第七届青年地学论坛

The North Tianshan Ocean (NTO) is one of the latest closed oceanic basins at the southwestern margin of the Paleo-Asian Ocean remnants. When, where, and how the NTO was finally consumed and closed have not been well addressed. Compared with the other arcs in the North Tianshan arc system, the Yamansu volcanic arc contains well-preserved late Carboniferous volcanic-sedimentary stratigraphic sequence. To better figure out the final magmatism before closure of the NTO, an integrated study of the volcanism in the Yamansu volcanic arc has been carried out.
Precise zircon U-Pb ages indicate that the arc volcanism ceased at the latest Carboniferous (325-305 Ma). The late Carboniferous volcanics in the Yamansu volcanic arc comprise of bimodal calc-alkaline and tholeiite arc lavas, which are covered by limited Permian to Jurassic terrestrial sedimentary. The late Carboniferous mafic volcanics could be further divided into two different types (low-LREE and high-LREE suites). The low-LREE suite displays fore-arc-like and tholeiite characteristics with N-MORB-like flat to depleted LREE patterns, implying that minor crustal contamination. They are likely from a moderately depleted mantle wedge that was metasomatized by subducted fluid-like component. The high-LREE suite is calc-alkaline and have high LREE/HREE ratios with (La/Yb)_N > 2 instead. Although more or less suffered crustal contamination, they should be from a depleted mantle wedge by the metasomatism of subducted sediment-derived melts. As the silicic member of the late Carboniferous bimodal volcanism, the felsic volcanics with I-type magmatic series, are probably derived from anatexis of different and Cambrian to Mesoproterozoic arc crustal rocks with the calculated one-stage Nd model ages (T_DM = 550-1150 Ma) in the region. In combination with previous studies, we suggest that the Kanggur Fault presumably represents the southernmost suture of NTO and the Yamanu volcanic arc and Kanggur accretionary wedge should be a fore-arc system of the Central Tianshan continental arc, probably related to southwards subduction of NTO. The final closure of NTO and termination of subduction is plausibly prolonged to around the boundary of Carboniferous-Permian (ca. 300 Ma).

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North Tianshan Ocean,,Late Carboniferous volcanism,the Central Asian Orogenic Belt