0.129
0
0.032
0.06
0.125
0.171
0.088
0.078
0
0.069
0.069
0.249
0.187
0.16
0.059
0.35
0.137
0.161
0.3
0.107
0.099
0.014
0.267
0.162
0.227
0.107
0.133
0.149
0.145
0
0.002
0
0.226
0.135
0.14
0.069
0.062
0.078
0.103
0.304
0.092
0.158
0.113
0.061
0.483
0.189
0.177
0.168
0.137
0.094
0.086
0.103
0.139
0.359
0.362
0.195
0.217
0.47
0.346
0.161
0.171
0.345
0.108
0.096
0.276
0.377
0.311
0.248
0.243
0.042
0.015
0.297
0.193
0.08
0.228
0.42
0.391
0.147
0.257
0.204
0.159
6.256
6.709
5.413
6.523
11.636
11.228
4.287
4.217
6.52
4.691
3.177
5.875
5.378
5.148
5.625
5.851
1.676
0.934
5.249
5.693
2.185
4.659
6.257
5.639
3.749
5.512
5.525
3.734
1.566
99.76
730
25
1.651
100.03
694
24
0
99.048
840
41
0.061
99.783
755
35
0.212
99.613
886
24
0.11
102.553
698
23
0.619 100.188
597
35
0.643
99.363
633
37
1.019
99.099
810
29
0
101.073
540
41
0.012
99.261
701
56
0.498
92.782
851
36
2.273
99.037
683
22
1.613
98.987
692
26
0
96.733
1,022
44
0.51
98.468
753
34
0.039
98.458
550
83
0.025
98.382
337
79
1.408
98.74
699
27
0
98.171
794
41
0.717 100.104
418
43
0.873 100.311
706
33
2.071
98.263
745
23
2.065
98.334
731
23
1.573
99.592
392
24
0.536
97.361
821
35
0.95
98.527
555
28
0.608
99.51
650
38
(Continued on following page)
Age of South Delhi Orogeny
February 2021 | Volume 8 | Article 594355
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22/1
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B5
1/1
2/1
3/1
4/1
5/1
6/1
8/1
9/1
10/1
11/1
12/1
13/1
14/1
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Al2O3
Singh et al.
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TABLE 4 | (Continued) EPMA analytical result of monazite for the samples CG-1 (26,014′12’’/,74,013′25″), B-4 (26,013′44’’/,74,013′12″), B-5 (26,013′27’’/,74,013′12″), and R-4 (26,012′56’’/,74,018′30″).
27
35
39
41
34
34
33
42
33
33
38
37
40
37
39
864
749
864
803
847
732
760
818
717
783
792
795
807
758
775
99.794
101.122
100.439
100.756
100.411
99.544
99.975
100.656
100.51
100.391
99.57
99.902
99.739
100.741
100.271
0.113
0.328
0.11
0.178
0.111
0.567
0.354
0.111
0.515
0.252
0.203
0.19
0.172
0.203
0.243
8.361
4.861
5.027
4.377
6.101
4.96
5.35
4.491
4.716
5.632
4.81
4.968
4.596
4.765
5.506
0.324
0.191
0.2
0.171
0.235
0.214
0.213
0.171
0.197
0.217
0.186
0.191
0.179
0.177
0.21
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
1.586
1.406
1.632
1.513
1.648
1.495
1.412
1.522
1.508
1.485
1.44
1.402
1.354
1.461
1.379
Zircon Geochronology
5.2.1 Sample No. TKB-1
The sample is collected from the gneissic part of the Sewariya
pluton near Bar, where high temperature solid state fabric is
prominently developed (Figure 2A). The zircon crystals are
subangular in shape. The CL image reveals oscillatory zoning
patterns, consistent with magmatic zircon (Figure 6, textural
details in Appendix). The isotopic data plot in a well-defined
cluster on Concordia, with two analyses within that
population plotting with larger errors and off Concordia,
corresponding to the two analyses with the highest f206
(Figure 7, TKB1-25, TKB1-26, TKB-1). The concordant
points correspond to a Concordia age of 878 ± 9 Ma, which
we take to be the best estimate for the emplacement age of the
granitoid. One analysis, TKB1-14, plots well away from the
Concordia age and corresponds to a darker CL zircon
(Figure 6), with a 207Pb/206Pb age of 1,634 ± 22 Ma (2σ
confidence, 101% concordant, Figure 7 TKB-1). This
represents the xenocrystic component age.
1.458
1.672
1.33
1.321
1.343
1.727
1.814
1.278
1.672
1.463
1.348
1.368
1.34
1.399
1.466
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
Sample No. TKB-2
The sample is collected from Rupnagar metarhyolite
(Figure 2C). The rock is foliated with close spaced S1
fabric. The zircon grains are dominantly subangular to
subrounded. The CL image reveals oscillatory zoning
patterns, consistent with magmatic zircon (Figure 6,
textural details in Appendix). The data plot in a welldefined cluster on Concordia (Figure 7), with the exception
of TKB-2–15 and TKB-2–17, which record the highest f206
values and plot off Concordia. The concordant population of
27 zircons defines a Concordia age of 982 ± 3 Ma, which is the
best approximation of the emplacement age of the granite
protolith.
