Anorthosite and associated gabbronorite plutons of Barabar hills in Chotanagpur granite gneiss complex (CGGC), eastern India: Estimation of parental melt for gabbronorite using equilibrium distribution method (EDM)

The small plutons of anorthosite and associated gabbronorite exposed near Barabar hills form a component of Chotanagpur granite gneiss complex (CGGC) in eastern India. Plagioclase (>90 vol %) make up the majority of anorthosite rock with minor mafic minerals (amphibole, mica), while orthopyroxene (>40 vol %), plagioclase (40-50 vol %) and clinopyroxene (>20 vol %) make up the associated gabbronorite. These are cumulate rocks with anorthosite and gabbronorite showing adcumulate and mesocumulate textures, respectively. Compositionally, plagioclase ranges from anorthite to labradorite (An_60-96) in anorthosite and from oligoclase to bytownite (An₅₀-₇₀) in gabbronorite. In gabbronorite, the clinopyroxene composition ranges from diopside to augite (En_36-43 Fs_12-15 Wo_43-47), and the orthopyroxenes are hypersthene (Wo_39-40 En_46-50 Fe_10–21).

Anorthosite show enrichment of LILE (Rb, Ba, Sr, Th, Pb) with respect to the HFSE (Zr, Ti, Nb and display enrichment in LREE ((La/Yb) _N = 2.78-15.29) with positive Eu anomaly (Eu/Eu* = 1.29-3.45) and variable MREE. A flat to depleted trend for HREE ((Sm/Yb) _N = 1.02-2.95) is observed for anorthosites. Associated gabbronorites show enrichment of LREE ((La/Yb) _N=1.99-4.93), depleted HREE ((Sm/Yb) _N = 0.88-3.24) with negative to positive Eu anomaly (Eu/Eu* = 0.78-2.95). Also, the gabbronorite shows enrichment of LILE (Rb, Ba, Sr, Th, Pb) compared to HFSE (Zr, Ti, Nb). Clinopyroxenes of gabbronorite have low REE abundances (53.29-60.29 ppm). Clinopyroxenes are depleted in light rare earth elements (LREEs) (La/Yb) _N = 0.75–0.80 and depleted in LILEs such as Ba, Sr. and also exhibit negative anomalies in Zr and Ti.

REE composition of gabbronorite clinopyroxene is constrained between TMF = 15-30% calculated using the equilibrium distribution method (EDM). This is substantiated by whole rock parental melt REE composition calculated using the concentration ratio approach (Nernst equation), the result of which is consistent with those made using EDM. In chondrite normalized plot, the estimated parental melt display (1) near-horizontal trend from Lu to Gd at rock/chondrite = ~100, (2) negative anomaly at Eu, (3) gradual rise from Sm to Ce and (4) slight dip from Ce to La.