Petrological and geochemical constraints on the origin of garnet peridotite in the North Qaidam ultrahigh-pressure metamorphic belt, northwestern China

Abstract

In the North Qaidam ultrahigh-pressure (UHP) metamorphic belt, NW China, a large garnet peridotite massif has been exhumed together with felsic gneisses and eclogite during Early Paleozoic continental collision between North China Craton and Qaidam–Qilian Block. Four rock types are recognized in this massif: dunite without garnet, dunite with garnet, garnet lherzolite and garnet pyroxenite. The bulk compositions of these rocks retain depleted major element characteristics, but possess elevated abundances of incompatible trace elements, especially the fluid-mobile large ion lithophile elements (LILE, e.g., Rb, Cs, Ba, Sr, U, and Pb). The unradiogenic Nd ( ε Nd[460 Ma] − 0.5 to − 6.8) isotopes of these rocks are consistent with their flat to light rare earth element (LREE)-enriched patterns. Petrological and geochemical data altogether suggest that the protoliths of the garnet peridotite massif represent rocks from a subarc system in the Early Paleozoic. These rocks include subarc lithospheric harzburgite and a subarc cumulate assemblage from the ascending mantle wedge melts. These rocks were then further metasomatized by the ascending melts before being transported to depths of 150–200 km or more by the subducting slab-induced mantle wedge corner flow. Their subsequent exhumation to the upper crust is likely to have been accomplished during continental collision with a recovery rate of about 7.5−10 mm/year.