Mesoproterozoic Reworking of Palaeoproterozoic Ultrahigh-temperature Granulites in the Central Indian Tectonic Zone and its Implications |
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Authors: | BHOWMIK, S. K. SARBADHIKARI, A. BASU SPIERING, B. RAITH, M. M. |
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Affiliation: | 1 DEPARTMENT OF GEOLOGY AND GEOPHYSICS, INDIAN INSTITUTE OF TECHNOLOGY, KHARAGPUR-721 302, INDIA 2 MINERALOGISCHPETROLOGISCHES INSTITUT, DER UNIVERSITÄT BONN, POPPELSDORFER SCHLOß, D-53115 BONN, GERMANY |
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Abstract: | In the southern periphery of the Sausar Mobile Belt (SMB), thesouthern component of the Central Indian Tectonic Zone (CITZ),a suite of felsic and aluminous granulites, intruded by gabbro,noritic gabbro, norite and orthopyroxenite, records the polymetamorphicevolution of the CITZ. Using sequences of prograde, peak andretrograde reaction textures, mineral chemistry, geothermobarometricresults and petrogenetic grid considerations from the felsicand the aluminous granulites and applying metamorphosed maficdyke markers and geochronological constraints, two temporallyunrelated granulite-facies tectonothermal events of Pre-Grenvillianage have been established. The first event caused ultrahigh-temperature(UHT) metamorphism (M1) (T 950°C) at relatively deepercrustal levels (P 9 kbar) and a subsequent post-peak near-isobariccooling PT history (M2). M1 caused pervasive biotite-dehydrationmelting, producing garnetorthopyroxene and garnetrutileand sapphirinespinel-bearing incongruent solid assemblagesin felsic and aluminous granulites, respectively. During M2,garnetcorundum and later spinelsillimanitebiotiteassemblages were produced by reacting sapphirinespinelsillimaniteand rehydration of garnetcorundum assemblages, respectively.Applying electron microprobe (EMP) dating techniques to monazitesincluded in M1 garnet or occurring in low-strain domains inthe felsic granulites, the UHT metamorphism is dated at 20402090Ma. Based on the deep crustal heatingcooling PTtrajectory, the authors infer an overall counterclockwise PTpath for this UHT event. During the second granulite event,the Palaeoproterozoic granulites experienced crustal attenuationto 6·4 kbar at T 675°C during M3 and subsequentnear-isothermal loading to 8 kbar during M4. In the felsic granulites,the former is marked by decomposition of M1 garnet to orthopyroxeneplagioclasesymplectites. During M4, there was renewed growth of garnetquartzsymplectites in the felsic granulites, replacing the M3 mineralassemblage and also the appearance of coronal garnetquartzclinopyroxeneassemblages in metamorphosed mafic dykes. Using monazites frommetamorphic overgrowths and metamorphic recrystallization domainsfrom the felsic granulite, the M4 metamorphism is dated at 15251450Ma. Using geochronological and metamorphic constraints, theauthors interpret the M3M4 stages to be part of the sameMesoproterozoic tectonothermal event. The result provides thefirst documentation of UHT metamorphism and Palaeo- and Mesoproterozoicmetamorphic processes in the CITZ. On a broader scale, the findingsare also consistent with the current prediction that isobaricallycooled granulites require a separate orogeny for their exhumation. KEY WORDS: Central Indian Tectonic Zone; UHT metamorphism; counterclockwise PT path; monazite chemical dating |
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Keywords: | : Central Indian Tectonic Zone UHT metamorphism counterclockwise P T path monazite chemical dating |
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