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

We report for the first time magmatic Ca-rich (CaO = 7-11.3 wt%) almandine garnet-bearing ferroan granites. The almandine (Alm(66.6)Grs(24.2)Pyr(4.9)Spes(3.9)Uvr(0.1)) garnet-bearing Tilaboni granite pluton was emplaced in a major regional shear zone of ChhotanagpurGneissic Complex, Eastern India. Garnets showpartial 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 4.91wt%), high total alkalis (average 7.5wt%), 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 ofmagnesian amphiboles/biotites enriched the FeO in the derivative melts. The granite magma had high liquidus temperature (800-950 degrees C), low oxygen fugacity (Delta QFM=+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 degrees C temperature, 6 kb pressure, and fO(2)-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. (C) 2020 Elsevier B.V. All rights reserved.