【作者】 李化启；

【导师】 许志琴；

【作者基本信息】 中国地质科学院 ， 构造地质学， 2009， 博士

【摘要】 本研究以松多榴辉岩带的发现和古特提斯洋盆的可能南移为线索,重点对松多榴辉岩构造带的造山作用、区域背景、发展演化以及高压-超高压变质岩石的折返动力学进行了研究。通过对拉萨地体中松多群变质岩系的变形构造、韧性剪切带的研究,剪切带中石英组构的EBSD测定,白云母石英片岩和榴辉岩及退变榴辉岩的白云母和角闪石的⁴⁰Ar-³⁹Ar年代学测定以及二长花岗岩岩体的SHRIMP U-Pb年代学分析,初步确认拉萨地体曾经历过一期规模宏大的印支期造山事件。本研究取得的进展和主要结论如下:1.从北向南将研究区划分为北拉萨古特提斯构造单元和冈底斯火山岩浆增生杂岩带两个构造岩性单元。初步认为松多群经历了三期区域构造变形。主变形期松多群内形成多条近东西向的逆冲型韧性剪切带,兼有自东向西的走滑,反应强变形阶段的挤压转换性质。由于榴辉岩或退变榴辉岩多沿韧性剪切带分布,与围岩接触位置有石榴子石流动变形等现象,显示榴辉岩在折返过程中经受了韧性剪切带的变形改造。2.松多群糜棱质白云母石英片岩的白云母⁴⁰Ar-³⁹Ar年代学以及榴辉岩和糜棱质退变榴辉岩的白云母和角闪石的⁴⁰Ar-³⁹Ar年代学测试表明拉萨地体经历过印支期造山事件,造山时间大约为220-240 Ma,这一事件为二叠纪古特提斯洋盆深俯冲（代表标志为拉萨榴辉岩带）之后南、北拉萨地体碰撞的产物。榴辉岩与糜棱质退变榴辉岩的白云母和角闪石⁴⁰Ar-³⁹Ar年代学年龄与白云母石英片岩反映的碰撞造山年龄基本一致,说明榴辉岩折返退变的时代和碰撞造山的时代一致,从年代学上证实了榴辉岩的折返和碰撞造山是同时代的产物,说明区域性的韧性挤出及挤压转换作用应是本区榴辉岩折返出露的主要原因和重要机制。3.本研究的二长花岗岩SHRIMP U-Pb年代学分析结合有关区域地质资料表明,拉萨地体内沿当雄-工布江达-波密一线分布一条形成于210-190Ma（高精度的单颗粒锆石U-Pb年龄）的晚印支期花岗岩带,这些花岗岩既有较基性的花岗闪长岩,又有较酸性的淡色花岗岩,形成时代低于松多群的碰撞造山年龄和榴辉岩的折返年龄,应是和拉萨地体内印支期造山事件相关的晚碰撞或后碰撞花岗岩,这种认识也得到岩石地球化学判别图解的支持。4.综合区域沉积学、火山岩浆活动以及蛇绿混杂岩等的证据,表明拉萨地体内部的印支造山带应是一条至少西起措勤（向西可追索至狮泉河）,向东经南木林北部、墨竹工卡、工布江达,然后绕过雅鲁藏布大拐弯直至波密境内的一条规模宏大的碰撞型造山带。拉萨地体内印支造山带的确定使青藏高原印支山链的范围从过去认为的羌塘地体向南扩大到拉萨地体之中,丰富了中国“T”字形印支造山系的研究内容。5.本研究初步建立起拉萨地体自早古生代中晚期直至早中生代的地质演化的时空格架。根据时空演化过程拉萨地体早古生代到早中生代的区域演化历史可划分为七个阶段,七个阶段共同构成了一个完整的威尔逊旋回,反映了古冈瓦纳大陆北缘与北拉萨地体之间古特提斯大洋演化的完整过程。更多还原

【Abstract】 In this study, the orogenetic process, regional geological background, development and evolution, and the exhumation machanism of high-ultrahigh pressure metamorphic rocks are analysed to the Sumdo eclogite belt. Based on the studies of the deformation characteristics and the ductile shear zones in Sumdo Group, the quartz fabric by EBSD, the data of muscovite or amphibole ⁴⁰Ar-³⁹Ar geochronology to the muscovite quartz schist, eclogite and retrograde eclogite, and the zircon SHRIMP U-Pb chronology of granites in Sumdo region, Lhasa terrane is thought to have experienced an important Indosinian orogenic event. The main advancements and conclusions are as followings:1 From north to south, the working area is divided into two structural-lithological units, such as the northern Lhasa Paleo-Tethys belt and Gangdise tectonic magmatic accretion zone. Sumdo Group is thought to experienced three periods of tectonic deformation. In the main deforming stage, several thrusting ductile shear zones were formed. In certain area, the characteristics of clockwise right-hand slipping is the extruding transform deformation. Retrograded eclogite lenses, with the long axes paralleled to the tectonic line, were found in some ductile shear zones. The flow deformations of garnets show that the retrograded eclogites experienced deformational transformation by the ductile shear zones.2 The muscovite ⁴⁰Ar-³⁹Ar chronological studies to the muscovite quartz schist of Sumdo Group, the eclogite and retrograde eclogite show that there has been a Indosinian orogenesis at 220 -240 Ma in Lhasa terrane, which caused the closure of the Paleo- Tethys ocean and the collision between northern part and southern part of Lhasa terrane. The consistence of the muscovite or amphibole ⁴⁰Ar-³⁹Ar age for the the eclogite, retrograde eclogite and muscovite quartz schist indicates that the exhumation of eclogites and the Indosinian Orogenesis Occurred at the same time, which reveals that the regional ductile extrusion should be the main mechanism for the eclogite exhumation.3 The zircon SHRIMP U-Pb chronology of 190Ma for biotite adamellite, with the regional geological data about granites in Lhasa terrane, indicates that there is a granite belt of late indochina epoch along Dangxiong -Gongbujiangda -Bomi county. The Indosinian granites consist of not only basic granodiorites, but also acidic theleucogranites, with the High precision single zircon U-Pb ages of 210Ma - 190Ma, which are lower than the collision orogenic age of Sumdo Group and the timing of the exhumation of eclogites. Based on the formation age and discrimination diagrams, these granites should be syn-collisional or late-collisional granites relate to Indosinian orogenesis occurred in inner Lhasa terrane.4 Based on the evidences of regional sedimentary, volcano-magmatic activity and ophiolite melange, the Indosinian orogenic belt in Lhasa terrane is thought at least starts from Coqen county, and goes eastward through the northern part of Nanmulin county, Mozhugongka county, Gongbujiangda county, and at last goes around the eastern Himalayan syntaxis into the Bomi county. The discovery of Indosinian orogenic belt in Lhasa terrane expanses the southern boundary of Indosinian orogenic belt in Qinghai-Tibet Plateau to Lhasa terrane from Qiangtang terrane, which changes the understanding about the distribution of Indosinian orogenic belt in Qinghai-Tibet Plateau and extends the "T" type Indosinian orogenic belt in China.5 Through the detailed studies, The temporal framework of geological evolution from early Palaeozoic to early Mesozoic for Lhasa terrane is built. Based on the evolutionary process, the geological development of Lhasa terrane from early Palaeozoic to early Mesozoic can be divided into seven stages. All of the seven stages make up a whole Wilson circle and reveal a perfect evolutionary process of the Paleo- Tethys ocean between northern part and southern part of Lhasa terrane更多还原