Diverse crustal components in pyroxenite xenoliths from Junan, Sulu orogenic belt: Implications for lithospheric modification invoked by continental subduction

摘要

Detailed mineralogical and geochemical studies have been carried out on a suite of pyroxenite xenoliths captured in a late Mesozoic basaltic dike from Junan, eastern Shandong, which is tectonically situated in the Sulu ultrahigh pressure orogenic belt. Two types of pyroxenites, namely websterite and garnet pyroxenite were identified according to their mineral assemblages. The equilibrium temperatures (828-935°C) of websterite and garnet pyroxenite xenoliths, falling within the temperature range of lower crustal xenoliths, suggest that they were derived from the lower crust rather than the lithospheric mantle. The websterite xenoliths are characterized by higher MgO and lower Al_2O_3; their convex upward REE patterns, along with lower concentrations of highly incompatible elements indicate that they were high pressure cumulates. Their extremely unradiogenic Nd and radiogenic Sr isotopic compositions imply the contribution of crustal materials to their sources. It is suggested that the precursor melts of websterite xenoliths were derived from a mantle source which had been intensely modified by the subducted lower continental crust of the Yangtze craton following the collision with the North China craton in the Triassic. The compositional features of garnet pyroxenites also suggest their cumulative origin, however, the sharp contrast in trace element concentrations and Sr-Nd isotopic compositions with those of websterite xenoliths indicate they have different precursor melts. Their Sr and Nd isotopic compositions are consistent with the suggestion that their precursor melts were derived from an enriched lithospheric mantle which had been metasomatized by melts released from the Proto-Tethyan oceanic crust which was subducted into the mantle prior to the subduction of continental crust of Yangtze craton. The occurrence of websterite and garnet pyroxenite xenoliths provide evidence that the lithospheric mantle of the North China craton had been significantly modified by the recycled oceanic and continental crust resulting from the collision between the Yangtze craton and the North China craton.