0.225
0.424
0.291
1.119
0.448
0.784
0.416
0.803
0.574
0.463
0.539
0.303
0.497
0.478
0.463
0.474
0.476
0.482
0
0.011
0.014
0.01
0
0.074
0.031
0.004
0.007
0
0
0.005
0.013
0
0
29.458
31.529
30.181
30.574
29.722
30.597
30.382
30.646
31.227
30.731
30.367
30.426
30.348
30.826
30.512
1.104
0.973
0.801
0.853
0.843
1.078
1.019
0.744
1.134
1.036
0.874
0.968
0.842
0.881
1
0.259
0.255
0.278
12.411
13.301
13.174
13.696
13.104
12.249
13.039
13.434
12.482
12.879
13.157
13.395
13.438
13.519
13.18
26.031
28.869
28.809
29.516
28.157
28.236
28.475
29.409
28.538
28.546
28.849
28.943
29.139
29.308
28.687
2.669
2.796
2.808
2.977
2.883
2.765
2.806
2.893
2.879
2.821
2.896
2.806
2.876
2.911
2.82
11.349
11.075
11.601
11.316
11.543
11.274
10.955
11.517
11.436
11.084
11.248
11.031
11.206
11.094
11.104
2.445
2.283
2.538
2.446
2.523
2.382
2.29
2.448
2.478
2.324
2.366
2.412
2.405
2.404
2.365
0.814
0.858
0.894
0.908
0.884
0.804
0.823
0.903
0.893
0.94
0.873
0.818
0.857
0.823
0.831
0
0
0
0.55
0.523
0.538
0.486
0.511
0.547
0.549
0.545
0.526
0.482
0.473
0.515
0.499
0.496
0.485
68
43
88
845
941
813
97.196
95.58
97.07
0.001
0.128
0.037
2.607
5.384
1.841
0.095
0.235
0.069
0.131
0.142
0.087
Sewariya Pluton
It indicates a higher % of SiO2 (75.35–78.68), moderate to high
Fe2 O3 (0.74–1.03), moderate to high Al2O3 (10.81–13.73), and
lower Na 2O compared to K2 O. The A/NK and A/CNK values
are >1.0. The CIPW norm shows normative quartz, diopside,
albite, corundum, and ilmenite (Table 2). The rock lies in the
per-aluminous field in the A/NK vs. A/CNK plot (Figure 5A)
and S type field in the Na2 O vs. K2O and TiO2 vs. Zr plot
(Figures 5B,C) (Chappell and White, 1992) and calc-alkaline
to calcic field in Na2 O + K2 O-CaO vs. SiO2 plot (Figure 5D)
(cf. Frost et al., 2001). The Na2O + K2 O–(CaO + MgO) *5
Fe2 O3 (t) * 5 ternary plot (Figure 5F) shows the sample lies in
“island arc to continental arc setting” with few in the rift
setting (Grebennikov, 2014). The Zr values range from 29 to
130 ppm indicating Zr saturation temperature is around 700
to 800 °C (cf. Watson and Harrison, 1983).
Geochemistry of the granites and metarhyolite can be
summarized as the Pratapgarh pluton is meta-aluminous, I
type, and emplaced in intraplate rift setting; metarhyolite is
peralkaline, I type, and emplaced in intracontinental rift
setting; these intruded during pre-D1 stage. The Sewariya
granite is peraluminous, S type, and emplaced in syncollisional
setting, syn-D1.
30/1
31/1
32/1
R4
1/1
2/1
3/1
4/1
5/1
6/1
7/1
8/1
9/1
10/1
46/1
47/1
48/1
49/1
50/1
0
0.072
0.023
29.364
29.315
29.58
0.567
1.11
0.712
0
0
0
15.687
12.606
15.367
31.8
28.872
32.073
3.048
3.081
3.097
10.326
10.519
10.544
2.028
2.122
2.028
0.234
0.364
0.287
0.735
0.854
0.673
0.09
0.096
0.082
Age
(Ma)
Total
UO2
ThO2
PbO
Tm2O3
Tb2O3
Y2 O 3
La2O3
Ce2O3
Pr2O3
Nd2O3
Sm2O3
Eu2O3
Gd2O3
Dy2O3
Age of South Delhi Orogeny
FeO
CaO
P2O 5
SiO2
Al2O3
CG1
Point
TABLE 4 | (Continued) EPMA analytical result of monazite for the samples CG-1 (26,014′12’’/,74,013′25″), B-4 (26,013′44’’/,74,013′12″), B-5 (26,013′27’’/,74,013′12″), and R-4 (26,012′56’’/,74,018′30″).
Age
err
Singh et al.
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24
February 2021 | Volume 8 | Article 594355
Singh et al.
Age of South Delhi Orogeny
Monazite Geochronology
Sample No. TKB-3
The sample belongs to the Pratapgarh granite pluton
(Figure 2B). The zircon grains are subangular in shape,
with some subrounded zircon. The CL image reveals
relatively low response, with dark oscillatory zoning
patterns, consistent with magmatic zircon (Figure 6,
textural details in Appendix). Twenty-one analyses were
conducted on twenty-one zircons and resulted in variable
f206 values between 0 and 21.7%. One zircon (TKB-3–16)
recorded very high U and Th values, but the Uranium and
Thorium content for all other zircons range between 100 and
1802 and 29 and 1,071 ppm, respectively (Table 3), resulting
in Th/U ratios between 0.02 and 1.96. The lowest Th/U values
were recorded on angular euhedral zircon with clear
oscillatory patterns, and the overall range of Th/U values
and CL patterns suggest the zircon to be magmatic in
character. Two zircon grains record older 207Pb/206Pb ages,
with the one near-concordant point (TKB-3–7) giving an age
of 2,765 ± 18 Ma. These are considered to record some
xenocrystic components in the granite protolith (Figure 7).
