Tectonothermal evolution of the gneiss complex at Salur in the Eastern Ghats Granulite Belt of India
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Textural relations, thermobarometry and petrogenetic grid considerations in the syn-tectonic granitoid massif and the enveloping metasedimentary gneisses at Salur are consistent with a counter-clockwise P-T-t path for the rocks. The low-P/high-T prograde sector is documented by the pre- to syn-D1 cordierite orthopyroxene garnet spinel-bearing metatexite leucosomes in metapelites. Heating and loading of the rocks (syn- to post-D1) resulted in the formation of garnet+ orthopyroxene cordierite-bearing diatexites, and decomposition of cordierite in metatexite leucosomes to orthopyroxene + sillimanite + biotitc + quartz symplectites. Near-peak temperature, 850C at 8.0 kbar, was reached syn- to post-D2. Post-peak cooling resulted in the stabilization of coronal grossular and anorthite + calcite symplectites at the expense of scapolite + wollastonite + calcite assemblages in calcsilicate gneisses, and the resetting of cation exchange temperatures at 700-750C. Near-isothermal decompression at c. 700-750C is manifested by the decomposition of garnet porphyroblasts in the granitoid gneisses to plagioclase + orthopyroxene/ilmenite/biotite two-phase coronas and restabilization of cordierite at garnet margins in metapelites. Subsequent low-P, near-isobaric cooling led to the overprinting of granulite facies assemblages by muscovite + calcite assemblages, and further resetting of cation exchange thermometers to lower temperatures c: 600C. The tectonothermal evolution of the Salur gneiss complex vis-a-vis the Eastern Ghats Belt is therefore consistent with high degrees of lower crustal melting, followed by prograde heating of the cover rocks due to magma invasion synchronous with crustal compression, and finally thermal relaxation over a protracted period punctuated by tectonic/erosional denudation of the thickened crust.
