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Metasomatism of ferroan granites in the
northern Aravalli orogen, NW India:
Geochemical and isotopic constraints, and its
metallogenic significance
ARTICLE in INTERNATIONAL JOURNAL OF EARTH SCIENCES · FEBRUARY 2014
Impact Factor: 2.09 · DOI: 10.1007/s00531-014-1005-x
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Int J Earth Sci (Geol Rundsch)
DOI 10.1007/s00531-014-1005-x
ORIGINAL PAPER
Metasomatism of ferroan granites in the northern Aravalli
orogen, NW India: geochemical and isotopic constraints,
and its metallogenic significance
Parampreet Kaur · Naveen Chaudhri ·
Albrecht W. Hofmann · Ingrid Raczek ·
Martin Okrusch · Susanne Skora · Jürgen Koepke
Received: 8 February 2013 / Accepted: 6 February 2014
© Springer-Verlag Berlin Heidelberg 2014
Abstract The late Palaeoproterozoic (1.72–1.70 Ga) ferroan granites of the Khetri complex, northern Aravalli orogen, NW India, were extensively metasomatised ~900 Ma
after their emplacement, at around 850–830 Ma by lowtemperature (ca. 400 °C) meteoric fluids that attained metamorphic character after exchanging oxygen with the surrounding metamorphic rocks. Albitisation is the dominant
metasomatic process that was accompanied by Mg and Ca
metasomatism. A two-stage metasomatic model is applicable to all the altered ferroan intrusives. The stage I is represented by a metasomatic reaction interface that developed
as a result of transformation of the original microcline–
oligoclase (An12–14) granite to microcline–albite (An1–3)
P. Kaur (*) · N. Chaudhri
Centre of Advanced Study in Geology, Panjab University,
Chandigarh 160 014, India
e-mail: param.geol@gmail.com
A. W. Hofmann · I. Raczek
Max-Planck-Institut für Chemie, Postfach 3060,
55020 Mainz, Germany
M. Okrusch
Lehrstuhl für Geodynamik und Geomaterialforschung, Institut für
Geographie und Geologie, Universität Würzburg, Am Hubland,
97074 Würzburg, Germany
S. Skora
Institute of Mineralogy and Geochemistry, University
of Lausanne, 1015 Lausanne, Switzerland
S. Skora
School of Earth Sciences, University of Bristol, Wills Memorial
Building, Bristol BS81JP, UK
J. Koepke
Institut für Mineralogie, Leibniz Universität Hannover,
Callinstr. 3, 30167 Hannover, Germany
granite, and this stage is rarely preserved. In contrast, the
stage II metasomatic reaction front, where the microclinebearing albite granite has been transformed to microclinefree albite granite, is readily recognisable in the field and
present in most of the intrusives. Some of them lack an
obvious reaction interface due to the presence of stage II
albite granites only. When studied in isolation, these intrusives were incorrectly classified and their tectonic setting
was misinterpreted. Furthermore, our results show that
the mafic mineralogy of metasomatised granites has a significant impact on the characterisation of such rocks in
the magmatic classification and discrimination diagrams.
Nevertheless, the stage I metasomatised granites can be
appropriately characterised in these diagrams, whereas the
characterisation of the stage II granites will lead to erroneous interpretations. The close spatial association of these
high heat producing ferroan granites with iron oxide–copper–gold (IOCG), U and REE mineralisation in the region
indicates a genetic link between the metasomatism and the
mineralisation. World-class IOCG, U and REE deposits are
associated with metasomatised ferroan granites, suggesting that such a relationship may act as a critical first-order
exploration target for undiscovered mineral deposits.
Keywords Ferroan granites · Metasomatism ·
Albitisation · IOCG metallogeny · Khetri complex ·
Aravalli orogen · NW India
Introduction
A-type granites, which are recommended to be more
appropriately termed as ferroan granites (Frost and Frost
2011), have high metallogenic potential and are associated
with many mineral deposits (Dall’Agnol et al. 2012 and
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Int J Earth Sci (Geol Rundsch)
references therein). These granites are enriched repositories
of Fe, REE, U and other incompatible elements, and if suitably metasomatised, can provide important constituents for
ore deposits. Metasomatised ferroan granites are, therefore,
known worldwide to be closely associated, in space and
time, with iron oxide–copper–gold (IOCG), U and REE
mineral deposits. Some of such well-studied occurrences
are located in the Gawler craton of southern Australia
where metasomatism of early Mesoproterozoic (1.59–
1.55 Ga) ferroan granites was responsible for the IOCG–
U–REE mineralisation (e.g. Williams et al. 2005; Pollard
2006). The temporal relationships between the intrusion of
ferroan granites and formation of IOCG–U–REE deposits
vary substantially, i.e. these deposits can form either immediately after the emplacement of the granites or sometimes
more than a billion years later as exemplified by the Olympic Dam and the Curnamona province deposits of southern
Australia, respectively (Elburg et al. 2012). This indicates
that the study of metasomatised ferroan granites is of great
importance in localising important ore deposits.
Ferroan granites are widespread in the Khetri and Alwar
complexes of the northern Aravalli orogen, NW India
(Fig. 1; e.g. Biju-Sekhar et al. 2003; Kaur et al. 2006a,
2007, 2011a). These late Palaeoproterozoic (1.72–1.70 Ga)
high heat producing granites in the Khetri complex, which
host IOCG and uranium mineralisation (Knight et al.
2002), are extensively metasomatised (Kaur et al. 2011a,
2012). Albitisation is the main metasomatic process that
affected ferroan granites of the Khetri complex. The subtle occurrence of the unaltered original granite, which has
so far been observed only in one intrusive, the Dosi pluton, reflects the pervasive nature of metasomatism in the
region. This enabled Kaur et al. (2012) to present a twostage metasomatic model for albitisation in the Khetri
complex that resulted in the formation of two metasomatic
fronts. These advanced through the original granite simultaneously, but at different rates. The leading metasomatic
front converted oligoclase of the original granite to albite,
producing microcline-bearing albite granite and a second, trailing front converted microcline to albite, forming
microcline-free albite granite (for details see Kaur et al.
2012).
This model is yet to be tested for other intrusions in the
northern Aravalli orogen where either both metasomatic
fronts or only the second metasomatic front is represented.
Moreover, in some of the intrusions, only the completely
albitised granites, without any metasomatic reaction front,
are exposed. For such severely metasomatised rocks, it
becomes difficult to ascertain their original protoliths.
When such ferroan granites were studied in isolation,
this led to their misclassification and to a misinterpretation of their tectonic setting. Furthermore, the nature and
volume of mafic phases in these ferroan rocks varies, as
13
metasomatism has produced either totally new mafic minerals or the same mafic phases with different composition,
or their complete disappearance in the severely albitised
rocks. For example, in the granites studied by Kaur et al.
(2012), biotite, although having remained annite during the
first stage of metasomatism, changed its composition from
XFe 0.71–0.74 to XFe 0.90–0.91 and nearly disappeared in
the second stage. Alongside, the original ferropargasite
amphibole was altered to hastingsite during the first stage
of metasomatism, and the hastingsite was subsequently
transformed to actinolite in the final stage. There are other
albitised ferroan granites in the region with either biotite
or clinopyroxene as the original mafic minerals that have
been transformed to amphiboles of varied compositions.
The significance and the impact of these mafic mineralogical transformations on the characterisation of metasomatic
granites have not been studied in detail.
This work complements our previous study (Kaur et al.
2012) by considering additional metasomatised intrusives
from the northern Khetri complex with entirely new objectives. In this study, we aim to (1) establish the temporal
relations between the ferroan granite intrusion, metasomatism and the IOCG–U mineralisation in the Khetri complex by taking into account recently available geological
and geochronological information; (2) present new petrological, geochemical and isotopic data for metasomatically
altered ferroan granites that show different original mafic
mineralogy; and in addition, (3) we demonstrate the pitfalls in the petrological and tectonic interpretations if such
intensely metasomatised rocks are not studied with extreme
care.
Geological setting
The Precambrian geology of the Aravalli orogen (Fig. 1a)
is characterised by two prominent Proterozoic cover
sequences, the Aravalli fold belt and the Delhi fold belt,
which unconformably rest over a Palaeo–Neoarchaean (ca.
3.3–2.5 Ga) basement (Fig. 1b; e.g. Sinha-Roy et al. 1998;
Roy and Jakhar 2002) known as the Banded Gneissic Complex or BGC (Heron 1953) in the southern part and the
Sandmata complex in the central part (Roy et al. 2012).
The present study is confined to the Khetri complex,
which is the westernmost entity of the northern Delhi fold
belt (Fig. 1b) and forms an important metallogenic terrane. Here, the rocks of the Delhi Supergroup form a cover
sequence over a Palaeoproterozoic basement. The Delhi
rocks are divisible into two lithostratigraphic units, an
arenaceous-dominated older sequence (the Alwar Group)
and an argillaceous-dominated younger unit (the Ajabgarh Group) with a gradational and conformable contact
between the two (e.g. Das Gupta 1968).
Int J Earth Sci (Geol Rundsch)
The basement, which is largely composed of 1.72–
1.70 Ga ferroan granites (Fig. 1c) and their cover sequence,
records multiple phases of deformation and medium- to
high-grade regional metamorphism (e.g. Sarkar and Dasgupta 1980; Lal and Ackermand 1981; Naha et al. 1988;
Gupta et al. 1998). Judging from microprobe dating of
monazites, this metamorphism likely took place between
ca. 950–910 Ma (Kaur et al. 2006b; Pant et al. 2008). The
metasomatism in the region is constrained to have occurred
at around 850–830 Ma as indicated by an U–Pb SHRIMP
titanite age (Knight et al. 2002) and a whole-rock–mineral
(clinopyroxene and scapolite) Sm–Nd isochron age (Kaur
Fig. 1 a Map showing the extent of the Aravalli orogen in NW India;
b simplified geological map of the Aravalli orogen showing major
Precambrian lithotectonic units (modified after Roy 1988; Roy and
Jakhar 2002) and location of the study area, and c generalised geolog-
ical map of the northern Khetri complex showing the distribution of
the ferroan granites (compiled after Heron 1923; Geological Survey
of India 1997; Gupta et al. 1998 and our own observations)
13
Int J Earth Sci (Geol Rundsch)
et al. 2013a). The north-westernmost part of the Khetri
complex represents an older (1.85–1.82 Ga) allochthonous
terrane of basement rocks (probably equivalent to the Aravalli Supergroup), which are now juxtaposed tectonically
along a shear zone with the younger Delhi Supergroup
sequences exposed in the east (Kaur et al. 2009, 2013b).
These ~1.82 Ga subduction-related magnesian granitoids
do not show any evidence of metasomatism, indicating
that this terrane was docked to its current position after the
metasomatic event in the region (for details see Kaur et al.
2009).
Field characteristics
Four granitoid plutons located around the villages of Tehara,
Bansiyal, Gorir and Gothara in the northern Khetri complex
(Figs. 1c, 2) were studied for the present work. Their petrological, geochemical and isotopic characteristics are compared
with other well-studied intrusives of Dabla, Biharipur and
Dosi (Chaudhri et al. 2003; Kaur et al. 2006a, 2012) that occur
in the north-eastern extremity of the same region (Fig. 1c).
It should be noted that, owing to the deformation during the metamorphic overprint, the Khetri granites attained
Fig. 2 Detailed geological maps of the a Tehara, b Bansiyal and c Gorir intrusives; d is the generalised geological map of the Gothara intrusive
(after Geological Survey of India 1997)
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Int J Earth Sci (Geol Rundsch)
a foliated structure and consequently should be designated
as meta-granites. The foliation was, however, widely obliterated by the subsequent metasomatic albitisation and
dissolution of mafic phases or replacement of biotite by
amphibole (see below) leading to a structural appearance of
undeformed granite. Therefore, for simplicity, we maintain
the traditional name granites for these rocks. Furthermore,
as the original oligoclase and in some cases, both oligoclase
and K-feldspar have been transformed to albite, these granites are attributed to the field of “normal granites” (field 3
of QAP diagram) as per the recommendations of the IUGS
(Streckeisen 1976), and the albitised granites are named
according to the nature of feldspars present in these rocks,
such as microcline–albite granite or albite granite. The term
alkali-feldspar granite has been discontinued for these rocks
(see Chaudhri et al. 2003; Kaur et al. 2006a, 2007, 2011a).
