Two distinct sources of 1.73–1.70 Ga A-type granites from the northern Aravalli orogen, NW India: Constraints from in situ zircon U-Pb ages and Lu-Hf isotopes

Abstract

The late Palaeoproterozoic A-type granites of the northern Aravalli orogen, NW India are moderately to completely metasomatised (albitisation), rendering conventional whole-rock isotope systems, such as Rb-Sr and Sm-Nd meaningless to understand petrogenesis of these rocks. In situ U-Pb-Hf isotope composition of zircon combined with previously published geochemical data from thirteen plutons reveal that the A-type granites intruded in an extensional setting at 1.73–1.70Ga, and that they were derived from two distinct sources. A-type magmas with subchondritic εHf values were predominantly formed by a relatively high degree of partial melting of 1.85Ga old calc-alkaline granitoids, and those with slightly superchondritic εHf by mixing of felsic crustal melts with abundant mafic melts derived from a depleted mantle source. Zircon xenocrysts and Hf model ages additionally point to substantial reworking of late Archaean-early Palaeoproterozoic (3.1–2.3Ga), and late Palaeoproterozoic (1.85 and 1.77Ga) crust at 1.73–1.70Ga. Ages of metamorphic zircon domains furthermore support previous results, indicating that the northern Aravalli orogen has been affected by overprints of metamorphic events at 1665–1600, 1400–1300 and 985–975Ma, and by a widespread metasomatic event at 900–850Ma.The new data suggest that A-type magmatism in the northern Aravalli orogen is the manifestation of lithospheric extension in a post-collisional setting triggered by slab break-off at 1.73–1.70Ga. Consequently, the asthenosphere flowed into the slab window and the sub-arc mantle wedge, leading to lithospheric extension and partial melting of predominately 1.85Ga granitoid crust along with injection of mantle-derived mafic melts.