Syn-exhumation magmatism during continental collision: Geochemical evidence from the early Paleozoic Fushui mafic rocks in the Qinling orogen, Central China

Mafic arc-like magmatism is rare during continental collision, but its products provide valuable information on the geochemical processes during plate convergence. Such rocks were found in the Fushui complex in the Qinling orogen, central China. In order to decipher their petrogenesis, we have carried an integrated study of geochronology and geochemistry. Zircon SIMS UPb dating yields consistent ages of 488 ± 4 to 490 ± 4 Ma for emplacement of mafic magmas. The ages are just slightly later than zircon UPb ages of 500–490 Ma for ultrahigh-pressure metamorphism of the continental crust in the North Qinling unit. Thus, the target rocks are the products of syn-exhumation magmatism during continental collision. These mafic igneous rocks exhibit arc-like trace-element patterns, such as enrichment in large ion lithophile elements (LILE), Pb, and light rare earth elements (LREE), and depletion in high field strength elements (HFSE). They show enriched whole-rock SrNd isotope compositions and variable zircon HfO isotope compositions, indicating involvement of the recycled oceanic basalt and terrigenous sediment in their mantle source. Such geochemical features suggest that the Fushui mafic magmas would originate from an enriched and fertile mantle source that was generated through the metasomatic reaction between the mantle wedge peridotite and the subducting Shangdan oceanic crust-derived liquid phase. With the tectonic transition from the lithospheric subduction for active compression to active extension for the crustal exhumation during the late Cambrian, the mantle source underwent decompression melting to produce mafic melts in synchronous with exhumation of the deeply subducted continental crust. Therefore, the syn-exhumation Fushui complex provides new geochemical insights into the nature of the orogenic lithospheric mantle and thus the tectonic evolution of this collisional orogen.