Tehara
The Tehara pluton, located about 2 km east of Khetri, is an
elongated intrusive (ca. 2 km × 1 km), exposed in the axial
depression of an anticline (Das Gupta 1968). A sedimentary
breccia, similar to that in the Gothara intrusion (see below),
containing granitic and quartzitic material in an iron oxide
ore matrix runs almost all along the periphery of the pluton. This is followed by the Alwar quartzite sequence and
other rocks of the Delhi Supergroup. Detailed mapping carried out during this study reveals that the albite granite is the
most widespread facies, whereas the microcline–albite granite is present as irregular, relic-like enclave bodies within
the albite granite (Fig. 2a). The microcline–albite granite is
foliated in a NE–SW direction with westerly dips; it crops
out as local topographic highs, separated by low-lying albite
granite (Fig. 3a, b). A small outcrop of the original granite, exposed in the eastern part, occurs in contact with the
microcline–albite granite. The albite granite is profusely
commingled, at the margins, with mafic rocks, similar to
what is observed at Biharipur (for details see Kaur et al.
2006a, 2013a). In addition, there are pegmatites, occurring
mainly in the northern part, and a few NW–SE trending dolerite dykes cutting through the granites, pegmatites and the
quartzite of the cover sequence. This phase of mafic magmatism postdates the regional metasomatic event because
the calcic plagioclase in these rocks has preserved its magmatic composition (An57.8±8.7; Kaur et al. unpublished
data) and is not albitised. The time of original emplacement
of this pluton was dated by the Pb–Pb zircon evaporation
method at 1,711 ± 0.5 Ma (Kaur et al. 2007).
Bansiyal
The Bansiyal body, located about 4 km SE of Tehara
(Fig. 1c), is contiguous with the Tehara pluton. It is an
oval-shaped small (ca. 1 km × 1 km) intrusive, which is
surrounded by quartzites in the north, unclassified metamorphics in the west, while its eastern and southern portions are covered by alluvium (Fig. 2b). The Bansiyal body
is essentially made up of albite granite, showing a small
outcrop of biotite-bearing foliated albite granite in the middle that is surrounded by amphibole-bearing albite granite.
The biotite-bearing albite granite has yielded a U–Pb zircon age of 1,727 ± 28 Ma (Kaur et al. 2011a), which is
considered as the time of crystallisation of this intrusive.
Gorir
The Gorir intrusive is located about 16 km ENE of Khetri,
and the granites here crop out as a number of isolated hillocks within alluvial cover trending in NE–SW direction and
covering a distance of at least 6 km (Fig. 2c); the south-western extension of these rocks is much larger than the mapped
area (see Dinkar et al. 2013). No previous work on these
granites is available. Most of these rocks are albite granite
with a small outcrop of microcline–albite granite occurring
to the south of Gorir (Fig. 2c). The latter is brick red in colour (Fig. 3c) and also occurs as relict patches or veins within
the grey coloured albite granite (Fig. 3d), defining a metasomatic reaction front between the two rock types. The albite
granites, north-west of Gorir, also contain bands of magnetite (Fig. 3e), and these rocks show extensive commingling
with mafic material, in places producing a banded structure.
The microcline–albite granite has provided a U–Pb zircon
age of 1,727 ± 8 Ma (Kaur et al. unpublished data).
Gothara
The Gothara pluton, situated about 8 km NE of Khetri,
extends over a distance of ~7 km in NW–SE direction as separate hillocks. The best exposures occur to the SW of Gothara,
covering a distance of about 1.5 km. The eastern margin of the
pluton is covered by alluvium, whereas a sedimentary breccia (Gupta et al. 1998) occurs all along its north-western margin. The breccia is followed by an albitic quartzite, containing iron bands (Fig. 2d), and shows a sharp contact with the
granite (Fig. 3f). An in situ laser ablation ICP–MS study on
detrital zircons from the albitic quartzite unit indicates that the
age of deposition of these sediments was <1.7 Ga (Kaur et al.
2011b). Previous workers described the Gothara intrusive as
quartz-hornblende granites with layers and patches of granophyres (Das Gupta 1968), an I-type K-feldspar–albite granite
of magmatic origin (Bhattacharyya and Dasgupta 1981; Gupta
et al. 1998), a hydrothermally altered granite (Knight et al.
2002) or even as unaltered plagiogranite of magmatic origin,
providing evidence of oceanic magmatism in the region (Kaur
and Mehta 2005). SHRIMP U–Pb zircon data yield an age of
1,691 ± 4 Ma for the Gothara pluton (Knight et al. 2002).
13
Int J Earth Sci (Geol Rundsch)
Fig. 3 Representative field photographs illustrating the general characteristics of the albitised ferroan granites. a Microcline–albite granite occurring as local topographic highs (marked by yellow arrows)
separated by the low-lying albite granite in the Tehara intrusive; b
close-up showing microcline–albite granite and the albite granite at
Tehara. The dashed line marked the contact between the two; c brick
red coloured microcline–albite granite at Gorir; d the same micro-
cline–albite granite occurring as relict patches within the grey albite
granite, illustrating the stage II metasomatic reaction interface; e iron
oxide band within the weathered Gorir albite granite and f Gothara
albite granite showing a sharp contact with the brecciated zone. The
coin is 2.5 cm across in Fig. 3c, the length of hammer’s handle is
28 cm in Figs. 3d and e and the length of the pen is about 14 cm in
Fig. 3f
Analytical procedures
Chemie, Mainz (JEOL Superprobe JXA-8200) and the
Mineralogisches Institut, Universität Würzburg (CAMECA
SX 50). In Würzburg, a CAMECA SX 50 electron microprobe was used, which was equipped with three independent WDS channels. Acceleration voltage was 15 kV, beam
Electron microprobe analyses of minerals were carried out
at the Institut für Mineralogie, Leibniz Universität Hannover (CAMECA SX 100), the Max-Planck-Institut für
13
Int J Earth Sci (Geol Rundsch)
current 10 or 15 nA and integration time 20 s (30 s for Fe);
beam size was 3–4 microns on feldspars and 1 micron on
other minerals. Metal oxides and synthetic mineral standards supplied by CAMECA were used for reference and
the PAP program for correction procedures (Pouchou and
Pichoir 1985). The analytical details for Hannover and
Mainz EMP analyses are provided in Kaur et al. (2006a).
In general, the relative analytical errors were approximately
1 % for element concentrations >2 wt% and 5 % for minor
elements (<2 wt%), and the detection limit was at about
0.1 wt%. Mineral abbreviations used are those given by
Whitney and Evans (2010).
Major and trace elements were analysed, using Philips
PW 1480 X-ray fluorescence (XRF) spectrometers in
the Department of Geosciences, University of Mainz (for
details see Kaur et al. 2009) and in the Mineralogisches
Institut, Universität Würzburg (see Kaur et al. 2006a). REE
as well as Hf, Ta and U concentrations of the samples were
obtained from the Activation Laboratories Ltd., Ontario,
Canada, by ICP–MS, using a PerkinElmer SCIEX ELAN
6000 ICP–MS. Duplicate measurements of sample GL-6
provided a precision generally better than 1 % (SD). The
accuracy of the analyses is monitored by many international reference materials, e.g. W2, WMG1, STM1, DNC1,
BIR1, SY3, etc. Based on the rock standard STM1, it is
usually better than 5 %, except for La, Er (<10 %) and Pr
(<30 %).
Sm–Nd and Rb–Sr isotopic analyses were carried out on
a Finnigan MAT 261 thermal ionisation mass spectrometer
in the Max-Planck-Institut für Chemie, Mainz as per the
procedures outlined by Kaur et al. (2009). Twenty analyses of the La Jolla Nd standard performed during this study
yielded a mean 143Nd/144Nd ratio of 0.511854 ± 7 (2σ),
whereas 17 analyses of NBS 987 provided a mean 87Sr/86Sr
ratio of 0.710231 ± 17 (2σ). For oxygen isotope analyses,
~1.5–2.5 mg whole-rock samples were extracted on a platinum sample holder in a F2 atmosphere, using a CO2 laser
system for heating (for details see Sharp 1992). Purified
O2 gas was analysed directly on a Finnigan MAT 253 mass
spectrometer at the University of Lausanne, Switzerland.
The Gore Mountain garnet standard (δ18O = 5.8 ‰) used
for reference (Valley et al. 1995) suggests an analytical
uncertainty (1σ) of 0.23 ‰. All oxygen isotope values are
reported relative to VSMOW.
Petrography
Full petrographic description of the granitoids is given in
the Appendix along with representative mineral analyses.
Here, we outline only some of the main features which
are particularly relevant in the context of this paper. Rock
referred to as “original granite” is the only rock type that
has not been affected by albitisation. It is only preserved
at Tehara in a small outcrop area and consists of the
major phases quartz, microcline, oligoclase and biotite.
The “microcline–albite granite” refers to the moderately
albitised granite in which there is a complete replacement
of oligoclase by albite and an incipient replacement of
K-feldspar by albite. This facies is well preserved at Tehara
only, with a minor outcrop at Gorir (for differences in mafic
minerals, see full description in the “Appendix”). The albite
granite is the completely albitised rock and is characterised
by almost complete replacement of K-feldspar and oligoclase by albite, often with small amounts of mafic minerals.
This rock type exclusively crops out at Gothara, covers a
large tract around Gorir and also forms a large part of the
Tehara intrusive.
Geochemical and isotopic results
The SiO2 contents in most of the intrusives are ≥70 wt%,
indicating their evolved nature (Tables 1, 2, 3). Overall,
the albite granites are more enriched in Si than the microcline–albite granites. All the albite granites can be distinguished from other granites by their disparity in Na and K
abundances, with Na2O as high as 8 wt% and K2O as low
as 0.1 wt%. The Rb, Ba and Pb abundances are in consonance with the extremely low K2O values. Using Na/K as
an index of albitisation, a progressive depletion in Rb from
178 ppm in the original granite to <1 ppm in the albite
granite occurred with advancing metasomatic replacement
of K-feldspar by albite and biotite by amphibole (Fig. 4).
The amphibole-bearing albite granites of Bansiyal and
Tehara as well as the Gothara albite granites show lower
Fe and higher Ca values than the biotite-bearing granites,
including the original granite at Tehara. No such particular trend is seen for the concentrations of Fe, Ca or Mg
in the Gorir microcline–albite granite and albite granites,
where Ca tends to be lower in the latter. Th/U ratios of the
original granite and the microcline–albite granite at Tehara
and some of the albite granites of Gothara are close to the
“normal” crustal Th/U ratio of about 4 (Tables 1, 2). By
contrast, the other albitised granites mostly show relatively
high Th/U ratios, especially those of Gorir (Table 3), indicating U loss. The internal heat production values of the
Tehara, Bansiyal and Gothara granites are high, mostly
around 5 µWm−3, but some albite granites show values as
low as 4.1 µWm−3 due to an extreme loss of K and also U
from these rocks. The relatively high loss of U in addition
to K in granites of the Gorir pluton may have resulted in
even lower heat production values of 3.8–2.6 µWm−3.
