JG vol. 117, no. 6 2009 Thursday Aug 27 2009 01:29 PM/80021/RITTERD CHECKED Unraveling Sedimentary Provenance and Tectonothermal History of High-Temperature Metapelites, Using Zircon and Monazite Chemistry: A Case Study from the Eastern Ghats Belt, India Dewashish Upadhyay, Axel Gerdes,1 and Michael M. Raith2 Department of Geology and Geophysics, Indian Institute of Technology, Kharagpur 721302, India (e-mail: dewashish@gg.iitkgp.ernet.in; upadhyay@uni-muenster.de) ABSTRACT q1 The geochemical behavior of detrital zircon and monazite during granulite facies anatexis in metapelites from the Eastern Ghats Belt (EGB), India, is explored using U-Pb geochronology, Hf isotopes, and trace elements. In a metapelite from the Ongole Domain, detrital zircon reequilibrated by coupled dissolution-reprecipitation and diffusion reaction during ultrahigh-temperature metamorphism at 1.63 Ga. The event completely reset the U-Pb systems, but Hf isotopes and trace elements were only partially reequilibrated. Overgrowths on the altered cores date migmatization at 1.61 Ga. Monazite yields metamorphic ages similar to those of zircon. In metapelites from the Eastern Ghats Province (EGP), detrital zircon grains give 2.44–1.40-Ga ages and metamorphic ones 1.2–0.5-Ga ages. Metamorphic components include detrital grains reequilibrated by coupled dissolution-reprecipitation in the presence of anatectic melt and newly crystallized overgrowths and grains. In reequilibrated domains, the U-Pb system was completely reset, but Hf isotope compositions of precursors were often retained. The 176Hf/177Hf of most zircon scatters between 2.7- and 1.9Ga crust evolution lines, indicating late Archaean to Mesoproterozoic juvenile provenance with major crust formation between 2.7 and 1.9 Ga and only minor perturbation of the Lu-Hf system during metamorphism. The 1.2–0.92- and 0.62–0.50-Ga metamorphic populations are related to Rodinia and Gondwana assembly, respectively. The 1.63-, 1.2– 0.92-, and 0.50–0.62-Ga ages allow correlation of the Ongole Domain and EGP with the Rayner Complex, suggesting that East Antarctica was contiguous with Proto-India in the Paleoproterozoic. Rifting in this terrane and sedimentation in the resulting basin deposited the EGP metapelites between 1.42 and 1.2 Ga, culminating in reamalgamation of East Antarctica with Proto-India during Rodinia assembly. The final crustal architecture of the belt was attained during Pan-African orogenesis when the EGB granulites were thrust westward over the cratons. Online enhancements: appendix tables. Introduction In polychronous crustal terranes, superposition of geological events and a nonhomogeneous distribution of metamorphism and deformation can make it difficult to constrain the regional crustal evolution. In such belts, zircon and monazite, being common accessory phases in paragneisses, serve as good candidates for unraveling sedimentary provenance and postdepositional thermal history. First, because of their highly refractory behavior, detrital Manuscript received July 15, 2008; accepted July 30, 2009. 1 Institut für Geowissenschaften (Facheinheit Mineralogie), Johann Wolfgang Goethe Universität, Altenhöferallee 1, Frankfurt am Main 60438, Germany. 2 Steinmann Institut, Universität Bonn, Poppelsdorfer Schloss, Bonn 53115, Germany. grains, notably zircon, can survive weathering and sedimentary recycling, as well as high-grade metamorphism, and thus retain provenance characteristics. Second, in metamorphic rocks these minerals often preserve microtextural records of repeated phases of consumption, growth, and reequilibration that can be used to reconstruct the tectonothermal histories of the rocks. The Eastern Ghats Belt (EGB) in southeastern India is one such composite granulite terrane that is dominated by ortho- and paragneisses having complex geological histories involving repeated hightemperature metamorphism and deformation from the Proterozoic to the early Phanerozoic. The belt defines the southeastern margin of the Singhbhum, [The Journal of Geology, 2009, volume 117, p. 000–000] 䉷 2009 by The University of Chicago. All rights reserved. 0022-1376/2009/11706-00XX$15.00. DOI: 10.1086/606036 CHECKED 1 CHECKED 2 D . U P A D H Y AY E T A L . Bhandara, and East Dharwar cratons and is correlated with the Rayner Complex in East Antarctica in several continental reconstruction models (e.g., Yoshida et al. 1992; Dalziel 1997; fig. 1). Both terranes have been commonly explained as having formed by orogenic collisions between Proto-India and East Antarctica during the assembly of Rodinia at ∼1.0 Ga (Hoffman 1991; Moores 1991; Condie 2003). The EGB thus serves as a crucial link in understanding Proterozoic and Phanerozoic IndoAntarctic tectonics. High-grade migmatitic metasedimentary rocks constitute a major component of the lithological packages making up the EGB (as reviewed by Dasgupta and Sengupta [2003]). Upadhyay (2008) proposed that some of these sedimentary units could Figure 1. Simplified geological map of the Eastern Ghats Belt (EGB) showing the different crustal provinces and the locations of the studied samples. Cratons: SB p Singhbhum; BC p Bhandara; EDC p Eastern Dharwar; RP p Rengali Province; JP p Jeypore Province; OD p Ongole Domain; UD p Udayagiri Domain; VD p Vinjamuru Domain; EGP p Eastern Ghats Province. Proterozoic basins: CG p Chattisgarh; KH p Khariar; AP p Ampani; IV p Indravati; SU p Sukma; GPG p Godavari Pranhita graben; MG p Mahanadi graben. The inset is a map of peninsular India showing the EGB and the neighboring Archaean cratons. JG vol. 117, no. 6 2009 Thursday Aug 27 2009 01:29 PM/80021/RITTERD CHECKED Journal of Geology H I G H - T E M P E R AT U R E M E TA P E L I T E S I N I N D I A have been deposited in a basin that may have formed during the opening of a rift along the southeastern margin of Proto-India in the Mesoproterozoic (1.5–1.3 Ga). Not much, however, is known about the provenance of the sediments or the postdepositional tectonothermal histories of these sedimentary units. This article is a detailed study characterizing the detrital and metamorphic zircon and monazite inventory of high-grade paragneisses from the EGB. In particular, the geochemical behavior of detrital zircon during granulite facies partial melting is explored using U-Pb geochronology, Hf isotope analyses, and trace element measurements. This is combined with the results of chemical dating of monazites to constrain the provenance of the sediments and the tectonometamorphic evolution of the belt. Metamorphic Zircon and Its U-Pb and Lu-Hf Isotope Composition A metamorphic zircon can be classified into a newly crystallized and altered type based on the process of its formation. During metamorphism, new zircon can nucleate from the breakdown of Zrbearing major, minor, or accessory phases (Fraser et al. 1997; Pan 1997). If a fluid or a melt phase is present, zircon may crystallize directly from the fluid (Williams et al. 1996) or from the melt (Roberts and Finger 1997). In partially molten rocks, older zircon grains may get dissolved and new ones precipitated as individual crystals or as overgrowths on older remnant cores. During metamorphism, preexisting zircon may get altered in response to the imposed pressuretemperature (PT) and fluid conditions. Three contrasting mechanisms