Pb, Sr and Nd isotope systematics of metavolcanic rocks of the Hutti greenstone belt, Eastern Dharwar craton: Constraints on age, duration of volcanism and evolution of mantle sources during Late Archean

The Hutti greenstone belt, hosting major gold mines, is made up of tholeiitic basalts, felsic volcanic rocks and sediments metamorphosed to amphibolite facies and surrounded by granitoid rocks, predominantly of granodiorite in composition. The granitoid rocks show intrusive contact relationship at places along the eastern and northern margins of the belt while such relationships could not be observed along the western margin as it is covered by soil. The tholeiitic basalts have flat to moderately LREE-enriched chondrite normalized REE patterns and show depletion in Nb relative to MORB. The Hutti metabasalts define a collinear array in the Rb–Sr isotope evolution diagram with a slope corresponding to an age of 2706 ± 130 Ma. The higher uncertainty is due to scatter caused by mobility of Rb and Sr during metamorphism and low temperature hydrothermal alteration. In the 206Pb/204Pb vs. 207Pb/204Pb diagram the metabasalts define a collinear array corresponding to an age of 2637 ± 150 Ma with significant scatter indicating that Pb was also mobilized by the post-crystallization alteration processes. The Sm–Nd isotope system in the Hutti metabasalts has been least disturbed giving an isochron age of 2662 ± 81 Ma which is suggested as the age of formation of their precursors. Combined with the published U–Pb SHRIMP ages on zircons from the felsic volcanic rocks it is inferred that the Hutti greenstone belt formed over a period of ⩾86 Ma. The major greenstone belts of the eastern Dharwar craton have evolved coevally at ca. 2700 Ma ago and are older than the surrounding granitoid rocks. The metabasalts have εNd values of +1.4 to +3.0 and εSr values of −23.4 to +2.0 and plot close to the mantle array. They were derived from long-term LREE depleted mantle source. The felsic volcanic rocks have εNd values of −1.3 and −1.4 and εSr values of −30.6 and +89.3, respectively. Their magmas were derived from older continental crustal sources.