Overall, the REE of all the granites are enriched and
generally fractionated; however, the degree of fractionation or the degree of REE enrichment differs. All the
13
Int J Earth Sci (Geol Rundsch)
Table 1 Whole-rock chemical compositions for the Tehara granitoids, Rajasthan, NW India
Sample no
Original granite
Microcline–albite granite
GL-7
GL-2
GL-5
GL-8
GL-1
GL-3
GL-4
GL-6
(Lat °N)
27°59′
57.2″
28°0′
22.2″
27°59′
50.2″
28°0′
7.0″
28°0′
15.2″
28°0′
17.2″
28°0′
7.8″
27°59′
56.2″
(Long °E)
75°49′
7.4″
70.9
0.65
12.58
5.11
0.03
0.57
1.00
3.26
4.55
0.13
0.61
99.4
75°48′
25.8″
70.7
0.62
12.60
4.87
0.03
0.66
1.01
3.63
4.35
0.13
0.88
99.5
75°48′
44.8″
69.4
0.69
12.60
5.43
0.02
0.67
1.21
3.99
3.83
0.14
1.04
99.0
75°49′
6.8″
71.6
0.61
12.54
4.40
0.02
0.75
0.70
4.40
3.43
0.13
0.81
99.4
75°48′
37.8″
71.8
0.65
12.86
4.24
0.05
0.75
1.51
7.07
0.09
0.13
0.46
99.6
75°48′
20.8″
72.6
0.63
12.64
2.91
0.05
0.99
1.61
6.92
0.10
0.13
0.65
99.2
75°48′
45.2″
73.0
0.58
12.69
3.48
0.05
0.72
1.44
6.94
0.13
0.12
0.44
99.6
75°48′
51.8″
72.7
0.66
12.57
3.35
0.05
0.81
1.61
6.79
0.11
0.12
0.53
99.3
SiO2 (wt%)
TiO2
Al2O3
Fe2Ot3
MnO
MgO
CaO
Na2O
K2O
P2O5
LOI
Sum
Albite granite
V (ppm)
30
31
34
38
32
34
28
30
Cr
Co
Ni
Zn
Ga
Rb
Sr
Y
Zr
Nb
Ba
Pb
Th
U
Hf
Ta
La
Ce
Pr
Nd
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
7
18
3
17
19
178
28
55
326
21
551
7
36.6
8.7
11.9
1.6
59.8
108
12.4
45.6
10.3
1.45
9.2
1.6
9.0
1.8
5.6
0.83
5.0
0.74
271.3
2
27
5
15
18
142
25
53
314
21
521
8
36.6
8.1
10
2.4
69.6
123
13.8
50.0
10.9
1.58
9.1
1.6
8.8
1.7
5.4
0.78
5.1
0.72
302.1
6
23
5
12
19
113
20
55
322
23
462
7
33.8
9.1
10.5
0.7
4
21
4
18
18
89
25
48
312
20
313
7
35.2
8.1
9.7
2.9
5
19
8
11
18
2
17
43
327
22
13
6
35.4
8.6
10.1
3.4
7
17
7
12
19
1.3
20
52
341
22
12
7
37.2
6
11.9
2.9
61.6
105
12.0
44.5
9.6
1.32
8.8
1.5
8.3
1.7
5.4
0.78
5.0
0.75
226.3
3
19
4
12
18
4
16
43
306
21
18
12
43.7
6.9
11.2
0.8
5
29
3
12
18
1.3
13
56
316
21
6
8
35.5
6.5
11.2
2.5
56.1
100
11.7
42.4
9.5
1.10
8.5
1.5
8.6
1.8
5.5
0.80
4.9
0.71
253.1
∑REE
13
Int J Earth Sci (Geol Rundsch)
Table 1 continued
Sample no
Original granite
Microcline–albite granite
GL-7
GL-2
GL-5
Albite granite
GL-8
GL-1
GL-3
GL-4
GL-6
(La/Yb)N
8.1
9.3
8.4
7.8
(La/Sm)N
3.6
4.0
4.0
3.7
(Gd/Yb)N
Eu/Eu*
Th/U
HPU
1.5
1.4
1.4
1.4
0.45
4.2
5.3
0.48
4.5
5.1
3.7
5.1
4.3
4.9
4.1
4.8
0.44
6.2
4.2
6.3
4.9
0.37
5.5
4.2
ASI
1.07
1.03
1.00
1.05
0.91
0.90
0.91
0.90
Eu/Eu* = EuN/(SmN × GdN)½. HPU = D[0.0967U (ppm) + 0.0263Th (ppm) + 0.035 K (wt%)] µWm−3 is heat production units (after
Beardsmore and Cull 2001) where D refers to density in gcm−3 and is taken as 2.7 for all the samples
Source: Data from Kaur et al. (2007). Bold Rb value: data from TIMS (see Table 4)
Tehara granites show exceptionally homogeneous REE
concentrations and profiles rather similar to that of the
original granite at Dosi. The patterns are fractionated with
(La/Yb)N = 7.8–9.3 and (La/Sm)N = 3.6–4.0, but nearly
flat heavy REE (HREE) profiles [(Gd/Yb)N = 1.4–1.5) and
prominent negative Eu anomalies with Eu/Eu* = 0.37–
0.48 (Fig. 5a). The Bansiyal albite granites display similar
REE patterns to those of Tehara (Fig. 5b), but are slightly
more fractionated [(La/Yb)N = 10.9–13.8], and the negative Eu anomalies are more distinct in the amphibolebearing albite granites (Eu/Eu* = 0.56–0.78). The REE
patterns of the Gorir granites (Fig. 5c) are also similar,
although relatively less fractionated than those of Tehara
[(La/Yb)N = 2.8–5.5], and are also nearly identical to those
of the original granite of Dosi. In contrast, the REE patterns of the Gothara albite granites are quite variable with
(La/Yb)N ratios ranging from 2 to 20 (Table 2; Fig. 5d).
These patterns fall within the range of the Biharipur, Dabla
and Dosi albite granites (Fig. 5d).
In a primitive mantle-normalised multielement diagram
(Fig. 5e), the original granite and the microcline–albite
granites of all studied plutons show similar patterns, characterised by variably pronounced negative spikes in Ba,
Sr, P and Ti, and variably marked enrichment in REE. The
completely albitised granites additionally display negative
spikes for K and Rb with more pronounced troughs for Ba,
Sr, P and Ti (Fig. 5f).
Figure 6 shows the Sm–Nd data of the newly analysed granites together with our previously published data
(Kaur et al. 2012). Although there is substantial scatter in
this “isochron” diagram, the combined dataset appears to
show two separate trends, one with a slope roughly corresponding to 2.6 Ga for the granites and most of the albite
granites, with 147Sm/144Nd < 0.15, the other one for four
samples of albite granites, all with 147Sm/144Nd > 0.15;
in other words, those samples (from the Dosi, Dabla and
Gothara intrusives) showing evidence for significant LREE
depletion. The slope of the latter trend corresponds to
about 1.4 Ga. We, therefore, speculate that selective LREE
loss occurred during an advanced substage of albitisation
through the partial loss of allanite, thought to be the major
host of LREE in these rocks (see also Kaur et al. 2012).
The apparent 2.6 Ga “isochron”, on the other hand, would
be older than the actual emplacement of the granites, but
would match the Nd model ages. Our tentative interpretation that allanite-related LREE loss occurred during albitisation, rather than via magmatic fractionation crystallisation of allanite, is consistent with the REE mineralisation in
the Khetri complex noted by Knight et al. (2002).
A severe disturbance in the Rb–Sr system is noted for
all the analysed granite samples because of the high mobility of both Rb and Sr during metasomatism (Kaur et al.
2012). In the albite granites samples, which are devoid of
both biotite and K-feldspar, the Rb/Sr ratios are close to
zero because of extreme loss of Rb during albitisation of
K-feldspar. Such samples show high initial 87Sr/86Sr ratios
(0.710–0.725), but low 87Rb/86Sr ratios (Table 4). Kaur
et al. (2012) inferred that these high “initial” Sr values in
these albite granites are due to the severe loss of Rb, which
effectively “froze” relatively radiogenic Sr at the time of
albitisation.
The whole-rock δ18O values (Table 4) for the original,
the microcline–albite granites of Tehara and the biotitebearing albite granites of Bansiyal are almost identical
(7.9–8.1 ‰). Generally, the albite granites have higher
δ18O values than the original granites or the microcline–
albite granites (Fig. 7), except for the Tehara albite granites, which possess somewhat lower δ18O values. Overall,
there is an increasing trend of whole-rock δ18O values
with progressive albitisation and Si enrichment (Fig. 7).
In particular, the whole-rock δ18O values at Gorir show a
significant increase from 8.1 ‰ in the microcline–albite
granite to 10.0 ‰ in the albite granite with concomitant increase in Si (Fig. 7b). A similar relationship has
also been observed for the Dosi, Biharipur and Dabla
counterparts.
13
Int J Earth Sci (Geol Rundsch)
Table 2 Whole-rock chemical compositions for the Bansiyal and Gothara albite granites, Rajasthan, NW India
Sample no
BS-4a
BS-7
BS-8
GO-1
GO-2
GO-3
GO-4
GO-5
(Lat °N)
27°59′
1.9″
27°59′
23.6″
27°59′
24.4″
28°03′
16.8″
28°03′
22.2″
28°03′
41.3″
28°03′
30.1″
28°03′
44.6″
(Long °E)
75°51′
41.9″
71.2
0.60
12.93
5.33
0.07
0.88
0.88
5.69
1.78
0.13
0.71
100.2
75°51′
46.0″
73.1
0.63
12.75
3.51
0.05
0.61
1.09
7.16
0.24
0.11
0.46
99.7
75°52′
7.2″
74.1
0.51
13.35
2.18
0.04
0.42
1.19
7.39
0.34
0.14
0.38
100.0
75°48′
18.9″
73.8
0.63
12.21
1.17
0.03
0.34
2.27
6.96
0.10
0.11
1.80
99.4
75°48′
27.4″
74.6
0.62
11.91
0.93
0.02
0.21
1.77
6.82
0.08
0.14
1.53
98.6
75°48′
31.6″
75.4
0.69
12.19
0.83
0.01
0.15
1.55
7.01
0.12
0.15
0.60
98.7
75°48′
40.5″
73.7
0.63
12.79
1.16
0.03
0.75
2.60
7.27
0.13
0.12
0.59
99.8
75°48′
34.5″
74.7
0.69
12.40
0.97
0.02
0.49
1.86
7.03
0.13
0.14
0.67
99.1
SiO2 (wt%)
TiO2
Al2O3
Fe2Ot3
MnO
MgO
CaO
Na2O
K2O
P2O5
LOI
Sum
V (ppm)
47
47
45
44
28
52
30
50
Cr
Co
Ni
Zn
Ga
Rb
Sr
Y
Zr
Nb
Ba
Pb
Th
U
Hf
Ta
La
Ce
Pr
Nd
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
∑REE
<10
20
6
7
14
29
16
38
283
17
93
7
38
4.9
7.9
1.8
69.1
118
12.9
46.1
9.4
1.36
7.8
1.2
6.5
1.2
3.9
0.57
3.4
0.50
281.5
<10
34
9
6
18
9.11
19
51
288
18
21
9
42
7.4
7.9
1.9
86.6
147
17.4
65.8
14.1
3.14
10.7
1.6
8.4
1.6
5.0
0.71
4.3
0.62
367.0
13
31
6
<5
21
6
13
58
251
19
20
<5
39
9.8
7.3
2.0
71.9
132
15.3
56.9
12.6
2.13
10.7
1.8
9.6
1.8
5.5
0.77
4.5
0.61
326.1
5
20
4
8
15
2.43
19
59
322
25
9
8
37
9.6
11.4
1
116
192
20.3
69.6
13.1
1.94
10.7
1.8
9.7
2.0
6.0
0.86
5.1
0.74
449.8
7
32
5
5
16
3
21
51
344
23
0
8
38.1
6.8
12.9
2.5
57.2
105
10.8
40.2
8.7
1.84
8.5
1.6
9.1
1.7
5.2
0.79
4.7
0.67
256.0
4
30
3
6
17
3
19
113
339
27
7
8
37.3
8.3
10.1
2.7
253
431
41.0
133
21.1
4.19
19.4
3.7
20.8
4.0
12.3
1.79
10.4
1.35
957.0
5
30
4
9
17
1.96
20
62
326
26
6
9
40.8
9.9
9.6
3.2
10.0
20.7
3.12
15.1
5.7
1.37
7.9
1.7
9.9
2.1
6.2
0.94
5.4
0.73
90.9
2
34
3
13
15
1.76
24
67
357
27
11
10
38.7
7.3
13.4
3.6
185
299
30.0
103
18.6
3.62
15.0
2.4
12.3
2.2
6.8
1.05
6.2
0.84
686.0
(La/Yb)N
13.8
13.7
10.9
15.5
8.3
16.5
1.26
20.3
13
Int J Earth Sci (Geol Rundsch)
Table 2 continued
Sample no
BS-4a
BS-7
BS-8
GO-1
GO-2
GO-3
GO-4
GO-5
6.2
(La/Sm)N
4.6
3.8
3.6
5.5
4.1
7.5
1.1
(Gd/Yb)N
Eu/Eu*
Th/U
HPU
1.9
2.0
1.9
1.7
1.5
1.5
1.2
2.0
0.48
7.8
4.1
0.78
5.7
4.9
0.56
4.0
5.4
0.50
3.9
5.0
0.65
5.6
4.4
0.63
4.5
4.7
0.62
4.1
5.4
0.66
5.3
4.6
ASI
1.03
0.93
0.93
0.79
0.84
0.86
0.77
0.84
Eu/Eu* = EuN/(SmN × GdN)½. HPU = D[0.0967U (ppm) + 0.0263Th (ppm) + 0.035 K (wt%)] µWm−3 is heat production units (after
Beardsmore and Cull 2001) where D refers to density in gcm−3 and is taken as 2.7 for all the samples
a
Data from Kaur et al. (2011a). Bold Rb value: data from TIMS (see Table 4)
Discussion
The occurrence of two kinds of albitised granites, viz. the
microcline–albite granite and the albite granite, reflects
the two different stages of progressive metasomatism. The
almost complete replacement of magmatic oligoclase to
albite and incipient alteration of K-feldspar in the microcline–albite granite indicate that these rocks are moderately
metasomatised. The more severely metasomatised albite
granites show nearly complete transformation of both microcline and oligoclase to albite. The albite granites are characterised by the whitening of feldspars, lack of foliation,
occurrence of chessboard albite, presence of microcline as
relics and exceptionally low K and high Na abundances. It is
this difference in mineralogy and chemistry between the two
variably metasomatised granites, which at times allows to
clearly recognise the metasomatism on outcrop scale in the
form of a reaction interface developed as a result of transformation of pink K-feldspar to white albite (Putnis and Austrheim 2010). That the original granite occurs in contact with
the microcline–albite granite and not with the albite granite
supports the previously proposed two-stage metasomatic
replacement model for the albitised granites in the northeastern part of the Khetri complex (Kaur et al. 2012). The
microcline–albite granite represents the first stage of albitisation (stage I), formed by the leading replacement front where
oligoclase is converted to albite, and the albite granite is an
outcome of the most advanced (second) stage of albitisation
(stage II) where K-feldspar is transformed to albite by the
second, slow-moving, trailing front.