The remaining points plot along a Pb-loss trend anchored at
an upper concordant cluster with a Concordia age of 992 ±
12 Ma (Figure 7). The Pb-loss pattern is consistent with
present-day loss.
Sample No. CG1
The sample belongs to mica schist mylonite of the Phulad thrust.
The mylonite is foliated containing dynamically recrystallized
elliptical quartz grains arranged oblique to the mylonitic foliation;
these oblique grains define mylonitic S foliation. The monazites
are elliptical in shape and occur oblique to the C fabric indicating
its synshearing recrystallization (Figure 8A). The monazite
grains contain domains; the lighter domain shows low Y and
high Th values and produces an age of 813 Ma to 716 Ma
(Figures 8B–D). This age corresponds to D2 events. The
darker domain contains fractures and produces an age of ca.
590 Ma suggesting the age of brittle deformation event. Isochron
plot and probability curve indicate three events 819–680–588 Ma
and 811–700–569 Ma, respectively (Figures 8E,F). The D2
deformation and thrusting belong to ca 811–680 Ma and the
brittle deformation is at ca. 588–569 Ma.
Sample No. B4
The sample is collected from pre-Delhi upper amphibolite facies
slivers emplaced within Phulad thrust; there are several elliptical
monazite grains aligned parallel to the mylonitic foliation
(Figures 8G,I). Grains are fractured along the edge and yield
ages as ca. 706 Ma and 550 Ma (Figure 8H), indicating age of the
D2 and D4 brittle deformation events, respectively. In addition,
several rounded grains (Figure 8J) resembling the rounded
garnet grains (Figure 4C) occur in the rock. These monazite
grains crystallized during pre-Delhi metamorphism and yield
higher ages as 1,595 to 1,638 Ma. Isochron and probability curve
indicate two age clusters (Figures 8K,L). The pre-Delhi
metamorphism is at ca. 1,611–1,609 Ma and the D2 is at ca.
704–686 Ma.
Sample No. TKB-4
The sample is collected from Sumel granite (Figure 2C). The
zircons range from euhedral crystals to irregular shape
indicating the presence of various xenocrystic components.
Faintly observable oscillatory zoning suggests magmatic
growth (Figure 6, textural details in Appendix). One
analysis yielded an older age of ca 2,890 ±87 Ga, but plots
slightly under Concordia (TKB- 4–3, Figure 7). This grain is
interpreted to reflect a xenocrystic component in the granite
dyke. The remaining analyses mainly plot along a broad Pb-loss
trend, with the highest U grains plotting inversely discordant
(Figure 7). The trend appears to be subparallel to Concordia
and most likely related to ancient Pb-loss, possibly during
intrusion of the regional granite suites or during subsequent
metamorphism. The oldest cluster of concordant grains in the
population and also the zircon with the lowest f206 provide a
Concordia age of 946 ± 18 Ma, which could reflect the
emplacement age of the granite dyke. One zircon (TKB4–7), analyzed twice during the session, provides a
concordant data point at 270 ± 12 Ma and corresponds to a
subangular zircon grain with faint sector zoning. This young
grain would put the age of emplacement of the granite dykes at
270 Ma, with the older populations reflecting inheritance of ca.
950 Ma aged zircon, recording Pb-loss due to a metamorphic
event either at or before 270 Ma.
Zircon geochronology suggests that the granites of the study
area were emplaced during two distinct periods, one at
992–946 Ma and the other at ca. 878 Ma (Table 1). The
former is pre-D1 while the latter is syn -D1. The granite
carries xenocrysts from the basement rock showing ages ca.
1,634 Ma, ca. 2,765 Ma, and ca. 2,890 Ma. The intrusion of
pegmatite veins in the granite along fractures is ca.270 Ma.
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Sample No. B-5
The sample is collected from the mica schist, west of Babra. The
monazite grains are equant in shape with moderate to low
ellipticity (Figures 9A–D). They occur parallel to S1 fabric in
the rock and developed during D1-M1. It contains domains; the
lighter domains, lying at the outer edge of the grain, are Y poor
and Th rich and yield higher age as ca. 886 Ma while the darker
domains, present in the center of the grain, show ages as ca.
783–597 Ma (Figure 9B). Fracture along the grain margin
channelized the fluid into the core of the grain resulting in
rejuvenation of the age. There are three isochrons indicating
as many events as 864 Ma for D1, 718 Ma for D2, and 588 Ma for
brittle deformation (Figure 9E). Probability graph indicates three
peaks at ca. 846, 697, and 564 Ma (Figure 9F), for D1, M1, and D2
and brittle deformation events, respectively.
Sample No. R4
The sample is collected from the mica schist (Figure 9G) close to
the Pratapgarh. The monazite grains occur within biotite and are
rounded to ellipsoidal with compositional domains (Figures
9H–J). The Y low/high Th domains yield higher ages as ca.
863 Ma corresponding to D1-M1, and Y poor domains yield ca.
792 Ma corresponding to D2 (Figure 9H). The isochrons indicate
two ages ca. 865 and 720 Ma (Figure 9K) and probability curve
25
February 2021 | Volume 8 | Article 594355
Singh et al.
