Petrogenesis of ca. 240 Ma intermediate and felsic intrusions in the Nan’getan: Implications for crust–mantle interaction and geodynamic process of the East Kunlun Orogen

Abstract

The East Kunlun Orogen in the Northern Qinghai–Tibet Plateau is an ideal region in which to investigate net crustal growth through a process of syn-collisional felsic magmatism. In this paper, we report zircon U–Pb geochronology, whole-rock geochemistry, and multiple isotope data from the Nan’getan quartz diorite porphyrites and syenogranites in the East Kunlun Orogen. Zircon U–Pb ages indicate that the quartz diorite porphyrites and syenogranites were synchronously emplaced at∼240Ma. The syenogranites are characterized by a high SiO2 (77.4–79.2wt%) and differentiation index (DI=95), and marked depletion in Sr, Ba and Eu, thereby indicating that they are highly fractionated I-type granitoids. These rocks yielded zircon εHf(t) of −4.8 to +3.0 and Hf crustal model ages (TDMc-Hf) of 1565–1075Ma. The quartz diorite porphyrites are characterized by a high MgO of 4.19–6.37wt% and Mg# of 59–61. These porphyrites have varying whole-rock εNd(t) (−13.2 to −10.8), (206Pb/204Pb) 240Ma of 18.325–18.511, (207Pb/204Pb)240Ma of 15.608–15.630, (208Pb/204Pb)240Ma of 38.167–38.614, and δ18OSMOW of 12.5‰–13.7‰. These signatures indicate that the Nan’getan syenogranites were most likely generated by an extensive differentiation of mixed parental magmas with both mantle-derived melt and crustal melt triggered by intrusion or underplating by mantle-derived basaltic magma. The Nan’getan Mg-rich diorites were likely derived from magma mixing between melts of ancient lower crustal materials and melts of an enriched mantle source that was metasomatized by fluids during ascent. In combination with the presence of the regional angular unconformity and the contemporaneous collisional metamorphism, the∼240Ma magmatism in the East Kunlun Orogen is inferred to have occurred in a compressional setting from subduction to syn-collision. The Nan’getan granitoids may have recorded the onset of collision between the Bayan Har–Songpan Ganze and East Kunlun terrane and net continental crust growth at∼240Ma.