[SCG60-P06] Clockwise pressure-temperature-time evolution of ultrahigh-temperature pelitic granulites from the Trivandrum Block, southern India: New insights from zircon U–Pb and REE analyses
Keywords:Southern India, Trivandrum Block, khondalite, UHT metamorphism, U–Pb geochronology, P–T–t evolution
The dominant lithologies of this quarry are leptynite (plagioclase + K-feldspar + quartz + garnet + biotite + ilmenite + magnetite) and khondalite. Peak, prograde, and retrograde mineral assemblages of the khondalite are quartz + garnet + plagioclase + K-feldspar + ilmenite + sillimanite + spinel, biotite + quartz + garnet + plagioclase + K-feldspar + ilmenite + sillimanite, and quartz + garnet + plagioclase + K-feldspar + ilmenite + magnetite + cordierite, respectively. Application of ternary feldspar geothermometry on mesoperthites in khondalites yielded peak temperatures of 1000–1100 °C at 8 kbar, suggesting UHT metamorphism recorded in the rocks. Application of mineral equilibria modeling in the system NCKFMASHTO indicates peak P–T condition of 920–1020 °C and 6.9–7.5 kbar, which further confirms UHT metamorphism of this region. Prograde and retrograde P–T conditions of 750 °C/7 kbar and 800 °C/4 kbar, respectively, were also obtained, suggesting a clockwise P–T evolution.
U–Pb and REE analyses of zircons suggest that low-HREE zircons grew together with garnet possibly during partial melting and melt crystallization stage in the rock at ca. 580 Ma at temperature conditions of >810 °C during prograde metamorphism. Relatively HREE-enriched zircons grew at ca. 530 Ma possibly by garnet decomposition related to partial melting of the rock during retrograde metamorphism. Growth of REE-enriched zircons at ca. 490 Ma might be related to fluid infiltration and hydration of garnet. These results suggest clockwise P–T evolution, and the high-grade metamorphism continued at least 55 Myr.