Age of South Delhi Orogeny
contains peaks at ca. 852 and 743 Ma (Figure 9L) corresponding
to D1 and D2, respectively.
The result of the monazite geochronology can be summarized
that the D1-M1 is ca. 865 Ma; it continued up to 846 Ma. The D2 is
ca. 811–680 Ma and D4 brittle deformation is ca. 588 to 564 Ma.
The pre-Delhi deformation and metamorphism were at 1,638 Ma
(Table 1). The D1 deformation age nearly coincides with D1 age
derived from zircon age of Sewariya granite (878 Ma). Combining
these ages, we define the D1-M1 age as 878–846 Ma. The D2 event
included both folding and thrusting. Therefor a spread in ages is
indicated (811–680 Ma). Thrusting is marked by fluid flow and
therefore resets the ages to ca. 680 Ma, in most cases. Therefore,
we are taking ca. 680 Ma as the lower age limit for thrusting. The
roll of D3 deformation cannot be ruled out for such wide spread of
ages. The age of D4 deformation is at ca 588–564 Ma.
More peraluminous nature implies there is little contribution
from mantle (Ray et al., 2015). Our study finds similar result as
the granite is peraluminous, S type, more calcic, and
syncollisional in nature (Figure 5). However, some of the
analysis lies in rift setting (Figure 5F). Collision/subduction of
the South Delhi Terrane resulted in the melting of the Delhi
metasediments and produced such granite.
Life Span of South Delhi Orogeny
The South Delhi orogeny is constrained by multiple proxies as
deformation, metamorphism, geochemistry, and geochronology
of pre- and synorogenic granites and tectonometamorphic fabric.
The study area is represented by greenschist facies rocks which
are intruded by two phases of granites; the Sewariya granite is the
youngest. All the plutons carry the imprint of D1 deformation;
hence the D1 deformation cannot be older than Sewariya pluton.
Geochemically it is S type and produced in a syncollisional
setting. In addition, we have illustrated that the Sewariya
pluton shows a transition from magmatic/submagmatic to
solid state deformation fabric S1, from core to the margin. At
the margin it shows perfect coupling with S1 fabric in the host
mica schist. This is an evidence of syntectonic (syn-D1-M1)
nature of granite intrusion. The granite yields a zircon age of
ca. 0.87 Ga that marks the initiation of the South Delhi orogeny.
We did the monazite dating of tectonometamorphic fabric which
further constrains the D1-M1 age at ca. 0.86–0.84 Ga. The D2
event is constrained at ca. 0.81 to 0.68 Ga. Though D3 event could
not be dated, the spread in D2 ages (nearly hundred years) may be
partly due to the D3 event. Considering all these ages, it is
suggested that the South Delhi orogeny is ca. 0.87–0.68 Ga old
and much younger than the Grenvillian orogeny (1.3 Ga to
1.0 Ga/Li et al., 2008) and may be synchronous with early
phase of Pan-African orogeny (ca. 0.9–0.7 Ga, Rogers and
Santosh, 2002; Kroner and Stern, 2005; Rino et al., 2008; ,
Singh et al., 2010; Tiwari and Biswal, 2019a). The
postorogenic brittle deformation in the SDT belongs to ca.
0.58–0.56 Ga that coincides with the period of Kuunga
orogeny (0.65–0.5 Ga, Meert, 2003; Pradhan et al., 2009)
representing the later part of Pan-African orogeny (Ambaji
area, 0.76–0.65 Ga, Tiwari and Biswal, 2019b; Pali area, ca.
0.6 Ga, Bhardwaj and Biswal, 2019).
Our study shows a similarity with other parts of the SDT. The
D1 event was dated at ca. 0.87–0.86 Ga and thrusting at ca.
0.81–0.78 Ga from Ambaji area (Tiwari and Biswal, 2019a)
and thrusting at ca. 0.81 Ga from Phulad area (Chatterjee
et al., 2017). Further, on the eastern flank of the SDT near
Srinagar, the granite bears post-0.98 Ga deformation imprint
(Ruj and Dasgupta, 2014; Bose et al., 2017). The Sirohi
Terrane in the west of SDT shows a deformation age between
0.89 and 0.8 Ga (Arora et al., 2017).
Earlier view that the South Delhi orogeny is Grenvillian comes
from 1.0 Ga age of Pratapgarh, Sumel, Chang, and Sendra plutons
(0.98 Ga, Tobisch et al., 1994; 0.96 Ga/Pandit et al., 2003; 0.97 Ga/
Tiwana et al., 2019), but geochemistry of these granites suggests
that these are I type, produced from remelting of basic rocks, and
intruded in postorogenic intracontinental extensional setting.
