【作者】 柯珊；

【导师】 莫宣学；

【作者基本信息】 中国地质大学（北京） ， 岩石学、矿物学、矿床学， 2006， 博士

【摘要】 青藏高原的形成被广泛解释为印度-亚洲大陆碰撞的结果,尽管吸引了大批国内外学者的注意,但对于高原的形成机制至今还远未取得共识。印度-亚洲大陆碰撞导致了青藏高原岩石圈的戏剧性巨大缩短和长期维持高海拔。在青藏高原许多重大的科学问题中,岩石圈的缩短过程与机制也是最具吸引力的科学问题之一。帕米尔构造结是青藏高原碰撞挤压表现最明显的地区之一,因而是揭示岩石圈缩短动力学过程的最佳场所;位于其中的塔什库尔干碱性杂岩带蕴含着岩石圈缩短过程的重要记录。 塔什库尔干新生代碱性杂岩带位于帕米尔构造结中东部,出露在中国—塔吉克斯坦边境一带,平行于喀喇昆仑大断裂展布。该岩带由若干岩体组成,其中以苦子干、卡日巴生和赞坎三个岩体为其典型代表。塔什库尔干碱性岩带主要由碱性正长岩类、碱性花岗岩类和碱性—偏碱性花岗岩类组成,代表性岩石类型为霓辉石正长岩、透辉石正长花岗岩和黑云母二长花岗岩。通过锆石SHRIMP定年,获得苦子干岩体正长岩和正长花岗岩、卡日巴生二长花岗岩的形成年龄均为～11Ma。研究表明,在～11Ma发生的强烈的构造-岩浆事件之后,于～8Ma又发生了一次构造热事件(喀喇昆仑断裂强烈走滑运动,公格尔山和木孜塔格山的构造抬升)。岩石地球化学研究显示,该岩带的各种岩石类型都具有富钾、富碱和富钙的特点,属于钾质碱性花岗岩类,可归属为A型花岗岩类。然而,这类A型花岗岩并非形成于造山作用结束阶段,而是形成于造山过程中局部伸展(如喀喇昆仑断裂带走滑运动)的环境。通过对三个岩体岩石学、矿物学、地球化学的综合研究及相平衡分析认为,形成碱性杂岩体的岩浆来源于斜长石不稳定区(>60km),及石榴石与金红石稳定区(>50km)。因此,岩浆起源于加厚地壳底部榴辉岩相镁铁质岩石的低度部分熔融,且岩浆源区受到了流体的影响。这表明,在～11Ma时帕米尔构造结地区已经具有加厚地壳,其厚度至少大于50km。但是,由于陆壳的增厚是一个冷却的过程,塔什库尔干碱性岩浆活动有赖于深部(如幔源岩浆)的热贡献。比较合理的解释是该区在～11Ma时发生了大规模的岩石圈拆沉作用。除了挤压增厚和块体挤出之外,岩石圈拆沉作用也是调节岩石圈缩短的重要机制。 在大陆碰撞挤压过程中,青藏高原岩石圈因挤压而缩短增厚。在这个过程中,地壳将发生分异,向下迁移的镁铁质物质将由于逐渐榴辉岩化而大大增加自身的密度,导致区域重力不稳定。当这种重力不稳定达到一定限度时,就会触发岩石圈拆沉作用,从而使岩石圈厚度大大减薄,导致软流圈上涌,产生岩浆活动,进一步导致岩石圈的弱化。继而,已经减薄的岩石圈可以通过软流圈顶面下降(散热引起)、岩浆底侵作用、或挤压等机制而再次加厚。拆沉-减薄-增厚这种深部过程的循环将可能会长期保持,导致高原地壳不断增厚和隆升。更多还原

【Abstract】 The Tibetan Plateau, caused by India-Asia continental collision, has attracted many researchers all over the world. However, there is still not a convincing mechanism to explain how the plateau formed. It is the India-Asia continental collision that gave rise to the large lithospheric shortening of the Tibetan Plateau and kept its high-elevation. Among many scientific problems of Tibetan Plateau, one of the best attractive problems is the process and mechanism of lithospheric shortening. Pamir syntax is an outstanding example of lithospheric extreme thickening during continental collision and thus would be a perfect place to unveil dynamics of lithospheric shortening. The Taxkorgan alkaline complex belt right in this area implies and records the process of lithospheric shortening.Taxkorgan alkaline complex belt, paralleling the Karakorum fault, is located in the middle-east Pamir syntax and along the boundary between China and Tajikistan. This belt is composed of several intrusions among which Kuzigan, Karibasheng and Zankan are typical ones. Taxkorgan alkaline belt is made up of alkaline syenites, alkaline syenitoids and alkaline to sub-alkaline granites, with aegirine-augite syenite, diopside syenitoid and biotite adamellite as their representative rocks. By using SHRIMP U-Pb zircon, we obtain accurate ages of Kuzigan and Karibasheng intrusions, and both of them are ~11Ma. Based on field observation, petrology, mineralogy, chronology of Ar-Ar and SHRIMP U-Pb zircon and former studies, we find that there was a strongly tectonic-magmatic event at ~11Ma and a tectonically thermal event at ~8Ma in Pamir area. Being rich in potassium, alkali and calcium, this belt is a series of potassic and alkalic granitoids and belongs to A-type granites. However, this kind of A-type granitoids was not formed at the end of orogeny. It is a magmatic product induced by the regional extension (e.g. large scaled strike-slip movement of the Karakorum) during the orogenic process. After researches on petrology, mineralogy, geochemistry and phase equilibrium, the three alkaline intrusions came from sources that had no plagioclase (>60km) but had garnet and rutile (>50km) in their melting residue. Therefore, the magmas were the product of low partial melting of mafic eclogite at the base of the thickened crust, and their magma sources were affected by fluids. These results suggest that the crust of the Pamir syntax had been thickened at -11 Ma, with a curst thickness over 50km. But, because crustal thickening is a cooling process, there must have some heat contribution of deep process (e.g. mantle magmatism) to Taxkorgan alkaline magmatism. A reasonable mechanism is delamination that had happened at ~11Ma in Pamir syntax. Hence, except convergent thickening and block extrusion, delamination of lithosphere is also a very important mechanism to accommodate lithospheric shortening.During continental collision, convergence induced the lithospheric shortening and thickening of the Tibetan Plateau. In this process, the crust will be fractionated: the moving down mafic rocks will be eclogitization gradually and thus have a higher density to result in regionally gravitational instability. When this instability comes to a certain limit, delamination would happen. Consequently, the lithosphere become thinning, the asthenosphere is upwelling, magmatism is playing and the mechanical properties is weakening. Then the thinning lithosphere could be thickened again by asthenosphere top descending (caused by heat elimination), magma underplating, compressing and so on. Delamination to thinning tothickening, a cycle of deep process, will happen again and again in a long period, and induce crustal thickening and uplifting in Tibetan Plateau.更多还原