北天山东段阿奇山组火山岩的地球化学特征及锆石U-Pb年龄

Geochemistry and zircons SHRIMP U-Pb age of volcanic rocks of Aqishan Formation in the eastern area of north Tianshan, China.

阿奇山组分布于北天山东段觉罗塔格构造带内部的雅满苏岛弧带中,属于原雅满苏组下部的火山岩部分,而上部火山岩则属于土古土布拉克组。在阿奇山地区,阿奇山组由中-酸性火山岩、火山碎屑岩和火山碎屑沉积岩组成。岩石地球化学特征显示为钙碱性岛弧火山岩系列,火山岩主体具有高Sr、Na_2O、Al_2O_3、SiO_2,低MgO、Y、Yb,及明显亏损HFSF、Nb、Ta等特征,与经典埃达克岩地球化学特征吻合,Nb、Sr、Sr/Y值显示为俯冲型埃达克岩。地球化学相关图解等表明岩浆同化混染作用弱、经历了辉石、斜长石、钛铁氧化物和磷灰石的分离结晶作用、且部分熔融发生在石榴石稳定域。与东部土屋-雅满苏地区阿奇山组火山岩的对比研究表明,东部地区的火山岩不具埃达克岩特征,说明埃达克岩分布的局限性。锆石U-Pb SHRIMP谐和年龄为341.7±2.7Ma,与首次在该组中采集的化石资料完全一致,代表了火山岩的形成年龄。与其北部大南湖岛弧带中的小热泉子组、企鹅山组形成年龄(325.1±3.2Ma、322.6±2.0Ma)对比研究暗示,准噶尔板块南缘石炭纪存在构造-岩浆活动随时间由南向北迁移。

Aqishan Formation is mainly distributed over the Yamansu island are bell. 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 pail of Yamansu island are belt. The petrochemical compositions of most volcanic rocks belong to the island are with calc-alkaline series. These volcanic rocks are characterized by high Na(2)O, Al(2)O(3), SiO(2) and Sr contents, low MgO, Y and Yb contents and high Sr/Y and La/Yb rations, arid 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 arid 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 are 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.