The granites lack magmatic fabric, deformed by all phases of
DISCUSSION
Tectonic Setting of Metarhyolite and
Granite
The Chang, Sendra, and Pratapgarh plutons (Figure 2C) intruded
the calcareous schist and folded the surrounding rocks in the core
of the SDT, showing similar geochemical character and isotopic
age (Tobisch et al., 1994; Pandit et al., 2003, Tiwana et al., 2019;
present study). Previous studies by Pandit et al. (2003) and
Tiwana et al. (2019) suggest that these are A2-subtype granites
(cf. Eby, 1992) and are generated from subcontinental lithosphere
or lower continental crust in postcollisional or postorogenic
settings, perhaps during late-stage extensional collapse. Based
on initial Sr isotope composition (0.710), the Chang pluton was
suggested to be derived from basic magmatic source, produced
from remelting of Archaean Banded Gneissic Complex in an
Andean type magmatic setting (Pandit et al., 2003). The plutons
were metamorphosed during South Delhi orogeny. Our study
suggests that the Pratapgath granite is meta-aluminous, I type,
and calc-alkaline (Figure 5) and was produced from the
remelting of basic igneous rocks at 750–800°C (zircon
saturation temperature). Further, Rb, NB, and Y values
indicate the granite to represent within plate magma intruded
in intracontinental rift setting. These led to implication that the
post-Aravalli (globally post-Columbia and Grenvillian) rifting of
the Marwar Craton formed the South Delhi basin and granite was
intruded in the basin in an extensional setting. Rupnagar
metarhyolite was studied for the first time by us. It occurs as
synsedimentary lava flows. Geochemical data indicates it is
peralkaline, I type, and alkalic to alkali-calcic composition
(Figure 5). The Rb- Nb-Y and Na2O + K2O–(CaO + MgO) *5
Fe2O3 (t) * 5 plot shows within plate magma intruded in a
continental rift setting. This implies that the rifting of the
Marwar Craton led to alkali volcanism in the South Delhi
basin. The Sewariya pluton exhibits a different tectonic setting
altogether. Previous study (Bhattacharjee et al., 1993; Ray et al.,
2015; Sivasubramaniam et al., 2019) shows that the granite is
peraluminous being produced from melting of Delhi
metasediments at higher water pressure (3 kb) at 750°C by
muscovite breakdown reaction in a subduction zone setting.
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Age of South Delhi Orogeny
deformation as the metasediments and metamorphosed in
greenschist facies. The plagioclase and K-feldspar retained
their magmatic habit. Based on these we interpret that these
plutons are preorogenic to SDT and intruded at the time of rifting
of South Delhi basin. As far as age of rifting and sedimentation are
concerned, previous study on SDT constrained it at ca. 1.7–1.0 Ga
(Wang et al., 2014) and ca. 1.2–0.86 Ga (Singh et al., 2010). Our
study on the Rupnagar metarhyolite that stands as an
unequivocal evidence of synsedimentary bimodal volcanism
like many in the SDT (Bhattacharjee et al., 1988) provides an
age of 0.98 Ga that lies within the above age range. Thus, the
rifting and formation of South Delhi basin overlap with Rodinia
amalgamation event (ca. 1.3–0.9 Ga). Hence, while many
continents were experiencing orogenic event, the SDT was
undergoing rifting.
Another viewpoint that advocates the South Delhi orogeny to
be Grenvillian is the 1.0 Ga metamorphic imprint on 1.7–1.5 Ga
old granulite of the Pilwa-Chinwali area. As these outcrops occur
in the north of the study area Bhowmik et al. (2018) considered
the granulite to be part of SDT. However, Fareeduddin et al.
(1994) suggested the granulite to be pre-Delhi and belongs to
Sandmata Terrane. Our finding suggests that the upper
amphibolite grade slivers of ca. 1.6 Ga (equivalent to PilwaChinwali granulite) occur within SDT as tectonic inclusions
along the Phulad thrust. The mineral assemblage is in sharp
contrast with the surrounding mica schist. We suggest that the
Pilwa-Chinwali granulite represents basement for the SDT and it
has been exhumed by thrusting and normal faulting along
Govindgarh-Jethana fault. Further, the xenocrysts in the
Pratapgarh and Sewariya granites yield similar ages (ca. 1.6 Ga,
2.7 Ga, and 2.8 Ga). Furthermore, the Beawar gneiss occurs as
exhumed basement block to the east of the study area and shows
the age as ca. 0.8 Ga, 1.0 Ga, 1.6 Ga, and 2.8 Ga old (Kaur et al.,
2020). Hence 1.0 Ga metamorphic event is an event in the
basement not in the SDT.
et al., 2008; Meert et al., 2013; Johansson, 2014; Pisarevsky
et al., 2014; Oriolo et al., 2017).
i. Before 1.0 Ga (Figure 10A): At 1.2 Ga India was in the polar
latitude (Pradhan et al., 2009) as a separate landmass, away
from Rodinia Supercontinent. By then, the Dharwar Craton
of India was separated from Sarmatia at 1.3 Ga, the eastern
margin of India was not connected to Antarctica and
Australia, and the Marwar Craton, Madagascar, and
Syechelles were bordering the western margin of India
(Pisarevsky et al., 2013). The Arabian-Nubian shield was
located to the south (Figure 10).
ii. Ca. 1.0–0.9 Ga (Figure 10B): Grenvillian orogeny took
place, consequently India amalgamated with Antarctica,
the Eastern Ghats Mobile Belt and Rayner Complex
formed through subduction and collision (Dasgupta et al.,
2013), and in turn Antarctica and Australia amalgamated
with the Laurentia completing the assembly of the Rodinia.
