Origin of vesuvianite-garnet veins in calc-silicate rocks from part of the Chotanagpur Granite Gneiss Complex, East Indian Shield: The quantitative P-T-X-CO2 topology in parts of the system CaO-MgO-Al2O3-SiO2-H2O-CO2 (+Fe2O3, F)

摘要

A calc-silicate rock from part of the Chotanagpur Granite Gneiss Complex, East India, develops veins and patches of vesuvianite (F: 2.3-3.9 apfu, Fe3+: 1.7-2.1 apfu) and garnet (Gr(71-80)Alm(12-17)Adr(1-9)) proximal to amphibole-bearing quartzo-feldspathic pegmatitic veins. The host calc-silicate rock exhibits a prominent gneissic banding that is defined by alternate clinopyroxene- and plagioclase-rich layers. The vesuvianite-garnet veins are both parallel and cross-cutting the gneissic banding of the host calc-silicate rock. Two contrasting mineralogical domains that are rich in garnet and vesuvianite, respectively, develop within the vesuvianite-garnet veins. Textural studies support the view that the garnet-and vesuvianite-rich domains preferentially develop in the clinopyroxene-and plagioclase-rich layers of the host calc-silicate rocks, respectively. Some of the vesuvianite-rich domains of the veins develop the assemblage vesuvianite + quartz + calcite + anorthite (as a result of the reaction diopside + quartz + calcite + anorthite = vesuvianite) that was deemed metastable in the commonly used qualitative isobaric T-X-CO2 topology in the system CaO-MgO-Al2O3-SiO2-H2O-CO2 (CMASV).