东昆仑东段阿拉思木辉长岩锆石U-Pb年代学、地球化学特征及洋盆闭合时限界定

摘要

Late Paleozoic intermediate-acid granitoids are widely distributed in the East Section of East Kunlun, but the records of basic magmatism are very insufficient. Alasimu mafic rocks on the southern margin of the arc magmatic rock belt in northern East Kunlun Mountains was selected as the study objective so as to provide a new basis for the research on the Late Paleozoic magmatic evolution of the east section of East Kunlun Mountains. LA-ICP-MS zircon U-Pb analysis yielded the crystallization age of 241± 1Ma, suggesting that Alasimu Mafic rocks were formed in the Middle Triassic. All the rock samples are characterized by relatively low TiO2 (0.17%-0.37%, averagely 0.25%) and high Mg# (72.94%-77.32%, averagely 74.95%) and high Al2O3 (13.08%-23.20%, averagely 18.9%), enrichment of large ion lithophile elements (LILE:Sr, Rb, Ba), apparent depletion of high field strength elements (HFSE, Nb, Ta, Zr and Hf). MgO shows some correlation with Fe2O3T, TiO2, Ni and Co in the diagram of Harker, which reveals some degree of fractional crystallization of olivine and Ti-Fe oxide. Alasimu mafic rocks have similar chemical composition to island arc basalts, and they formed earlier than the collision and post-collision time of tectonic-magmatic activity. Combined with regional studies, the authors hold that Alasimu gabbro rocks ought to be the latest magmatism records in the East Kunlun region related to Middle Triassic Paleo Tethys oceanic crust subduction, which indicated that the tectonic transition from ocean subduction to collisional orogeny might have commenced in Middle Triassic.%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TiO2(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,MgO与Fe2O3T、TiO2、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.