西昆仑奇台达坂一带晚三叠世中酸性侵入岩锆石LA-ICP-MS年龄、地球化学特征及其构造意义

LA-ICP-MS Zircon U-Pb Ages,Geochemical Characteristics of Late Triassic Intermediate—Acid Intrusive Rocks in Qitaidaban Area,Western Kunlun and Their Tectonic Significance

奇台达坂位于新疆西昆仑新藏公路540km处,在其周围分布有小规模的中酸性侵入岩,岩体严格受郭扎错—金沙江缝合带控制呈北西向展布,岩性主要为英云闪长岩、石英闪长岩和花岗闪长岩。锆石环带结构明显,Th/U值介于0.15~0.73之间,为典型的岩浆锆石。LA-ICP-MS锆石U-Pb同位素测年结果表明,其形成年龄分别为214.7±2.0 Ma、211.3±3.3 Ma和210.6±2.8 Ma,时代为晚三叠世。主微量元素英云闪长岩→石英闪长岩→花岗闪长岩,岩石中SiO_2(58.96%~67.52%)含量逐渐增高,Al_2O_3(14.79%~16.37%)、CaO(3.41%~5.25%)和Mg O(1.47%~3.42%)逐渐降低;里特曼指数1.03~1.79,K_2O+Na_2O全碱含量为4.36%~6.4%,属钙碱性系列。稀土元素中等(∑REE=101.27×10-6~271.04×10-6),轻重稀土分异明显(LaN/YbN平均为10.69),Eu异常明显(δEu平均值0.60)。大离子亲石元素(LILE)Rb、K、Th富集、相对亏损Ba,高场强元素(HFSE)Nb、Zr、Hf、Sr、P、Ti亏损。对研究区侵入岩进行投图分析,认为它是形成于大陆边缘弧环境的未分异I型花岗岩,物质来源主要为岩浆弧地壳的角闪岩相变玄武岩,并卷入少量的地幔物质。根据岩体的成因类型并结合区域构造环境演化认为,在晚三叠世末,古特提斯洋南缘局限洋盆(巴颜喀拉中生代边缘裂陷海洋盆)发生自NE向SW的持续俯冲消减,洋—陆转换开始,该套中酸性侵入岩侵入就位。

Objectives： Based on Petrology study and method of zircon LA-ICP-MS ages and geochemistry of Intrusive Rocks in Qitaidaban Area,to dicuss the age of the Paleo-Tethys closure and to explore It’s direction subduction. Methods：Zircon trace element analysis by the Northwest University of State Key Laboratory of Continental Dynamics completed,this instrument was the United States Agilent Company produced Agilent7500a type quadrupole mass spectrometer and Germany Microlascompany’s Geolas200M type laser ablation system.Calculating rhe zircon age used the standard ziecon 91500 as the external standard,elemental content used NISTSRM610 as an external standard,29Si was calibrated as an internal standard element.Data acquisition and processing used GLITTER,And used the Andersen to test the data for ordinary Pb correction,Ages calculation and concatenation drawing used ISOPLOT. Main quantity elements,rare earth elements,trace elements are completed by Industry Analysis test center of No.203 Researth Institute of Nuclear.Main quantity elements and Trace elements （ such as P, Ba, V, Cr, Rb, Sr, Zr, Sc） was analyzed by XRF method （FeO was analyzed by volumetric method）,this instrument was an Axios X ray spectrometer manufactured in the Netherlands; all rare earth and trace elements such as Co, Ni, Nb, Hf, Ta, Th, U by ICP MS analysis,This instrument was an XSERIES2 type ICP MS manufactured by Thermo Fisher Scientific. Results： The intrusive rocks of Qitaidaban area were composed of tonalite, quartz diorite and granodiorite. LA ICP MS zircon U Pb LA ages results show that their were 2147±2 Ma、2113±33 Ma and 2106±28 Ma,their belongs to Late Triassic. Geochemmical analysis shows that：SiO2=5896%～6752%,Al2O3= 1637%～1479%,CaO=341%～525%,MgO=147%～342%, K2O ＋ Na2O ）=436% ～64%,They belong to calc alkaline series. ∑ REE = 10127× 10-6～27104 × 10-6,REE fractionation is obvious （ LaN / YbN average was 1069 ）,Which whith Eu is obviously abnormal.LILE（Rb, K and Th）are quite enrichment,but relative loss Ba;Strong loss of high field strength elements（ HFSE） such as Nb, Zr, Hf, Sr, P, Ti. Conclusions： Analysis of the intrusive rocks in the study area,results show that they are formed in the continental margin arc environment of Non I type granite,the material derived from the crust source for amphibolite facies basalt, involved in a small amount of mantle material.According to the genetic types of the intrusives and combined with the regional tectonic environment evolution.in the Late Triassic,the southern margin of the Paleo Tethys occurred continuously underthrust from NE to SW, began to ocean—continent transition. At the same time,the intrusives was produced under such mechanism.