[S-CG61_2PO1] Petrology, Mineralogy and Resource Geology

[SCG61-P09] Infiltration of CO₂-H₂O binary fluid and formation of patchy charnockite from Southern India

*Takahiro ENDO¹, Toshiaki TSUNOGAE¹, Santosh M.² (1.Univ. Tsukuba, 2.China University of Geosciences Beijing)

Keywords:incipient charnockite, metamorphic fluid, pseudosection, geothermobarometry, Trivandrum Granulite Block, southern India

Since the first discovery of patches, veins and ladders of coarse-grained orthopyroxene-bearing felsic granulite (incipient charnockite) within foliated amphibolite-facies gneiss from Kabbal in Karnataka, southern India, by Pichamuthu (1960), the origin and petrogenesis of charnockite and its implications for granulite processes in lower crust have been the focus of many petrologists. According to previous studies, charnockite formation in the SGT is considered to have resulted by the infiltration of CO₂-rich anhydrous fluids along structural pathways within upper amphibolite-facies gneisses, resulting in the lowering of water activity and stabilization of orthopyroxene through breakdown of biotite (e.g. Janardhan et al., 1979; Newton et al., 1980; Hansen et al., 1987; Santosh et al., 1990; Newton, 1992; among others). This study presents new petrological data of ‘incipient’ charnockite developed within garnet-biotite (Grt-Bt) gneiss from Kakkod with the western Trivandrum Granulite Block (TGB), India. In this locality, bulk rock compositions of charnockite and the host Grt-Bt gneiss are almost equivalent. The result of conventional geothermobarometry using Grt-Opx-Pl-Qtz assemblage shows the peak metamorphic condition of 860-960℃ and 6.9-8.4 kbar, which is consistent with the results of mineral equilibrium modeling. The metamorphic condition certainly corresponds to granulite-facies event, and it is higher than those reported from other incipient charnockite localities in the TGB and adjacent Nagercoil Block. Furthermore, the estimated metamorphic condition is too high for the stability of the host Grt-Bt gneiss that contains a mineral assemblage formed at amphibolite-facies condition. In addition, althought pseudosecondary fluid inclusions are composed of pure CO₂, secondary fluid inclusions contain CO₂-H₂O binary fluid. Therefore, patchy chanorckite in Kakkod from the TGB is considered to have formed by infiltration of CO₂-H₂O binary fluid during a retrograde stage. The petrogenetic model of incipient charnockite formation proposed in this study is therefore different from reported petrogenesis from other localities.