Three distinct Archean crustal growth events as recorded from 3.48 Ga migmatite, 2.70 Ga leucogranite, and 2.54 Ga alkali granite in the Bundelkhand Craton, Central India

摘要

Field relations, microstructures, phase petrology, elemental geochemistry, and zircon U-Pb geochronology of migmatites and (undeformed) granites (sensu lato) from the Central Bundelkhand Tectonic Zone (CBTZ) of the Bundelkhand Craton, central India are investigated to understand the age and origin of these rocks. Anatexis of felsic proto-crust produced in situ leucocratic melts with entrained residual assemblages, forming the metatexite and diatexite in the migmatite. The chemistry of biotite (X-Fe = 0.65-0.72) from the leucosome of migmatite supports its crystallization in a peraluminous felsic melt more likely in a syn-collisional tectonic setting. The residual amphibole in the leucosome from migmatites belongs to metaluminous (I-type) calc-alkaline protocrust source that originated in a subduction setting. Biotites (X-Fe = 0.80-0.81) and amphiboles (X-Fe = 0.84-0.87) from granites exposed proximal to the CBTZ, suggest alkaline (A-type) nature of the host magma, emplaced at midcrustal depth (3.79-4.73 +/- 0.5 kbar). Whole-rock geochemistry of migmatite reveals the magnesian nature of melt formed by differential degrees of melting of heterogeneous sources (i.e., TTG and mafic crust), at 12-14 kbar. The alkali granite was likely formed by the anhydrous melting of ancient felsic crustal rocks. U-Pb geochronology of inherited magmatic zircon cores (Th/U = 0.06-0.48) from migmatite yielded an upper intercept age of 3478 +/- 48 Ma interpreted as zircon crystallization age in the protolith. The zircon rims of magmatic origin (Th/U = 0.06-0.72) grown over the 3.48 Ga old zircon cores provide an upper intercept age of 2700 +/- 16 Ma; interpreted as zircon crystallization age in a leucocratic melt. This is synchronous with the timing of intense deformation/collision and the melting event in the CBTZ. Zircons (Th/U = 0.26-0.92) from alkali granite yielded a weighted mean crystallization age of 2539.9 +/- 6.6 Ma without any noticeable zircon inheritance. The Bundelkhand Craton probably experienced at least two phases of collision-accretion orogens, the first one initiated at ca. 2.8 Ga with arc-related magmatism and ended with ca. 2.7 Ga collision-related felsic magmatic record. The second phase began with ca. 2.58 Ga subduction-related magmatism and culminated at ca. 2.54 Ga with a minor amount of post-collisional, alkali granite magmatism.