Geochemistry and dating of E-MORB type mafic rocks from Dalabute ophiolite in West Junggar, Xinjiang and geological implications
新疆西准噶尔达拉布特蛇绿岩E-MORB型镁铁质岩的地球化学、年代学及其地质意义

The West Junggar region in northwestern China is an important part of Central Asian Orogenic Belt, which consists of successive subduction-accretion of ophiolites, island arc, oceanic ialands, seamounts, accretionary wedges in Paleozoic. A number of ophiolites or ophiolitic melanges were emplaced within this orogenic belt, Dalabute ophiolite, which is one of the largest ophiolite belts in West Junggar, represents the remnants of Paleoasian oceanic crust. Geochemically, the mafic rocks of Dalabute ophiolite display normal mid-ocean ridge basalt (N-MORB), enriched mid-ocean ridge basalt (E-MORB) and ocean island basalt (OIB) signatures. This paper reports that the LA-ICP-MS zircon U-Pb dating result for E-MORB type leucocratic gabbros from Akebasitao terrane of Dalabute ophiolite, yielding an age of 302±1.7Ma. Since the more complexity petrogenesis of E-MORB and OIB mafic rocks in ophiolites, combining previous studies, we propose two hypotheses for explanation of gabbro age: (1)E-MORB and OIB type mafic rocks were formed in late stage of back arc extension, the dating represents the formation age of the ophiolite, suggesting the Junggar ocean possibly closed after Late Carboniferous; (2) E-MORB and OIB type mafic rocks were forearc seamount in accretionary prism during the death or emplacement stage of ophiolite, which formed in response to collision of spreading center with Dulabute ophiolite subduction zone. They post-dated formation of main ophiolite sequence, but have significant tectonic implications for the origin and evolution of the Dalabute ophiolite. We favor the latter explanation and propose that there was an active margin during the Late Carboniferous-Early Permian in North Xinjiang. The important cause of tectonic-magmatic-metallogenic-orogenic climax is the subdution of spreading center, by which the large volume of continental crust increased through transfer granitic melt, which were from partial melting of accretionary wedge, subducted slab and recycled sediments triggered by underplating of mantle-derived basaltic magmas, to orogenic belt. This process may be caused by slab window effects following spreading center subduction.