Search

• Granitoids of Singhbhum Craton. • Two generation of TTGs at 3.42 and 3.32 Ga formed by melting of juvenile mafic crust. • Later K-rich, silicic granites at 3.28 and 3.25 Ga formed by melting of tonalites. • Recurring crustal melting in a gradually thickening oceanic plateau. • Final cratonisation of the Singhbhum Craton at 3.25 Ga. A significant part of many Archean cratons formed during Paleoarchean. Yet, the mechanism and tectonic setting of formation of Paleoarchean continental crust remain highly debated. In this contribution, we present field, petrographic, geochemical, zircon U-Pb age and Hf isotope data on Paleoarchean granitoids from west-central part of the Singhbhum Craton (Champua-Hat Gamharia corridor), India. The whole process starting from nucleation of a juvenile continent to its evolution and final stabilization is documented. The core of the craton nucleated with formation of 3.45–3.40 Ga TTGs showing juvenile character (zircon ɛHf t = +0.6 to +7.1). These rocks show slightly depleted HREE and Y, negligible Eu-anomaly (Eu/Eu* = 0.90 to 1.00) and moderate Sr/Y (25–64), consistent with derivation from a low-K mafic crust at a pressure near the lower end of the garnet stability field, causing subordinate garnet retention in the residue and negligible role of plagioclase. A second generation of TTG formation took place at 3.32 Ga in the area by deeper melting of a juvenile mafic crust (zircon ɛHf t = +1.3 to +5.7) as suggested by strongly depleted HREE and Y, and high Sr/Y (52–155) implying significant amount of garnet retention in the residue. Subsequently, the area witnessed intracrustal melting at 3.28 and 3.25 Ga which tapped moderately old to juvenile (zircon ɛHf t = −1.9 to +4.5), mostly TTG sources at variable depths generating potassic, LILE-enriched, high-silica granites. Intrusion of these potassic granites marks the final cratonization of the Singhbhum Craton. The sequence of events is interpreted in terms of repeated crustal melting and granitoid generation in a gradually thickening oceanic plateau with a progressive change in granitoid source from mafic to felsic in composition. A synthesis of rock assemblage, regional geological setting and structural pattern also supports intraplate nature of the magmatism in Singhbhum Craton, which might have been a significant mechanism of crustal growth worldwide during Paleoarchean. Further, a comparison of juvenile crustal growth and crustal reworking events of the Singhbhum and other Indian cratons show that these cratons record distinct evolutionary histories and were probably nucleated at different sites. [ABSTRACT FROM AUTHOR]

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 (εHf t = +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 εHf t (+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 εHf t (+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. [ABSTRACT FROM AUTHOR]

Highlights: [•] Granitoid magmatism changed in composition with evolution of Kadiri greenstone belt. [•] Initial sanukitoid magmatism was followed by transitional TTG, leucogranite and A-type granite. [•] A convergent setting characterized by repeated slab break-off and crustal reworking. [•] Finally post-subduction anorogenic granite magmatism. [•] Reworking of Mesoarchaean crust in eastern Dharwar craton identified. [Copyright &y& Elsevier]