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Neoarchean high-pressure metamorphism from the northern margin of the Palghat-Cauvery Suture Zone, southern India Petrology and zircon SHRIMP geochronology [查看] YohsukeSaitoh、ToshiakiTsunogae、M.Santosh、T.R.K.Chetty、KenjiHorie

We report the metamorphic pressure–temperature (P–T) history of mafic granulites from two localities in southern India, one from Kanja Malai in the northern margin and the other from Perundurai in the central domain of the Palghat–Cauvery Suture Zone (PCSZ). The PCSZ is described in recent models as the trace of the suture along which crustal blocks were amalgamated within the Gondwana supercontinent during Late Neoproterozoic–Cambrian. The mafic granulite from Kanja Malai yields P–T conditions of 750–800℃ and 8–12 kbar reflecting the partially retrograded conditions following a peak high-pressure(HP) metamorphic event. The common Grt + Cpx + Qtz assemblage in these rocks and lack of decompression texture suggest that peak metamorphism was probably buffered by Grt + Cpx + Opx + Pl + Qtz assemblage, following which the rocks were exhumed through a gradual P–T decrease. The mafic granulite from Perundurai (Grt + Cpx + Pl) contains Opx + Pl symplectite commonly occurring between garnet and clinopyroxene, suggesting the progress of reaction: Grt + Cpx z+ Qtz?Opx + Pl, with the Grt + Cpx + Qtz representing the peak metamorphic assemblage. The reaction microstructures and calculated P–T conditions suggest that the mafic granulites from Perundurai underwent peak HP metamorphism at P > 12 kbar and T = 800–900 C and subsequent isothermal decompression along a clockwise P–T path, in contrast to the P–T path inferred for Kanja Malai. The contrasting P–T paths obtained from the two localities suggest that whereas Perundurai is a part of the metamorphic orogen developed within the PCSZ during Gondwana assembly, the high-pressure granulites of Kanja Malai belong to a different orogenic regime.In order to evaluate this aspect further, we analyzed zircons in a charnockite and garnet-bearing quartzo-feldspathic gneiss associated with the HP granulites from Kanja Malai which yielded mean 207Pb/206Pb magmatic protolith emplacement ages of 2536.1 ± 1.4 Ma and 2532.4 ± 3.7 Ma, and peak metamorphic ages of 2477.6 ± 1.8 Ma and 2483.9 ± 2.5 Ma, respectively. These results closely compare with the available magmatic (2530–2540 Ma) and metamorphic (2470–2480 Ma) ages reported from charnockites in the Salem Block at the southern fringe of the Archean Dharwar craton, immediately north of the PCSZ. The Neoarchean/Paleoproterozoic ages obtained from Kanja Malai correlate with the tectonic history at the margin of the Archean craton. Although no age data are available for the Perundurai mafic granulite, the close correspondence of their P–T data and exhumation path with those reported for Late Neoproterozoic–Cambrian HP–UHT metamorphism within the PCSZ suggest that these rocks form part of the Gondwana-forming orogen.

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