Early Paleozoic high-Mg diorite-granodiorite in the eastern Kunlun Orogen, western China: Response to continental collision and slab break-off

Abstract

Abstract Phanerozoic high-Mg diorites and granodiorites are known to be related to diverse subduction styles at convergent margin settings but have not been linked to collision processes. The Santonggou complex in the eastern Kunlun Orogen is composed of a dominant granodiorite intruded by small stocks and dykes of porphyritic granodiorites and diorites, which are all characterized by high MgO and Ni but low Yb and Y contents. Mafic microgranular enclaves (MMEs) occur in the granodiorite and the porphyritic granodiorites. LA-ICPMS U-Pb analyses of zircon from the granodiorite, porphyritic granodiorite, and porphyritic diorite yield similar ages of 432 ± 2 Ma, 432 ± 2 Ma, and 427 ± 3 Ma, respectively. Plagioclase in the granodiorite exhibits complex zonations with resorbed and patchy cores. Plagioclase and hornblende phenocrysts in the porphyritic granodiorites and diorites also show textural and compositional zonations. The highest e Nd ( t ) value of 0.6 in MMEs suggests asthenosphere-derived mafic magma. These lines of mineralogical and geochemical evidence together with results from geochemical modeling suggest that the porphyritic diorites were derived from mantle metasomatized slab melts. The porphyritic granodiorites and the granodiorite were generated by simultaneous assimilation and fractional crystallization of the magma approximated by the porphyritic diorites, plus mixing of asthenosphere-derived mafic and evolved granodioritic magmas. Pressure estimates from the Al-in-hornblende geobarometer suggest that the granodiorite had been uplifted to near the surface before the emplacement of the porphyritic granodiorites. Partial melting of the slab, upwelling of the asthenosphere, and rapid orogenic uplift at ~ 430 Ma suggest that the cessation of subduction and the initiation of collision as well as slab break-off occurred almost simultaneously in the eastern Kunlun Orogen.