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

发布时间：2019-02-17 08:30

【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,推测阿尔金北缘脆-韧性剪切带的形成时代应介于早奥陶世至晚志留世早期之间,这一年龄范围与北阿尔金洋闭合的时代具有良好的一致性。因此,笔者认为阿尔金北缘脆-韧性剪切带可能是塔里木板块和阿中地块斜向碰撞汇聚事件的构造响应。
[Abstract]:The brittle-ductile shear zone in the northern margin of Altun is an important tectonic boundary in the northern Altun area. A thorough study of its formation age and shear characteristics is beneficial to a deeper understanding of the early Paleozoic tectonic framework and tectonic evolution in the northern Altun area. In this paper, a set of deformed diorite involved in the brittle ductile shear zone has been found in Kaladawan area, and detailed field geological survey, microstructural observation, geochemistry and its chronology have been carried out. Field observation shows that the deformable diorite has a tendency of 114 掳~ 137 掳, a dip angle of 25 掳~ 43 掳, a trend of 98 掳~ 120 掳and a SW, dip angle of 60 掳~ 75 掳, which is consistent with the extension direction of the brittle ductile shear zone. Microscopic observation shows that mica fish and S-C fabric are developed in the deformed diorite, and the macula of amphibole is faulted, showing the right direction of shear. The geochemical characteristics of the deformed diorite show that the Si O _ 2 of the deformed diorite is 57.596.39, the Na2O K _ 2O is 4.79 and 5.29, the value of Mg# is 4.04% and the Mg# value is 49.9%. The aluminum saturation index (A/CNK) is 0.81g 0.89, which belongs to the quasi-aluminous calc-alkaline rock and is characterized by I-type granite. The samples are rich in large ion lithophile elements (such as Rb,Ba,K), light rare earth (LREE) and active incompatible elements (Th,U), while relatively depleted heavy rare earth elements (HREE) and high field strength elements (Nb,Ta,Zr,Hf,Ti). There are weak Eu negative anomalies (未 Eu=0.75~0.84), similar to island arc magmatic rocks. The petrogenetic study indicates that the rock body should be formed in the active continental margin island arc environment associated with oceanic crust subduction, and its magma should be derived from partial melting induced by subduction plate fluid metasomatic mantle wedge. The LA-ICP-MS zircon U-Pb isotopic analysis shows that the 206Pb/238U weighted mean age is 478.1 卤2.1Ma (MSWD=0.27). It represents the age of crystallization of deformed diorite, which not only indicates that the North Altun Ocean is already in the subduction stage in early Ordovician, but also limits the lower limit of the brittle-ductile shear zone in the northern margin of Altun. Based on the regional geological data and the previous study on the porphyry monzogranites and biotite monzonitic granites (Abei rock mass, zircon SHRIMP U-Pb age 413 卤5Ma~427.3 卤5.7Ma), the deformed diorite is exposed to the northeast side of the deformed diorite. It is inferred that the brittle-ductile shear zone in the northern margin of Altun is between early Ordovician and early late Silurian, and this age range is in good agreement with the age of the closure of the Northern Altun Ocean. Therefore, the authors believe that the brittle-ductile shear zone in the northern Altun margin may be the tectonic response of the oblique collision convergence event between the Tarim plate and the Azhong block.
【作者单位】：中国地质科学院地质力学研究所;东华理工大学;中国地质大学地球科学与资源学院;
【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
【分类号】：P588.122

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