Abstract

The Meso- to Neoarchaean Singhbhum Granitoid Complex (SGC), Eastern India has been intruded by NW-SE, NE-SW, N-S and E-W trending Newer dolerite dykes (NDD). A few ultramafic intrusives are spatially associated with NDD. In the studied samples olivine and orthopyroxene makes a major portion. Serpentine as alteration product of olivine is also present in these rocks. On the bases of petrography they are classified as harzburgites. These representative samples have high MgO (>30.0wt.%) and low SiO2 (< 45.0wt.%), Al2O3 (< 5.0wt.%) and alkalies (< 1.0wt.%). On various variation diagrams studied samples follow up normal crystallization trends. Their geochemical characteristics such as SiO2, Na2O+K2O, CaO/Al2O3 (0.85) values and enriched light rare earth element (LREE) patterns suggests that they are picritic in nature rather than komatiitic or komatiites. On Primitive mantle normalized diagrams they show enriched patterns of large ion lithophile elements (LILEs) and LREEs and depletion of high field strength elements (HFSEs) (e.g., Nb,Ta,Zr,Ti). Such characteristics resemble to that of subduction related basaltic rocks. On chondrite-normalized REE diagram studied samples display least to moderate LREE fractionated patterns {(La/Sm)n=2.62-2.97} and almost unfractionated /flat HREE {(Gd/Lu)n=1.12-1.58} with negative Eu anomalies {Eu/Eu*=0.77-0.84}., indicates removal of plagioclase. On tectonic setting diagrams such as Zr/Y vs Ti/Y and Y vs Nb/Thstudied samples plot in plate margin (arc related) tectonic setting. The low values of (La/Yb)n< 12.01 and (La/Sm)n< 3.39 and high values of Ba/Nb (38 to119) and Sr/P (0.30 to 1.49) parameters in the studied dykes also indicate their arc setting relation.

Keywords: Singhbhumcraton; Petrogenesis; Subduction zone; Ultramafic dykes