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岩石学报 2009
Geochemistry and zircons SHRIMP U-Pb age of volcanic rocks of Aqishan Formation in the eastern area of north Tianshan, China
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Abstract:
Aqishan Formation is mainly distributed over the Yamansu island arc belt within Jueluotage tectonic belt in the eastern part of the North Tianshan, China, belonged to the volcanic unit of lower-part of original Yamansu Formation, while Tugutubulake Formation is the volcanic unit of upper-part of it. Aqishan Formation is composed of the middle-acidic volcanic rock, pyroclastic rocks and tuffaceous sedimentary rocks in Aqishan area of the western part of Yamansu island arc belt. The petrochemical compositions of most volcanic rocks belong to the island arc with calc-alkaline series. These volcanic rocks are characterized by high Na2O, Al2O3, SiO2 and Sr contents, low MgO, Y and Yb contents and high Sr/Y and La/Yb rations, and clearly depleted of high field strength elements (Nb, Ta), which similar to the characteristics of the classical adakite. These Nb, Sr contents and Sr/Y ration are also similar or close to those of most subduction-type adakites. Geochemical characteristics indicated that the magma undergone a weak assimilation-contamination, the fractional crystallization of clinopyroxene, plagioclase, Fe-Ti-oxides and apatite, and partial melting took place in garnet stabilizing-area. Contrasted it with Aqishan Formation in Tuwu-yamansu areas, the latter don`t possess the characteristics of adakite, implies that the distribution of adakites has its limitation. The zircon SHRIMP U-Pb dating is 341.7±2.7Ma, according with the fossils data, which stands for volcanics age. A comparison between ages (325.1±3.2Ma, 322.6±2.0Ma) of Xiaorequanzi, Qiershan Formation in Dalanhu island arc belt and Aqishan Formation in Yamansu island arc belt, indicates that the tectonic-magma-activities migrated from south to north along the south edge of Junggar plate in Carboniferous Period.
