Subduction-related metasomatism in the thinning lithosphere: Evidence from a composite dunite-orthopyroxenite xenolith entrained in Mesozoic Laiwu high-Mg diorite, North China Craton

The North China Craton (NCC) lost its Archean keels in the Phanerozoic. Prevalent and intensive magmatism, mineralization, and development of extensional basins in the late Mesozoic NCC imply that the late Mesozoic could be the key stage for this transformation. Ultramafic xenoliths in the Early Cretaceous high-Mg diorites of Shandong province might provide key information about the transformation of subcontinental lithospheric mantle (SCLM) beneath the NCC. Here we present a unique composite dunite-orthopyroxenite xenolith from Tietonggou, one of the high-Mg diorite-dominated plutons in Laiwu, Shandong province. The petrography and mineral chemistry of the xenolith suggest complicated metasomatic processes, which occurred before its entrainment in the host magma. Early stage metasomatism includes the growth of intergranular phlogopite and clinopyroxene and the development of a phlogopite- and amphibole-bearing clinopyroxenite veinlet. Late-stage metasomatism (termed Si (Na) metasomatism) is characterized by the growth of secondary orthopyroxene, Na-rich plagioclase and amphibole with resorption of olivine and clinopyroxene, and the decomposition of phlogopite. The xenolith has exceptionally high concentrations of Na2O and Al2O3 and shows enrichments in Cs, Rb, Th, U, K, and the light rare earth elements. It also shows positive Pb and Sr anomalies and negative Nb, Ta, P, and Ti anomalies in a primitive mantle normalized spider gram. The geochemistry, as well as the elevated delta O-18, suggests that this Si (Na) metasomatism is associated with subduction. The secondary orthopyroxene in the orthopyroxenite portion of the xenolith has exceptionally low Mg# values, which may be the result of reaction between silica-rich melts and olivine with high melt: rock ratios. Mg-Fe disequilibrium of the minerals in the orthopyroxenite indicates that Si (Na) metasomatism may have been introduced shortly before entrainment of the xenolith in the host magma. Correlation of the Sr-Nd-Pb isotopic compositions of ultramafic xenoliths, high-Mg diorites, and adakitic granites of the Tietonggou indicates there may be a genetic relationship between these rocks. Thus we propose that the SCLM beneath Shandong may have been metasomatized by a slab-derived melt. Since Si (Na) metasomatism occurred at the key stage of lithospheric thinning, oceanic subduction might have been involved in the thinning process.