Compositional polarity of Triassic granitoids in the Qinling Orogen, China: Implication for termination of the northernmost paleo-Tethys

The Qinling Orogen was formed from the closure of the northernmost paleo-Tethys sea and the tectonic suturing of the Yangtze and North China Cratons. The timing of this collision and the tectonic framework are debated. The widely developed Triassic granitoids in the western Qinling Orogen offer a key to understand the tectonic evolution of this region. Here we compile the geological, geochemical and geochronological data of the Triassic granitoids from the Qingling Orogen and conclude that the granitoids north of the Mian-Lue Suture were emplaced in an active continental margin related to the northward subduction of the Mian-Lue oceanic plate during 248 − 200 Ma. The granitoids can be classified into I- and S-types, with the former constituting the major variety. Northward from the Mian-Lue Fault, an S-type granite belt appears, followed by and locally overlapping with an I-type granite belt. The I-type granite belt can be subdivided into four sub-belts from south to north, with increasing contents of K2O, K2O + Na2O, SiO2, Th and U, and the ratios of K2O/Na2O, Rb/Sr and (87Sr/86Sr)i, but decreasing contents of Na2O, Al2O3, Mg#, and εHf(t), except for the northernmost belt which occurs in the Huaxiong Block of the North China Craton. This geochemical polarity of I-type granitoids and the zoned distribution of S- and I-types granitoids cannot be explained through continental collision orogeny (including syn- to post-collision), but can be well interpreted by considering the progressive subduction of the northernmost branch of the paleo-Tethys, as represented by the Mian-Lue Ocean. Thus, the termination of the northernmost paleo-Tethys and the onset of the continental collision between the Yangtze and the North China plates are considered to have occurred at about 200 Ma, during the transition from Triassic to Jurassic. Our model is also supported by the available data from other studies and provides a revised framework for the timing and tectonics of assembly of the Yangtze and the North China Cratons.