Isocon analysis
In order to assess the chemical changes that resulted from
metasomatism during the two stages, we present isocon
diagrams (Fig. 8), following the approach of Grant (1986,
2005) and as more specifically outlined in Kaur et al.
(2012). Only at Tehara, both stages (stage I and stage II)
of metasomatism are preserved, whereas at Gorir, merely
stage II can be observed. In the Bansiyal and Gothara
intrusives, it is not possible to apply isocon analysis, as
they completely consist of albite granites that are the final
product of stage II metasomatism. As Al has remained conserved during the metasomatism of the Khetri ferroan granites (Kaur et al. 2012), conforming to the common immobility of Al during a metasomatic event (Grant 1986), the
isocons are drawn through the data point of this element
and the origin. Two isocon diagrams are presented for the
Tehara intrusive, with the first (Fig. 8a) representing the
chemical changes during alteration of the original granite
(sample GL-7) to the microcline–albite granite (samples
GL-2) and the second (Fig. 8b) documenting the changes
during the transformation of the microcline–albite granite
to the albite granite (average of samples GL-3 and GL-6).
Element mobility during the stage I metasomatic alteration
At Tehara, the stage I microcline–albite granite shows addition of H (for H2O, determined by loss on ignition, LOI), relative to the original granite, indicating hydration at this stage
of metasomatism. This may be partly related to the formation
of epidote and, more abundantly, to sericitisation of plagioclase in the microcline–albite granite. The minor gain in Na is
reflected by the transformation of oligoclase to albite, whereas
the slight Rb loss may be related to the incipient alteration of
K-feldspar, although K has remained virtually immobile. The
slight gain in Mg may be related to the relatively Mg-rich biotite in the microcline–albite granite (sample GL-2; Table 7).
The minor gain in La is more likely to be a result of progressive fractional crystallisation in the original granite. The other
elements plot either on or close to the isocon, indicating no
significant losses or gains for most of the elements.
Element mobility during the stage II metasomatic
alteration
The gains and losses are more pronounced during the
stage II, i.e. the formation of the albite granite from the
13
Int J Earth Sci (Geol Rundsch)
Table 3 Whole-rock chemical compositions for the Gorir albitised granites, Rajasthan, NW India
Sample no
Mc–Ab granite
Albite granite
GR-27
GR-1
GR-4
GR-5
GR-13
GR-15
GR-16
GR-17
GR-18
GR-20
GR-22
(Lat °N)
28°0′
43.9″
28°2′
3.4″
28°01′
34.2″
28°01′
37.7″
28°0′
48.5″
28°0′
11.3″
28°0′
11.2″
28°0′
1.4″
27°59′
56.2″
27°59′
36.1″
28°0′
14.8″
(Long oE):
75°57′
4.9″
69.0
0.60
13.50
3.69
0.02
0.47
2.30
5.32
4.15
0.13
0.61
99.8
75°57′
19.7″
72.1
0.48
13.32
3.47
0.02
0.47
1.21
6.69
1.81
0.09
0.20
99.9
75°57′
8.6″
72.4
0.56
13.74
1.56
0.02
0.71
2.03
8.13
0.11
0.07
0.36
99.7
75°57′
18.5″
72.8
0.45
13.27
3.32
0.02
0.48
1.17
7.11
1.15
0.08
0.42
100.3
75°57′
1.5″
68.1
0.69
13.44
5.84
0.03
0.47
1.71
7.86
0.15
0.15
0.64
99.1
75°56′
31.5″
67.3
0.73
13.50
6.39
0.02
0.63
2.41
7.99
0.13
0.16
0.47
99.7
75°56′
38.8″
73.1
0.35
13.41
1.72
0.02
0.56
0.93
7.98
0.13
0.01
0.59
98.8
75°56′
46.4″
75.9
0.27
12.90
1.14
0.01
0.24
0.52
7.66
0.11
0.02
0.33
99.1
75°56′
22.5″
74.7
0.40
13.65
1.12
0.01
0.24
0.60
7.77
0.63
0.02
0.46
99.6
75°56′
17.4″
72.7
0.40
12.87
4.32
0.01
0.39
0.76
7.70
0.09
0.07
0.25
99.6
75°56′
57.8″
69.0
0.61
13.49
5.37
0.02
0.56
1.98
7.36
0.98
0.06
0.30
99.7
SiO2 (wt%)
TiO2
Al2O3
Fe2Ot3
MnO
MgO
CaO
Na2O
K2O
P2O5
LOI
Sum
V (ppm)
20
29
30
29
38
35
10
6
10
36
36
Cr
Co
Ni
Zn
Ga
Rb
Sr
Y
Zr
Nb
Ba
Pb
Th
U
Hf
Ta
La
Ce
Pr
Nd
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
<10
34
<5
<5
21
67
22
82
372
18
272
<5
22
6
9.5
1.5
33.3
84
11
46.8
11.3
1.97
12.1
2.1
13.3
2.8
7.9
1.18
8
1.24
237.0
<10
44
<5
<5
16
42
34
72
339
17
343
<5
31
<5
11
32
<5
<5
17
<5
29
67
318
19
33
<5
32
3.7
8.7
1.6
47.6
97.7
12.0
45.9
10.7
1.75
10.2
1.8
10.0
2.1
6.4
0.98
5.9
0.86
253.9
<10
30
<5
<5
17
26
32
73
300
14
208
<5
29
<5
3
31
10
7
23
1.09
23
77
371
19
7
3
22.6
3.9
12.2
1.8
51.0
111
13.7
52.1
12.3
1.95
12.1
2.1
12.2
2.5
7.9
1.20
7.2
1.06
288.3
3
25
13
7
23
0.64
37
88
373
22
20
3
23.5
4
13.1
bdl
4
24
6
6
20
0.43
20
58
249
15
12
2
26.3
3.3
6.8
2.1
28.1
64.2
8.3
33.4
8.4
0.85
8.4
1.6
9.3
2.0
6.0
0.92
5.4
0.74
177.6
3
32
2
3
21
3
15
76
244
18
9
3
33.2
5.3
6.8
2.1
1
24
4
4
20
14.21
16
68
350
18
49
3
33.4
3.4
11.9
2.5
<10
31
6
<5
22
<5
17
71
466
21
<20
<5
31
<5
<10
27
9
<5
21
14
47
68
365
17
153
<5
22
<5
∑REE
13
Int J Earth Sci (Geol Rundsch)
Table 3 continued
Sample no
Mc–Ab granite
Albite granite
GR-27
GR-1
GR-4
GR-5
GR-13
GR-15
GR-16
GR-17
GR-18
GR-20
GR-22
(La/Yb)N
2.8
5.5
4.8
3.5
(La/Sm)N
1.8
2.8
2.6
2.1
(Gd/Yb)N
Eu/Eu*
Th/U
HPU
1.2
1.4
1.4
1.3
0.51
3.7
3.5
>6
3.4
0.51
8.6
3.2
>6
3.2
0.49
5.8
2.6
5.9
2.7
0.31
8.0
2.7
6.3
3.8
9.8
3.3
>6
3.3
>4
2.7
ASI
0.79
0.89
0.81
0.89
0.85
0.78
0.90
0.95
0.94
0.92
0.81
Eu/Eu* = EuN/(SmN × GdN)½. HPU = D[0.0967U (ppm) + 0.0263Th (ppm) + 0.035 K (wt%)] µWm−3 is heat production units (after
Beardsmore and Cull 2001) where D refers to density in gcm−3 and is taken as 2.7 for all the samples
Bold Rb value: data from TIMS (see Table 4)
microcline–albite granite, which is seen only in the Tehara
and Gorir intrusives. At Tehara, the high field strength elements (HFSE) and the REE have remained immobile during this stage, whereas there are significant gains in Na
and losses in Ba, Rb and K. These gains and losses can be
attributed to the almost complete replacement of K-feldspar
to albite by an allochemical, Al-conservative replacement
reaction (e.g. Moore and Liou 1979):
+ K+ aq
+ Na+ aq ⇔ NaAlSi3 Osolid
KAlSi3 Osolid
8
8
(1)
In addition, the gains in Mg and Ca and the attendant
losses in Fe and H are reflected by the transformation of
annite to magnesiohornblende in the albite granite. This
may also point out that the granites experienced Mg and
Ca metasomatism as the formation of magnesiohornblende requires more Ca and Mg, and lesser Fe and H2O
than the annite. This can be shown below by a simplified, Al-conservative, allochemical reaction considering
the biotite of sample GL-2 and the amphibole of sample
GL-6:
K2.0 Mg1.0 Fe2+ 3.9 AlVI 0.5 Ti0.3 (OH)4 AlIV 2.5 Si5.5 O20 + 1.4Na+
annite
+ 3.4Ca+2 + 3.2Mg+2 + 2.7Fe3+ + 10.9H4 SiO4 =
3+ VI
IV
2Na0.3 Ca1.7 Mg2.1 Fe2+
1.6 Fe1.2 Al0.1 (OH)2 Al1.0 Si7 O22
magnesiohornblende
+ 2K+ + 0.7Fe2+ + 0.3FeTiO3 + 0.8Na[AlSi3 O8 ] +
ilmenite
+
12.3H2 O + 19.0H
Fig. 4 Rb (ppm) versus Na/K (cationic) diagram showing the depletion of Rb with progressive albitisation of K-feldspar. Data sources
Dabla, Chaudhri et al. (2003), Biharipur, Kaur et al. (2006a) and
Dosi, Kaur et al. (2012)
albite
(2)
Reaction (2) mirrors, in a schematic way, the decomposition of annite to form magnesiohornblende with
minor albite and ilmenite and that a part of the K
loss can also be related to the breakdown of biotite to
amphibole.
For the Gorir rocks, we have carried out an isocon analysis between the representative albite granite sample GR-13
and the microcline–albite granite sample GR-27 because
both occur in close proximity in the same outcrop (Fig. 2c).
In addition to gains and losses observed at Tehara, the isocon diagram (Fig. 8c) suggests loss of Ca and significant
gains in Fe and La with minor gain in Ti and P. These compositional changes are likely related to the partial transformation of diopside to amphibole (see above) because the
resultant calcic amphibole is richer in Fe, Ti and poorer in
Ca than its parent diopside. The gain in LREE is attributed
13
Int J Earth Sci (Geol Rundsch)
Fig. 5 Chondrite-normalised
REE patterns for the a Tehara,
b Bansiyal, c Gorir, and d
Gothara granites; primitive
mantle (PRIMA)-normalised
multielement patterns for the e
Tehara original granite and the
microcline–albite granites of
Tehara and Gorir, and f albite
granites of the studied intrusives. Chondrite-normalising
and the primitive mantlenormalising values are after
McDonough and Sun (1995).
Data sources are the same as in
Fig. 4
Fig. 6 Sm–Nd isochron diagram for all analysed granites.