Thus Eastern Ghats Mobile Belt is considered to be a
Grenvillian Mobile Belt. Further, the NW India witnessed
discrete metamorphic events, e.g., Sandmata Terrane
(Bhowmik and Dasgupta, 2012) and North Delhi Terrane
(Pant et al., 2008) of the ADMB. Following this, the NW
India underwent extension and rifting reflecting a top-down
process (e.g., Cawood et al., 2016).
iii. After Rodinia (Figure 10B inset-1): Extension led to rifting
in the Marwar-Aravalli-Bhilwara-Bundelkhanda Craton in
NW India forming South Delhi basin. A simple shear
extension model (cf. Wernicke, 1985) with westward
inclined detachment probably created the South Delhi
basin (Biswal et al., 1998a, b). In the simple shear model,
the rotation of the faulted blocks is easier due to listric nature
of the detachment fault which is steeper at the surface and
gradually becomes gentler and horizontal at brittle-ductile
boundary. This type of model has been applied to Basin and
Range province, Bay of Biscay, and northwestern European
continental shelf (Montadert et al., 1979; Kusznir et al., 1987;
Lister et al., 1991). However, a pure shear model as proposed
by McKenzie (1978) cannot be ruled out. In the pure shear
model, lithospheric stretching takes place through conjugate
planar faults. The slips along conjugate faults oppose each
other (Ramsay and Huber, 1987) which is not there in a
listric detachment fault postulated in the simple shear
model. However, mechanism of basin formation is
beyond the scope of the paper. The sediments were
deposited in fault-bounded basin. Thick erosional
unconformity with conglomerate formed at several parts
of the SDT. Biomodal volcanism produced rhyolite and
basalt flows (Rupnagar metarhyolite) which are dated at ca.
0.98 Ga in the study area. Granites emplaced in extension
setting, namely, Bilara granite and Sendra-ChangPratapgarh-Sumel plutons at ca. 1.0–0.9 Ga. With
progressive lithospheric stretching, the MOR developed
on the east (Figure 10C inset −1); the Phulad ophiolites,
Ranakpur diorite, and Sirohi plagiogranite (ca. 1.0 Ga,
Volpe and Macdougall, 1990; Dharma Rao et al., 2013)
were part of the oceanic crust. This represents the
South Delhi Orogeny in Relation to East
Gondwana Tectonics
The East Gondwana includes much of the continents of Australia,
India, and Antarctica, which underwent several phases of
amalgamation and separation in the Proterozoic period
(Fitzsimons, 2000; Collins and Pisarevsky, 2005; Pradhan
et al., 2009). It was not a coherent mass during much of
Rodinia and Gondwana amalgamation; they finally joined
together in Neoproterozoic-Cambrian period in bits and
pieces. Thus, individual continents in the assembly were not
even completely amalgamated; this is proven from India (Collins
and Pisarevsky, 2005). For example the Eastern Ghats Mobile Belt
provides evidence for final suturing between India and Antarctica
during Cambrian (ca. 0.51 Ga/Biswal et al., 2007). The present
study on SDT points to amalgamation of Marwar Craton with
Bundelkhanda Craton during Neoproterozoic (ca.0.87 Ga). Based
on our finding we are proposing a model for the SDT in reference
to amalgamation of Rodinia and Gondwana, using the
reconstruction of Supercontinents proposed by several
researchers (Figure 10, cf. Powell and Pisarevsky, 2002; Li
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Age of South Delhi Orogeny
FIGURE 10 | Cartons showing position of India during continental assembly and breakup. (A) 1.3–1.1 Ga, Rodinia was under amalgamation; India separated from
Baltica, drifted to join Antarctica and Australia. (B) East Gondwana continents amalgamated to complete Rodinia assembly by 1.0 Ga. Eastern Ghats Mobile Belt
developed in the SE margin of India by subduction/collision between India and Antarctica. Synchronously, the NW margin of India underwent internal rifting that led to the
formation of South Delhi basin. A simple shear model has been proposed for basin opening (Figure 10B, inset 1) (e.g., Wernicke, 1985, model drawn cf.; Allen and
Allen, 2013). The sediments were laid down with a prominent erosional unconformity over granitic basement, marked by conglomerate and paleosol. Further, bimodal
volcanism (982 Ma) occurred and granite plutons (992–946 Ma) intruded in the basin, (C) Ca 880 Ma, Rodinia broke up; East Gondwana continents were drifted apart.
(Continued )
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Age of South Delhi Orogeny
FIGURE 10 | Synchronously, the South Delhi basin closed by subduction/collision, to form SDT, pro- and retro-wedge thrust belts formed. The study area belongs to
retro-wedge part (C, insets 1, 2, 3, model of subduction, collision, and extension, cf. Allen and Allen, 2013), (D) Ca. 600 -530, East and West Gondwana continents
amalgamated to form Gondwana. The SDT experienced brittle tectonics during this period. Number of faults developed (D, inset 1). The Eastern Ghats and several other,
Columbia-Rodinia related mobile belts of Peninsular India were overprinted by ca. 0.5 Ga brittle as well as ductile deformation. Brittle tectonics continued till 270 Ma
when Gondwana broke up; pegmatite veins in the SDT bear testimony for that.
introversion tectonics as the juvenile crust formed out of
such rifting is younger than the age of the breakup (cf.
