PHASE EQUILIBRIA MODELING INTEGRATED WITH GEOCHRONOLOGY IN HIGHLY RESIDUAL UHT GRANULITES FROM THE EASTERN GHATS PROVINCE, INDIA

Melt loss from UHT granulites implies melt percolation through them promoting reaction that erases evidence of the prograde evolution. To determine the prograde evolution requires an assumption about the protolith composition or inversion by step-wise reintegration of melt into the residual granulite to create a series of progressively less-depleted model compositions down temperature. Step-wise reintegration of melt has been used to evaluate the prograde to peak P–T evolution of UHT granulites from several localities in the central Eastern Ghats Province (EGP). The prograde evolution was counter-clockwise in all cases followed by near-isobaric post-peak cooling to the elevated solidi. Peak T >>950°C (up to 1020°C) at P ~0.8GPa was constrained using phase assemblage fields in T/P–X(H₂0/O) and P–T pseudosections, and accessory phase thermometry. As melt flux declined the percolation threshold determined the volume of melt retained and the amount of reaction during cooling. Reaction between osumilite and melt at close to peak or during initial cooling produced Crd–Kfs (± Qtz, Pl, Opx, Sill, Bt) intergrowths aligned in the foliation at several localities demonstrating the crust remained weak at peak P–T and during initial cooling. Emplacement of regionally extensive enderbitic–charnockitic melts into suprasolidus crust during metamorphism is consistent with crustal heating being partly due to lithosphere thinning that accompanied crustal thickening approaching UHT conditions. SHRIMP U–Pb ages from metamorphic zircon and monazite in 10 samples of residual granulite, migmatite and enderbite are integrated with the results of the phase equilibria modeling to quantify the late Mesoproterozoic–early Neoproterozoic P–T–t evolution of the central EGP. A spread of concordant 207Pb/206Pb ages between ca 1040 and ca 980 Ma is interpreted as the timing of peak UHT metamorphism and initial cooling. Weighted mean 207Pb/206Pb ages between ca 980 and ca 930 Ma in several samples record post-peak close to isobaric cooling from peak UHT conditions to the elevated solidi. The variability in the calculated weighted mean ages across the region is due mainly to differences in the temperature of the final solidus from sample to sample, suggesting a slow cooling rate (~1°C/My) during a long-lived tectono-metamorphic event.