Tehara et al. include the newly
studied intrusives of Tehara–
Bansiyal–Gorir–Gothara and
Dosi et al. comprise granites of
Dosi–Biharipur–Dabla intrusives, and their data are taken
from Kaur et al. (2012)
13
Sm (ppm)
Nd (ppm)
147
143
Tehara pluton
Original granite
GL-7e
Mc–Ab granite
GL-2e
Mc–Ab granite
GL-5e
Ab granite
GL-3e
Ab granite
GL-6e
10.79
10.94
10.29
9.78
9.63
54.83
56.67
50.37
50.21
48.69
0.1189
0.1167
0.1235
0.1177
0.1195
0.511446 ± 8
0.511456 ± 8
0.511566 ± 7
0.511496 ± 7
0.511498 ± 8
−6.3
−5.7
−5.0
−5.1
−5.5
Bansiyal pluton
Ab granite
BS-4f
BS-7
Ab granite
9.33
13.94
50.09
70.67
0.1125
0.1192
0.511371 ± 9
0.511438 ± 8
Gothara pluton
GO-1
Ab granite
GO-4
Ab granite
GO-5
Ab granite
12.43
5.71
17.60
70.32
16.06
100.75
0.1068
0.2151
0.1056
Gorir pluton
GR-27
Mc–Ab granite
GR-13
Ab granite
12.00
GR-15
Ab granite
13.14
GR-16
Ab granite
7.89
56.40
59.71
33.50
GR-18
47.67
Sample
a
Rock type
Ab granite
10.53
Sm/144Nda
Nd/144Ndb
εc
Nd(t)
Td(Ga)
DM
Rb (ppm)
Sr (ppm)
87
Rb/86Sr
87
Sr/86Sr
δ18O WR
2.59
2.54
2.49
2.50
2.52
180.54
144.33
115.60
1.30
1.30
26.04
23.10
17.65
18.08
10.27
20.94
18.78
19.74
0.21
0.37
1.158711 ± 10
1.102403 ± 7
1.131751 ± 13
0.726102 ± 8
0.729020 ± 15
−6.4
−6.6
2.60
2.61
30.61
9.11
13.72
14.62
6.62
1.81
0.860200 ± 15
0.746105 ± 10
8.1
9.0
0.511374 ± 10
0.512555 ± 7
0.511328 ± 8
−5.1
−5.7
-5.7
2.50
2.54
2.55
2.43
1.96
1.76
16.65
17.01
22.05
0.42
0.33
0.23
0.720563 ± 12
0.722464 ± 14
0.726434 ± 11
7.9
0.1286
0.1330
0.1423
0.511704 ± 11
0.511773 ± 9
0.511897 ± 9
−3.4
−3.0
−2.6
2.36
2.34
2.31
1.09
0.64
0.43
19.86
32.75
16.45
0.16
0.06
0.08
0.727634 ± 14
0.723522 ± 14
0.727682 ± 14
0.1335
0.511760 ± 13
−3.4
2.36
14.21
12.99
3.18
0.756933 ± 15
8.0
7.9
7.5
8.1
10.0
Estimated error for 147Sm/144Nd is less than 0.5 %
b143
Nd/144Nd normalised to 146Nd/144Nd = 0.7219; Within run error expressed as 2σ in the least significant digits
c
Initial εNd values calculated using the present-day values of (143Nd/144Nd) = 0.512638 and (147Sm/144Nd) = 0.1966 for the chondritic uniform reservoir (Jacobsen and Wasserburg 1980), and
the crystallisation age of 1,700 Ma for the granites
d
Depleted mantle model ages calculated using the two-stage model of Liew and Hofmann (1988)
e
Nd data from Kaur et al. (2007)
f
Data from Kaur et al. (2011a); Mc microcline, Ab albite, WR whole rock
Int J Earth Sci (Geol Rundsch)
Table 4 Nd, Sr and O isotopic systematics for the ferroan granites, Rajasthan, NW India
13
Int J Earth Sci (Geol Rundsch)
Fig. 7 Whole-rock δ18O values against a an index of albitisation,
the Na/K (cationic), and b SiO2 (wt%), displaying increasing values
during progressive albitisation. Data source for Dosi, Biharipur and
Dabla, Kaur et al. (2012) and symbols as in Fig. 4
to fractional crystallisation as in the case of the Tehara
pluton.
The Bansiyal intrusion consists only of albite granites, but
their contiguous occurrence with the Tehara pluton suggests
that the protolith of Bansiyal albite granites should be similar to that of the Tehara. This assumption is also supported
by the similar ages of both intrusives (see above, field characteristics section) as well as almost identical REE patterns
(Fig. 8c). Moreover, the biotites of the Bansiyal albite granite
(sample BS-4) and the Tehara microcline–albite granite (GL8) have almost identical compositions (Fig. 11a). Despite this,
it would not be appropriate to consider the microcline–albite
granites of Tehara as protolith of the Bansiyal albite granites
for isocon analyses because the latter intrusive shows two
types of albite granites with different mafic mineralogy, i.e.
either containing amphibole or biotite. The occurrence of
amphibole-bearing albite granite at the margins and the biotite-bearing counterpart in the core of Bansiyal intrusive may
indicate that the metasomatic fluid, when infiltrated from the
margins towards the core, transformed biotite to amphibole
at the margins. Subsequently, the Mg/Ca ratio became high
13
Fig. 8 Isocon diagrams (Grant 2005), illustrating the whole-rock
chemical changes during the a stage I metasomatic alteration in the
Tehara intrusive, i.e. transformation of the original granite to the
microcline–albite granite; b stage II metasomatic alteration in the
Tehara intrusive, i.e. transformation of the microcline–albite granite
to the albite granite; and c stage II metasomatic alteration in the Gorir
intrusive. Major oxides are given in wt% and trace elements in ppm;
labels for major oxides are abbreviated to the elements
Int J Earth Sci (Geol Rundsch)
enough not to transform the biotite to amphibole in the core
of the intrusive. Thus, the different nature of the mafic phases
in Bansiyal albite granites is most likely due to variation in
the fluid composition on a local scale.
The proximity of the Gothara to Tehara intrusive may
suggest that the albite granites of Gothara might have
been transformed from a Tehara–like protolith. At the
same time, it is noteworthy that the breakdown of biotite
at Tehara has yielded amphibole of largely magnesiohornblende composition, whereas the Gothara albite granites
are dominantly composed of actinolite (Fig. 11b), like the
amphiboles of the Biharipur and Dabla albite granites. The
variations in amphibole chemistry, however, most likely
reflect the local compositional gradients in the evolving fluid phase. This is because the cationic composition
of amphibole is dictated by crystal chemical constraints,
induced on the crystal structure by Cl incorporated on the
anion side (Kullerud 1996). Hence, the compositional variation in the secondary amphibole is principally controlled
−
by the fluid activity ratio a−
Cl/aOH (Kullerud and Erambert
1999). Therefore, it is difficult to ascertain the protolith of the Gothara pluton, which may have been derived
from a Dosi-like protolith, and it is also equally possible
that these rocks have a similar protolith which crops out at
Tehara.
Magmatic characterisation and magmatic misclassification
Based on the arguments presented above, the original and
the microcline–albite granites can be characterised in magmatic classification and discrimination diagrams. However,
geochemical characterisation of severely metasomatised
rocks, like the albite granites where any reaction front is
absent, can cause misleading geodynamic interpretations.
In the CIPW normative Ab–Or–An granitoid classification diagram of Barker (1979), the microcline–albite
granites of Tehara and Gorir can be classified as granite
(Fig. 9a). The original granite of Tehara also falls in the
same field, but more towards the Or corner, reflecting the
higher amount of biotite in this rock. The same holds true
for the biotite-bearing albite granite of Bansiyal. On the
other hand, amphibole-bearing Bansiyal samples, which
are devoid of significant amount of biotite, cluster at the
Ab corner. Due to the extreme gain in Na and the loss in
K during the second stage of metasomatism, the normative composition of the albite granites from Tehara, Gorir
and Gothara isshifted towards, or tightly cluster at, the Ab
corner, providing an erroneous notion that these rocks are
trondhjemites.
The original and the microcline–albite granites of Tehara
are peraluminous (Fig. 9b) with aluminium saturation indices molar Al2O3/(CaO-3.3P2O5 + Na2O + K2O) (ASI or
A/CNK) ≥ 1. In contrast, the albite granites of the same
as well as other plutons, including the microcline–albite of
Gorir, are metaluminous, having ASI < 1. The lowering in
the ASI values at Tehara can be explained by the transformation of biotite (of the microcline–albite granite) to calcic amphibole (in the albite granite). During this stage of
metasomatism, the albite granites gained Ca (reaction 2)
and Na (reaction 1) with the concomitant additional loss of
K due to decomposition of biotite, finally leading to relatively high CNK values resulting in their lower ASI. In the
other studied granites, however, the original mafic phases,
such as amphibole or clinopyroxene, were not K-bearing,
and therefore, K was lost only as per reaction (1) without
much affecting their ASI values. For example, the amphibole-bearing original, moderately and completely albitised
granites at Dosi maintained their metaluminous character
(Kaur et al. 2012).
The classification diagrams discussed above demonstrate
the significant role of mafic mineralogy of a metasomatised
granite in its characterisation. For instance, an albite granite, derived from a Tehara-like peraluminous protolith, will
be misinterpreted as a metaluminous trondhjemite in the
absence of any reaction fronts between the original/microcline–albite granite and the albite granite.
In a Fe-number [Fe* = wt% FeOt/(FeOt + MgO)] versus SiO2 diagram (Frost et al. 2001; Frost and Frost 2008),
the original granite, all the microcline–albite granites
and a few albite granites are classified as ferroan granites
(Fig. 9c). The majority of the amphibole-bearing Tehara
and Gothara albite granites extend towards the magnesian
field because of their relatively low Fe*. This is reflected
by the metasomatic transformation of biotite (XFe ~ 0.80) to
amphibole (XFe ~ 0.55) in Tehara. An analogous assumption seems quite reasonable for Gothara, if actinolite
(XFe ~ 0.30) was derived from a mafic phase with high XFe,
from a Tehara-like or Dosi-like protolith. Some of the Gorir
albite granites are ferroan, while others are magnesian as
reflected by the degree of replacement of clinopyroxene by
amphibole. In the albite granites that plot in the magnesian
field, the clinopyroxene shows minor alteration to amphibole, whereas those falling within the ferroan field show
relatively more alteration. In the microcline–albite granites
extending to the ferroan field, clinopyroxene is considerably altered to Fe-rich amphibole. Therefore, in the absence
of a reaction interface between the original granite and the
microcline–albite granite, it becomes difficult to constrain
the composition of pristine clinopyroxene and classify
these rocks either as ferroan or magnesian. Similarly, the
biotite-bearing Bansiyal albite granite is ferroan, whereas
its amphibole-bearing counterparts straddle the boundary
between ferroan and magnesian. This shows how misleading it can be to classify the metasomatised rocks when their
modified geochemical characteristics are viewed as primary signatures.
13
Int J Earth Sci (Geol Rundsch)
Fig. 9 Whole-rock chemical compositions of the Khetri ferroan
granites in the a normative Ab–Or–An ternary diagram (Barker
1979); b Shand Index plot with discrimination fields after Maniar and
Piccoli (1989); c FeOt/(FeOt + MgO) versus SiO2 (wt%) classification diagram (Frost et al. 2001). The Fe-number (Fe*) dividing line
is after Frost and Frost (2008); d Rb (ppm) versus Y + Nb (ppm) tectonic discrimination diagram (Pearce et al. 1984); e Nb (ppm) versus
Y (ppm) tectonic discrimination diagram (Pearce et al. 1984), and f
Zr (ppm) versus 104 × Ga/Al classification diagram (Whalen et al.
1987). Data sources are the same as in Fig. 4
Although Rb is a highly mobile element, Pearce et al.
(1984) used this element in their tectonic discrimination diagrams Rb versus SiO2 and Rb versus Nb + Y.
The extreme loss of Rb in the Khetri albitised granites
(see Fig. 4) gives rise to some interesting and potentially
useful interpretations (Fig. 9d). The original granites of
Tehara and Dosi fall in the within plate granite (WPG)
field. The microcline–albite granites of Tehara and Gorir
13
Int J Earth Sci (Geol Rundsch)
are slightly shifted vertically downward because of some
Rb loss (see Fig. 8a) due to incipient K-feldspar alteration, but still plot well within the WPG field. The albite
granites experienced substantial loss in Rb, together with
K, as reflected by nearly complete replacement of K-feldspar and biotite, while Y and Nb remained immobile during the second stage of metasomatism (see isocon analysis
section). Therefore, the albite granites display a vertical
drop in Rb with respect to their protolith and thus plot in
the ocean ridge granite (ORG) field. Some of the Gorir
albite granite samples plot in the WPG field because they
still retain some Rb substituting for K in minor biotite or
relict K-feldspar or both. Similarly, the Bansiyal albite
granites, which are biotite-bearing, plot in the WPG field,
although below the original and microcline–albite granites of other intrusives, because they still retain some Rb
and, with varying degree of alteration of biotite to amphibole, they range towards the ORG field. The same holds
true for the transitional granites of Dosi and Dabla, which
mostly plot close to or at the boundary between the WPG
and ORG. The ORG affinity for the Gothara albite granites
is an apparent case of “mistaken ORG identity” because
Kaur and Mehta (2005), who studied these rocks in isolation, (mis)interpreted these as a plagiogranite occurrence
of magmatic origin. The present study, in conjunction with
the findings of Knight et al. (2002), demonstrates amply
that the Gothara intrusive is an extremely metasomatised
equivalent of the ferroan/A-type granites. Therefore, the
existence of an oceanic magmatic activity in the Khetri
complex is untenable. Finally, when the data of original,
moderately albitised and completely albitised granites are
plotted in an immobile trace element discrimination diagram (Fig. 9e), all the granites confirm their WPG affinity. Besides Al, the trace elements Zr and Ga behaved as
immobile elements during metasomatism of these granites
as shown by the isocon analysis (Fig. 8). The discrimination diagram of Whalen et al. (1987), using Zr versus Ga/
Al, classifies all the studied samples as A-type granites
(Fig. 9f). Note the tight cluster shown by the Tehara and
the Gothara granites.
Nature of fluid
The whole-rock δ18O of the studied plutons range between
7.5 and 10 ‰ (Fig. 7). It is noteworthy that during the transformation of the original granite to the microcline–albite
granite, there is either virtually no increase (in Tehara) or
a significant increase (in Dosi) in whole-rock δ18O values
along with the increase in Si content (Fig. 7). Oxygen isotope analyses for mineral separates are not available for the
studied intrusives. The conclusions presented below are
therefore based on oxygen isotope patterns detailed in Kaur
et al. (2012) for the Dosi, Biharipur and Dabla plutons.
