P-T-t evolution of sapphirine-bearing semipelitic granulites from Vadkampatti in Eastern Madurai Domain, southern India: Insights from petrography, pseudosection modelling and in-situ monazite geochronology

Abstract

Migmatitic semipelitic granulites from the Eastern Madurai Domain of southern India preserves reaction textures indicating a clockwise P-T evolution, with near isothermal decompression. The prograde history is recorded by included phases within porphyroblastic garnet and orthopyroxene. Development of high XMg garnet (up to 0.69) + orthopyroxene (Al2O3 up to 9.6 wt%) assemblage indicates attainment of ultrahigh temperature (UHT) condition at the peak stage. Subsequent formation of a series of symplectic assemblages (Opx2 + Spr2 + Sil2 + Pl2 and Opx2 + Crd2) with aluminous orthopyroxene (Al2O3 up to 9.5 wt%), at the expense of garnet indicates decompression at ultrahigh temperatures. Conventional thermobarometry and feldspar thermometry indicate peak metamorphic P-T conditions of ~970 ± 30 °C, 10.3 ± 0.4 kbar and formation of symplectites at ~960 ± 20 °C, 9.2 ± 0.3 kbar for Opx2 + Spr2 + Sil2 and at ~890 ± 40 °C, 7.3 ± 0.6 kbar for Opx2 + Crd2. Obtained P-T conditions are in agreement with the P-T estimates from pseudosection modelling. Texturally controlled in-situ monazite dating shows that the Th-poor and extremely HREE depleted dark cores of matrix monazite with a weighted age of 586 ± 8 Ma formed simultaneously with garnet growth during the prograde evolution. On the other hand, Th-rich and HREE depleted bright rims with a weighted age of 532 ± 7 Ma formed from the dissolution-reprecipitation due to melt crystallization at or near T-max, dating the peak of metamorphism. The compositionally homogeneous HREE + Y enriched monazite in the symplectite with a weighted age of 475 ± 12 Ma, formed along with the symplectites during garnet breakdown. Our results are suggestive of a late Neoproterozoic to Cambrian long-lived (ca. 55 million years) hot orogenic plateau in the Madurai Block. UHT conditions were likely achieved by conductive heating, before being rapidly uplifted to shallower crustal depth during late Cambrian to early Ordovician (475 ± 12 Ma). This is consistent with the hot orogen model for the attainment of UHT conditions, which demonstrates that crusts that are moderately rich in heat producing elements can reach temperatures >900 °C when the duration of orogeny is sufficiently long (i.e. slow erosion rates) to allow conductive heating through radioactive decay.