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

Abstract

In this paper we present a new model that can explain the large zircon age spectrum of ~510 – 420Ma 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 ~500Ma, followed by slab steepening and the ultimate slab break-off at ~450Ma. These granitoids have a large major and trace element compositional variation, but limited initial Sr (ISr[450]=0.709 to 0.715), Nd (ԐNd[450]=−6.5 to −3.7), Hf (ԐHf[450]=−4.3 to 1.5) and Pb (206Pb/204Pb[450]=17.70 to 17.17; 207Pb/204Pb[450]=15.60 to 15.69; 208Pb/204Pb[450]=38.04 to 38.73) isotopic variation. The small negative whole rock ԐNd[450] and ԐHf[450] values are most consistent with the granitoid source being dominated by subducted seafloor materials. The inherited zircons with large negative ԐHf[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.