北阿尔金拉配泉地区齐勒萨依岩体的年代学、地球化学特征及其构造意义

阿尔金北缘拉配泉地区的齐勒萨依岩体主要由辉长岩、闪长岩组成。地球化学特征表明,辉长岩具有高的Al2O3(13.47%~15.34%)、FeOT含量(10.77%~16.89%)以及中等的MgO含量(5.15%~7.49%);闪长岩属钙碱性系列,具有较低SiO2含量(52.06%~54.23%)和高的Mg#值(56.43~62.45)。二者在球粒陨石标准化稀土元素配分曲线上呈REE近平坦型或者LREE轻微富集型,均显示Eu弱的亏损或富集。在原始地幔标准化蛛网图上,二者均富集Rb、Ba等大离子亲石元素(LILE),相对亏损Nb、Ta、Zr和Hf等高场强元素(HFSE),具明显消减带岩浆岩的特征。辉长岩和闪长岩的锆石LA-ICP-MS U-Pb定年结果分别为477.5±3.3 Ma和469.7±3.4 Ma,表明它们属于早中奥陶世岩浆活动的产物。结合岩石的地球化学、微量元素构造环境判别图解、年代学和区域地质背景的特征,判定齐勒萨依岩体形成于与俯冲有关的活动陆缘弧的构造环境,二者的侵位与阿尔金北缘早古生代的古大洋向南俯冲消减到米兰河-金雁山活动陆缘地块之下密切相关,从而为北阿尔金早中奥陶世的陆缘弧和红柳沟-拉配泉蛇绿混杂岩带俯冲极性的进一步确定提供了佐证。

LA-ICP-MS zircon U-Pb geochronology and geochemistry of gabbro and diorite from Qilesayi pluton in Lapeiquan area of northern Altun Mountains and their tectonic implications

Located in Lapeiquan area,northern Altun Mountains of Xinjiang,the Qilesayi pluton consisting mainly of gabbro and diorite intruded into the Lapeiquan Group.This paper has studied petrographic,geochemical and zircon U-Pb chronological characteristics of gabbro and diorite from Qilesayi pluton as well as their tectonic significance.Geochemical analyses suggest that the gabbro has high Al2O3(13.47%~15.34%),FeOT(10.77%~16.89%) and moderate MgO(5.15%~7.49%) values,whereas the diorite samples are characterized by low SiO2 content(52.06%~54.23%) and high Mg# values(58.63~62.45),belonging to the calc-alkaline series.The gabbro and diorite samples have features of low ΣREE content with low fractionation of LREE/LREE and no or slightly negative Eu anomalies,depletion of HFSE(Nb,Ta,Zr,Hf) and enrichment of LILE(Rb,Ba).These geochemical features imply an origin similar to that of the subduction-related tectonic setting.Zircon U-Pb ages obtained by LA-ICP-MS analyses from the gabbro and diorite samples are 477.5±3.3 Ma and 469.7±3.4 Ma,respectively,implying that these rocks were emplaced in the Early-Middle Ordovician period.Their petrogenic ages are similar to rock-forming ages of the adjacent Bashenkaogong gigantic phenocryst granitoid(474 Ma),Bashenkaogong-Xiemierbulake quartz diorite(481.6±5.6 Ma) and Jinyanshan granodiorite intrusive bodies(467.1±6 Ma),suggesting that they may be products of the same magmatism and constitute the basic-intermediate-acidic magmatic evolution series.Integrated with geochemistry,trace elements discrimination,geochronology and regional background,it is inferred that that the gabbro and diorite were formed in a subduction-related active continental margin arc.Meanwhile,this study proposes that the gabbro and diorite from Qilesayi pluton were derived from the southward subduction of an oceanic crust to Milanhe-Jinyanshan active continental margin during the Early Paleozoic period,and this provides the corroborating evidence for the further ascertaining of the existence of the Early-Middle Ordovician continental margin arc and subduction polarity of Hongliugou-Lapeiquan ophiolite mélange zone in the North Altun Mountains.