High-pressure and ultrahigh-temperature metamorphism at Komateri, northern Madurai Block, southern India |
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| Authors: | Toshiaki Tsunogae M Santosh Hiroyuki Ohyama Kei Sato |
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| Institution: | aGraduate School of Life and Environmental Sciences (Earth Evolution Sciences), University of Tsukuba, Ibaraki 305-8572, Japan;bDepartment of Natural Environmental Science, Faculty of Science, Kochi University, Akebono-cho 2-5-1, Kochi 780-8520, Japan;cDepartment of Earth and Planetary Sciences, Graduate School of Science and Engineering, Tokyo Institute of Technology, 2-12-1 Ookayama, Tokyo 152-8551, Japan |
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| Abstract: | We report for the first time the evidence for prograde high-pressure (HP) metamorphism preceding a peak ultrahigh-temperature (UHT) event in the northernmost part of the Madurai Block in southern India. Mg–Al-rich Grt–Ged rocks from Komateri in Karur district contain poikiloblastic garnet with numerous multi-phase inclusions. Although most of the inclusion assemblages are composed of gedrite, quartz, and secondary biotite, rare staurolite + sapphirine and spinel + quartz are also present. The XMg (=Mg/Fe+Mg]) of staurolite (0.45–0.49) is almost consistent with that reported previously from Namakkal district in the Palghat–Cauvery Shear Zone system (XMg = 0.51–0.52), north of the Madurai Block. The HP event was followed by peak UHT metamorphism at T = 880–1040 °C and P = 9.8–12.5 kbar as indicated by thermobarometric computations in the Grt–Ged rock and associated mafic granulite. Symplectic intergrowth of spinel (XMg = 0.50–0.59, ZnO < 1.7 wt.%) and quartz, a diagnostic indicator of UHT metamorphism, probably formed by decompression at UHT conditions. The rocks subsequently underwent retrograde metamorphism at T = 720–760 °C and P = 4.2–5.1 kbar. The P–T conditions and clockwise exhumation trajectory of the Komateri rocks, comparable to similar features recorded from the Palghat–Cauvery Shear Zone system, suggest that the Madurai Block and the Palghat–Cauvery Shear Zone system underwent similar HP and UHT metamorphic history probably related to the continent–continent collision during the final stage of amalgamation of Gondwana supercontinent. |
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| Keywords: | High-pressure and ultrahigh-temperature metamorphism Granulite Magnesian staurolite Spinel + quartz Geothermobarometry Palghat– Cauvery Shear Zone system Gondwana Supercontinent |
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