Palaeozoic multiphase magmatism at Barleik Mountain, southern West Junggar, Northwest China: Implications for tectonic evolution of the West Junggar

The West Junggar lies in the southwest part of the Central Asian Orogenic Belt (CAOB) and consists of Palaeozoic ophiolitic melanges, island arcs, and accretionary complexes. The Barleik ophiolitic melange comprises several serpentinite-matrix strips along a NE-striking fault at Barleik Mountain in the southern West Junggar. Several small late Cambrian (509503 Ma) diorite-trondhjemite plutons cross-cut the ophiolitic melange. These igneous bodies are deformed and display island arc calc-alkaline affinities. Both the melange and island arc plutons are uncomfortably covered by Devonian shallow-marine and terrestrial volcano-sedimentary rocks and Carboniferous volcano-sedimentary rocks. Detrital zircons (n=104) from the Devonian sandstone yield a single age population of 452517million years, with a peak age of 474 million years. The DevonianCarboniferous strata are invaded by an early Carboniferous (327 Ma) granodiorite, late Carboniferous (315311 Ma) granodiorites, and an early Permian (277 Ma) K-feldspar granite. The early Carboniferous pluton is coeval with subduction-related volcano-sedimentary strata in the central West Junggar, whereas the late Carboniferousearly Permian intrusives are contemporary with widespread post-collisional magmatism in the West Junggar and adjacent regions. They are typically undeformed or only slightly deformed. Our data reveal that island arc calc-alkaline magmatism occurred at least from middle Cambrian to Late Ordovician time as constrained by igneous and detrital zircon ages. After accretion to another tectonic unit to the south, the ophiolitic melange and island arc were exposed, eroded, and uncomfortably overlain by the Devonian shallow-marine and terrestrial volcano-sedimentary strata. The early Carboniferous arc-related magmatism might reflect subduction of the Junggar Ocean in the central Junggar. Before the late Carboniferous, the oceanic basins apparently closed in this area. These different tectonic units were stitched together by widespread post-collisional plutons in the West Junggar during the late CarboniferousPermian. Our data from the southern West Junggar and those from the central and northern West Junggar and surroundings consistently indicate that the southwest part of the CAOB was finally amalgamated before the Permian.