Geochemistry of adakites from Neoarchaean active continental margin of Shimoga schist belt, Western Dharwar Craton, India: Implications for the genesis of TTG

Abstract

Adakites are identified for the first time from a Neoarchaean active continental margin (ACM) preserved in the eastern part of the Shimoga schist belt, near Ranibennur, Western Dharwar Craton, India. These adakites consists mainly of quartz, plagioclase and minor amphiboles and are interlayered with the turbidites. They exhibit typical adakitic low Y (8.35–14.70 ppm) and Yb (0.85–1.47 ppm), may be classified as high SiO 2 and low Mg#, Ni and Cr type adakites. Other constituents such as Na 2O, K 2O, Nb, Zr, LREE and HREE and their ratios such as Na 2O/K 2O, Zr/Sm, Nb/La, (La/Yb) n , (Gd/Yb) n and Th/U also closely resemble with the high silica adakites (HSA) of Martin et al. [Martin, H., Smithies, R.H., Rapp, R., Moyen, J.-F., Champion, D., 2005. An overview of adakite, tonalite–trondhjemite–granodiorite (TTG), and sanukitoid: relationships and some implications for crustal evolution. Lithos 79, 1–24]. Sr/Y values are moderate, generally lower than the high Sr–low silica Cenozoic adakites, but within the range of Archaean adakites. The negative Nb, Ta, and Ti anomalies characteristic of Cenozoic adakites are found in Ranibennur adakites also. Zr/Zr* and Hf/Hf* ratios are also similar to HSA. These adakites further depict compositional similarities with Meso and Neoarchaean tonalite–trondhjemite–granodiorites (TTGs). Such types of adakites have been experimentally generated by the partial melting of hydrous basalts, that are similar to adakitic glass veins (low MgO, CaO, Cr and Ni) found in peridotite xenoliths of Kamchatka arc [Xiong, X.L., Xia, B., Xu, J.F., Niu, H.C., Xiao, W.S., 2006. Na depletion in modern adakites via melt/rock reaction within the sub-arc mantle. Chem. Geol. 229, 273–292]. Since Archaean had higher geothermal regime, subduction of the basaltic slab was most probably shallow and significant mantle wedge was not available to react with adakitic melt. Hence Archaean adakites in general, and particularly from Ranibennur are relatively depleted in the peridotiitic mantle wedge components. Their identification from an Archaean ACM and their compositional similarities with the Paleo-Meso Archaean TTGs suggest the possibility of identical origin, i.e. the melting of a shallow hydrous basaltic slab. This mechanism during Archaean was widespread and rapid, resulted in faster continental crustal growth during the early history of the Earth. Identification of Archaean adakites in association with island arc turbidites from a greenstone belt sequence supports the rapid subduction mechanism for the crustal growth that coincides with 2.7 Ga peak (episode) of almost all geological processes.