Petrology and U–Th–Pb geochronology of the sapphirine–quartz-bearing metapelites from Rajapalayam, Madurai Block, Southern India: Evidence for polyphase Neoproterozoic high-grade metamorphism

Abstract

Strongly migmatitic sapphirine-bearing granulites occur as dm- to m-sized enclaves in garnetiferous leucogranites in the southeastern part of the Madurai Block (MB), the largest section of Neoproterozoic crust in the Southern Granulite Terrain of India (SGT). Early mineral assemblages (biotite–sillimanite–quartz, garnet–biotite–quartz) break down via successive dehydration melting reactions to high- T and high- P phase assemblages (e.g. garnet-melt, orthopyroxene–sillimanite-melt, orthopyroxene–cordierite-melt). Melt-consuming reactions along the retrograde path led to partial resorption of garnet to orthopyroxene–sillimanite, orthopyroxene–sapphirine and/or orthopyroxene–cordierite as well as orthopyroxene–sillimanite assemblages to cordierite–biotite–quartz symplectites. The occurrence of UHT metamorphism is indicated by the presence of small inclusions of sapphirine–quartz intergrowths in garnet. This reaction sequence is interpreted in a KFMASH partial petrogenetic grid taking both melt-producing and melt-consuming reactions into account which yields a clockwise heating–cooling–decompression P– T path culminating at 8–11 kbar and 1000–1100 °C. In-situ U–Th–Pb monazite dating has been performed to more precisely constrain the ages given by chemical electron-microprobe on monazite in various textural positions. Monazite inclusions in garnet often yield a bimodal distribution of early (950–850 Ma) and late Neoproterozoic (600–500 Ma) ages which is in good agreement with the results of U–Pb and EPMA zircon dating in other places of the southeastern Madurai Block. By contrast, monazite inclusions in other mafic minerals (orthopyroxene, cordierite, biotite) or as interstitial phase exclusively yield ages between 550 and 480 Ma. The early Neoproterozoic population is interpreted to reflect the timing of migmatisation and peak UHT metamorphism, whereas the late Neoproterozoic ages are inferred to date cooling and exhumation with granitic magmas to a higher crustal level of the Madurai Block.