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Mafic–ultramafic rocks; Chhotanagpur Gneissic Complex (CGC); physico-chemical parameters; magma composition

In the Chhotanagpur Gneissic Complex (CGC) of Eastern India a suite of mafic and ultramafic rocksoccurs 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 $998 \pm 10 \rm{Ma}$ ago. Field occurrence, petrology and mineral chemistry of the mafic–ultramafic rocks have been studied. Both the mafic $\rm{(Pl + Hyp + Di + Hbl + Bt + Mag + Spn \pm Ol \pm Spl)}$ and ultramafic rocks $\rm{(Di + Hyp + Bt \pm Hbl \pm Ol \pm Pl \pm Spl \pm Ep \pm Spn)}$ 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 $f\rm{O}_{2}$ of the magma. Liquidus temperature $(975–1088^{0}\rm{C})$ 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 $(782–819^{0}\rm{C})$ of crystallization were determined using the amphibole–plagioclase geothermobarometry. Similar range of values of pressure, temperature and $f\rm{O}_{2}$ values were obtained using other thermobarometers. High $\rm{H_{2}O}$ and $f\rm{O}_{2}$ (>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 $\rm{SiO_{2}}$ (48.16–67.64 wt%), high $\rm{CaO}$ (3.01–11.73 wt%), high $\rm{K_{2}O}$ (1.34–4.49 wt%) and low $\rm{TiO_{2}}$ (0.04–2.71 wt%) contents and intermediate Mg# (46.73 and 59.78).

SUSMITA DAS¹ BAPI GOSWAMI¹ CHITTARANJAN BHATTACHARYYA¹

1. Department of Geology, University of Calcutta, 35, Ballygunge Circular Road, Kolkata 700 019, India

Published

12 March 2020

Manuscript received

1 March 2019

Manuscript revised

2 August 2019

Accepted

16 October 2019

Early published

Unedited version published online

Final version published online

11 March 2020