Melting of subducted continental crust during collision and exhumation: Insights from granitic rocks from the North Qaidam UHP metamorphic belt, NW China

Abstract

The formation and reworking of continental crust have close connections with orogeny. Syn- and post-collisional magmatism is key to understanding crustal growth and reworking during continental collision and post-collisional extension. Here we report a series of syn-collisional felsic magmatic rocks intruded into migmatized gneiss and amphibolite/eclogite in the North Qaidam ultrahigh-pressure (UHP) metamorphic belt, to provide insights into melting/reworking of UHP metamorphosed continental crust during continental collision and exhumation. The syn-collisional magmas are mainly tonalite-trondhjemite and mica granite that occur as plutons and dykes, and high potassium granite veins in migmatites. They were formed by decompression melting of subducted oceanic and continental crust at different levels during exhumation, without any discernible mantle contribution. The tonalite-trondhjemites show adakitic signatures of both oceanic crust and mafic continental crust with varying Nd isotopic compositions. The mica granite plutons have high silica and alkali contents, are weakly to strongly peraluminous, and have crustal SrNd isotope compositions. These mica granites were formed by partial melting of previously subducted continental crust, and formed mostly in high-pressure conditions, in the garnet stability field. The protolith is predominantly granitic gneiss, with minor meta-sediment and metabasite. A compilation of published ages and our results show that these syn-collisional magmatic rocks formed between 437 and 397 Ma, overlapping with UHP metamorphic ages (440–420 Ma), and suggests that exhumation occurred synchronously with continental subduction. Individual zircon grains show metamorphic cores and magmatic rims with a ≤ 20 Myr time interval, interpreted as the time between peak UHP metamorphism and melting during exhumation. Such long-lived and episodic melting of UHP metamorphic continental crust should result from periodic processes, involving melt weakening and decompression by upward movement within the subduction channel, rather than coherent motion of the entire UHP belt. This process represents crustal reworking during collision and exhumation, without input of mantle melts.