have been proposed to explain this reequilibration process: (1) dry solid-state recrystallization (Hoskin and Black 2000), (2) diffusion reaction, and (3) coupled dissolution-reprecipitation process (Geisler et al. 2007). During solid-state recrystallization, structural strain in a trace element–rich zircon is dissipated by the expulsion of rare earth elements (REEs) from sites of high lattice strain by a process of thermally activated particle and defect volume diffusion. The recrystallized domains may inherit a relict igneous zoning and have diffuse, transitional, or concordant boundaries to the precursor zircon (Hoskin and Black 2000; Hoskin and Schaltegger 2003). Reequilibration by diffusion reaction is also a solidstate process but is assisted by a fluid phase. It operates only in radiation-damaged crystals and can take place at any PT condition. The reequilibrated zircon is usually characterized by high concentraJG vol. 117, no. 6 2009 CHECKED 3 tion of nonformula elements such as Ca, Ba, Al, Fe, and Mn and may preserve some memory of the parent isotopic composition (Geisler et al. 2007). In contrast, reequilibration through a coupled dissolution-reprecipitation process requires a fluid or a melt phase to be present and can operate even in crystalline zircon. The process involves the dissolution of a parental zircon and a contemporaneous nucleation and growth of new zircon along an inward-moving interface (Geisler et al. 2007). The reequilibration is driven by the solubility difference between the zircon solid solution (greater solubility) and a pure zircon end member (lower solubility) and leads to a complete resetting of the isotope systems, producing zircon with lower minor and trace element concentrations compared with those of the parent. The U-Pb and Lu-Hf isotope composition of a metamorphic zircon would be determined by several factors, including its mechanism of formation and the extent and nature of chemical and isotopic reequilibration in the rock. This can be understood by considering the whole rock to be consisting of two subsystems, i.e., zircon and matrix (cf. Zeh et al. 2007; Liu et al. 2008; Gerdes and Zeh 2009). The zircon has high U/Pb and low Lu/Hf, whereas the matrix (all other phases excluding zircon) has lower U/Pb and higher Lu/Hf. The growth of radiogenic Pb and Hf isotopes in these two subsystems would thus be different. While the 176Hf/177Hf in zircon would remain practically constant over time, the 176 Hf/177Hf of the matrix would increase from radiogenic ingrowth. In the case of Pb isotopes, the two subsystems would behave exactly the opposite, with zircon accumulating more radiogenic Pb than the matrix. Assuming a closed-system behavior on a whole-rock scale, the difference in 176Hf/177Hf (t) between reequilibrated or newly grown metamorphic and inherited zircon would depend on the relative proportion of matrix and dissolved primary zircon involved in the metamorphic reactions, their respective Lu/Hf, and the time elapsed since the last isotopic equilibration. Unless affected by later reequilibration events, newly crystallized metamorphic zircons should give concordant U-Pb ages that date the timing of metamorphism. The 176Hf/177Hf of such newly crystallized zircon would depend on the proportion of radiogenic (matrix) and inherited (zircon) Hf that it acquires. Because primary zircon usually dissolves only partially during metamorphism (cf. Watson and Harrison 1983), the 176Hf/177Hf of newly crystallized zircon is usually more radiogenic than the primary one because most of its Hf is sourced from the matrix. A zircon recrystallizing in the solid Thursday Aug 27 2009 01:29 PM/80021/RITTERD CHECKED q2 q3 CHECKED 4 q4 D . U P A D H Y AY E T A L . state is most likely to give discordant U-Pb ages because the loss of radiogenic Pb loss is usually only partial, owing to the slow diffusion of cations in the zircon lattice (cf. Lee et al. 1997; Möller et al. 2002; Cherniak and Watson 2003). The recrystallized zircon will always inherit the Hf isotope composition of its precursor. Zircon reequilibrating by a diffusion-reaction process is also usually discordant because of partial Pb loss. The Hf isotope composition of such a zircon will again depend on the degree of reequilibration between matrix and primary zircon, their Lu/Hf, the time elapsed since the last isotopic equilibration, and the Hf isotope composition of the fluid or melt. In contrast, a zircon forming by coupled dissolution-reprecipitation is expected to give concordant U-Pb ages that should date the reequilibration event. The Lu-Hf isotope system in this zircon would behave similarly to that in a newly crystallized one. The Hf and U-Pb isotope composition of reequilibrated zircon can be meaningfully interpreted only if the reequilibration mechanism and its effects on the Lu-Hf and U-Pb isotope systems are traceable. Two contrasting scenarios can be visualized. (1) The U-Pb decay systems get completely reset, and the Hf isotopes reequilibrate completely with the matrix during metamorphism. Because the matrix has Lu/Hf higher than that of the zircon, the 176Hf/177Hf of the reequilibrated zircon would be more radiogenic than that of its precursor. (2) The altered zircon domain retained its Hf isotope composition but its age changed because the U-Pb system was disturbed during metamorphism. In this case, the 176Hf/177Hf of the zircon is still close to the initial composition of the crust from which it formed because of the low Lu/Hf (usually !0.007) in zircon. Thus, in a plot of time versus 176Hf/177Hf, the zircon would evolve on a virtually horizontal line between metamorphic resetting of the U-Pb system and original formation. Without knowing the true crystallization age to which the 176Hf/177Hf corresponds, the average crustal residence time (e.g., Hf model ages) will be overestimated and calculated ␧Hf values too low. Geologic Background Dobmeier and Raith (2003) proposed that the EGB is an amalgamation of several crustal units (called “domains” and “provinces”) that had distinct geological histories. They subdivided the belt into the Jeypore, Rengali, and Eastern Ghats provinces and the Ongole Domain on the basis of their contrasting geological history, which is only briefly summarized below (fig. 1). JG vol. 117, no. 6 2009 The Jeypore Province mostly comprises Neoarchaean (TDM, 3.9–3.0 Ga [Rickers et al. 2001]; U-Pb zircon ages, ∼2.7 Ga [Kovach et al. 2001, 2004]) high-grade metaigneous rocks, including enderbites, charnoenderbites, and basic granulites metamorphosed at ∼2.5 Ga. The Rengali Province is dominated by an association of medium- to highgrade quartzofeldspathic gneisses, biotite schists, amphibolites, migmatitic hornblende gneisses, quartzites, and pelitic schists. Neoarchaean magmatism in the province has been dated at ∼2.8 Ga (207Pb/206Pb zircon ages; Mishra et al. 2000; Crowe et al. 2001) and confirmed by similar Rb-Sr wholerock ages (2.9–2.7 Ga) from biotite granite and charnockite (Sarkar et al. 1998; Mishra et al. 2000). To the south of the Godavari-Pranhita graben, the Ongole Domain consists of a high-grade metasedimentary package of interlayered diatexitic pelitic granulites, quartzitic to psammitic gneisses, and calc-silicate rocks of unknown depositional age (TDM Nd, 2.8–2.6 Ga; Rickers