Syn-exhumation magmatism during continental collision: Evidence from alkaline intrusives of Triassic age in the Sulu orogen

Abstract

A combined study of zircon U–Pb ages, Lu–Hf isotopes, mineral O isotopes, whole-rock elements and Sr–Nd isotopes was carried out for alkaline intrusive rocks (including gabbro, syenite and granite) in the Sulu orogen. The results provide insights into crust–mantle interaction in the continental subduction zone. SIMS and LA-ICPMS zircon U–Pb dating yield Late Triassic ages of 201±2 to 212±1Ma for their crystallization. These ages are younger than Middle Triassic ages for ultrahigh-pressure (UHP) metamorphism of country rocks, corresponding to syn-exhumation magmatism during continental collision. The alkaline rocks are characterized by the arc-like patterns of trace element distribution, with relative enrichment of LILE and LREE but relative depletion of HFSE. They have high initial 87Sr/86Sr ratios of 0.7064 to 0.7114 and highly negative εNd(t) values of −16.4 to −13.8 with two-stage Nd model ages of 2.11 to 2.33Ga for whole-rock. Zircon Lu–Hf isotope analyses also show highly negative εHf(t) values of −20.9±0.5 to −14.1±0.9, with two-stage Hf model ages of 2.10±0.06 to 2.56±0.03Ga. The zircon exhibits relatively consistent δ18O values of 5.6 to 6.2‰, slightly higher than normal mantle values. The enrichment of radiogenic Sr–Nd–Hf isotopes in the gabbro indicates its origination from an isotopically enriched mantle source, whereas the arc-like pattern of trace element distribution for it suggests a fertile mantle source with enrichment of LILE and LREE. Thus, it is part of the orogenic lithospheric mantle that would be generated by underplate reaction of the subcontinental lithospheric mantle with hydrous felsic melts derived from the subducted continental crust during the Triassic continental collision. On the other hand, there are general similarities in trace element and radiogenic isotope characteristics between the syenite–granite and UHP metaigneous rocks in the Dabie–Sulu orogenic belt, suggesting its genetic link to the subducted continental crust itself. Therefore, the alkaline rocks are derived from partial melting of the orogenic lithospheric mantle and the subducted continental crust in the stage of exhumation. They provide a petrological record of recycling the subducted continental crust into mantle depths with consequent crust−mantle interaction during the continental collision.