Murphy and Nance, 2013).
iv. Rodinia breakup and South Delhi orogeny (0.88–0.65 Ga,
Figure 10C). Rodinia broke up and India was detached from
Antarctica and Australia along Eastern Ghats and was
placed near the north pole by 0.82 Ga (Powell and
Pisarevsky, 2002). Breakup of Rodinia gave rise to the
opening of major oceans, namely, Mozambique Ocean
(Figure 10C) and Iapetus Ocean (Figure 10D) (e.g.,
Cawood et al., 2001; Meert and Torsvik, 2003; Li et al.,
2008). However, the SDT witnessed subduction/collision
between Bundelkhanda + Aravalli Craton and Marwar
Craton. The oceanic crust was subducted westward
forming continental arc with granite magmatism ranging
in ca 0.87–0.76 Ga (Figure 10C, inset 1,2) (Erinpura granite,
Ambaji granite, and Sewariya granite, Malani Igneous Suite;
Sinha Roy, 1988; Sugden et al., 1990; Synchanthavong and
Desai, 1977; Biswal et al., 1998b; Singh et al., 2010). Possibly,
the detachment zone in the basin converted to a
northwesterly dipping subduction zone/collision zone.
Seismic reflection data support a NW dipping thrust
plane at the contact between SDT and Sandmata Terrane
(Satyavani et al., 2004). The magmatic arc was extending
upto Madagascar in the south (Singh et al., 2010) and South
China in the north (Zhao et al., 2018) as indicated by the
presence of similar age granite (ca. 0.7 Ga, Singh et al., 2010)
in those far-off continents (Figure 10C). The Marwar
Craton, which was forming the overriding plate, was
completely metacratonized by intrusion of these granitic
rocks. The metacratonization was so extensive that Archean
outcrops are hardly survived in Marwar Cratonic domain;
the Azania block in Madagascar may be the survived part of
extensive Marwar Craton (Singh et al., 2010). Along the
magmatic arc, several low grade metasedimentary outcrops
(roof pendants) occur within expansive granite outcrops,
which constitute the Sirohi Terrane (Figure 1B). With
progressive subduction/collision, compression led to
exhumation of rocks at 0.81–0.68 Ga by thrusting (cf.
Tiwari and Biswal, 2019a). The SDT is marked by pro- as
well as retro-wedge thrust belts (cf. Naylor and Sinclair,
2008). The study area Beawar-Babra sector belongs to retrowedge thrust belts while the eastern part around Shyamgarh
(Figure 2A) belongs to pro-wedge thrust belt, together
forming a doubly-vergent orogen (Hahn et al., 2020)
v. The SDT type Tonian-Cryogenian (1.0–0.72 Ga) rifting
occurred between landmasses in the West Gondwana
(Oriolo et al., 2017). The Arabian-Nubian shield shows
signature of ca. 0.88–0.7 Ga accretion of island arcs and
juvenile crust (Kroner and Stern, 2005); the Dahomey Belt
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has subduction with associated back-arc extension and
development of island arc (Ganade et al., 2016);
Borborema Province has ca. 0.85–0.75 Ga juvenile
material (Ganade de Araujo et al., 2014) and the São
Gabriel Block has ca. 0.9–0.7 Ga old intraoceanic
subduction (Fortes de Lena et al., 2014).
vi. Ca.0.6–0.53 Ga
(Amalgamation
of
Gondwana
Supercontinent): Collision and strike-slip shearing led to
amalgamation of different Gondawana blocks along PanAfrican orogens (Figure 10D). The Mozambique ocean
closed through Kuunga orogeny (0.65–0.5 Ga), bringing
together the West and East Gondwana continents along
East African Orogen (Figure 10D, Collins and Windley,
2002; Meert, 2003). India initially joined with Australia at ca.
0.6 Ga through strike-slip shearing (Powell and Pisarevsky,
2002) and later at 0.53 with Antarctica through collision.
The Eastern Ghats Mobile Belt thrusts over Bastar Craton
along the terrane boundary shear zone at 0.51 Ga (TBSZ,
Biswal et al., 2007). During this period, the SDT experienced
extensional tectonics (Figure 10D inset-1) that produced
several prominent faults, namely, Kui-Chitraseni fault,
Surpagla-Kengora fault, and Govindgarh-Jethana fault.
Extensional basins like Sindreth and Punagarh developed
in the intra-arch region (Figure 10C, inset-3, cf. Schobel
et al., 2017). The brittle tectonics in the ADMB continued till
the breakup of Gondwana. The N-S fractures that host
0.27 Ga pegmatite veins in the study area probably belong
to Gondwana breakup period.
CONCLUSION
(1) We applied the tectonic fabric and geochemistry of the
granite and metarhyolite in combination with zircon and
monazite geochronology to date the South Delhi orogeny.
The Pratapgarh-Sumel granite plutons intruded in
extensional setting during rifting, Rupnagar metarhyolite
extruded during sedimentation, and all these yielded an
age of ca. 992–946 Ma. The Sewariya granite is
syncollisional and produced an age of ca. 878 Ma. It is
syntectonic with D1 as indicated by coplanar attitude of
magmatic Sm and S1 fabrics. The monazite geochronology
estimated the age of D1-M1 metamorphism at 865–846 Ma,
the D2 at 810–680 Ma, and brittle deformation D4 at
588–564 Ma. We suggest that the South Delhi orogeny is
ca. 878 to 680 Ma old and coeval with early part of the PanAfrican orogeny.
(2) The South Delhi orogeny brought amalgamation of the
Marwar Craton with Aravalli-Bhilwara-Bundelkhanda
Craton that led in process to forming a united Indian
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Singh et al.