As compared to Dosi, there is no noticeable evidence
of (re-)crystallisation of isotopically heavy new quartz at
stage I in Tehara, and therefore, it does not show any appreciable increase in whole-rock δ18O values. This conclusion
is based on the strong tendency of quartz to concentrate 18O
at any given temperature (Zheng 1993), as well as isotopic
bulk rock/quartz patterns presented in Kaur et al. (2012).
The replacement of oligoclase by heavy albite has not contributed much to 18O enrichment.
On the contrary, there is a significant increase in wholerock δ18O values during stage II of metasomatism, specifically in case of Gorir and Bansiyal, similar to that observed
in the Dosi, Biharipur and Dabla plutons (Kaur et al. 2012).
Also the Si enrichment is quite appreciable at this stage
in all the above-mentioned intrusives. Hence, the increase
in whole-rock δ18O values may be principally linked to
the crystallisation of isotopically heavier Si-rich phases
(amphiboles and albite), (re-)crystallisation of isotopically
heavy quartz and to some extent to the replacement of
microcline by isotopically heavy albite. Therefore, part of
the whole-rock δ18O increase is the reflection of mineralogical transformation.
During the stage II in Tehara, there is, however, a slight
decrease in whole-rock δ18O values. This can be related, in
parts, to the fact that amphibole forming from biotite was
the significant mineralogical change that contributed to
bulk Si enrichment. This is in contrast to the other intrusives where the breakdown of mafic phases is accompanied
by albite crystallisation. The tendency of albite to concentrate 18O values is much stronger as compared to amphibole
(Zheng 1993), resulting into a net increase in Si as well as
18
O in albite granites. Amphibole, however, still tends to be
isotopically heavier than biotite (Zheng 1993); hence, the
mineralogical change should have still resulted in a slight
net increase in bulk 18O. One possibility is that the amount
of biotite to amphibole transformation was too small to significantly alter the bulk rock composition. In the absence
of 18O data specific for mafic phases, it can also not be
excluded that slight local differences in the isotopic composition of the fluid phase, and/or alteration temperature,
could have contributed to the isotopic pattern observed
here. Moreover, biotite is also sometimes altered to chlorite, which could further lower the whole-rock δ18O values
(especially if chlorite was formed as a response to an even
later alteration event that involved an isotopically lighter
fluid phase).
Nevertheless, the overall enrichment in the whole-rock
δ18O values with progressive albitisation and silicification
(Fig. 7) and the fact that the metasomatism occurred immediately after the regional metamorphic event point to the
metamorphic nature of the metasomatising fluid because
meteoric or surface-derived waters are depleted in δ18O
(δ18O = ~ 0 to −10 ‰, Sheppard 1986). Considering the
13
Int J Earth Sci (Geol Rundsch)
pervasive nature of the regionally widespread metasomatism, Kaur et al. (2012) suggested that the required high
“water–rock” ratios may ultimately point to a meteoric origin for the fluid, which acquired the metamorphic character
after exchanging oxygen with the surrounding low-temperature metamorphic rocks.
Metasomatism, metallogeny and ferroan granites
In the northern Khetri complex Cu mineralisation, associated with minor Au and Ag, extends over a strike length
of 10 km (Fig. 1c), and thus, this metallogenic province is
more commonly known as the Khetri Copper Belt. In the
southern domain, iron sulphide (pyrite) with minor Cu–Au
constitutes an important ore deposit. Besides, U mineralisation occurs throughout the belt along with minor REE,
Mo, Fe, fluorite and ilmenite–magnetite deposits (e.g. Das
Gupta 1970; Ray 1987; Sarkar 2000; Singh 2000). The copper and iron sulphide mineralisation in the region was considered either to be sedimentary diagenetic (e.g. Sarkar and
Dasgupta 1980; Sarkar 2000) or hydrothermal epigenetic
(e.g. Roy Chowdhury and Das Gupta 1965; Das Gupta,
1970) in origin. Knight et al. (2002), for the first time, proposed that the Khetri complex is an important IOCG-mineralised province, covering a large area of 150 × 150 km.
These deposits are comparable to the well-known IOCG
sensu stricto deposits worldwide (Groves et al. 2010). In
such deposits, Fe oxides are associated with Cu, Au, Ag, U,
Ba, F and LREE and these are formed due to metasomatic
activity where fluids and ore components are likely sourced
from magmatic bodies (e.g. Pollard 2006).
The results of our studies (Kaur et al. 2012; this study)
demonstrate that during the second stage of metasomatism,
a significant amount of Fe, Ti, U and REE were mobilised
from the Khetri ferroan granites due to the breakdown of
the mafic and accessory phases. It should be noted, however, that the REE mobility documented in the north-eastern Khetri ferroan granites (Kaur et al. 2012) is found to
be negligible in the newly studied intrusives, except for
Gothara. The loss of these elements is manifested by the
ore formation in and around the ferroan granites. For example, a breccia with iron-rich bands occurs all along the
western margin of the Gothara and almost along the entire
margin of the Tehara intrusives. Earlier, these breccias were
Temperature of albitisation
We have used two-feldspar geothermometry to estimate
the temperature of albitisation; for this, microcline–albite
granites of Tehara and Gorir were considered. A number of
feldspar pairs of pure albite and microcline were used to
calculate equilibration temperatures (Table 5), employing
the program SOLVCALC (Wen and Nekvasil 1994) and the
thermometer of Fuhrman and Lindsley (1988). The other
details of the method are outlined in Kaur et al. (2012).
The coexisting feldspars yield exceptionally concordant
temperatures for the Or, Ab and An components of all the
analysed feldspar pairs, indicating that the feldspar compositions represent equilibrium (for more discussion on this
aspect, see Kaur et al. 2012).
The microcline–albite granites yield a discordant temperature (Tdis) range of 267–288 °C with very small errors
of ±3 °C (Table 5). As lattice constants determined on an
albite crystal from the Dosi granite conform to a completely
ordered structural state (Kaur et al. 2012), this temperature
was increased by about 100 °C to account for the approximation of the equilibrium temperature (Tord) for ordered
feldspar pairs (Brown and Parsons 1989). The corresponding “ordered” temperature range of 370–390 °C conforms
well to the Tord estimate of 360–395 °C obtained by Kaur
et al. (2012) for the north-eastern Khetri intrusives. Moreover, quartz-feldspar oxygen thermometry yielded, within
error, a nearly identical temperature range of 350–400 °C
for the albitisation of the granites in the Khetri complex
(Kaur et al. 2012).
Table 5 Temperature (°C) estimates from the two-feldspar geothermometer for the ferroan albitised granites, Rajasthan, NW India
Sample
Rock type
Tehara pluton
GL-2
Mc-Ab granite
GL-5
Mc-Ab granite
GL-8
Mc-Ab granite
Plagioclase
K-feldspar
Ab
Or
An
Ab
Or
An
TAb
TOr
TAn
95.6
97.3
96.9
0.4
0.4
0.5
4.0
2.3
2.6
5.4
5.5
6.0
94.6
94.5
94.0
0.0
0.0
0.0
280
278
288
280
278
288
280
278
288
98.4
0.5
1.1
5.1
94.9
0.0
267
267
267
n
TDis
TOrd
9
9
9
280 ± 3
278 ± 2
288 ± 3
380
380
390
10
267 ± 2
370
Gorir pluton
GR-27
Mc–Ab granite
n = Number of feldspar pairs used to calculate the temperatures; TDis = Equilibrium temperature for disordered feldspars; TOrd = Approximate
equivalent temperatures for ordered feldspars, which is ~100 °C more than the TDis (Brown and Parsons 1989)
Mc microcline, Ab albite
13
Int J Earth Sci (Geol Rundsch)
thought to be of sedimentary origin (Gupta et al. 1998), but
they are now interpreted as hydrothermal breccia, typical of
many IOCG deposits (Williams et al. 2005). Small magnetite deposits also occur within some of the albitised granites
(e.g. Gorir and Dhanota). Part of Fe, mobilised during this
metasomatism, might have also been involved in the formation of Cu–Au ores, as most of these intrusives are spatially
associated with these ore deposits. A very close spatial
association of the Gothara intrusive with the main copper
deposit (Madan Kudan mine) of the region is noteworthy
(Fig. 1c); besides, numerous old workings of Cu occur in
and around Bansiyal, Tehara and Dhanota. Although the
U–REE–Mo–F mineralisation is regionally widespread,
the major ore deposits are located in the southern part of
the complex in association with severely albitised granites.
Under oxidising conditions, the immobile U4+ is converted
into highly mobile U6+, which can be easily leached out
of the rocks during hydrothermal alteration. The occurrence of U deposits along with Cu–Au–REE and others
exclusively in the regional-scale albite–haematite and calc
silicate alteration zones in the region (Knight et al. 2002)
supports that albitisation was accompanied by oxidation,
which lead to the hydrothermal U loss. Knight et al. (2002)
established a genetic and temporal relationship between
the widespread Cu ± Au ± Ag ± Co ± Fe ± Mo ± REE
± U mineralisation and the regional metasomatic alteration
(also see Yadav et al. 2000), and thus, there is an apparent
synchronicity between the metasomatic activity and the
metallogenic mineralisations in the Khetri complex.
This metasomatic event is Early Cryogenian in age
(850–830 Ma), which occurred nearly 900 Ma after the
emplacement of the Khetri ferroan granites, and it closely
followed the metamorphic activity (950–910 Ma) in the
region (e.g. Knight et al. 2002; Kaur et al. 2013a). As this
alteration event is associated with significant mobility of
elements on a large scale, it requires long-term horizontal
thermal gradients in the upper crust to serve as heat pumps
for hydrothermal fluid circulation (for details see Kaur
et al. 2012). The high U–Th contents of ferroan granites
result in high internal heat production of such intrusives.
For example, the original ferroan granite of Tehara has
high present-day internal heat production of 5.3 µWm−3
(Table 1). The abnormally high heat flow values in the
Khetri and adjoining regions, induced by the high internal
heat production of the granites (Gupta et al. 1967; Sundar
et al. 1990), support the existence of permanently elevated
crustal temperatures in the region. Thus, the widespread
occurrence of high heat producing ferroan granites in
the northern Khetri complex may have provided the heat
source for driving the metasomatising fluid circulation long
after their emplacement. Additionally, a granitoid emplacement event at 830–820 Ma recorded in the adjoining areas
(Kaur et al. 2013a and references therein) might have acted
as an additional heat pump for the ore formation in the
region. In this regard, the Khetri IOCG deposits are similar
to the Fe–Cu–U–REE deposits of the Curnamona Province
of South Australia, where the 1,557–1,579 Ma old ferroan
granites were extensively metasomatised, more than 1 Ga
later, at ~455 Ma and these granites provided the essential
chemical constituents for the IOCG mineralisation in the
province (Elburg et al. 2012). The present study in conjunction with the new results on the origin of IOCG deposits
at Curnamona Province, therefore reiterates that ferroan
granites, which are enriched repositories of Fe, U and REE,
if suitably metasomatised can act as potential sources for
important ore deposits.
Conclusions
Detailed mapping, petrography, whole-rock elemental and
isotope geochemistry of the ferroan granites of the Khetri
complex, in conjunction with previously published data,
lead to the following conclusions:
• All the 1.72–1.70 Ga ferroan granites of the northern Khetri complex are metasomatised in two-discrete
steps during a single metasomatic event at around
850–830 Ma by the infiltration of a low-temperature
(~400 °C) meteoric fluid that acquired metamorphic
character by exchanging oxygen with the surrounding
metamorphic rocks. This produced two types of granites with different extents of albitisation: moderately
albitised stage I and completely albitised stage II granites. During the stage I of metasomatism, the original
(K-feldspar-oligoclase) granite was transformed into a
microcline–albite granite, and during the stage II, the
microcline–albite granite was finally converted to an
albite granite.
• The rare preservation of the stage I metasomatic reaction front signifies the pervasive nature of metasomatism; the stage II is present in most of the plutons and is
decipherable by the characteristic whitening of granite
outcrops and by destruction of foliation.
• Isocon analyses reveal that most of the elements preserved their near-primary abundances during the first
stage of metasomatism, except for some minor loss of
Rb or a slight gain in Na. More pronounced losses in K,
Rb, and Ba, and gains in Na, Mg, and Ca are recorded
for the second stage of metasomatism, whereas Fe displays gain or loss depending on the original composition of the mafic phase involved.