et al. 2001) intruded by felsic plutonic rocks (enderbitic-charnockitic and leucogranitic orthogneisses) at ∼1.72 Ga. The major high- to ultrahigh-temperature (UHT) metamorphism in the region is thought to have occurred at ∼1.61 Ga. This was followed by localized ductilebrittle deformation and hydration at 1.45–1.35 Ga and a moderate thermal overprint during the early Neoproterozoic (∼1.10 Ga; Mezger and Cosca 1999; Kovach et al. 2001; Simmat and Raith 2008). The western margin of the domain was subsequently reworked at amphibolite facies conditions during the late Neoproterozoic to early Phanerozoic (∼0.50 Ga). The Eastern Ghats Province (EGP) is considered to be a part of the sensu stricto early Neoproterozoic orogen related to the assembly of the supercontinent Rodinia (Dobmeier and Raith 2003; fig. 1). It comprises mostly voluminous megacrystic, garnet- and orthopyroxene-bearing granitoid plutons (0.98–95 Ga; Paul et al. 1990; Kovach et al. 1998) and anorthosites intruded into an intensely deformed and metamorphosed supracrustal (TDM Nd, 2.6–2.0 Ga; Rickers et al. 2001) garnet-sillimanite (khondalites) and migmatitic quartzofeldspathic gneisses. The rocks of the province underwent an early UHT metamorphism that has not been dated but the record of which is preserved in sapphirine-spinel-bearing granulites (Dasgupta and Sengupta 2003; Das et al. 2006). Reconnaissance electron-microprobe monazite U-Th-Pb (Dobmeier and Simmat 2002; Upadhyay and Raith 2006; Upadhyay et al. 2006a; Simmat and Raith 2008) and preliminary U-Pb isotopic zircon dating (Shaw et al. 1997; Mezger and Cosca 1999) indicate early Thursday Aug 27 2009 01:29 PM/80021/RITTERD CHECKED q5 Journal of Geology H I G H - T E M P E R AT U R E M E TA P E L I T E S I N I N D I A Neoproterozoic (1.1–0.95-Ga) polyphase deformation and granulite facies metamorphism, usually related to Rodinia assembly. In the Chilka Lake region, renewed intense high-grade reworking of the crust during late Neoproterozoic compressional tectonics (0.80–0.66 Ga) has been documented by Dobmeier and Simmat (2002). The available geochronological data also suggest that the internal structure of the belt was considerably modified during an early Phanerozoic (∼0.50-Ga) ductile deformation that reactivated older tectonic boundaries. The provinces within the EGB are separated from each other and the adjoining cratons by major tectonic boundaries interpreted as crustal-scale sutures (Chetty and Murthy 1994; Gupta et al. 2000; Bhadra et al. 2004; fig. 1). These sutures are usually marked by a network of interconnected shear zones (cf. Biswal and Jena 1999; Gupta et al. 2005; fig. 1), some of which are superimposed on an earlier (∼1.5–1.3-Ga) continental rift at the craton margin (Upadhyay and Raith 2006; Upadhyay et al. 2006a, 2006b). The crustal structure of the contact regions is commonly interpreted to be the result of thrust (westward) or strike-slip movement between the granulites and the cratonic basement along these shear zones. Sample Description and Petrography Zircon and monazite grains from three representative samples of migmatitic quartz-rich metapelitic granulites from the EGP and one from the Ongole Domain were chosen for detailed analyses using a laser ablation inductively coupled mass spectrometer (LA-ICPMS) and electron probe microanalyzer (EPMA; fig. 1). The selected samples come from different parts of the EGB and are representative of the metapelitic lithologies in their respective crustal units. The age populations obtained from these samples can hence be argued to be representative of the crustal terranes in the EGB. The sample from near Vijayawada (KR-8) is a strongly deformed stromatic metapelite (khondalite) and was collected from the gneissic unit at the Vijayawada temple hill (lat 16⬚10⬘522N, long 80⬚14⬘229E; fig. 1). Although altered by weathering, the paragneiss has a distinct finely banded mylonitic fabric that documents strong overprinting of the stromatic migmatite by ductile shear deformation. Microtextures indicate that biotite had been completely consumed during incongruent volatile phase–absent melting. The restitic layers are dominated by garnet and sillimanite, while the leucosomes are composed primarily of alkali feldspar JG vol. 117, no. 6 2009 CHECKED 5 and quartz. Secondary biotite growth can be seen along grain boundaries. The metasedimentary rock from near Phulbani (KR-87/1; lat 20⬚18⬘557N, long 84⬚12⬘611E) is a typical metapelite (khondalite) from the north-central part of the EGP (fig. 1). It is a strongly deformed (mylonitic) quartz-rich migmatitic rock. The mylonitic foliation, defined by sillimanite, platy quartz grains, minor biotite, and plagioclase, wraps around garnet and alkali feldspar porphyroclasts. Spinel occurs as inclusions in garnet and in the matrix, where it is mantled by sillimanite. The metapelite sample from the Chilka Lake (010201-5) region is a stromatic khondalite collected from an active quarry (lat 20⬚6⬘238N, long 85⬚35⬘109E) to the northeast of the Chilka Lake anorthosite complex (fig. 1). It has a distinct mylonitic foliation, defined by sillimanite, alkali feldspar, and quartz, that wraps around garnet-bearing leucosomes. The garnet grains contain sillimanite, quartz, and alkali feldspar inclusions, indicating that they are the product of incongruent vapor phase–absent melting of biotite. The sample from the Ongole Domain (KR-12/6) is a migmatitic metapelite exposed in an active quarry southwest of Guntur (lat 16⬚19⬘04N, long 80⬚21⬘03E; fig. 1). The diatexitic structure is evidence of very extensive granulite facies anatexis experienced by the rock. Dominant leucosomal (leucogranitic) domains (mesoperthite, quartz, garnet, Ⳳplagioclase) host largely resolved restitic layers, streaks, and nebulitic schlieren (garnet, spinel, corundum, sillimanite, and cordierite). Alteration of feldspars can be seen along grain boundaries and cracks and is accompanied by the formation of secondary biotite. Analytical Techniques Zircon grains were analyzed for the concentrations of U, Th, and Pb and 25 other trace elements and U-Pb isotopes using a Thermo-Finnigan Element II sector field ICPMS coupled to a New Wave UP213 ultraviolet laser system at the Mineralogical Institute, University of Frankfurt. Laser spot (12–40-mm size) locations were selected on the basis of the internal structures of the grains as seen in cathodoluminescence (CL) images of the mounted and polished grains. Further details of the analytical protocol can be found in Gerdes and Zeh (2006). Reported uncertainties were propagated by quadratic addition of the external reproducibility (SD) obtained from the standard zircon GJ-1 (0.6% and 0.7% for 207Pb/206Pb and 206Pb/238U ratios, respectively; n p 18) during the analytical session and the Thursday Aug 27 2009 01:29 PM/80021/RITTERD CHECKED CHECKED 6 q6 q7 D . U P A D H Y AY E T A L . within-run precision of each analysis (SE). Concordia and age probability diagrams were made using Isoplot/Ex 2.49 (Ludwig 2001). For age probability histogram plots, a 10% discordance cutoff was used as a filter to exclude unreliable data. Trace element data were reduced offline using the GLITTER 4.0 program, with 29 Si for internal standardization and National Institute of Standards 612 as an external calibration