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Sapphirine granulites from Panasapattu,eastern ghats belt,India:Ultrahigh-temperature metamorphism in a Proterozoic convergent plate margin

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Geoscience Frontiers 2012年第1期3卷 9-31页

作者： C.V.Dharma Rao M.Santosh Reia M.Chmielowski National Disaster Management Authority Government of India A-1 Safdarjung EnclaveNew DelhiIndia Department of Interdisciplinary Science Faculty of Science Kochi UniversityAkebono-choKochi 780-8520Japan Dipartimento di Scienze della Terra Universita degli Studi di Milano Italy

We report equilibrium sapphirine ＋ quartz assemblage in biotite-orthopyroxene-garnet granulites from a new locality in Panasapattu of Paderu region in the eastern ghats granulite belt, which provide new evidence for ultrahigh-temperature （UHT） metamorphism at 1030-1050 ℃ and 10 kbar in this region. The development of migmatitic texture, stabilization of the garnet-orthopyroxene- plagioclase-K-feldspar association, prograde biotite inclusions within garnet and sapphirine as well as sapphirine and cordierite inclusions within garnet in these granulites indicate that the observed peak assemblages probably formed during prograde dehydration melting of a Bt-Sill-Qtz assemblage, and constrain the prograde stage of the p--T path. The core domains of orthopyroxene porphyroblasts have up to w（Al203） 9.6%, which suggest that the temperatures reached up to 1150 ~C suggesting extreme crustal metamorphism. These conditions were also confirmed by the garnet-orthopyroxene thermobar- ometery, which yields a p--T range of 1012--960 ~C and 9.4 kbar. The p--T phase topologies computed using isochemical sections calculated in the model system Na20-CaO-K20--FeO-MgO-A1203-SiO2 --H2O （NCKFMASH） for metapelites, garnet-free sapphirine granulites and garnet-bearing sapphirine granulites match the melt-bearing assemblages observed in these rocks. Isochemical sections constructed in the NCKFMASH system for an average sub-aluminous metapelite bulk composition, and contoured for modal proportions of melt and garnet, as well as for the compositional isopleths of garnet, predict phase and reaction relations that are consistent with those observed in the rocks. Garnet and orthopyroxene contain Ti-rich phlogopite inclusions, suggesting formation by prograde melting reactions at the expense of phlogopite during ultrahigh-temperature conditions. These p--T results underestimate ＇peak＇ condi- tions, in part as a result of the modification of garnet compositions in the domains where some melt was retai

关键词： Panasapattu eastern ghats belt Melting Sapphirine + quartz Ultrahigh-temperaturemetamorphism Tectonics

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