• The results show that completely metasomatised granites should be studied with extreme caution, especially
in the absence of any reaction interface either between
original protolith and its relatively less altered deriva-
13
Int J Earth Sci (Geol Rundsch)
tive or between the two albitised derivatives differing
in the extent of albitisation. We demonstrate that the
stage I microcline–albite granites can be correctly characterised in magmatic classification and discrimination
diagrams, whereas the albite granites of the stage II, if
studied in isolation, will lead to incorrect interpretations. The mafic mineralogy of the albitised granites
can play a significant role in the characterisation of such
granites.
• The close association of the high heat producing U–Thrich ferroan granites with Cu–Au, Fe-oxide and U mineralisation in the region suggests that such rocks, which
are enriched repositories of Fe, REE and U, if suitably
metasomatised, would act as a potential magmatichydrothermal system for driving the IOCG and U mineralisation.
Acknowledgments We are grateful to Dmitry V. Kuzmin (Mainz),
Uli Schuessler (Würzburg), Nora Groschopf (Mainz) and Doris Neuhäuser (Mainz) for their assistance in microprobe work, XRF analyses
and clean laboratory, and Rosemarie Baur (Würzburg) for performing some of the XRF analyses. We thank two anonymous reviewers
and the Editor Ingo Braun for their comments that led to improvement
of the manuscript. Fruitful discussions with, and valuable suggestions
by, Hartwig Frimmel (Würzburg) are warmly acknowledged. This
work was supported by grants from the Department of Science and
Technology, New Delhi (DST, SR/S4/ES-388/2008), and the German
Academic Exchange Service, Bonn (DAAD, A/03/02882).
Appendix: Detailed petrographic description
Tehara
At Tehara, the original granite is a medium-grained, pinkish grey coloured foliated granite and shows a subhedralgranular microstructure (Fig. 10a). It is composed of
quartz, plagioclase, K-feldspar and biotite as the major
minerals; titanite and apatite are dominant accessories,
while zircon, tourmaline, muscovite, ilmenite and magnetite are subordinate. At places, biotite and feldspars are
deformed. The plagioclase (An14.3±0.7, n = 10; Table 6)
mostly shows albite twinning and is usually sericitised.
K-feldspar (Or94.5±0.4, n = 10) largely displays tartan twinning typical of microcline and in places shows perthitic
intergrowths. In contact with plagioclase, myrmekite intergrowths are occasionally noticed. Biotite, which imparts
the foliation to the rock, is annite (Fig. 11a) and is quite
uniform in composition with XFe [=Fet/(Fet + Mg)] of
0.80 (Table 7).
Megascopically, the microcline–albite granite appears
to be almost identical to the original granite with the
same colour, microstructure and mineral content. However, except for minor relics of magmatic oligoclase
(An9.7–14.0, n = 5), plagioclase in this rock is virtually
13
Fig. 10 Photomicrographs illustrating the textural and mineralogi-▸
cal aspects of the Khetri ferroan granites. a Original granite at Tehara
showing a subhedral-granular microstructure and the foliation defined
by biotite (crossed nicols); b Microcline (Mc) displaying its incipient alteration to untwinned albite (Ab) in Tehara microcline–albite
granite (crossed nicols); c albite showing deformed twin lamellae in
Tehara albite granite (crossed nicols); d chessboard–twinned albite,
which typically develops due to complete replacement of K-feldspar
by albite in Tehara albite granite (crossed nicols); e Bansiyal amphibole-bearing albite granite displaying evidence of transformation of
biotite (Bt) to amphibole (Amp); (one nicol); f Gorir microcline–
albite granite, illustrating partial replacement of clinopyroxene (Cpx)
to amphibole (Amp); Zrn: zircon (one nicol); g Gorir albite granite,
showing replacement of clinopyroxene (Cpx) to amphibole (Amp,
bottom left) as well as that of amphibole to biotite (Bt); Ttn titanite
(one nicol) and h Chessboard–type albite in Gothara albite granite,
displaying graphic microstructure (crossed nicols)
pure albite (An3.0±1.2, n = 29). The composition of microcline (Or94.2±0.8, n = 26) is almost identical to that of the
original granite, and the mineral also shows its incipient
replacement by untwinned albite (Fig. 10b). Biotite, which
is still annite, shows more varied compositions (Fig. 11a),
with the average XFe ranging between 0.80 (in sample GL2) and 0.73 (sample GL-8). Accessories are the same as
in the original granite, while calcite and epidote occur as
secondary minerals.
The albite granite is megascopically medium grained,
non-foliated and is distinctly recognised due to whitening
of outcrop, which is an important field indicator of severe
albitisation (e.g. Kaur et al. 2012; Petersson et al. 2012).
K-feldspar is absent, and almost pure albite (An2.0±0.6,
n = 40) is the dominant felsic phase; instead of biotite,
amphibole is the main mafic mineral. Although this rock
lacks foliation because of the transformation of biotite
to amphibole (see below, the section on isocon analysis),
the presence of bending in twin lamellae of albite that has
formed from deformed plagioclase (Fig. 10c) indicates that
the original rock was deformed prior to albitisation. Besides
albite twinning, the albite also shows chessboard twinning
(Fig. 10d), which is considered to have formed due to complete replacement of K-feldspar by Na-feldspar (e.g. Moore
and Liou 1979; Slaby 1992; Kaur et al. 2012). In places,
this chessboard-type albite also contains cuneiform quartz,
which most likely represents earlier graphic intergrowths
between K-feldspar and quartz; this feature is better developed and common at Gothara (see below). There is no
compositional difference between chessboard- and albitetwinned albite. Bluish green amphibole is largely magnesiohornblende, with minor ferrohornblende and actinolite
with XFe values of 0.46–0.65 (Fig. 11b; Table 8). In places,
the amphibole has altered to form chlorite. The amounts of
titanite and apatite are lower than in the microcline–albite
granite, while the other accessories are the same. Allanite
appears as an additional accessory mineral in the albite
granite.
Int J Earth Sci (Geol Rundsch)
13
13
Table 6 Average plagioclase and K-feldspar compositions for the ferroan granites, Rajasthan, NW India
Pluton
Tehara
Rock type Or. granite
(GL7)
Bansiyal
Gorir
Gothara
Tehara
Gorir
Mc–Ab granite Mc–Ab granite Ab granite (GL1, Ab granite
(GL2, 5 & 8)
(GL2 & 8)
3, 4 & 6)
(BS4 & 7)
Mc–Ab granite Ab granite
(GR27)
(GR4 & 16)
Ab granite (GO1, Or. granite
2, 3 & 4)
(GL-7)
Mc–Ab granite Mc–Ab granite
(GL2, 5 & 8)
(GR27)
n
10 (Pl)
29 (Pl)
5 (Pl)
40 (Pl)
35 (Pl)
10 (Pl)
15 (Pl)
35 (Pl)
10 (Kfs)
26 (Kfs)
8 (Kfs)
SiO2
64.73 ± 0.31
0.01 ± 0.01
20.07 ± 0.16
0.09 ± 0.04
0.03 ± 0.03
0.00
2.98 ± 0.14
9.78 ± 0.21
0.11 ± 0.01
99.66
64.33 ± 0.99
0.01 ± 0.01
21.95 ± 0.33
0.04 ± 0.04
0.01 ± 0.01
0.00
0.63 ± 0.26
11.31 ± 0.26
0.09 ± 0.08
99.49
64.57 ± 0.45
0.04 ± 0.00
21.27 ± 0.51
0.14 ± 0.06
0.00
0.01 ± 0.01
2.44 ± 0.30
10.32 ± 0.21
0.07 ± 0.01
98.83
68.09 ± 0.60
0.01 ± 0.01
19.84 ± 0.18
0.04 ± 0.03
0.01 ± 0.02
0.01 ± 0.01
0.42 ± 0.13
11.43 ± 0.13
0.04 ± 0.02
99.87
68.21 ± 0.79
0.01 ± 0.01
19.91 ± 0.26
0.04 ± 0.02
0.01 ± 0.01
0.01 ± 0.01
0.60 ± 0.32
11.72 ± 0.20
0.07 ± 0.02
100.57
67.67 ± 0.49
67.73 ± 0.32
0.01 ± 0.01
19.92 ± 0.31
0.04 ± 0.04
0.01 ± 0.02
0.01 ± 0.02
0.45 ± 0.09
11.31 ± 0.31
0.07 ± 0.04
99.55
68.43 ± 0.13
0.01 ± 0.01
18.30 ± 0.15
0.01 ± 0.02
0.01 ± 0.01
0.00
0.01 ± 0.01
0.59 ± 0.05
15.69 ± 0.12
99.45
64.60 ± 0.37
0.01 ± 0.01
18.32 ± 0.15
0.04 ± 0.04
0.01 ± 0.01
0.00
0.00
0.63 ± 0.09
15.68 ± 0.15
99.29
63.71 ± 0.20
0.23 ± 0.15
11.43 ± 0.14
0.13 ± 0.05
99.05
68.54 ± 0.66
0.01 ± 0.02
19.27 ± 0.30
0.08 ± 0.04
0.01 ± 0.01
0.01 ± 0.01
0.22 ± 0.07
11.75 ± 0.29
0.12 ± 0.03
100.03
0.00
0.56 ± 0.05
15.88 ± 0.19
98.35
Cations per 32 oxygens
Si
11.44 ± 0.03
Al
4.57 ± 0.03
0.01 ± 0.01
Fe+3
Ca
0.56 ± 0.03
Na
3.35 ± 0.06
K
0.02 ± 0.00
Sum
19.96
An
14.3 ± 0.7
Ab
85.1 ± 0.8
11.84 ± 0.07
4.16 ± 0.08
0.01 ± 0.01
0.12 ± 0.05
3.86 ± 0.09
0.02 ± 0.02
20.01
3.0 ± 1.2
96.5 ± 1.5
11.51 ± 0.05
4.47 ± 0.06
0.02 ± 0.01
0.46 ± 0.05
3.56 ± 0.10
0.02 ± 0.00
20.04
11.5 ± 1.5
88.1 ± 1.5
11.92 ± 0.04
4.09 ± 0.04
0.01 ± 0.00
0.08 ± 0.02
3.88 ± 0.04
0.01 ± 0.00
19.98
2.0 ± 0.6
97.8 ± 0.2
11.88 ± 0.07
4.09 ± 0.06
0.01 ± 0.00
0.11 ± 0.06
3.96 ± 0.07
0.02 ± 0.00
20.06
2.7 ± 1.5
96.9 ± 1.6
11.94 ± 0.04
4.06 ± 0.04
0.01 ± 0.01
0.04 ± 0.03
3.98 ± 0.04
0.03 ± 0.01
19.99
1.1 ± 0.7
98.3 ± 0.6
11.99 ± 0.03
3.97 ± 0.04
0.01 ± 0.01
0.04 ± 0.01
3.99 ± 0.12
0.03 ± 0.01
20.03
1.0 ± 0.3
98.3 ± 0.4
11.89 ± 0.04
4.12 ± 0.06
0.01 ± 0.01
0.08 ± 0.02
3.85 ± 0.10
0.02 ± 0.01
19.97
2.1 ± 0.4
97.4 ± 0.4
12.02 ± 0.02
4.00 ± 0.03
0.00
0.00
0.21 ± 0.02
3.71 ± 0.03
19.94
0.0
5.5 ± 0.4
12.00 ± 0.03
4.01 ± 0.04
0.01 ± 0.01
0.00
0.23 ± 0.03
3.72 ± 0.04
19.96
0.0
5.7 ± 0.8
11.98 ± 0.02
4.01 ± 0.03
0.02 ± 0.01
0.00
0.20 ± 0.02
3.81 ± 0.04
20.02
0.0
5.1 ± 0.5
Or
0.5 ± 0.5
0.4 ± 0.1
0.2 ± 0.1
0.4 ± 0.1
0.6 ± 0.3
0.7 ± 0.2
0.5 ± 0.2
94.5 ± 0.4
94.3 ± 0.8
94.9 ± 0.5
TiO2
Al2O3
FeO
MnO
MgO
CaO
Na2O
K 2O
Sum
n number of analyses, Or. original, Mc–Ab Microcline–albite, Ab Albite
18.11 ± 0.16
0.10 ± 0.06
Int J Earth Sci (Geol Rundsch)
0.6 ± 0.1
19.51 ± 0.23
0.08 ± 0.04
Int J Earth Sci (Geol Rundsch)
Fig. 11 Mineral compositions
of the Khetri ferroan granites.
a Classification of analysed
biotite in the annite–siderophyllite–eastonite–phlogopite
quadrilateral. Data for Dosi
original granite from Kaur et al.
(2012); b IMA classification
of analysed amphiboles (Leake
et al. 2004). Data for Dabla and
Biharipur albite granites from
Chaudhri et al. (2003) and Kaur
et al. (2006a), respectively, and
c IMA classification of analysed
pyroxenes (Morimoto et al.