standard. Repeated analyses of the Geostandards 91500 zircon (30-mm spot size) give an external reproducibility of 5%–10% for the REEs (110 ppb), Y, Ti, Sr, Nb, Ta, Th, and U and are in agreement (Ⳳ10%) with data reported from other laboratories (Liu et al. 2006). Hafnium isotope ratios in zircon were measured on a Thermo-Finnigan Neptune multicollector (MC)-ICPMS at the University of Frankfurt using the same laser system and ablation cell as described above, with a spot size of 40 mm and 60-s analysis time. The isotopes 172Yb, 173Yb, and 175Lu were simultaneously monitored during each analysis to correct for isobaric interferences of Lu and Yb isotopes on mass 176. Further details of the analytical technique and data processing can be found in Gerdes and Zeh (2006, 2009). All Hf isotope analyses were adjusted relative to the JMC 475 176Hf/177Hf of 0.282160. Multiple laser ablation MC-ICP-MS analyses of the reference zircons 91500 and GJ-1 over a period of 6 mo yielded 176Hf/177Hf of 0.282298 Ⳳ 0.000026 (2j; n p 88) and 0.282003 Ⳳ 0.000018 (2j), respectively. Monazites were analyzed in thin sections and polished mounts using a WDS Cameca Camebax EPMA at the Mineralogy and Petrology Institute, University of Bonn, operating at an accelerating voltage of 20 kV and a beam current of 50 nA. The intensities of U (Mb), Th (Ma), Pb (Ma), and Y (La) peaks were analyzed with counting times of 100 s for the peak and 50 s on the background for U, Th, and Pb and 40 and 20 s, respectively, for Y. UO2, ThO2, a pyromorphite (Pb5 [Cl (PO4)3]) crystal, and a synthetic YPO4 from the Smithsonian Institution (Jarosewich and Boatner 1991) were used as standards. Data quality was monitored by interspersing analyses of the sample with internal laboratory standard F6. The standard, dated by Paquette et al. (1994) at 545 Ⳳ 2 Ma, gives a mean age of 552 Ⳳ 47 Ma (2j; n p 19). Calculation of individual ages and age populations followed the procedure described by Montel et al. (1996). Results Zircon Morphology and Internal Structure. In the Vijayawada metapelite (KR-8), the analyzed zircon JG vol. 117, no. 6 2009 grains were of medium size. CL images indicate that most have xenocrystic cores surrounded by narrow to broad rims (fig. 2, zircons A–F). The cores are usually subrounded but occasionally have irregular margins (e.g., zircon A). Some lack any internal structure and have poor CL (e.g., zircon A), while others show truncated oscillatory (e.g., zircons C, E) or patchy (e.g., zircons B, D) zonation. The rims are usually more luminescent and display faint concentric oscillatory or sector zoning (e.g., zircons A, C, E, and F). Zircon grains in the Phulbani (KR-87/1) metapelite exhibit a range of morphologies from subrounded to more elongate subhedral crystals (fig. 2, zircons G–J). CL images reveal that many grains have luminescent cores surrounded by weakly luminescent rims, both usually showing a faint internal zonation (e.g., zircons G–I). In some grains, the cores and rims show concentric oscillatory zonation (e.g., zircon J), while in a few others, concentrically zoned cores are truncated by oscillatory zoned rims (e.g., zircon I). Most zircon grains in the Chilka Lake metapelite (010201-5) have a core and mantle structure (fig. 2, zircons K–O). Some of the cores are concentrically zoned and overgrown by concordant rims (e.g., zircon K). Others may be embayed (e.g., zircons L, O), display poor CL and lack any internal structure (e.g., zircons L, N, O), or show weak fir tree–like/ patchy zonation (e.g., zircon M). In such grains, a luminescent oscillatory zoned rim mantles the dark cores (e.g., zircon M). CL images reveal complex internal structures in zircon from the Ongole Domain metapelite (KR12/6; fig. 2, zircons P–T). The internal structure in all grains is characterized by poorly luminescent cores that are mantled by one (e.g., zircons Q–T) or two (e.g., zircon P) more luminescent rims. The cores usually appear homogeneous, with some having embayed margins (e.g., zircons Q, R). They appear either structureless (e.g., zircons P, Q) or show chaotic/patchy (e.g., zircon T) or relict oscillatory (e.g., zircons R, S) zonation. The mantles, in contrast, are more luminescent and often preserve weak oscillatory or sector zoning (e.g., zircons P– T). U-Pb (LA-ICPMS) Zircon Geochronology. U-Pb spot dating of zircon cores from the Vijayawada metapelite (KR-8) yields both concordant and discordant ages. The 207Pb/206Pb ages from concordant domains range from 1.64 to 0.92 Ga, defining peaks at 1.64, 1.49, 1.38, 1.20, 1.01, and 0.92 Ga on a probability density plot. Spot ages from the rims give concordia ages between 0.98 and 0.50 Ga, with 207Pb/206Pb age probability peaks at 0.98 and 0.85 Ga (fig. 3). In the Thursday Aug 27 2009 01:29 PM/80021/RITTERD CHECKED Journal of Geology H I G H - T E M P E R AT U R E M E TA P E L I T E S I N I N D I A CHECKED 7 Figure 2. Cathodoluminescence (CL) images of some of the sectioned zircon grains showing their internal structure. A–F, Vijayawada metapelite (KR-8); G–J, Phulbani metapelite (KR-87/1); K–O, Chilka Lake metapelite (010201-5); and P–T, Ongole Domain metapelite (KR-12/6). The spot locations (smaller than their actual size), their unique identification numbers, and their 207Pb/206Pb ages are marked on the CL images. Phulbani metapelite (KR-87/1), spot ages from cores and rims in the analyzed zircon grains are mostly concordant (fig. 4a), with the 207Pb/206Pb ages ranging from 2.44 to 0.95 Ga (fig. 4b, 4c). Most cores are older than 1.20 Ga, with the concordant ones defining peaks at 1.80, 1.60, 1.48, 1.37, 1.21, and 0.95 Ga on a probability density plot. Spot ages from the rims range from 1.50 to 0.98 Ga, with probability peaks at 1.10 and 0.98 Ga (fig. 4). Zircon cores and rims in the Chilka Lake metapelite (010201-5) give mostly concordant ages younger than ∼1.20 Ga. The 207Pb/206Pb ages from cores range from 1.77 to 0.62 Ga, the concordant ones of which define peaks at 1.20, 1.10, 0.97, 0.80, and 0.62 Ga on a probability density diagram. The rims give concordia ages between 1.05 and 0.80 Ga, with JG vol. 117, no. 6 2009 most clustering at 0.80 Ga (fig. 5). In the Ongole Domain metapelite (KR-12/6), zircon cores give concordant ages defining a single cluster with a mean concordia age of 1629 Ⳳ 4 Ma (2j). Rim regions of the grains are also concordant at a slightly younger mean age of 1612 Ⳳ 5 Ma (2j; fig. 6). The U-Pb isotope composition and age data for all the samples are listed in table A1, available in the online edition or from the Journal of Geology office. U-Th-Total Pb (EPMA) Monazite Ages. Spot analyses of monazite in the Vijayawada metapelite (KR8) yield ages ranging from 1.20 to 0.60 Ga. These define peaks at 1.18, 1.00, 0.85, and 0.64 Ga on an age probability density diagram (fig. 7a). The 1.18-, 1.00-, and 0.85-Ga monazite age clusters are Thursday Aug 27 2009 01:29 PM/80021/RITTERD CHECKED CHECKED 8 D . U P A D H Y AY E T A L . rim zircon ages, but the ∼0.82-Ga population seen in monazite is absent in zircon. This Neoproterozoic monazite population is, however, clearly identifiable in the Vijayawada metapelite sample. Spot analyses of monazite from the Chilka Lake meta- Figure 3. a, Concordia diagram showing the range of UPb ages from detrital and metamorphic zircons in the