阿尔金北缘脆-韧性剪切带内变形闪长岩的年代学、地球化学特征及其对北阿尔金早古生代构造演化的指示  

Geochronological and geochemical characteristics of the deformed diorite from the North Altyn brittle-ductile shear zone and its constraint on the Early Paleozoic tectonic evolution of the North Altyn Tagh

作　　者：吴玉[1] 陈正乐[1,2] 陈柏林[1] 王永[1] 孟令通[1] 何江涛[1,3] 韩梅梅[1,3] 王斌[3] 

机构地区：[1]中国地质科学院地质力学研究所,北京100081 [2]东华理工大学,南昌330013 [3]中国地质大学地球科学与资源学院,北京100081

出　　处：《岩石学报》2016年第2期555-570,共16页Acta Petrologica Sinica

基　　金："十二.五"国家科技支撑计划项目(2011BAB06B08-04);国家自然科学基金项目(41502086;U1403292);中国地质调查局地质调查项目(12120113095800;1212011220936;12120114032401;12120115033801)联合资助

摘　　要：阿尔金北缘脆-韧性剪切带是北阿尔金地区一条重要的构造边界,对其形成时代及剪切特征进行深入的研究有利于加深对北阿尔金地区早古生代构造格局与构造演化的认识。本文在喀腊大湾地区发现了一套卷入该脆-韧性剪切带内的变形闪长岩,对其开展了详细的野外地质调查、显微构造观察、地球化学及其年代学研究。野外观察该变形闪长岩发育片麻状面理及拉伸线理,线理的倾向为114°~137°,倾角为25°~43°;面理产状为走向98°~120°,倾向SW,倾角60°~75°,与脆-韧性剪切带延伸方向一致。显微构造观察显示变形闪长岩发育有云母鱼和S-C组构,并可见角闪石残斑被错断,显示出右行剪切的指向。地球化学特征研究表明变形闪长岩的Si O2为57.59%~62.39%,全碱(Na2O+K2O)为4.79%~5.29%,具有较高的Mg O(3.10%~4.04%)、Mg#值(49.9~51.2),铝饱和指数(A/CNK)为0.81~0.89,属准铝质钙碱性岩石,具有I型花岗岩特征。样品富集大离子亲石元素(如Rb、Ba、K)、轻稀土(LREE)和活泼不相容元素(Th、U),而相对亏损重稀土(HREE)和高场强元素(Nb、Ta、Zr、Hf、Ti),具有弱的Eu负异常(δEu=0.75~0.84),与岛弧岩浆岩类似。岩石成因研究表明岩体应形成于与洋壳俯冲有关的活动大陆边缘岛弧环境,其岩浆应来源于俯冲板片流体交代地幔楔诱发的部分熔融,并在岩浆上升过程中混染了上地壳物质或/和在俯冲过程混入了俯冲沉积物。LA-ICP-MS锆石U-Pb同位素分析,获得206Pb/238U加权平均年龄为478.1±2.1Ma(MSWD=0.27),代表了变形闪长岩的结晶年龄,这不仅表明北阿尔金洋在早奥陶世已处于俯冲消减阶段,而且限定了阿尔金北缘脆-韧性剪切带形成时代的下限。结合区域地质资料和本项目组以往对变形闪长岩体东北侧出露的未变形似斑状二长花岗岩和黑云母二长花岗岩(即阿北岩体,锆石SHRIMP U-Pb年龄为413±5Ma^427.3±5.7Ma)的研究,推测阿尔金北The North Altun brittle-ductile shear zone as an important tectonic boundary in the North Altyn Tagh is important to be investigated. In particular,detailed studies on its deformation timing is significant to improve understandings on the tectonic evolution history of the North Altyn Tagh. According to the current study,a set of deformed diorites involved in the brittle-ductile shear zone in the Kaladawan area were found,focusing on which comprehensive studies consisting of field investigation,microstructure observation,major and trace element geochemical analysis and LA-ICP-MS zircon U-Pb dating were performed. Field observations noticed that the deformed diorite have weak SE-trending stretching lineation,with plunge of 25° ~ 43°,and well-developd foliation with attitudes from ca. 98° to 120° and the SE-diping with an angle of 60° ~ 75°,which are parallel to the extension of the brittle-ductile shear zone. By microstructure observation,dislocation amphibole porphyroclasts,S-C fabrics and mica-fish textures developed in the deformed diorite indicate the dextral strike-slip shearing. The geochemical analysis results show that the deformed diorite is characterized by relatively high contents of Mg O（ 3. 10% ~ 4. 04%） and Mg#values（ 49. 9 ~ 51. 2）,with the Si O2 content ranges from 57. 59% to 62. 39%,ALK（ Na2 O ＋ K2O） content ranges from 4. 79% to 5. 29% and aluminium saturation index（ A / NCK） = 0. 81 ~ 0. 89,belonging to metaluminous and calc-alkaline I-type granites. In addition,The diorite plutons exhibit enrichment of large-ion lithophile elements（ LILEs,e. g.,Rb,Ba,K）,light rare-earth elements（ LREEs） and more mobile incompatible elements（ e. g.,Th,U）,as well as depletion of high field strength elements（ HFSEs,e. g.,Nb,Ta,Zr,Hf,Ti） and heavy rare earth elements（ HREEs）,and weakly negative Eu anomalies（ δEu = 0. 75 ~ 0. 84）,indicating they have arc magmatic signature. Petrogenesis research suggests that the diorite plutons were probably formed in an active contin

关 键 词：阿尔金北缘脆-韧性剪切带 变形闪长岩 锆石U-PB定年 构造演化 早古生代 北阿尔金 

分 类 号：P588.122[天文地球—岩石学] P597.3[天文地球—地质学]