1988). Ab albite, Mc microcline
Bansiyal
At Bansiyal, the biotite-bearing albite–granite is medium
grained, foliated, like the Tehara granite or microcline–
albite granite, but is dark grey because of absence of pink
feldspars. Accessory phases are apatite, zircon and secondary calcite along with relict K-feldspar, which shows
irregular and diffused boundaries with albite (Kaur et al.
2011a). The amphibole-bearing albite granite is nonfoliated and shows whitening of feldspars. Moreover,
13
Int J Earth Sci (Geol Rundsch)
Table 7 Average biotite and clinopyroxene compositions for the ferroan granites, Rajasthan, NW India
Pluton
Tehara
Rock
Or. Granite
(GL-7)
Mc–Ab granite
(GL-2)
Mc–Ab granite
(GL-5)
n
10 (Bt)
16 (Bt)
SiO2
33.62 ± 0.32
2.98 ± 0.13
15.31 ± 0.12
29.15 ± 0.61
0.15 ± 0.04
3.93 ± 0.12
0.03 ± 0.02
0.06 ± 0.02
9.01 ± 0.22
94.23
TiO2
Al2O3
FeO
MnO
MgO
CaO
Na2O
K2O
F
Cl
Sum
F=O
Cl=O
Sum
Cations per 22 oxygens
Si
5.48 ± 0.02
Ti
0.37 ± 0.02
Al
2.94 ± 0.03
3.97 ± 0.05
Fe+2
Mn
0.02 ± 0.01
Mg
0.95 ± 0.03
Ca
0.01 ± 0.00
Na
0.02 ± 0.01
K
1.87 ± 0.03
F
Cl
Sum
15.63
En
Fs
Wo
XFe
0.81 ± 0.00
Bansiyal
Gorir
Gorir
Mc–Ab granite
(GL-8)
Ab granite
(BS-4)
Mc–Ab granite
(GR-27)
Ab granite
(GR-4 & 16)
17 (Bt)
10 (Bt)
35 (Bt)
5 (Cpx)
15 (Cpx)
33.86 ± 0.28
2.48 ± 0.12
16.10 ± 0.18
29.30 ± 0.35
0.19 ± 0.03
4.19 ± 0.14
0.01 ± 0.01
0.07 ± 0.03
9.18 ± 0.16
34.26 ± 0.60
3.28 ± 0.22
15.96 ± 0.74
28.63 ± 0.49
0.13 ± 0.05
4.57 ± 0.14
0.01 ± 0.02
0.06 ± 0.03
9.46 ± 0.15
34.46 ± 0.35
3.07 ± 0.18
15.65 ± 0.15
26.65 ± 0.42
0.12 ± 0.03
5.55 ± 0.13
0.03 ± 0.01
0.06 ± 0.03
9.27 ± 0.11
51.43 ± 0.12
<0.05
0.40 ± 0.17
12.86 ± 0.59
0.21 ± 0.03
10.27 ± 0.52
23.47 ± 0.26
0.76 ± 0.08
52.65 ± 0.35
0.05 ± 0.03
0.59 ± 0.16
12.30 ± 0.68
0.18 ± 0.06
10.74 ± 0.32
20.18 ± 1.00
2.14 ± 0.50
95.38
96.37
94.86
34.73 ± 0.43
3.30 ± 0.06
15.56 ± 0.31
26.28 ± 0.36
0.35 ± 0.03
5.27 ± 0.14
0.01 ± 0.01
0.07 ± 0.05
9.34 ± 0.12
0.12 ± 0.03
0.46 ± 0.02
95.49
0.05 ± 0.01
0.10 ± 0.01
95.34
99.40
98.83
5.45 ± 0.02
0.30 ± 0.01
3.05 ± 0.03
3.94 ± 0.05
0.03 ± 0.00
1.01 ± 0.04
0.00
0.02 ± 0.01
1.88 ± 0.03
5.44 ± 0.07
0.39 ± 0.02
2.98 ± 0.10
3.80 ± 0.08
0.02 ± 0.01
1.08 ± 0.04
0.00
0.02 ± 0.01
1.91 ± 0.03
5.50 ± 0.03
0.37 ± 0.02
2.94 ± 0.03
3.56 ± 0.07
0.02 ± 0.01
1.32 ± 0.03
0.00
0.02 ± 0.01
1.89 ± 0.02
15.68
15.65
15.61
0.80 ± 0.01
0.78 ± 0.01
0.73 ± 0.01
Cations per 6 oxygens
5.53 ± 0.05
1.98 ± 0.00
0.40 ± 0.01
2.92 ± 0.06
0.02 ± 0.01
3.50 ± 0.05
0.41 ± 0.02
0.05 ± 0.00
0.01 ± 0.00
1.25 ± 0.03
0.59 ± 0.03
0.00
0.97 ± 0.01
0.02 ± 0.01
0.06 ± 0.01
1.90 ± 0.02
0.06 ± 0.01
0.12 ± 0.01
15.75
4.04
29.8 ± 1.2
21.3 ± 1.2
48.9 ± 0.3
0.74 ± 0.01
0.41 ± 0.02
2.02 ± 0.01
0.00
0.03 ± 0.01
0.39 ± 0.02
0.01 ± 0.00
0.61 ± 0.02
0.83 ± 0.04
0.16 ± 0.04
4.05
33.2 ± 0.6
21.7 ± 1.5
45.0 ± 1.3
0.39 ± 0.02
n number of analyses, Or. original, Mc–Ab Microcline–albite, Ab albite
in the latter rock, biotite occurs as a minor relict phase,
and sometimes displays evidence of its transformation
to amphibole (Fig. 10e); traces of apatite, titanite, allanite and secondary calcite occur as accessory minerals. In
both the rocks, albite is the main felsic phase in addition
to quartz, showing a compositional range of An3.4±1.3
(n = 21) to An1.6±0.7 (n = 14) in the biotite- and amphibole-bearing varieties, respectively. Biotite is sometimes
deformed in the biotite-bearing albite granite; it is annite
with XFe at 0.72–0.75 (Table 7) and has nearly the same
composition to that of the microcline–albite granite of
13
Tehara (Fig. 11a). In the amphibole-bearing albite granite, amphibole is largely magnesiohornblende with minor
actinolite (Fig. 11b).
Gorir
The microcline–albite granite at Gorir is brick red in
colour due to haematite precipitation that accompanies
albitisation (Engvik et al. 2008; Kaur et al. 2012); it is a
medium-grained and non-foliated rock. The major minerals are quartz, nearly pure albite (An1.1±0.7, n = 10),
Int J Earth Sci (Geol Rundsch)
Table 8 Average amphibole compositions for the ferroan granites, Rajasthan, NW India
Pluton
Tehara
Bansiyal
Gorir
Gothara
Rock type
Ab granite (GL1, 3, 4 & 6)
Ab granite (BS4 & 7)
Mc–Ab granite (GR27)
Ab granite (GO1, 2, 3 & 4)
n
48
22
10
50
SiO2
47.31 ± 2.34
0.28 ± 0.10
6.26 ± 1.89
0.01 ± 0.02
20.79 ± 1.53
0.30 ± 0.07
9.76 ± 1.26
11.16 ± 0.14
1.05 ± 0.31
0.18 ± 0.08
50.91 ± 1.08
0.32 ± 0.10
3.67 ± 0.89
0.01 ± 0.01
16.37 ± 0.80
0.45 ± 0.04
13.00 ± 0.66
10.98 ± 0.22
0.75 ± 0.20
0.23 ± 0.09
0.08 ± 0.03
0.08 ± 0.03
96.84
0.03 ± 0.01
0.02 ± 0.01
96.79
37.11 ± 1.44
0.20 ± 0.14
9.57 ± 0.74
<0.05
30.14 ± 0.59
0.21 ± 0.04
3.10 ± 0.31
10.80 ± 0.29
1.37 ± 0.16
2.62 ± 0.27
53.06 ± 1.67
0.07 ± 0.07
1.93 ± 1.24
0.02 ± 0.02
13.88 ± 1.52
0.37 ± 0.09
14.93 ± 1.23
12.58 ± 0.31
0.33 ± 0.20
0.06 ± 0.05
95.13
97.22
7.44 ± 0.13
0.56 ± 0.13
6.14 ± 0.15
1.84 ± 0.15
7.70 ± 0.18
0.30 ± 0.19
8.00
8.00
8.00
8.00
Al
Ti
Fe+3
Mg
Fe+2
Mn
0.16 ± 0.09
0.03 ± 0.01
0.80 ± 0.31
2.17 ± 0.26
1.79 ± 0.27
0.04 ± 0.01
0.07 ± 0.04
0.04 ± 0.01
0.72 ± 0.12
2.83 ± 0.13
1.28 ± 0.13
0.05 ± 0.00
0.03 ± 0.03
0.03 ± 0.02
0.97 ± 0.16
0.76 ± 0.07
3.19 ± 0.14
0.02 ± 0.01
0.04 ± 0.04
0.01 ± 0.01
0.24 ± 0.15
3.23 ± 0.24
1.45 ± 0.18
0.04 ± 0.02
ΣC
5.00
5.00
5.00
5.00
Mg
Fe+2
Mn
Ca
Na
0.00
0.00
0.00
1.79 ± 0.09
0.19 ± 0.05
0.00
0.00
0.00
1.72 ± 0.04
0.21 ± 0.05
0.00
0.01
0.01
1.92 ± 0.06
0.07 ± 0.06
0.00
0.00
0.01
1.96 ± 0.04
0.04 ± 0.03
ΣB
1.98
1.93
2.01
2.00
Na
K
0.11 ± 0.10
0.04 ± 0.02
0.15
0.01 ± 0.10
0.04 ± 0.00
0.05
0.37 ± 0.05
0.55 ± 0.06
0.92
0.05 ± 0.04
0.01 ± 0.01
0.06
TiO2
Al2O3
Cr2O3
FeO
MnO
MgO
CaO
Na2O
K2O
F
Cl
Sum
F=O
Cl=O
Sum
97.10
Cations per 23 oxygens
Si
7.06 ± 0.28
0.94 ± 0.28
AlIV
ΣT
VI
ΣA
Σ T ALL
15.13
14.97
15.91
15.06
XFe
0.55 ± 0.05
0.41 ± 0.02
0.85 ± 0.01
0.34 ± 0.04
n number of analyses, Or. original, Mc–Ab Microcline–albite, Ab albite
K-feldspar (Or94.9±0.6, n = 8) and clinopyroxene. Secondary amphibole and calcite occur as minor phases, whereas
titanite, apatite, zircon and fluorite are important accessories. K-feldspar occurs as microcline and is similar in
composition to that of the Tehara counterparts. Green
coloured clinopyroxene is diopside (Fig. 11c), uniform
in composition with En31Fs20Wo49 to En28Fs23Wo49 and
XFe at 0.39–0.44 (Table 7). The mineral generally shows
its partial transformation to deep bluish green amphibole
(Fig. 10f), which is quite variable in composition. Although
it is largely potassic hastingsite (K > 0.50 apfu; Leake et al.
1997) to potassian Fe-edenite (with K = 0.43–0.44 apfu),
13
Int J Earth Sci (Geol Rundsch)
Fe-actinolite, Fe-hornblende and potassian katophorite also
occur in minor amounts (Fig. 10b).
The albite granites are greyish white, non-foliated,
quartz-albite rocks with clinopyroxene as the major mafic
phase. K-feldspar, secondary amphibole and biotite occur
in minor to accessory amounts. The other accessory minerals are same as in the microcline–albite granite. Albite is
identical in composition (An1.1±0.3, n = 15) to that of the
microcline–albite granite. Clinopyroxene is largely diopside (XFe 0.33–0.41), but has a higher Na/Ca ratio than
diopside of the microcline–albite granite (Table 7). The
mineral invariably shows its alteration to amphibole, which
varies from minor to almost negligible in some samples
(e.g. GR-4 and GR-16). In part of the samples, the amphibole is further replaced by biotite (Fig. 10g), and in such
cases, titanite often occurs in close association with biotite.
Gothara
The Gothara pluton is entirely made up of medium-grained,
non-foliated albite granites, which are greenish white to grey
in colour. In some of the samples, albite forms porphyroblasts in a matrix consisting of quartz, amphibole and albite.
Commonly, albite displays chessboard and albite twinning
and both types revealed identical and exceptionally homogenous compositions (An2.1±0.4, n = 48). Well-developed
intergrowths of albite with cuneiform quartz resembles a
graphic microstructure (Fig. 10h). Amphibole in these rocks
is largely actinolite, similar to the amphiboles of BiharipurDabla albite granites (Fig. 11b) with XFe 0.30–0.44 (Table 8),
and very few analyses yielded compositions of magnesiohornblende. Important accessory phases are allanite and
titanite, commonly associated with amphiboles, apatite, zircon, magnetite and ilmenite; secondary calcite is formed at
the expense of amphibole. In GO-4, the amount of allanite,
titanite and apatite is smaller than in the other samples.
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