Syn-exhumation melting of the subducted continental crust: Geochemical evidence from early Paleozoic granitoids in North Qaidam, northern Tibet

Syn-exhumation partial melting of the deeply subducted continental crust commonly occurs in collisional orogenic settings. The resulting felsic melts not only form granitoids, but can also modify the composition of the mantle wedge above continental subduction zones. This study investigates crustal anatexis and its role in granitoid formation and crust–mantle interaction during continental collision using new whole-rock geochemical, Sr–Nd–Hf isotope, and zircon U–Pb geochronological and Lu–Hf isotope data for early Paleozoic granitoids of the North Qaidam area of northern Tibet, a collisional orogen that contains ultrahigh-pressure (UHP) metamorphic rocks. Zircon U–Pb dating of the granitoids in this area yielded ages between 416 ± 6 Ma and 393 ± 2 Ma, coeval with the exhumation of the deeply subducted continental crust. This indicates that these granitoids formed as a result of syn-exhumation magmatism during the continental collision. Relict zircons in the granitoids yield concordant Neoproterozoic and early Paleozoic U–Pb ages that are consistent with the protolith and metamorphic ages of UHP metaigneous rocks in the orogen, respectively. The granitoids have calc-alkaline to high-K calc-alkaline compositions, with total alkali contents of 4.07–8.09 wt%. They are also metaluminous to weakly peraluminous with A/CNK ratios of 0.86–1.14. They are enriched in large ion lithophile elements (LILE) and depleted in high field strength elements (HFSE), yielding arc-like trace element patterns. They also have high (87Sr/86Sr)i ratios (0.7058–0.7195) and negative εNd(t) values (−7.1 to −1.4) and have whole-rock and zircon εHf(t) values of −2.8 to +2.7 and − 8.6 to +4.7, respectively. These data indicate that the granitoids are compositionally similar to the UHP metaigneous rocks in the collisional orogen, suggesting a genetic link. This in turn indicates that the granitoids in the study area formed from magmas generated by the partial melting during the exhumation of deeply subducted continental crust. The resulting granitic melts not only generated the syn-exhumation granitoids in this area, but also caused metasomatism associated with crust–mantle interaction in continental subduction channels.