A Geochemical perspective on A-type granites of Nilgiri area, Southeastern Singhbhum Craton, Odisha India: Implication for petrogenesis

Abstract

<p>Singhbhum Craton is one of the oldest cratonic nuclei of Indian shield that represents a complex evolutionary history that includes granite emplacement at different episodes, episodic sedimentation and polyphase magmatism. A wide variety of granitoids are exposed in the south-eastern part of Singhbhum Craton, in and around the Nilgiri area, Odisha, India. However, their petrogenesis is poorly understood due to the lack of high precision geochemical, geochronological and isotope data.</p><p>Nilgiri granite are characterised by their ferron nature and high K<sub>2</sub>O concentration along with high TiO<sub>2</sub>/MgO and 1000*Ga/Al ratio. Nilgiri granite also show enriched HFSE and total REE and marked depletion in Ni, Cr, V and MgO. Nilgiri granite samples fall within the “within plate granite” field in the Nb Vs Y tectonic discrimination diagram. In the 1000*Ga/Al vs Zr bivariate diagram these granite plot as A type granite. Thus from the salient geochemical characteristics and discrimination diagrams the Nilgiri granite suite appear as A-type granite. In the Y-Nb-Ce ternary space these granites are classified as the A2 type suggesting the probable origin through partial melting of the prexisting  continental crust in an extensional rifting system.</p><p> Well fractionated LREE/REE, flat HREE along with strongly negative anomaly in Eu indicate a shallow depth of source rock melting for the formation of these granitoids. The depletion in (HFSE) Nb, Ta and Ti in the primitive mantle normalized multi-element variation diagrams is probably inherited from the source rocks. The negative anomaly in Sr is in agreement with the negative Eu-anomaly and corresponds to its low pressure origin.</p>