Vijayawada metapelite (KR-8). b, c, Relative probability and histogram plots of concordant age populations from cores and rims. Detrital zircons have 1.38–1.64-Ga ages, while the metamorphic ones are younger than 1.20 Ga. similar to the 1.20–0.85-Ga zircon core and rim ages. In the Phulbani metapelite (KR-87/1), spot ages from monazite range from 2.40 to 0.80 Ga, with most clustering between 1.20 and 0.82 Ga (fig. 7b). Among these, the 1.20–0.91-Ga age populations are comparable to the 1.21–0.95-Ga core and JG vol. 117, no. 6 2009 Figure 4. a, Concordia plot of U-Pb isotope ratios from detrital and metamorphic zircons in the Phulbani metapelite (KR-87-1). b, c, Relative probability and histogram plots of age populations from zircon cores and rims, respectively. Ages from detrital domains range from 1.37 to 1.80 Ga. Metamorphic zircons are younger than 1.20 Ga. Thursday Aug 27 2009 01:29 PM/80021/RITTERD CHECKED Journal of Geology H I G H - T E M P E R AT U R E M E TA P E L I T E S I N I N D I A CHECKED 9 Spot ages from some of these grains and a few others from the matrix define four clusters with peaks at 3.45, 2.44, 1.58, and 1.48 Ga on an age probability diagram (fig. 7d). The data for all the samples are listed in table A2, available in the online edition or from the Journal of Geology office. Zircon Trace Element Characteristics. In the Ongole Domain metapelite (KR-12/6), core and rim regions of zircon have distinct trace element compositions. The cores themselves can be grouped into two types, on the basis of their trace element characteristics. One group (type 1) has low HREE abundances, low Nb/Ta (0.23–0.36) and Th/U (0.022–0.049), variable GdN/YbN (0.63–2.25), and high U concentrations (1818–2777 ppm; fig. 8). The other group (type 2) has elevated HREEs and higher Nb/Ta (1.04–1.19) and Th/U (0.15–0.18) but lower U contents (762–1078 ppm) and a narrow range of GdN/YbN (0.09–0.12; fig. 8). The rims in all grains Figure 5. a, Concordia diagram showing the range of UPb ages from metamorphic zircons in the Chilka Lake metapelite (010201-5). b, c, Relative probability and histogram plots of age populations from zircon cores and rims, respectively. All zircons are younger than 1.20 Ga and hence of metamorphic origin. pelite (010201-5) yield 1.68–0.68-Ga ages that define peaks at 1.01, 0.88, 0.77, and 0.68 Ga on an age probability diagram (fig. 7c). These are similar to the 1.10–0.62-Ga core and rim zircon age populations. In the Ongole Domain metapelite (KR-12/ 6), monazites are scarce, with most grains occurring as armored inclusions in porphyroblastic garnet. JG vol. 117, no. 6 2009 Figure 6. Concordia plot of U-Pb isotope ratios from zircon cores (a) and rims (b) in the Ongole Domain metapelite (KR-12/6). Core ages define a single concordant population with a mean age of 1629 Ⳳ 4 Ma. The rims are also concordant at 1612 Ⳳ 5 Ma. Thursday Aug 27 2009 01:29 PM/80021/RITTERD CHECKED CHECKED 10 D . U P A D H Y AY E T A L . Figure 7. Relative probability plots illustrating the range of U–Th–total Pb ages from monazites in the Vijayawada (a), Phulbani (b), Chilka Lake (c), and Ongole (d) metapelites. The !1.20-Ga age populations are similar to those from metamorphic zircons and interpreted to date multiple tectonothermal events in the Eastern Ghats Province. q8 have elevated HREE concentrations, higher Th/U (0.17–0.26), and a narrow range of GdN/YbN (0.18– 0.28; fig. 8). For the Vijayawada metapelite (KR-8), only the ∼1.0-Ga age zones could be analyzed for trace elements. These are mostly depleted in HREEs and have low Th/U (0.02–0.16) and GdN/ YbN (0.07–0.54; fig. 8). In the Chilka Lake metapelite (010201-5), zircon grains or zones younger than 1.0 Ga were analyzed for trace elements. These domains have low HREE abundances and low Th/U (0.01–0.06) and show large variations in U concentrations (340–2192 ppm) and GdN/YbN (0.65–3.06; fig. 8). The trace element data for all the analyzed zircon grains are given in tables A4 and A5, available in the online edition or from the Journal of Geology office. Hf Isotope Composition of Zircon. The 176Hf/177Hf (t) from zircon cores (1.24–0.92 Ga) in the Vijayawada metapelite (KR-8) is highly variable (0.28207– 0.28159), while that from the rims (1.0–0.85 Ga) clusters tightly (0.28202–0.28196; fig. 9b, 9c). The JG vol. 117, no. 6 2009 initial ␧Hf of cores varies from ⫺1.3 to ⫺19.0, corresponding to apparent DM Hf model ages of 1.87– 2.80 Ga. In the Phulbani metapelite (KR-87/1), the 176 Hf/177Hf (t) of zircon cores ranges from 0.28200 to 0.28165, corresponding to initial ␧Hf of ⫹7.8 to ⫺6.2 and DM Hf model ages between 1.83 and 2.7 Ga. In comparison, six of the analyzed rims have a relatively uniform 176Hf/177Hf (t) of 0.28197 Ⳳ 0.00003 (2 SD), while three others are much less radiogenic (0.28182–0.28148; fig. 9b, 9c). The 176Hf/ 177 Hf (t) of zircon cores (1.0–0.6 Ga) and rims (∼0.80 Ga) from the Chilka Lake metapelite (010201-5) ranges from 0.28166 to 0.28155, corresponding to an apparent ␧Hf of ⫺22 to ⫺29 and apparent DM Hf model ages of 2.8–3.0 Ga (table A3, available in the online edition or from the Journal of Geology office; fig. 9b, 9c). In the Ongole Domain metapelite (KR12/6), type 1 cores and rim regions of zircon have a relatively uniform 176Hf/177Hf (t) of 0.28160 Ⳳ 0.00004 (2j), with apparent ␧Hf of about ⫺5.6 and apparent DM Hf model ages of about 2.6 Ga. In Thursday Aug 27 2009 01:29 PM/80021/RITTERD CHECKED q9 q10 Figure 8. LA-ICPMS trace element data from zircon cores and rims. CHECKED 12 D . U P A D H Y AY E T A L . Figure 9. The 176Hf/177Hf (t) and ␧Hf (t) of detrital and metamorphic zircons plotted against their 207Pb/206Pb ages. The Hf isotope ratios indicate that the crustal sources of detrital zircons were extracted from a depleted mantle during major crust-forming events between 2.5 and 1.9 Ga. type 2 cores, the 176Hf/177Hf (t) ranges between 0.28153 and 0.28140, corresponding to apparent DM Hf model ages of 2.68–2.77 Ga (fig. 9; for the data, see table A3). Discussion Processes of Metamorphic Zircon Formation in EGB Metapelites. In the Ongole metapelite (KR-12/6), detrital zircon cores and rims surrounding them JG vol. 117, no. 6 2009 give ∼1.63- and ∼1.61-Ga ages, respectively. These ages closely correspond to the timing of high/ultrahigh-temperature metamorphism (1900⬚C) and extensive partial melting in the Ongole terrane (Simmat and Raith 2008), indicating that detrital grains in the rock strongly reequilibrated during metamorphism. This inference is supported by the poor CL, the lack of internal zonation, and the presence of chaotic/relict zoning in detrital cores (fig. 2). The uniform Mesoproterozoic ages of these domains can be reconciled with complete resetting of the U-Pb isotope system in zircon grains from multiple provenances during ∼1.63-Ga high-grade metamorphism. The rims surrounding the reequilibrated cores are oscillatory or sector zoned and can be interpreted as overgrowths crystallizing from the anatectic melt phase at ∼1.61 Ga, in consonance with field and microtextural evidence of extensive prograde dehydration melting of biotite during the Mesoproterozoic metamorphism. In contrast to U-Pb isotopes, reequilibration of trace elements and Hf isotopes in the detrital cores during metamorphism was variable. The type 1 cores and