Abstract:
Mafic volcanic rocks and dykes associated with Na-rich TTG and K-rich granites in the Kudremukh greenstone belt of western Dharwar Craton provides a window to Precambrian crustal evolution in the southern Peninsular India. Here we present zircon U-Pb geochronology and geochemistry of these rocks to constrain the greenstone-basement relationship and crustal growth events in the western Dharwar Craton. TTGs exhibit fractionated REE patterns [(La/Yb) sub(N) = 11.8-50.3; (Gd/Yb) sub(N) = 2.1-4.9] whereas the granites with K sub(2) O contents of 3.84-5.20 wt percent are peraluminous calc-alkaline with moderate enrichment of LREE over HREE and depleted MREE abundances. The metabasalts are characterized as arc basalts with moderate to high Fe sub(2) O sub(3), CaO, SiO sub(2), MgO, LILE, LREE and low HFSE contents. The higher Nb contents (6.2-11.4 ppm) in some of them, classified as Nb-enriched basalts, were erupted at 2498 plus-minus 43 Ma, indicating a 2.5 Ga greenstone volcanism in the western Dharwar Craton. These are characterized by slightly fractionated REE along with negative Nb, Zr, Hf and Ti anomalies. The amphibolitic and doleritic dykes have overlapping chemistry with moderate Al sub(2) O sub(3), moderate to high Fe sub(2) O sub(3) and MgO displaying coherent chondrite normalized REE patterns. The 3068 plus-minus 34 Ga TTG were generated through high pressure melting of mafic crust in a garnet-spinel peridotite mantle regime with the assimilation of upper crustal material in a subduction zone environment. The granites of 2936 plus-minus 23 Ma were the products of partial melting and fractional crystallization of TTG melts which display intrusive relationship with the thickened TTG crust. The mafic volcanism took place at 2498 plus-minus 43 Ma subsequently followed by the dyke emplacement at 2484 plus-minus 29 Ma. Therefore, two major episodes of crustal growth are identified from this region: i) TTG-granite association approx. 3.0-2.9 Ga and ii) mafic volcanism - dyke emplacement between 2.49 and 2.48 Ga. The geochemical signatures of arc basalts and Nb enriched basalts indicate variable slab-mantle interactions including fluid-fluxed metasomatism and melt-fluxed hybridization of mantle wedge in a subduction zone environment followed by the emplacement of amphibolitic dykes during post-subduction collision phase. Our results document the episodic growth of continental crust in western Dharwar Craton from 3.0 to 2.5 Ga.
