【作者】 罗改；

【导师】 何明友；

【作者基本信息】 成都理工大学 ， 地球化学， 2009， 硕士

【摘要】 在大陆地壳形成发展的地史中,花岗岩类占十分重要的地位。长期以来,花岗岩类一直是地质科学家的主要研究对象之一。花岗岩类的形成、演化与大地构造环境和地壳演化有密切关系,其活动明显受到板块构造活动的控制;它是大陆地壳的主要组成部分,是构造运动的直接产物,其地球化学特征可以反映源区性质,并且也是构造发生时间重要的历史记录;大量的研究结果表明,不同成因类型的花岗岩形成于不同的构造环境,并显示出不同的地球化学特征。花岗岩的成因类型和构造环境一直是地球科学研究中具有重要理论和实际意义的问题,长期以来,观点分歧,争论激烈。20世纪70年代,Chappell和White划分出I型花岗岩的和S型花岗岩。80年代,许多学者将花岗岩成因及构造环境结合,提出了许多花岗岩的分类方案;我国的徐克勤根据物质来源、形成方式及构造位置将花岗岩划分为陆壳改造型、同熔型及幔源型。到了90年代,人们更多的考虑到壳—幔间的相互作用,并认为花岗岩成分是受构造环境、源岩成分、熔融条件、组分间物化反应、地壳混染、岩浆演化机理等诸多因素制约。因此,对花岗岩的成因及构造环境研究不仅要考虑源岩和经典的板块构造动力学特征,而且还需要从区域和时间演化上去认识。松潘-甘孜造山带位于青藏高原东部,经历了古特提斯和新特提斯两个连续的造山事件,素有中国“地质百慕大”和“地质博物馆”的称号。造山带内构造形迹复杂,但主要变形过程发生在晚三叠世;造山带内广泛出露印支期中酸性侵入岩,这些岩体侵入到三叠系西康群中,是松潘—甘孜造山带构造发展过程中的一个重要组成部分,区内复杂的构造演化特点也广泛受人关注。通过对松潘—甘孜造山带花岗岩类的研究可以揭示地壳源区性质、岩石成因,对恢复造山带内复杂的构造演化历史及动力学机制提供重要信息。目前,前人对松潘-甘孜造山带内许多花岗岩体进行了比较详细的研究,包括花岗岩类的地球化学特征、地质年代学、岩石成因机制等;但对羊拱海地区花岗岩类的研究大多仅建立在地质年代学基础之上,对其地球化学特征及成因方面等系统研究目前还是比较缺乏。论文依托四川省地质调查院承担的“中国地质调查局项目:四川1:25万若尔盖县、红原县、炉霍县、马尔康县幅区调修测”项目,选择羊拱海地区花岗岩类为主要研究对象(包括羊拱海主岩体及其邻区的达盖寨、二道桥、剑科等岩体),结合区域地质背景,综合运用岩石学、矿物学、地球化学及地质年代学等多学科研究手段,着重研究了该区各岩体主微量元素地球化学特征,并对羊拱海岩体锆石U-Pb年代学特征进行了详细分析;论文引用了前人若干数据成果,如胡建民对羊拱海岩体的一件Sr-Nd同位素和两件锆石SHRIMP U-Pb定年数据,以及赵永久若干微量元素数据等,此外还与华南花岗岩地球化学特征进行了对比分析。在这些研究基础上,着重探讨了区内花岗岩类的成因机制及形成的构造环境。通过以上研究,得出以下主要认识:1.从主量元素地球化学特征可以看出,研究区花岗岩类大致可以分为二长花岗岩及花岗闪长岩两类,两者均显示高钾钙碱性、偏铝质～弱过铝质的特征;微量元素主要表现为Rb、Ba、Sr、Th等大离子亲石元素富集,而高场强元素Nb、P、Ti常表现为负异常的特点;稀土元素分布模式曲线均表现为向右陡倾的LREE富集模式,较平滑,δEu、δCe值基本相近,大部分具有不明显或弱负异常特征;相似的微量元素模式及稀土元素配分模式表明区内的两类花岗岩具有同源性质,应为同源岩浆分异演化的产物,岩石演化顺序为花岗闪长岩→(黑云母)二长花岗岩;2. LA-ICP-MS锆石U-Pb定年分析结果为211.7±1.3Ma,表明羊拱海花岗岩体形成于晚三叠世,该结果与前人在区域上许多岩体的测年结果(晚印支期～早燕山期)较为一致,可能也代表羊拱海地区各岩体侵位的时代大致在晚印支期;此外还捡获到了元古代锆石(899Ma～1505 Ma),推测松潘—甘孜造山带下部可能存在古老陆壳基底;3.羊拱海地区花岗岩类具有I型与S型花岗岩的双重特征,且显示出存在壳-幔间的相互作用,这种相互作用可能主要为地幔岩浆的底侵作用。因此可以认为,羊拱海地区花岗岩主要来自下地壳的部分熔融,并伴有地幔岩浆的底侵作用。此外,稀土元素HREE及Y/Yb特征可以反映其源区可能为角闪石残留相;4.对羊拱海地区花岗岩类的构造环境判别,显示出后碰撞花岗岩的特性;另外,区内花岗岩所显示的高钾钙碱性特点、壳幔源双重特征及同时期区域上大规模岩浆侵位及强烈的褶皱造山运动,这都反映出了后碰撞的特点。由此可以推测,研究区花岗岩类可能形成于主碰撞之后的陆内后碰撞环境,属于同构造期花岗岩,是区域大规模造山运动的产物。更多还原