the overgrowths have similar 176Hf/177Hf (fig. 9), an indication that they were in equilibrium with the anatectic melt phase and inherited their Hf isotope signatures from it. Depleted HREE concentrations in these cores (fig. 8) relative to the overgrowths, however, suggests that type 1 cores may have reequilibrated when garnet was crystallizing as an incongruent phase during dehydration melting of biotite, while the overgrowths formed later after garnet crystallization. This would also explain the 20-Ma age difference between the cores and the overgrowths, with the type 1 cores reequilibrating during prograde dehydration melting and the overgrowths crystallizing during the solidification of anatectic melts. The type 1 cores thus appear to have lost their detrital trace element and Hf isotope information during the Mesoproterozoic metamorphic event. Type 2 cores, in contrast, have less radiogenic initial 176Hf/177Hf and higher HREE abundances (figs. 8, 9). They could not have been in equilibrium with garnet or the anatectic melt. Their trace element and Hf isotope signatures appear to have been at least partially inherited from the precursor zircon. Contrasting behavior of trace elements and Hf isotopes during reequilibration of type 1 and type 2 cores suggests that they may have been altered by different mechanisms. The resetting of the U-Pb, Hf, and trace element systematics in type 1 cores indicates that they may have reequilibrated by a coupled dissolution-reprecipitation process. In contrast, type 2 cores may have reequilibrated by a diffusion reaction mechanism because Thursday Aug 27 2009 01:29 PM/80021/RITTERD CHECKED Journal of Geology H I G H - T E M P E R AT U R E M E TA P E L I T E S I N I N D I A their Hf isotope and trace element signatures were partially inherited from the parental zircon. They have high U concentrations (1819–2777 ppm) and could have easily accumulated enough radiation damage for such a mechanism to proceed. The “parental” trace element and Hf isotope signatures were retained even though the U-Pb isotope system was reset because radiogenic Pb occupies mostly intralattice spaces and could have been easily lost during reequilibration, while the trace elements occupy lattice positions and thus were more compatible in the structure. Preliminary age data from the EGP (Shaw et al. 1997; Mezger and Cosca 1999; Dobmeier and Simmat 2002; Upadhyay and Raith 2006; Upadhyay et al. 2006a; Simmat and Raith 2008) indicate that the terrane underwent polyphase deformation and granulite facies metamorphism between ∼1.20 and ∼0.50 Ga. Zircon grains younger than 1.20 Ga in the EGP metapelites (KR-87/1, KR-8, 010201-5) are therefore of metamorphic origin. In the Chilka Lake sample (010201-5), all zircon grains are younger than 1.20 Ga and hence of metamorphic origin. They have extremely low Th/U (0.01–0.06; fig. 8), which could be a result of most Th residing in metamorphic monazite or having been extracted from the system by segregation and removal of monazite-saturated anatectic melt phase. This is supported by the observation that monazite grains occur as armored inclusions in garnet but are almost absent in the restitic matrix. The zircon grains in rock are oscillatory/chaotically zoned or have no visible internal structure (fig. 2). They consist of newly crystallized as well as reequilibrated domains. Of these, the ∼1.0-, 0.80-, and 0.60-Ga domains are depleted in HREEs (fig. 8), indicating that they grew or reequilibrated in the presence of garnet. Because the formation of garnet in the rock is ascribed to incongruent vapor phase– absent melting of biotite, these zircon grains either crystallized from the melt or reequilibrated with it. This is supported by the similar initial 176Hf/177Hf of the cores and rims (fig. 9), a clear indication that the cores reequilibrated with the melt before the overgrowths crystallized. The resetting of the U-Pb and Hf isotope system and the loss of detrital trace element signatures in the reequilibrated zones suggest that most of these zircons may have been altered by a coupled dissolution-reprecipitation mechanism in the presence of voluminous amounts of anatectic melt. However, a few of the reequilibrated domains have high U concentrations and may have suffered enough radiation damage to be altered by a diffusion reaction process. Metamorphic zircons (i.e., !1.20 Ga old) in the JG vol. 117, no. 6 2009 CHECKED 13 Phulbani (KR-87/1) metapelite define concordant age peaks at 1.20, 1.10, and 0.95–0.98 Ga on probability density diagrams (fig. 4). Among these, the 1.20-Ga domains comprise reequilibrated relict cores (e.g., fig. 2, zircon G) and newly crystallized overgrowths (e.g., fig. 2, zircon I), while the 1.10and 0.95–0.98-Ga zones include reequilibrated rims and overgrowths. The 176Hf/177Hf of these zircon domains is quite variable, even within zones having the same age (fig. 9). This indicates that the different zones may have inherited their Hf isotope signature from different sources: the overgrowths from the anatectic melt and the reequilibrated rims from their parental zircon grains. The isotopic signatures can best be explained by a coupled dissolution-reprecipitation reequilibration mechanism at low melt/rock ratio, allowing the altered domains to retain the Hf isotope composition of the parental zircon. A diffusion reaction process appears to be unlikely in this case because these zircons may not have accumulated enough radiation damage, as a result of their low U contents (Nasdala et al. 2004). In the sample from Vijayawada (KR-8), metamorphic zircon grains have ages that span a wide range from 1.20 to 0.50 Ga (fig. 3). Among these, the cores usually show relict chaotic zonation or have no visible internal structure (fig. 2) and give concordant U-Pb ages (fig. 3). They represent reequilibrated detrital grains whose U-Pb isotope system was completely reset during metamorphism. These altered zones, however, have variable 176Hf/ 177 Hf (fig. 9), an indication that although they lost their detrital age information, they inherited the Hf isotope composition of their parental zircon. The rims in contrast show faint oscillatory or sector zonation and have a fairly uniform initial 176Hf/ 177 Hf (fig. 9). They may have crystallized from the anatectic melt that formed during granulite facies migmatization. This inference is supported by trace element measurements on the ∼1.0-Ga domains that indicate that they are depleted in HREEs (fig. 8). This can be explained by reequilibration in the presence of garnet produced as an incongruent phase during vapor phase–absent melting of biotite. Because the reequilibrated grains mostly have relatively low U concentrations, they could not have been altered by a diffusion reaction mechanism. Rather, their geochemical signatures can be best explained by a coupled dissolution-precipitation process at a low melt/rock ratio that allowed them to retain the Hf isotope composition of the precursor zircon. Sediment Provenance and Tectonometamorphic History. The EGP. The 1.20- to 0.50-Ga age specThursday Aug 27 2009 01:29 PM/80021/RITTERD CHECKED CHECKED 14 D . U P A D H Y AY E T A L . trum of metamorphic zircons from the four metapelite samples indicates that the EGP is a polymetamorphic belt. This large spread of the