Characterization of green garnets from an Archean calc-silicate rock, Bandihalli, Karnataka, India: evidence for a continuous solid solution between uvarovite and grandite

Abstract

Green garnets in a low-pressure metamorphic Archean calc-silicate rock from Bandihalli, Karnataka were characterized by optical microscopic, electron microprobe analysis, powder x-ray diffraction, ICP-mass spectroscopic and Fourier Transform Infrared (FT-IR) spectroscopic techniques. Earlier studies on these garnets indicated that these garnets are chrome grossularite (with the variation of uvarovite content from 2.6 to 44 mol%). Present studies on these garnets show that they have a wide range of uvarovite content from 27 to 72 mol%, bearing testimony to a complete range of solid solutions between uvarovite and grandite. As no IR data appear to be available for naturally occurring uvarovite, we also present here, the first FTIR spectroscopic studies on the uvarovite-rich garnet, along with the IR data on the grossularite from the same calc-silicate rock. The analysis on a single grain of uvarovite garnet (72 mol% Uv) shows an increase in the grossular content towards the rim pointing to the influence of calcareous composition of metasedimentary host rock on the garnet composition. The pyrope content in the studied samples is very low varying from 0.12 to 0.66 mol%. The composition of all the garnets when plotted in a MgO-FeO-CaO diagram (after Dawson and Stephens, 1975) suggests that these garnets are unlike kimberlitic garnets in their composition, thus ruling out the possibility of their being detrital garnets derived from a mantle rock like kimberlite as surmised by Radhakrishna (1994).