ALBERT

Description: In the western part of the Singhbhum Shear Zone (SSZ), East Indian Shield, borosilicate-bearing veins of variable thickness (tens of micrometers to 1 m thick) are hosted in kyanite-quartzite and kyanite-mica schist. The veins have been classified into three types, which are, from oldest to youngest, generation I (tourmaline), II (dumortierite + tourmaline), and III (tourmaline) veins. Alkali- and Mg-rich tourmaline [XMg = Mg/(Mg + Fe) = 0.68 {+/-} 0.09; X = Na, Ca, K, {square} (vacancy) = 0.40 {+/-} 0.12] is the sole borosilicate in generation I veins, which have been folded in response to regional deformation. Generation II veins were emplaced along shear bands (1 mm to 1 m thick) developed parallel to the axial planes of these folds. Long axes of fibrous dumortierite and prismatic tourmaline of generation II veins are oriented along the shear bands and have been bent around lenticular remnants of host kyanite-quartzite. Generation III veins have a dendritic pattern, crosscut generation II veins and show aggregates of fibrous to acicular tourmaline. Prismatic tourmaline in generation II veins is optically zoned with a green tourmaline core that is variably replaced and rimmed by blue tourmaline. Fibrous to acicular tourmaline in generation III veins is comprised up of blue tourmaline with compositions similar to the rim composition of prismatic tourmaline in generation II veins. Green and blue tourmaline is aluminous (Al total 〉7 apfu) and alkali-deficient (X = 0.71 {+/-} 0.08). High YAl content, high X, low XMg (0.19 {+/-} 0.10), and excess cation charge indicate tourmaline in generation II veins is rich in an "oxy-foitite" component. Foitite-rich tourmaline in generation III veins has tetrahedral Al and a slightly lower Mg-content and X than those of generation II veins. Optical zoning in prismatic tourmaline corresponds to an abrupt compositional change with paragenetically older green tourmaline having higher Al and XMg, but lower alkali content in the X-site than the blue tourmaline rim. The compositional variation in green and blue tourmaline can be explained by a combination of coupled substitutions represented by AlO[R(OH)]-1 and Al(NaR)-1, where R = (Fe2+ + Mg). Pseudosections in the system Na2O-K2O-Al2O3-SiO2-H2O constructed from bulk chemical compositions of the studied rocks and the P-T slopes of two isochors computed from brine-rich inclusions trapped in quartz grains indicate that borosilicate formation in generation II and III veins occurred within 4.1 {+/-} 0.5 kbar and 377 {+/-} 21 {degrees}C. The mineral assemblages and textures suggest that the borosilicate-bearing veins formed from infiltration-driven alteration of host kyanite-quartzite and kyanite-mica schist along structurally controlled conduits by more than one batch of chemically distinct boron-rich aqueous fluids.

Description: 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 (Gr71-80Alm12-17Adr1-9) 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-XCO2 topology in the system CaO-MgO-Al2O3-SiO2-H2O-CO2 (CMASV). Using an internally consistent thermodynamic database, quantitative petrogenetic grids in the P-T and isobaric T-XCO2 spaces have been computed in the CMASV system. The influence of the non-CMASV components (e.g., Na, Fe3+, F) on the CMASV topologies have been discussed using the published a-X relations of the minerals. Our study shows topological inversion in the isobaric T-XCO2 space that primarily depends upon the composition of the vesuvianite. The quantitative CMASV topologies presented in this study successfully explain the stabilities of the natural vesuvianite-bearing assemblages including the paradoxical assemblage vesuvianite + quartz + calcite + anorthite. Application of the activity-corrected CMASV topology suggests that infiltration of F-bearing oxidizing aqueous fluids into the calc-silicate rocks develop the vesuvianite-garnet veins in the studied area. A genetic link between quartzo-feldspathic pegmatites and the vesuvianite-garnet veins seems plausible. This study demonstrates controls of topological inversion in the complex natural system, owing to which certain mineral assemblages develop in nature that are otherwise deemed metastable in one set of reaction geometry.