Matti Kurhila

The diversification of granitoids and plate tectonic implications at the Archaean–Proterozoic boundary in the Bundelkhand Craton, Central India

Abstract The Bundelkhand Craton in Central India holds a large Archaean granitoid complex consisting of cores of TTG (tonalite–trondhjemite–granodiorite) gneisses of island arc or oceanic origin surrounded by abundant younger high-K calc-alkaline granitoids. Major and trace element groupings and ion probe U–Pb zircon datings of the groups show a time gap of 130 Ma between the main formation episodes of the TTGs (3.5/3.3–2.7 Ga) and the emplacement of the first high-K granitoids (2.57–2.54 Ga). Based on their geochemical diversity, the high-K calc-alkaline granitoids can be divided into low-silica high-magnesium (LSHM) granitoids such as sanukitoids and Closepet-type granitoids, and high-silica low-magnesium (HSLM) monzogranites with low-HREE and low-Eu subgroups. The former group points to mantle or mixed mantle and crustal sources, and the latter to pure crustal sources. All the varieties of the high-K granitoids formed within a narrow time span, which indicates large-scale partial melting and fluid activity in the mantle and crust, possibly resulting from a slab breakoff or delamination at the margin of an Archaean TTG continent. Supplementary material: Major and trace element concentrations and U-Pb results of granitoids from the Bundelkhand Craton are available at https://doi.org/10.6084/m9.figshare.c.3576377

Geochronology of Neoarchaean granitoids of the NW eastern Dharwar craton: implications for crust formation

Abstract The Neoarchaean Era is characterized by large preserved record of continental crust formation. Yet the actual mechanism(s) of Neoarchaean crustal growth remains controversial. In the northwestern part of the eastern Dharwar craton (EDC) granitoid magmatism started at 2.68 Ga with gneissic granodiorites showing intermediate character between sanukitoid and tonalite–trondhjemite–granodiorite (TTG). This was followed by intrusion of transitional (large-ion lithophile element-enriched) TTGs at 2.58 Ga. Finally 2.53–2.52 Ga sanukitoid and Closepet-type magmatism and intrusion of K-rich leucogranites mark the cratonization in the area. These granitoids mostly display initial negative ϵNd and Mesoarchaean depleted mantle model ages, suggesting presence of older crust in the area. Available data show that most of the Neoarchaean sodic granitoids in the EDC are transitional TTGs demonstrating the importance of reworking of older crust. It is suggested that the various c. 2.7 Ga greenstone mafic–ultramafic volcanic rocks of EDC formed in oceanic arcs and plateaus which accreted to form continental margin environment. Subsequent 2.7–2.51 Ga granitoid magmatism involved juvenile addition of crust as well as reworking of felsic crust forming transitional TTGs, sanukitoids and K-rich leucogranites. Microcratons were possibly the source of older crustal signatures and their accretion appears to be one of the important processes of Neoarchaean crustal growth globally. Supplementary material: Analytical techniques are available at https://doi.org/10.6084/m9.figshare.c.3470724