Metamorphic signatures

Multiple cycles of metamorphism are hallmarks of the EGB where at least two events occurred under granulite facies conditions. Considerable amount of petrological data have accumulated from studies in different parts of EGB over the last three decades (summarized in Dasgupta and Sengupta, 2003; Mukhopadhyay and Basak, 2009 and references therein). The apparently contrasting P-T data and conflicting nature of P-T paths can now be treated with domainal or provincial basis. It is now realized that crustal provinces/domains with contrasting evolutionary histories are likely to show such variability and one needs to understand the story in a fragmental way before being able to visualize the entire story in a collative manner.

The earliest event of granulite-grade metamorphism in EGB is reported from the Jeypore Province (or domain 1B of Rickers et al., 2001) where the enderbitic gneiss with protolith age of ~3.90-3.00 Ga (Nd-model age) shows metamorphism at ~2.80 Ga (U-Pb zircon age after Kovach et al., 2001). The nature of metamorphism, deformational pattern and field relations of this unit are not clear. No subsequent metamorphic imprints are identified from this province, which is partly caused by paucity of data. If we accept the fact that the Jeypore Province has antiquity beyond any known tectonothermal event in rest of the EGB, we are left with no alternative but to exclude this from the EGB. Moreover, since the southern tip of this province is conjectural (as discussed by Dobmeier and Raith, 2003); the apparent signatures of Grenvillian (~1.1-1.0 Ga) and Pan African (~500 Ma) events on Kunavaram Alkaline Complex can not be ascribed to that of Jeypore Province with certainty.

In the southern part of the EGB (Ongole domain or domain 1A of Rickers et al., 2001), an early event of UHT metamorphism has been documented from enclaves of metasedimentary rocks within mafic magma (Sengupta et al., 1999). UHT metamorphism has been interpreted as the fallout of regional-scale contact metamorphism of the mafic magma, now represented by mafic granulite. Peak temperature of metamorphism exceeded 1000°C at deep crustal conditions (9-10 kbar pressure). The resultant P-T path is a heating-cooling one and the post-UHT condition is marked by prolonged isobaric cooling and hydration. Timing of UHT metamorphism is not well-constrained so far, but it should be older than ~1.72 Ga emplacement age of enderbitic gneiss. Bose et al. (2008) identified a zircon SHRIMP U-Pb concordia age of ~1.76 Ga, which is the most likely timing of UHT metamorphism vis-à-vis emplacement age of basic magma in the basement of EGB. Evidence of metamorphic reworking is not prominent in this rock, yet zircon and monazite grains from different rock-types of the Ongole domain show a prominent growth at ~1.65-1.60 Ga (Simmat and Raith, 2008; Upadhyay et al., 2009). Since this broadly coincides with emplacement of a pegmatoidal enderbite (Mezger and Cosca, 1999), it may be viewed at least as a thermal imprint. A somewhat similar heating-cooling type P-T path was also deduced from further south (Dasgupta et al., 1997) which implies that the early UHT metamorphism in the Ongole domain was chiefly thermal in nature. The lower pressure (~6 kbar) of metamorphism in the latter suggest magma emplacement at much shallower level. Rocks of this domain did not undergo any granulite-grade reworking after ~1.60 Ga, but 40Ar-39Ar data from hornblende suggest weak amphibolite grade reworking during the Grenvillian (~1.1 Ga) orogeny. It is reasonable to postulate that the Ongole domain was far from the Grenvillian orogenic front and was mostly cratonized during ~1.60 Ga.

