The syncollisional granitoid magmatism and continental crust growth in the West Kunlun Orogen, China – Evidence from geochronology and geochemistry of the Arkarz pluton

Abstract

The West Kunlun orogenic belt (WKOB) at the northwest margin of the Greater Tibetan Plateau records seafloor subduction, ocean basin closing and continental collision with abundant syncollisional granitoids in response to the evolution of the Proto- and Paleo-Tethys Oceans from the early-Paleozoic to the Triassic. Here we present a combined study of detailed zircon U-Pb geochronology, whole-rock major and trace elements and Sr-Nd-Hf isotopic geochemistry on the syncollisional Arkarz (AKAZ) pluton with mafic magmatic enclaves (MMEs) exposed north of the Mazha-Kangxiwa suture (MKS) zone. The granitoid host rocks and MMEs of the AKAZ pluton give the same late Triassic age of ~225Ma. The granitoid host rocks are metaluminous granodiorite and monzogranite. They have initial 87Sr/86Sr of 0.70818 to 0.70930, εNd(225Ma)= −4.61 to −3.91 and εHf(225Ma)= −3.01 to 0.74. The MMEs are more mafic than the host with varying SiO2 (51.00–63.24wt.%) and relatively low K2O (1.24–3.02wt.%), but have similar Sr-Nd-Hf isotope compositions to the host ((87Sr/86Sr)i=0.70830–0.70955, εNd(225Ma)= −4.88 to −4.29, εHf(225Ma)= −2.57 to 0.25). Both the host and MMEs have rare earth element (REE) and trace element patterns resembling those of bulk continental crust (BCC). The MMEs most likely represent cumulate formed from common magmas parental to the granitoid host. The granitoid magmatism is best explained as resulting from melting of amphibolite of MORB protolith during continental collision, which produces andesitic melts with a remarkable compositional similarity to the BCC and the inherited mantle-like isotopic compositions. Simple isotopic mixing calculations suggest that ~80% ocean crust and ~20% continental materials contribute to the source of the AKAZ pluton. Thus, the hypothesis “continental collision zones as primary sites for net continental crust growth” is applicable in the WKOB as shown by studies in southern Tibet, East Kunlun and Qilian orogens. In addition, we also propose a new view for the tectonic evolution of the Paleo-Tethys Ocean in geological regions recorded and represented by the MKS.