PRESSURE-TEMPERATURE-TIME EVOLUTION OF SAPPHIRINE BEARING SEMIPELITIC GRANULITES FROM EASTERN MADURAI DOMAIN: INSIGHTS FROM PETROGRAPHY, PSEUDOSECTION MODELLING AND MONAZITE GEOCHRONOLOGY

Migmatitic semipelitic granulite near Vadkampatti village in the Eastern Madurai Domain of southern India preserves conspicuous reaction textures indicating a clockwise P-T evolution, with near-isothermal decompression, after attaining the peak of metamorphism. The P-T-t evolution of the rocks is constrained using petrography, mineral chemistry, P-T pseudosection modelling, thermobarometry and monazite geochronology. The prograde history is recorded by numerous monophase and multiphase inclusions within garnet and orthopyroxene. Development of high X_Mg garnet (X_Mg upto 0.69) + orthopyroxene (Al₂O₃ upto 9.5 wt%) assemblage is indicative of attainment of UHT condition at the peak stage. It is followed by near-isothermal decompression, leading to development of symplectitic assemblages of orthopyroxene + cordierite, orthopyroxene + sapphirine + sillimanite, and orthopyroxene + plagioclase + sapphirine at the expense of garnet. Using conventional thermobarometry, peak metamorphic P-T conditions of ca. 1050 °C at 10 kbar and symplectite formation at ca. 946 °C at 7 kbar is deduced, taking mineral compositions from the core and symplectite, respectively. Estimates of peak P-T conditions from pseudosection modelling in NCKFMASH system as calculated from isopleths thermobarometry, using isopleths of X_Mg garnet, X_Al and X_Mg of orthopyroxene, are in agreement with conventional thermobarometry. Texturally controlled in-situ monazite dating provides time constraints to the P-T evolution of the rock. The monazite grains present in different textural settings can also be distinguished by a systematic variation in grain size and chemistry. Monazite present in the matrix are coarse grained (20–100µm) and shows moderate to extreme depletion in HREE content indicating their growth along with the garnet at the peak stage, whereas, the ones present in the symplectites are fine grained (>10µm-70µm) with enriched HREE content, indicating their formation during the breakdown of garnet along with the symplectites. Spot ages from the core of the matrix monazite and some inclusions in garnet yield weighted mean ages of 586 ± 8 Ma defining the peak of metamorphism, while the monazite grains in the symplectites yield a weighted mean age of 475 ± 12 Ma defining the timing of the symplectite formation.