Considerable petrological data are available from the Eastern Ghats Province (combining Domains 2 and 3 of Rickers et al., 2001). Although two isotopic domains of Rickers et al. (2001) are grouped under a single Province (Eastern Ghats Province of Dobmeier and Raith, 2003), careful inspection identifies several problems with this line of argument. The metamorphic history of the Domain 2 was initiated by a high-T/low-P progressive metamorphism and deformation that eventually led to UHT (~1000°C) peak (M1-D1) under deep-crustal conditions (8-9 kbar). This was followed by isobaric cooling. This entire process followed a single orogenic cycle when the lower crust evolved following a counter-clockwise P-T path (reviewed in Dasgupta and Sengupta, 2003). A second granulite-grade metamorphism and associated deformation (M2-D2) strongly reworked the deep-crustal granulites and exhumed them to mid-crustal level as evident from decompression-dominated retrogressive segment (Dasgupta and Sengupta, 2003; Dobmeier and Raith, 2003 and references therein). M3 is weak amphibolite-grade overprint and mostly localized along ductile shear zones. Thermal imprint associated with this is manifested by emplacement of pegmatite crosscutting the M2-D2 foliation. The timeframes for this three-fold metamorphic cycle are more or less constrained from available data. The timing of M1 UHT metamorphism is a long-drawn controversy as it was previously constrained to be ~1100 Ma (Jarick, 1999) based on zircon common Pb method. Texturally-constrained monazite studies later portray a much wider timeframe (~1250-1100 Ma) (Simmat and Raith, 2008). This is still problematic since a time span in the tune of ~150 m.y. is too large for a specific tectonothermal event. Careful study using monazite and/or zircon grains by in-situ methods would probably resolve the issue. The timing of M2 overprinting is ~950-900 Ma as all available geochronological studies seem to converge to this window (Grew and Manton, 1986; Shaw et al., 1997; Mezger and Cosca, 1999). U-Pb zircon ages using LA-ICPMS (Upadhyay et al., 2009) and SHRIMP methods (Bose et al., 2008) also confirm this scenario. The timing of M3 metamorphism and associated pegmatite emplacement is constrained to be ~550-500 Ma (Kovach et al., 1997; Mezger and Cosca, 1999).

Metamorphic history of the granulites occurring in northern domains of Eastern Ghats Province (Phulbani, Angul and Chilka domains) is somewhat different from that in Domain 2 to the south. All the three domains are characterized by ~950 Ma high-grade granulite event (Dobmeier and Raith, 2003; Simmat and Raith, 2008). The most discernable metamorphic signature of the Chilka Domain is the decompression-dominated P-T path. In Chilka Lake area, Sen et al. (1995) documented step-wise decompression and cooling related retrograde path, the interpretation of which was later refuted by later work (Dasgupta and Sengupta, 2003). Nevertheless, an early decompression-dominated P-T trajectory is strikingly different from cooling-dominated one from that in the Domain 2. Geochronological data do not furnish any unique answer to this. Although monazite dating identified ages in the time frame of ~1250-950 Ma (e.g. Simmat and Raith, 2008), a more pronounced cluster of metamorphic ages is found in the time window of ~800-500 Ma (Simmat and Raith, 2008 and references therein). This is vindicated by more precise zircon U-Pb ages (Bose et al., 2008; Upadhyay et al., 2009). Moreover, the so-called “protracted” tectonothermal history of the Chilka Domain needs to be clarified as it is difficult to assume a single orogeny persisting over 300 Ma. Apart from regional-scale metamorphism, the Chilka Domain also witnessed contact metamorphism due to the emplacement of anorthosite magma. In the southern fringe of the anorthosite body, the enclosing metasediments underwent UHT metamorphism (~1000°C) at mid-crustal (6-7 kbar) level (Raith et al., 2007; Sengupta et al., 2008). Timing of this event is problematic as controversy over the emplacement age of the anorthosite still persists. The newly computed zircon ID-TIMS and monazite age (~983 Ma by Chatterjee et al., 2008) contrasts the earlier age of ~792 Ma (Krause et al., 2001; Dobmeier and Simmat, 2002). This again requires a proper reappraisal. The preponderance of Pan-African age granulite metamorphism in this domain certainly is a key feature and its implication will be explored later in the discussion section. The tectonothermal imprints in the time frame of ~800-500 Ma are characteristically absent in the Angul Domain whose metamorphic history is least known.

Rengali Province is situated at the northern extremity of EGB close to its contact with the Singhbhum Craton. This fault-bounded terrain has petrological, structural and geochronological signatures transitional between EGB and Singhbhum Craton. An early event of granite emplacement at ~2.80 Ga is the first tangible magmatic event in this terrain. Metamorphic signatures are of amphibolite to weakly granulite grade and scattered geochronological data suggest two thermal events at ~800-700 Ma and ~500 Ma respectively (reviewed in Dobmeier and Raith, 2003). This distribution of metamorphic ages is surprisingly similar to that of the Chilka Domain of the Eastern Ghats Province.