Petrogenesis of tholeiite associations in Kudi ophiolite (western Kunlun Mountains, northwestern China): implications for the evolution of back-arc basins

The Kudi ophiolite in the western Kunlun Mountains comprises harzburgites, dunites, cumulate dunites, cumulate pyroxenites and gabbros, diabase dikes, and pillow and massive lavas, and are fragments of a supra-subduction zone (SSZ) ophiolite from the Early Paleozoic. The extrusive rocks can be classified into three groups of tholeiites: back-arc basin (BAB) tholeiites, low-Ti island arc tholeiites (IAT), and LREE-enriched IAT, as shown by their distinctive geochemical characteristics. The SSZ-type mantle peridotites, the cumulate complex with arc tholeiite affinity, and BABB-type diabase dikes and basalts constitute an upper mantle and crustal section of a back-arc basin formed by coupling of MORB-type mantle upwelling with fluid efflux from slab devolatilization. The low-Ti IAT are characterized by low Ti and HFSE, and slightly U-shaped or LREE-depleted chondrite-normalized REE patterns, and represent melts derived from a depleted mantle source region (extraction of BABB magma) modified compositionally by fluids and/or melts from the subducting lithospheric slab during propagation and extension of the back-arc basin. We interpret the LREE-enriched IAT as products of closure of the back-arc basin because an interaction between the parental magma of this IAT and the mantle peridotites (formerly the upper mantle of the basin) in a newly formed mantle wedge had occurred.