Petrology and Geochemistry of the Ultramafic Intrusion of Keshargaria, Singhbhum, Bihar

An ultramafic body flanked on either side by tholeiitic dolerite occurs as a minor intrusion in the Singhbhum granite, southwest of Keshargaria (22°36' : 85°56'). The ultramafic body varies from pyroxene-rich harzburgite to olivine-rich poikilitic harzburgite, the latter being conspicuous in the central part of the intrusion. A crude planar structure defined by enstatite crystals is developed towards the contact walls. The constituent mafic silicates, arranged in paragenetic order, are: forsteritic olivine, enstatite, diopsidic augite (malacolite), phlogopite and plagioclase, the last three phases appear to represent intracummulus crystallisation.

The central part of the intrusion is relatively enriched in olivine whereas the marginal parts are in an advanced stage of differentiation relative to the central zone, indicated by lower Mg/Fe and Ba/Rb ratios in the former. This spatial variation in degree of differentiation is considered to have been induced during flowage of the crystal-iquid mush. The ultramafic intrusion is non-uniform in chemical and mineralogical compositions and does not represent unaltered mantle material. The significant deviation from the mantle composition is in higher aluminium and ferric iron and lower magnesium in the investigated peridotite. The Keshargaria ultramafic body which is one of several such bodies associated with the Newer Dolerite intrusions, represents a stage of fractionation of mantle derived magma.