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

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 rocics. 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 1.73-1.70 Ga, and that they were derived from two distinct sources. A-type magmas with subchondritic sHf values were predominantly formed by a relatively high degree of partial melting of 1.85 Ga old talc-alkaline granitoids, and those with slightly superchondritic epsilon Hf 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 (3.1-2.3 Ga), and late Palaeoproterozoic (1.85 and 1.77 Ga) crust at 1.73-1.70 Ga. 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 985-975 Ma, and by a widespread metasomatic event at 900-850 Ma. 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 1.73-1.70 Ga. Consequently, the asthenosphere flowed into the slab window and the sub-arc mantle wedge, leading to lithospheric extension and partial melting of predominately 1.85 Ga granitoid crust along with injection of mantle-derived mafic melts. (C) 2017 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.