Petrology, Geochemistry, Model Sm-Nd Ages and Pedogenesis of the Granitoids of the Northern Block of Western Dharwar Craton

Gianitoids comprising the northern block of Western Dharwar Craton (WDC), enclosed within the 225 km long and 40 km wide transect corridor stretching from East of Gadag to the Goa coast, have been examined in the field, mapped and interpreted on the basis of petrological, geochemical and Sm-Nd isotopic data. The rocks constitute 43% of the corridor lithology and include 5 large plutons of batholithic dimensions and two Stock-Like bodies. Those are classified as Anmod Ghat trondhjemite(AGT), Chandranath granite(CNG), Dudhsagar granite(DSG) Ramnagar migmatite (RNM), Ramnagar granite gneiss(RNGn), Ramanagar porphyntic granite(RNPG), Annigen-Majjigudda granite(AMG) and Hatalgen-Naregal migmatite(HNM) Of these, only RNM and HNM show the typical polyphase migmatitic character, the remaining are lather homogeneous with only localized occurrence of tectomcally emplacedmahc rocks and pegmatiteaphte. Modal mineral compositions coupled with geochemical parameters especially the combination of K, Rb, Sr, Sr/Ba, La_N/Y_N and Eu/Eu^* values, serve as the distinctive features of the granitoids. The granitoids and greenstone belts of the corridor show overall structural conformity and evidence of Low-Grade metamorphism in the range of Greenschistlow-Amphibohte facies. There is no obvious evidence of their diapinc emplacement and contact metamorphism the granitoids bear essentially a basement relation towards the associated greenstone belts. The results of Sm-Nd isotopic analyses have indicated existence of mainly three groups viz , (I) the older trondhjemite (AGT) - Tonahte (RNM) - Monzogranite (HNM) providing model ages (T_DM) of around 3 3 Ga, (ii) a suite of granitoids viz CNG, DSG, RNGn and RNPG with T_DM between 2 96 and 2 83 Ga and (in) younger granite viz AMG, dating 2 68 Ga The isotopic data suggests that the protohth of AGT was different from that of CNG, RNGn and RNPG Higher soda and Na₂O K₂0 ratio of 1 1 or more and marked depletion in REE of all the younger granitoids of the tiansect are distinctive chemical features as compared to Closepet Granite, Hyderabad Gianite and other younger K rich granites in the Eastern Dharwar Craton The geochemical characteristics recorded place the granitoids of the corridor closer to those constituting Peninsular Gneissic Complex in the Southern part of WDC.

Various chemical parameters suggest derivation of the transect granitoids from predominantly igneous protohths and their emplacement in syncolhsional volcanic arc regime In addition, their lithogeochemical features suggest that they constitute separate plutons AGT is related to a 3 3-3 4 Ga old crust forming event RNM and HNM are products of reworking of old crust whereas the remaining granitoids of the corridor appear to be plume related.