NEW OPPOSITE SUBDUCTION MODEL FOR LATE PALEOZOIC NORTH TIANSHAN, CAOB: IMPLICATIONS FROM GEOCHRONOLOGY, GEOCHEMISTRY EVIDENCE ON VOLCANIC-SEDIMENTARY SEQUENCES ON SHICHANG AREA, NORTH XINJIANG, CHINA

Abstract: Late Paleozoic volcanic rock is widely distributed in the northwestern Tianshan with highly controversial tectonics background, which leads to different opinions on the tectonic evolution process. This paper carried out systematic studies from field geology, geochemistry and geochronology on the volcanic rock and associated highly deformed turbidite of Shichang area to constrain the late Paleozoic tectonic evolution of the North Tianshan. The volcanic rock is calc-alkaline serie, charactered with enriched Light Rare Earth Elements (LREEs), Large Iron Lithophile Elements (LILEs, e.g. Rb, Ba, K), flat Heavy Rare Earth Elements (HREEs), and depleted High Field Strength Elements (HFSEs, e.g. Nb, Ta, Ti). These geochemical characteristics suggest an island arc origin of the volcanic rock, with U-Pb age of 311 ± 2 Ma. The turbidite yields wide distribution spectrum from 267 to 369 Ma, with no Devonian or Precambiran peaks, which are essential in the Central Tianshan unit. With previous geophysical data on the Junggar basin, it is suggested that the volcanic rock on Shichang area is generated from the Mosuowan intro-oceanic arc with no Devonain or Precambrian detrital materials, related to a northward subduction setting of the North Tianshan Ocean (NTO) on late Carboniferous to middle Permian, while a southward subduction beneath the Central Tianshan generates the North Tianshan Arc simultaneously. These two arcs amalgamated together after late Permian to Mesozoic and then formed the present North Tianshan tectonic framework.