A Grenvillian magmatic almandine garnet-bearing ferroan granite intrusion in the Chhotanagpur Gneissic complex, Eastern India: Petrology, petrochemistry, petrogenesis and geodynamic implications

Abstract

We report for the first time magmatic Ca-rich (CaO = 7–11.3 wt%) almandine garnet-bearing ferroan granites. The almandine (Alm66.6Grs24.2Pyr4.9Spes3.9Uvr0.1) garnet-bearing Tilaboni granite pluton was emplaced in a major regional shear zone of Chhotanagpur Gneissic Complex, Eastern India. Garnets show partial dissolution, corona, and symplectitic textures. Petrography and composition of minerals suggest the garnets are magmatic. Mineral chemistry of these garnets differs from those of calc-alkaline I-type, S-type, A-type, mantle-type, and metamorphic garnets. Geochemically, the host granites show high 10,000Ga/Al ratios (average 3.2), high K2O (average 4.91 wt%), high total alkalis (average 7.5 wt%), high Ta + Yb (average 10.2 ppm), high Ce + Nb + Zr + Y (average 668 ppm), high Ce/Yb (average 27.3) and strong negative Eu-anomalies (average 0.3). These granites are classified as ferroan, calc-alkalic to alkalic, metaluminous to weakly peraluminous, and highly fractionated I-type. Early crystallization of magnesian amphiboles/biotites enriched the FeO in the derivative melts. The granite magma had high liquidus temperature (800–950 °C), low oxygen fugacity (ΔQFM = +1 to −1.6), and solidified at around 5 to 6 kb pressure (Al-in hornblende barometers). Pseudosection modelling shows that the garnets crystallized from a hydrous melt (H2O = 6–9 wt%) at around 760 °C temperature, 6 kb pressure, and fO2–15 log unit (bar). The pluton emplaced in a post-collisional tectonic setting. Low Mg# (average 0.24), low Nb/U (average 9.8), and Ce/Pb (average 7.1) ratios but high Th/U (average 9.8) ratios of the Tilaboni granites strongly suggest their crustal source. The granite magma was derived by the 20–40% partial melting of an old high-K high alumina shoshonitic hornblende granulite protolith at around 7 kb. The shear zone facilitated the fast upward movement of the magma and incomplete dissolution of the garnets before solidification at lower pressure.