新疆东天山博格达造山带西段基性侵入岩的U-Pb年代学、地球化学及其构造意义

U-Pb Dating,Geochemical and Tectonic Signficances of Basic Intrusive Rocks from the West of Bogda Orogenic Belt,Eastern Xinjiang

博格达造山带广泛分布着基性侵入岩体,其成因对正确认识该区地球动力学背景具有重要意义。对位于博格达造山带西段的基性侵入岩体分别进行了岩石学、岩石地球化学以及LA-ICP-MS锆石U-Pb年代学的研究,从而探讨其岩石成因及构造意义,以期为解决博格达造山带的地质构造演化提供依据。岩石地球化学分析结果显示,基性侵入岩w(SiO2)介于48.08%~50.88%之间,全碱(ALK)质量分数为3.09%~4.39%(小于5%),富CaO(6.87%~11.41%)、贫Al2O3(12.59%~18.49%),w(MgO)介于3.78%~7.66%之间,Mg#值为31.75~65.73(均值为51.29),铝饱和指数值A/CNK为0.60~0.80,属准铝质中钾钙碱性岩石系列。岩石总稀土元素质量分数较低,且轻重稀土元素之间以及轻重稀土元素内部不存在明显的分馏过程;微量元素中Th、Ta、Nb元素明显亏损,Ti元素亏损不明显以及U元素轻微富集,反映岩石具大陆裂谷岩浆活动特点。有关微量元素、稀土元素的相关图解及其比值,指示该区基性侵入岩岩浆源于亏损的岩石圈地幔,侵位过程中受到了地壳物质的混染,分异演化程度较低,为原始尖晶石相橄榄岩经较高程度的部分熔融的产物,形成于板内裂谷的构造环境。此外,辉长岩LA-ICP-MS锆石U-Pb定年结果表明,其形成年龄为(305.0±1.6)Ma(晚石炭世末期),处于博格达裂谷火山活动期晚期,为博格达西段裂谷作用由全面伸展向局限伸展转换的标志。综合对比区域研究成果,认为晚古生代期间(早石炭世-早二叠世),博格达造山带存在大面积与真正裂谷演化有关的岩浆活动,造山带是在裂谷基础上发展起来的,其形成可能与古亚洲洋沿克拉麦里碰撞带向准-吐-哈地块斜向俯冲作用有关。

The Bogda orogenic belt is widely distributed with basic intrusive rock mass,and its origin is of great significance to correctly understand the geodynamic background of the area.This paper studies systematically petrology,geochemistry and zircon U-Pb geochronology of basic intrusive rocks in the west of Bogda orogenic belt in order to define its petrogenesis and tectonic significance,and then to provide a basis for solving geological tectonic evolution of the belt.Rock geochemical analyses show that its SiO2 ranges from 48.08% to 50.88%,and ALK from 3.09% to 4.39%（less than 5%）,rich in CaO（6.87%-11.41%）,and depleted in Al2 O3（12.59%-18.49%）.It has MgO ranging from 3.78% to 7.66%,and Mg#=31.75-65.73（the average is 51.29）,A/CNK=0.60-0.80,indicating that it belongs to metaluminous middle K calc-alkaline rock series.The analysis also shows that the belt has low REE（36.30×10-6-101.41×10-6）,between light and heavy rare earth and heavy rare earth internal no obvious fractionation process.Obvious loss of Th,Ta and Nb element,no obvious loss of Ti and slight enrichment in U element,show the characteristics of continental rift magmatism.The relevant diagram and the ratio about the trace elements and rare earth elements,indicate that the basic intrusive rock magma originates from the loss lithospheric mantle,suffering contamination of the crust,with low differentiation evolution degree,as the original phase spinel peridotite higher degree of partial melting,formed in the intraplate rift environment.Moreover,gabbro LA-ICP-MS zircon U-Pb dating results show that the forming age of（305.0±1.6）Ma（Latest Carboniferous）,in the late period of the Bogda rift volcano activity,indicates the west of Bogda rift underwent a conversion from the overall extension stage to the confined extension stage.The analysed of regional geological datas claim that there is a large rift evolution related to magmatism and during Late Paleozoic Era（Early Carboniferous-Early Permian）,the orogenic belt developed on the basis of rifting.Propably,its formation is related to the oblique subduction of the Paleo-Asian Ocean plate towards the Junggar-Turpan-Kumul plates along the Kalameili subduction belt.