Metamorphic phase equilibria modelling and zircon U-Pb geochronology of ultrahigh-temperature cordierite granulites from the Madurai Block, India: implications for hot Gondwana crust

摘要

The Madurai Block (MB) is the largest Precambrian crustal block in the Southern Granulite Terrane (SGT) of India and hosts rare cordierite- and orthopyroxene-bearing granulites. Investigations based on field study, petrology, metamorphic P-T estimation, and detrital zircon geochronology of these granulites are crucial for understanding the ultrahigh-temperature (UHT) metamorphism and crustal evolution in this block. Here we investigate the petrology and zircon U-Pb geochronology of two new localities of cordierite granulites at Kottayam (southern MB; SMB) and Munnar (central MB; CMB). Petrographic observations and phase equilibria modelling results indicate that these rocks experienced UHT metamorphism with the peak temperature exceeding 950? and involving clockwise P-T paths. The prograde mineral assemblages define the P-T conditions of 6.8-8.7kbar and 750-875?. The peak conditions are estimated using pseudosection modelling and geothermometry, which yield P-T estimates of 7.1-9.1kbar and 955-985?. The retrograde cooling and decompression are inferred at 860-790? and 6.5kbar, respectively. Partial melting played an important role during metamorphism and contributed to the overgrowth around detrital zircons. The melt production process was probably related to biotite dehydration melting, and was mainly triggered by heating, with or without the effect of decompression. Detrital zircons in cordierite granulite samples from the two localities show similar age distributions and have dominantly Neoproterozoic ages (1024-760Ma). The zircon cores show oscillatory zoning with a wide range of Th/U ratios (0.01-0.96), implying complex protoliths from multiple Neoproterozoic provenances from both southern and central domains of the MBs. Zircon rims and homogeneous bright zircons yield mean ages of 549 +/- 5Ma, 536 +/- 6Ma, and 544 +/- 6Ma, which are interpreted to represent zircon overgrowths during the post-peak cooling and decompression process. The timing of peak UHT metamorphism is constrained as 549-599Ma, which coincides with the assembly of the Gondwana supercontinent.