【Abstract】 In the history of continental crust formation and development, granitoids played a very important role and it was one of the object in geology science research. The formation and evolution of granitoids, whose activity was controlled by the activities of plate tectonics, was closely related to tectonic environment and the evolution of the crust. The granitoids were major component of continental crust and the direct result of tectonic movement, and their geochemical characteristics can reflect the the nature of the source region, also they were important magma records of tectonic time.It was found that granites with varied causes of formation were formed in different tectonic environments and showed different geochemical characteristics. Granite types and tectonic setting has been always an important theoretical and practical significance in earth science research, with different opinions and heated debate for a long time. In the 20th century, 70s, Chappell and White divided into two types of granites: I-type granites and S-type granites. In 1980s, a number of scholars related causes of granites and tectonic environment and put forward many granites classification programs. According to material sources, modes of the formation and tectonic location, Xu Keqin divided granites into continental crust transformation type, syntectic type and mantle-derived type. In 1990s, it was more thouhgt about the interaction between crust and mantle, and that the components of granites were affected by many factors such as tectonic environment, source rock composition, melting conditions, physico-chemical reaction, crustal contamination, magmatic evolution mechanism. Therefore, the causes of granites and tectonic environment research should be considered not only the source of rocks and classical dynamic features of plate tectonics, but also from the regional and temporal evolution. Songpan-GarzêOrogenic Belt, which had been known as Chinese“Geological Bermuda”and“Geological Museum”, located in the east of Qinghai-Tibet Plateau, and went through Paleotethys Orogenesis and Neo-Tethys Orogenesis. Tectonic trackway was so complicated, but principal distortion occurred in Late Triassic Epoch. In Songpan-GarzêOrogenic Belt, intermediate-acidic intrusive rocks, which outcropped extensively and emplaced in Triassic Xikang Group, were one of the important part in the development of orogenic belt structure, and complex tectonic evolution of the region was paid a wide attention. Researching on granitoids in Songpan-GarzêOrogenic Belt, it can be revealed the nature of the crust source region, petrogenesis, and many important informations in complex history of tectonic evolution and dynamic mechanism. As yet, many granite bodies in this Orogenic Belt has been researched in detail, which involved geochemical characters, geochronology, petrogenesis mechanism etc. However, the research on granitoids in Yanggonghai area were merely built on the basis of age of geochronology, and it was lack of geochemical characters, genesis etc.This paper relyed on the“Project of China Geological Survey: 1:25 million Ruoergai County, Hongyuan County, Luhuo County, Barkam County Geological Survey measured the regional repair”which assumed by Sichuan Geological Survey. Investigating on the granite bodies in Yanggonghai area (involve Yanggonghai body and Dagaizhai body, Erdaoqiao body, Jianke body etc. in its neighboring area), combining regional geological setting, using petrology, mineralogy, geochemistry, geochronology etc., it was researched geochemical characters of main elements and trace elements in the area, analysised Zircon U-Pb geochronology characters. Moreover, this paper cited a number of data on previous results, such as one Sr-Nd isotope and two zircon SHRIMP U-Pb dating data of Yanggonghai body by Hu Jianmin, as well as several trace elements data by Zhao yongjiu; in addition, it was analyzed and compared with geochemical characters of southern China granitoids. Based on these studies, it was discussed the formation mechanism and tectonic environment of the granitoids in the area.By this research, it was recognized that:1. From the geochemical characteristics of main elements, granites in the area can be roughly divided into two classes: monzonitic granites and granodiorites, both of which showed high-K calcium alkaline and metaluminous to weak peraluminous characters. Trace elements were enriched in LILE (large-ion lithophile element) such as Rb, Ba, Sr, Th, with negative anomaly in HFSE (high field strength element) such as P, Ti Nb. REE distribution patterns were steeply right tilted and smooth, and enriched in LREE, with similarδEu andδCe, which mostly had not obvious anomaly or weak negative anomaly. Similar trace element model and REE distribution patterns showed that the two kinds of granitoids were productions of comagmatic evolution, and evolved from granodiorites to (biotite) monzonitic granites.2. By LA-ICP-MS zircon U-Pb dating, it was found that Yanggonghai granite body were formed in Late Triassic Epoch (211.7±1.3Ma) probability, and the result was in accordance with age-dating results (Late Indosinian to Early Yanshanian) of many bodies in the area, so that the result may indicate that magmatic emplacement in Yanggonghai area might be happened in Late Indosinian. Moreover, based on the picked Proterozoic era zircon (899Ma～1505 Ma), it can be speculated that ancient continental crust fundus may existed in the under-part of Songpan-GarzêOrogenic Belt.3. Granitoids in Yanggonghai area possessed both characters of I-type granites and S-type granites, and showed the interaction between crust and mantle, which was mainly and probably the underplating by mantle magma. So it can be considered that granites in Yanggonghai area mainly formed from partial melting lower crust, with underplating by mantle magma. In addition, HREE and Y/Yb characteristics may reflect that the source region were mainly hornblende residual phases.4. Tectonic environment of granitoids showed post-collision granite characters in Yanggonghai area. Additionally, high-K calcium alkaline characters, crust-mantle dual characteristics, regional magma emplacement and intense folding orogenesis, all reflected the characteristics of post-collision. So it was can be speculated that granitoids in research area were probably formed in intracontinental post-collision environment after major collision; they belonged to syn-tectonic granites, and were the productions of regional large-scale orogenic movement.更多还原