LAUSR

Abstract Palaeoarchaean granitoids are exposed over wide area in the Singhbhum craton whose origin and role in crustal evolution are not well constrained. This study presents whole-rock and mineral chemical data coupled with zircon U-Pb dating and Hf isotope results on such granitoids from the central part of the craton to understand their petrogenesis, tectonic setting and role in continental crustal evolution. The first phase of granitoid magmatism in this area is represented by a 3.47 Ga Na-rich, gneissic tonalite belonging to the Archaean TTG (tonalite-trondhjemite-granodiorite) suite. This rock is characterized by high-HREE (heavy rare earth elements), negative Eu anomaly, low Sr/Y ratio and positive zircon Hf isotope signature (eHft = +2.1 to +4.8). It is interpreted to be formed by shallow melting of a juvenile mafic source. At 3.35 Ga a silicic, ferroan porphyritic biotite granite formed. It shows variable K/Na, low Y and high Sr/Y, moderately fractionated HREE and positive zircon eHft (+1.8 to +4.0), and is explained as a product of high-temperature melting of a heterogeneous, juvenile source consisting of tonalites and mafic rocks at lower crustal depth. The final phase of granitoid magmatism is marked by a 3.30 Ga non-porphyritic ferroan, silica-rich biotite granite. Geochemical characteristics like moderately high K, moderately fractionated HREE, low Ca and Sr/Y, and zircon eHft (+0.8 to +3.7) suggest that the granite was formed by high-pressure melting of a tonalite-dominated source with short crustal residence. All the three granitoid phases display low Mg, Ni and Cr contents and magnesium number (Mg#) precluding direct involvement of mantle in their origin. Rather, crustal reworking caused by episodic plume-related mafic-ultramafic magma underplating and intraplating in an oceanic plateau setting is suggested as the possible mechanism for formation of the granitoids. Successive crustal reworking events involved progressively greater amount of previously formed felsic crust generating more evolved, K-rich granitoids. It appears to be a globally important process which led to effective crustal differentiation and maturing of the cratons during Palaeoarchaean.