ages cannot be explained by variable Pb loss during a single metamorphic event because the individual data points are mostly concordant and have relatively small analytical uncertainties (table A1). The 1.2-, 1.0–1.1-, 0.92–0.98-, 0.80–0.85-, 0.62-, and 0.50-Ga age clusters can therefore be argued to date multiple tectonothermal events in the region. The oldest metamorphic zircon ages (∼1.2 Ga) are interpreted to date reequilibration and growth of zircon during partial melting related to the earliest UHT metamorphic event (M1), relics of which have been identified in sapphirine-spinel-bearing granulites (cf. Dasgupta and Sengupta 2003; Das et al. 2006). Our data confirm the high-grade nature of that event and are in agreement with reconnaissance EPMA ages of 1.26–1.18 Ga obtained from garnet-hosted monazites in metapelites of the Vijayawada and Angul regions, also interpreted to date early high-grade metamorphism (Simmat and Raith 2008). The predominance of 1.1–0.92-Ga ages in the metamorphic zircon population indicates that the province experienced major tectonometamorphic events during the late Mesoproterozoic and the early Neoproterozoic. The 1.1–1.0- and 0.98–0.92-Ga age clusters are interpreted to date reequilibration and growth of zircon during polyphase-pervasive deformation and granulite facies metamorphism. These ages are a characteristic feature of the EGP rocks and have been reported in several other studies (e.g., Grew and Manton 1986; Aftalion et al. 1988; Paul et al. 1990; Shaw et al. 1997; Sarkar and Paul 1998; Mezger and Cosca 1999; Lisker and Fachmann 2001; Simmat and Raith 2008). Of these, the 1.1–1.0-Ga ages can be related to the fabric-defining event coeval with intense tectonometamorphic reworking (M2) and melting of quartzofeldspathic lithologies in the crystalline complex at granulite facies conditions. This early Neoproterozoic tectonothermal event was followed by a 0.98–0.92-Ga high-temperature thermal overprint (M3), contemporaneous with synto posttectonic emplacement of voluminous megacrystic granitoids (Simmat and Raith 2008 and references therein). Tectonothermal activity in the province continued well into the late Neoproterozoic and early Paleozoic. The most prominent among these is a 0.85–0.80-Ga event, conspicuously identifiable in the Chilka Lake and Vijayawada metamorphic zircon and monazite population and also recorded as a prominent age peak in the Phulbani monazite (but not the zircon). Our JG vol. 117, no. 6 2009 data for the first time conclusively establish the pervasive and high-grade nature of this event in the EGP. The last major high-temperature metamorphic event recorded by zircon and monazite was at 0.62–0.50 Ga. Similar late Neoproterozoic to early Paleozoic ages have been reported from a wide variety of minerals in different rock types from the province (Kovach et al. 1997; Mezger and Cosca 1999; Upadhyay and Raith 2006; Upadhyay et al. 2006a; Simmat and Raith 2008). This thermal imprint reached amphibolite to near granulite facies conditions in many regions. From a geodynamic perspective, the Neoproterozoic (1.1–0.92-Ga) age populations clearly document the involvement of the EGP in orogenic events associated with the assembly of the supercontinent Rodinia at ∼1.0 Ga. Early Neoproterozoic ages similar to those from the EGP have also been identified from the Rayner Complex of East Antarctica (Young and Black 1991; Manton et al. 1992; Kinny et al. 1997; Dunkley 1998; Boger et al. 2000; Carson et al. 2000; Kelly et al. 2002). The EGP together with the Rayner Complex thus clearly formed a part of the early Neoproterozoic orogen system associated with the assembly of Rodinia. The late Neoproterozoic to early Paleozoic (0.62– 0.50-Ga) zircon and monazite ages can be related to tectonometamorphic events during Pan-African orogenesis when the EGP granulites were transported westward over the cratonic foreland along a series of Pan-African thrusts at the craton-EGB suture. Comparable ages have also been reported from the Rayner Complex of East Antarctica (Sandiford 1985; Shiraishi et al. 1997; Fitzsimons 2000; Carson et al. 2002), suggesting that the EGP and the Rayner Complex had similar geological histories from Mesoproterozoic to early Paleozoic times. The tectonic implications of the early UHT (∼1.20 Ga; M1) and the 0.85–0.80-Ga high-grade events are not clear and need further study. Our interpretations are in conformity with a recent geodynamic model (Upadhyay 2008) that proposes that the EGP sedimentary sequences may have been deposited between 1.4 and 1.2 Ga in a rift-related basin that opened between Proto-India and the Rayner Complex (Upadhyay and Raith 2006; Upadhyay et al. 2006a, 2006b). Before this rifting and the deposition of the EGP sediments, the Ongole Domain at the then margin of eastern India may have been continuous with the Rayner Complex, as seen from the preponderance of ∼1.6Ga ages from the two terranes (Kelly et al. 2002; Simmat and Raith 2008; this study). Closure of the basin and cessation of sedimentation at 1.20 Ga Thursday Aug 27 2009 01:29 PM/80021/RITTERD CHECKED Journal of Geology H I G H - T E M P E R AT U R E M E TA P E L I T E S I N I N D I A may have been related to the onset of collisional tectonics before and during the assembly of Rodinia. The above geodynamic scenario implies that eastern India and parts of East Antarctica likely had a common geological history from the Mesoproterozoic to early Paleozoic times. Several possibilities exist for the provenance of the EGP sediments. Ages from detrital zircon grains/domains indicate that they were sourced from late Archaean/early Paleoproterozoic to Mesoproterozoic provenances. This is also supported by their relatively young TDM Hf (2.7–1.8 Ga; table A3) and whole-rock TDM Nd (2.5–1.9 Ga; Rickers et al. 2001) model ages. The late Archaean/early Paleoproterozoic (2.44–2.10Ga) sediments were possibly derived from the eastern Dharwar Craton and the Napier Complex. The 1.64–1.60-Ga detritus may have sourced from the Ongole Domain or the Rayner Complex. Possible provenances for the 1.80-Ga populations in the Phulbani sample (KR-87/1) include the 1.87–1.77Ga felsic volcanics in the Vinjamuru Domain (Dobmeier and Raith 2003) at the eastern margin of the East Dharwar Craton, the southern part of the Bhandara Craton (French et al. 2004), or the ∼1.80Ga Central Indian Tectonic Zone (Bhowmik et al. 1999; Acharyya 2003). The 1.50–1.40-Ga age clusters in the Vijayawada (KR-8) and Phulbani (KR-87/ 1) zircon populations are similar to those reported from monazite cores (detrital?) in metapelitic granulites from near the Kunavaram alkaline complex (Upadhyay and Raith 2006). These detrital components can be correlated with sources involved in rifting, Mesoproterozoic crustal extension, and associated alkaline plutonism along the eastern margin of Proto-India. The Ongole Domain. Our zircon age data constrain the pervasive UHT metamorphism and deformation event affecting the Ongole Domain terrane at 1.63–1.61 Ga. This event led to the reequilibration of the U-Pb isotope system in all zircons. Kovach et al. (2001) have reported older zircon ages (∼1.7 Ga) from orthogneisses, interpreted as dating the emplacement of granitoid complexes. In an exposure near Kondapalle, these charnoenderbitic gneisses (presumably intruded at ∼1.7 Ga, though not dated) host large metapelite xenoliths taken up as already high-grade migmatitic rocks. This older metamorphism has not yet been dated. Thus, in the Ongole Domain, at least two high-grade metamorphic events can be postulated: pre-1.7 and 1.63–1.61 Ga. From the early Mesoproterozoic onward the Ongole Domain was not affected by any other pervasive tectonometamorphic JG vol. 117, no. 6 2009 CHECKED 15 event. Available age data show that there was an early Neoproterozoic (1.1-Ga) moderate thermal overprint (Mezger and Cosca 1999) and a late Neoproterozoic (∼0.50-Ga) amphibolite facies tectonometamorphic event (Dobmeier et al. 2006; Upadhyay et al. 2006b). The Pan-African deformation was mostly concentrated along the tectonic boundaries with the other crustal blocks. In contrast to zircon, monazite included in garnet preserves both detrital and metamorphic age populations. The detrital components, though sparse, define two clusters at ∼2.50 and ∼3.45 Ga (fig. 6). Spot ages from matrix monazite cluster around 1.58 and 1.48 Ga, the older of which closely corresponds to the zircon core and rim ages interpreted as dating the Mesoproterozoic high-grade tectonothermal event in the domain. The ∼1.48-Ga population falls in the range of 1.55–1.45-Ga monazite ages reported for the Ongole Domain granulites (Simmat and Raith 2008). Microtexture indicates that these younger ages date repeated fluid/strain-induced recrystallization of monazite during episodes of localized ductile brittle deformation probably associated with Mesoproterozoic rifting along the margin of Proto-India. The detrital monazite ages point to mid- to late Archaean and early Paleoproterozoic sources for the sediments. This finding, however, needs to be confirmed with more analyses. Late Archaean/ early Paleoproterozoic (∼2.50-Ga) detritus could have been derived from the East Dharwar Craton (Jayananda et al. 2000) and the Napier Complex in East Antarctica, respectively. An East Dharwar Craton provenance for the Ongole sediments has been proposed by Rickers et al. (2001) on the basis of Nd model ages (TDM 2.8–2.7 Ga) and Pb isotope ratios of feldspars. If Proto-India was in the proximity of East Antarctica during the Mesoproterozoic, as several continental reconstruction models suggest, the late Archaean/early Paleoproterozoic zircon detritus could also have been derived from the 2.55– 2.48-Ga para- and orthogneisses of the Napier Complex (Kelly and Harley 2005). Hf Isotope Systematic of Zircon and Crust Extraction The 176Hf/177Hf (t) of most zircons from the EGB metapelites scatters along or between 1.9- and 2.7Ga reference lines, with slopes corresponding to a 176 Lu/177Hf of 0.01, similar to that for the average continental crust (Wedepohl 1995; fig. 9b). The positive correlation of the ␧Hf (t) with the 207Pb/206Pb age (fig. 9c) indicates that the progressively lower Thursday Aug 27 2009 01:29 PM/80021/RITTERD CHECKED CHECKED 16 D . U P A D H Y AY E T A L . 176 Hf/177Hf of the younger zircon can be explained by crystallization from material evolving with a crustal 176Lu/177Hf. Although some of the reequilibrated metamorphic zircons (with ages !1.2 Ga) plot somewhat below the 1.9- and 2.7-Ga reference lines and have apparently higher crustal residence time (11.9 and 12.7, respectively), it can be argued that, in general, they conform to the 1.9- and 2.7Ga crustal trends. This can be explained to be generally reflecting a scenario indicating disturbance of the U-Pb system but at least partial preservation of Hf isotope composition. The trends of 176Hf/177Hf (t) versus time thus point to major crust extraction events between 2.7 and 1.9 Ga in the source regions. Such an inference is supported by the Hf isotope composition of detrital zircons from the Phulbani metapelite (KR-87/1), which have 176Hf/177Hf (t) close to or even more radiogenic than chondrite uniform reservoir (positive ␧Hf), an indication that they represent detritus derived from juvenile sources with Paleoproterozoic to late Archaean crustal residence time (fig. 9b). This is in agreement with other studies (e.g., Kemp et al. 2006) that indicate global crust-forming episodes at these times. ACKNOWLEDGMENTS We are grateful to B. Spiering (Bonn) for her assistance during EPMA dating of monazites and Dettmer from Bochum for preparing superbly polished grain mounts for LA-ICPMS analyses. 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Thursday Aug 27 2009 01:29 PM/80021/RITTERD CHECKED Journal of Geology H I G H - T E M P E R AT U R E M E TA P E L I T E S I N I N D I A Young, D. N., and Black, L. P. 1991. U-Pb zircon dating of Proterozoic igneous charnockites from the Mawson coast, east Antarctica. Antarct. Sci. 3:205–216. Zeh, A.; Gerdes, A.; Klemd, R.; and Barton, J. M. 2007. JG vol. 117, no. 6 2009 CHECKED 19 Archaean to Proterozoic crustal evolution in the central zone of the Limpopo belt (South Africa–Botswana): constraints from combined U-Pb and Lu-Hf isotope analyses of zircon. J. Petrol. 48:1605–1639. Thursday Aug 27 2009 01:29 PM/80021/RITTERD CHECKED CHECKED 20 D . U P A D H Y AY E T A L . QUERIES TO THE AUTHOR 1 I have provided a temporary running head for your article. Please revise as necessary. 2 When you use “cf.” throughout the article, did you mean “see” or “compare”? 3 OK to change Gerdes and Zeh 2008 to 2009 to match references cited? See also below. 4 The phrasing “the loss of radiogenic Pb loss” seems to be confusing. Please revise. 5 Please define TDM. 6 Does JMC need to be defined? parts, and because they did not contain the same information in the top rows (sample, spot ID, and age in the first part of the table vs. sample, zone, spot, and age in the second part, I was not able to combine the two tables. Instead, I created a new table, table A5, per JG preferences. Please see references to tables A4 and A5 throughout. Also please revise the title for table A5 as necessary (I merely copied the title from table A4). Finally, should age be shown in Ma, not m.yr., in both tables? 9 Please define DM. 10 In footnote d of table A3, you cite Tayler and McLennan 1985, but it is not listed in the references cited. Please provide reference information. Also in table A3, there is a hyphen for the last entry under KR871 core. What does this indicate? 7 Please spell out WDS, if appropriate. 11 Please provide a first initial for Dettmer. 8 Table A4 seemed to consist of two different JG vol. 117, no. 6 2009 Thursday Aug 27 2009 01:29 PM/80021/RITTERD CHECKED The Journal of Geology The University of Chicago Press 1427 E. 60th St. Chicago, IL 60637 FAX (773) 753-3616 PAGE CHARGE FORM x x x x x x x x To encourage the widest distribution of the Journal by keeping subscription costs low, we charge $75.00 per Journal page. Papers in excess of 22 printed pages are assessed an additional $30.00 per excess page. For those authors whose articles include color figures in their print versions, we charge $150.00 per color Journal page. Please note that this color fee is non-negotiable. Send forms and billing questions to Cindy Garrett, Billing Manager: o (773) 753-8028, fax (773) 753-3616, cgarrett@press.uchicago.edu. MAKE CHECKS AND PURCHASE ORDERS PAYABLE TO: The University of Chicago Press. 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