Syn-collisional granitoids in the Qilian Block on the Northern Tibetan Plateau: A long-lasting magmatism since continental collision through slab steepening

In this paper we present a new model that can explain the large zircon age spectrum of similar to 510-420 Ma within a single sample from the Gangcha (Gcha) biotite granodiorite and the Huangyuan (HY) two-mica monzogranite on the northern Tibetan Plateau. The large age spread recorded in zircons is characteristic of granitoid samples from the studied region, which is best explained by the long-lasting magmatism since the onset of continental collision at similar to 500 Ma, followed by slab steepening and the ultimate slab break-off at similar to 450 Ma. These granitoids have a large major and trace element compositional variation, but limited initial Sr (I-sr[450] = 0.709 to 0.715), Nd (epsilon(Nd[450]) = -6.5 to -3.7), Hf (epsilon(Hf[450]) = -4.3 to -1.5) and Pb (Pb-206/Pb-204([450]) = 17.70 to 17.17; Pb-207/Pb-204([450]) = 15.60 to 15.69; Pb-208/Pb-204([450]) = 38.04 to 38.73) isotopic variation. The small negative whole rock epsilon(Nd[450]) and epsilon(Hf[450]) values are most consistent with the granitoid source being dominated by subducted seafloor materials. The inherited zircons with large negative epsilon(Hf[450]) values (e.g. -50) are indicative of input from the lower continental crust and subducted sediments. The correlated variations among major elements, trace elements and radiogenic isotopes are best interpreted as reflecting melting-induced mixing of a compositionally heterogeneous source with superimposed effect of varying extent of fractional crystallization and crustal assimilation. The inherited zircons of Palaeo-Proterozoic age and the Archean crustal model ages signify the involvement of ancient basement rocks. (C) 2016 Elsevier B.V. All rights reserved.