Late Triassic high Mg diorites of the Wulong pluton in the South Qinling Belt, China: Petrogenesis and implications for crust-mantle interaction

High Mg diorites provide important insights into crust-mantle interaction during tectonic evolution of orogenic belt. An integrated study including zircon U-Pb dating, whole-rock geochemistry and isotopic compositions of Sr and Nd was carried out on high Mg diorites of the Wulong pluton located in the narrowest part of the South Qinling Belt. Crystallized ages of 218-210 Ma are obtained in combination with the published data. These rocks are characterized with relatively high MgO (2.76–7.17%), Cr (68.7-310 ppm) and Ni (21.3–76.3 ppm) contents. Most diorites exhibit high Sr/Y and La/Yb ratios with negligible Eu anomaly. Isotopic compositions of whole-rock show enriched attributes (87Sr/86Sr(i) = 0.7050–0.7071, εNd(t) = −7.56 to −3.45). Geochemical models indicate that these rocks did not result from melting of subducted slab, magma mixing, fractional crystallization or melts derived from delaminated lower crust reacted with mantle. Instead, high Mg diorites were generated through basaltic melts derived from asthenosphere mantle assimilating crustal components with crystallized phase of garnet-bearing amphibolite (AFC). Together with regional geological setting of the South Qinling Belt, we propose that the parental magma of these high Mg diorites was probably originated from upwelled asthenosphere mantle at the base of thinned crust after delamination of thickened crust during Late Triassic. The continental crust of the South Qinling Belt was thickened as a result of collision with the Yangtze Block during Middle Triassic. Then eclogitized lower crust collapsed and foundered into asthenosphere mantle due to partial melting of thickened crust during Late Triassic. In response, the upwelling and underplating of asthenosphere mantle took place at the base of continental crust. Moreover, upwelling of hot mantle may provide the heat source for migmatites and initiate the doming of Foping area.