GEOCHEMISTRY OF DISTINCT MAFIC DYKES FROM THE DAMODAR VALLEY GONDWANA BASINS AND CHHOTANAGPUR GNEISSIC TERRAIN, EASTERN INDIA: IMPLICATIONS FOR THEIR PETROGENESIS AND TECTONIC SETTING

A number of mafic dykes were emplaced in the Damodar valley Gondwana sedimentary basins and Chhotanagpur gneissic terrain, at the northern-most margin of the Singhbhum craton during the Mesoproterozoic and Cretaceous. There are a number of geological evidences that suggest the CGT is a cratonic block rather a mobile belt. Field relationships of these mafic dykes reveal that Mesoproteozoic mafic dykes are intruded on Precambrian basement of CGT, trending EW to WNW, whereas a majority of Cretaceous mafic dykes are intruded only in Damodar valley basins trending in NE to ENE and NNW to WNW directions; some of them are also intruded in CGT too. Four distinct mafic dyke swarms, two from the Mesoproterozoic mafic dykes and two from the Cretaceous mafic dykes, are identified from the region. Petrographic characteristics of the studied mafic dykes classify Mesoproterozoic mafic dykes as metabasites and metadolerite dykes, whereas Cretaceous mafic dykes are classified as high-Ti dolerite (HTD) and low-Ti dolerite (LTD) dykes. Available ages on Cretaceous mafic dykes suggest their emplacement ~110-115 Ma and association with the Kerguelen mantle plume activities. All four types of mafic dykes identified from the present study have very distinct geochemical characteristics. Geochemistry of the Mesoproterozoic dykes suggests their derivation from primary melt; however involvement of subduction related processes in their genesis is not discounted. The geochemical characteristics the Cretaceous mafic dykes suggest that these dykes are emplaced in an intracratonic rift-setting and fed from two different melts generated through plume tectonics.