Physico-chemical conditions of crystallization and composition of source magma of the Grenvillian post-collisional mafic–ultramafic rocks in the Chhotanagpur Gneissic Complex, Eastern India

Abstract

In the Chhotanagpur Gneissic Complex (CGC) of Eastern India a suite of mafic and ultramafic rocks occurs as sills, dykes and enclaves within porphyritic granitoid pluton. These mafic and ultramafic rocks and host porphyritic granitoids were emplaced in a post-collisional setting around 998 ± 10 Ma ago. Field occurrence, petrology and mineral chemistry of the mafic–ultramafic rocks have been studied. Both the mafic (Pl + Hyp + Di + Hbl + Bt + Mag + Spn ± Ol ± Spl) and ultramafic rocks (Di + Hyp + Bt ± Hbl ± Ol ± Pl ± Spl ± Ep ± Spn) are composed of same minerals but in different modal proportions. Plagioclase, clinopyroxene, orthopyroxene, amphibole, biotite and rarely olivine and spinel are important primary minerals of mafic–ultramafic suite. Primary amphiboles, biotites and pyroxenes show their affinity with shoshonitic lamprophyres. Chemically these rocks are similar to the kentallenite (of appinite suite) and are enriched in both compatible (Fe, Mg, Ni, and Cr) and incompatible (K, Ba, Rb, and LREE) elements and show crust-like trace element patterns. Crystallization of clinopyroxene before labradorite and presence of primary hornblende and biotite suggest high water content while biotite–magnetite–sphene assemblage suggests high fO2 of the magma. Liquidus temperature (975–1088°C) of the parental magma of the mafic–ultramafic rocks was obtained by two-pyroxene thermometer. The pressure (2.9–5.7 kbar) and near-solidus temperature (782–819°C) of crystallization were determined using the amphibole–plagioclase geothermobarometry. Similar range of values of pressure, temperature and fO2 values were obtained using other thermobarometers. High H2O and fO2 (>NNO buffer) of the magma are characteristics of convergent setting. The mafic–ultramafic rocks of the suite probably crystallized from a magma which had high SiO2 (48.16–67.64 wt%), high CaO (3.01–11.73 wt%), high K2O (1.34–4.49 wt%) and low TiO2 (0.04–2.71 wt%) contents and intermediate Mg# (46.73 and 59.78).