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东昆仑东段广泛分布晚古生代的中酸性花岗岩,基性岩浆岩记录较少。本文选择位于东昆仑北弧岩浆岩带南缘的阿拉思木基性岩进行研究,为东昆仑东段晚古生代岩浆演化研究提供新的依据。通过LA-ICP-MS锆石U-Pb定年方法获得阿拉思木基性岩形成年龄为(241±1)Ma,为中三叠世岩浆活动的产物。岩石地球化学特征显示岩石具有低Ti O2(0.17%~0.37%,平均值0.25%)和较高的Mg#值(72.94~77.32,平均值为74.95)及Al2O3值(13.08~23.20%,平均值18.9%),富集大离子亲石元素(LILE:Sr、Rb、Ba),明显亏损高场强元素(HFSE:Nb、Ta、Zr、Hf)。Harker图解表明,Mg O与Fe2O3T、Ti O2、Ni、Co呈明显的正相关,暗示阿拉思木基性岩浆在形成过程中有一定程度的橄榄石、钛铁氧化物的分离结晶。阿拉思木辉长岩具有岛弧玄武岩的地球化学特征,并且其结晶年龄明显早于东昆仑碰撞及后碰撞构造岩浆时限。综合区域地质资料认为,阿拉思木辉长岩可能代表了东昆仑地区中三叠世洋壳俯冲的最晚期岩浆记录,进而确定东昆仑古特提斯洋俯冲结束碰撞开始的时间可能为中三叠世。
In the eastern part of East Kunlun, the Late Paleozoic medium-acid granites are widely distributed, and the basic magmatic rocks have few records. In this paper, we selected the Alaithsian basic rocks located in the southern margin of the northern arc arc magmatic belt of the East Kunlun Mountains to provide new evidences for the Late Paleozoic magmatic evolution in the eastern segment of East Kunlun Mountains. The LA-ICP-MS zircon U-Pb dating method was used to obtain the age-average age of (241 ± 1) Ma of the Allasimen basic rock, which is the product of the Middle Triassic magmatism. The geochemical characteristics of rocks show that the rocks have low Ti O2 (0.17% -0.37%, average 0.25%) and higher Mg # (72.94-77.32, average 74.95) and Al2O3 (13.08-23.20% 18.9%) enrichment of large ion lithophile elements (LILE: Sr, Rb, Ba), obvious depletion of high field strength elements (HFSE: Nb, Ta, Zr, Hf). The Harker diagram shows that MgO is positively correlated with Fe2O3T, Ti O2, Ni and Co, indicating that there is a certain degree of separation and crystallization of olivine and titanium iron oxide in the formation of Allaisian basic magma. The Arathi gabbro has the geochemical characteristics of island-arc basalts, and its crystallization age is earlier than that of the East Kunlun collisions and post-collision tectonic magmatism. According to the comprehensive regional geological data, it is considered that the Alameic gabbro may represent the latest magmatic record of the Middle Triassic subduction of the Middle Triassic in the East Kunlun area, and thus the timing of the end collision of the East Tethys Ocean subduction of the East Kunlun may be confirmed as Middle Third Stacked.