Age of South Delhi Orogeny
subcontinent, in the East Gondwana assembly. This strengthens
the view that the East Gondwana blocks were sutured in bits and
pieces till Neoproterozoic-Cambrian period.
(3) The Marwar Craton was extensively metacratonized by
≤878 Ma granites during South Delhi orogeny so that the
Archaean rocks are totally lost from the cratonic domain.
Similar metacratonization is also observed in Mangalwar and
Sandmata terranes during Aravalli orogeny. Migmatization
of Archean rocks and intrusion of charnockitic magma
completely reset the age of the rocks in those terranes to
Mesoproterozoic.
AUTHOR CONTRIBUTIONS
All authors listed have made a substantial, direct, and intellectual
contribution to the work and approved it for publication.
ACKNOWLEDGMENTS
The authors acknowledge the Department of Earth Sciences, IIT
Bombay, for financial support during field work. Reviewers are
profusely thanked for their constructive comments that helped in
improving the MS.
DATA AVAILABILITY STATEMENT
SUPPLEMENTARY MATERIAL
The original contributions presented in the study are included in
the article/Supplementary Material, further inquiries can be
directed to the corresponding authors.
The Supplementary Material for this article can be found online at:
https://www.frontiersin.org/articles/10.3389/feart.2020.594355/
full#supplementary-material.
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Conflict of Interest: Author BDW was employed by the company SRK Consulting
(Australasia) Pvt Ltd.
The remaining authors declare that the research was conducted in the absence of
any commercial or financial relationships that could be construed as a potential
conflict of interest.
Copyright © 2021 Singh, De Waele, Shukla, Umasankar and Biswal. This is an openaccess article distributed under the terms of the Creative Commons Attribution
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Sample TKB-3. Zircons from sample TKB-3 range in size from
50 to 150 µm and have length to width ratios of 1:1 to 3:1. The
zircon grains are subangular in shape, with some subrounded
zircon. CL image reveals relatively low response, with dark
oscillatory zoning patterns, consistent with magmatic zircon
(Figure 6). Twenty-one analyses were conducted on
twenty-one zircons and resulted in variable f206 values between
0 and 21.7%. One zircon (TKB-3–16) recorded very high U and
Th values, but the Uranium and Thorium content for all other
zircons range between 100 and 1802 and 29 and 1,071 ppm,
respectively (Table 3), resulting in Th/U ratios between 0.02 and
1.96. The lowest Th/U values were recorded on angular euhedral
zircon with clear oscillatory patterns, and the overall range of Th/
U values and CL patterns suggest the zircon to be magmatic in
character.
Sample TKB-4. Zircons from sample TKB-4 range in size
from 50 to 200 µm and have highly variable length to width
ratios between 1:1 and 4:1 (Figure 6). The zircons range from
euhedral crystals, to irregular and rounded shapes, interpreted
to possibly reflect the presence of various xenocrystic
components. CL image reveals very low response for all
zircons, with very faintly observable oscillatory zoning
suggestive of magmatic growth (Figure 6). Twenty-eight
analyses were conducted, including two analyses on a single
zircon (TKB-4–7). Except for a few zircons, the analyses show
high f206 values, ranging from 0 to 18.70%. The higher values
correspond to high U and high Th zircon, and the U and Th
values in the zircon are in the ranges 30–4,903 and
4–3,773 ppm, respectively, with Th/U ratios between 0 and
2.14. Nineteen analyses display extremely low Th/U values
consistent with metamorphic zircon, but these analyses
correspond to euhedral elongate zircon grains and are thus
unlikely related to metamorphic growth.
APPENDIX
Textural interpretation of zircon:
Sample TKB-1. Zircons from this sample range in size from 50 to
150 µm and have length to width ratios of 1:1 to 3:1. The zircon
crystals are subangular in shape. CL image reveals oscillatory
zoning patterns, consistent with magmatic zircon (Figure 6). A
small number of zircons have small homogenous overgrowths,
but none were large enough to allow measuring on the
instrument. Seventeen analyses were conducted on seventeen
separate grains and indicate low levels of f206 (proportion of
nonradiogenic 206Pb in total 206Pb), between 0 and 2.02 (Table 3).
Uranium and Thorium values are in the ranges 118–434 and
55–470 ppm, respectively, resulting in Th/U ratios between 0.24
and 1.69, typical for magmatic zircon.
Sample TKB-2. Zircons from this sample range in size from 50
to 150 µm and have length to width ratios of 1:1 to 2:1. The zircon
grains are dominantly subangular to subrounded. CL image
reveals oscillatory zoning patterns, consistent with magmatic
zircon (CL Figure 6). Some zircons show an inner
medium-CL domain of oscillatory zoning, overgrown by an
outer higher-CL domain, also oscillatory zoned. The internal
zoning patterns in the zircon are consistent with growth from
magmatic fluids. A small number of zircons have small
homogenous dark-CL overgrowths, possibly related to
metamorphic overgrowth, but none were large enough to
allow measurement. Thirty analyses were conducted on thirty
grains, all on oscillatory zoned domains. One analysis resulted in
extremely high U and Th counts and plots inversely discordant
(TKB-2–20, Table 3). The 29 remaining analyses yielded f206
values between 0 and 9.51%, with U and Th in the ranges 115–483
and 101–681 ppm, respectively. Th/U ratios are in the range
0.79–1.73, consistent with magmatic zircon.
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