西藏拉萨地体南缘中生代火山岩与碎屑沉积岩的年代学、地球化学及构造意义

发布时间：2018-06-12 15:32

本文选题：年代学 + 地球化学　；参考：《中国地质大学(北京)》2017年博士论文

【摘要】：拉萨地体是欧亚大陆最南缘的构造单元,是青藏高原的重要组成部分之一。研究拉萨地体前碰撞的构造演化,不仅可以更好理解印度-欧亚大陆的碰撞过程,还可以对青藏高原成矿作用提供重要的基础资料。然而,对于该演化历史,学术界一直存在争议;对拉萨地体南缘新生地壳的增生机制亦缺乏足够的认识。在此背景之下,本文对拉萨地体南缘早侏罗世火山岩和白垩纪碎屑沉积岩进行了全面的年代学和地球化学研究,意在为拉萨地体在中生代的构造和地壳生长机制提供新的约束。拉萨地体南缘早侏罗世火山岩主要分布于叶巴组和桑日群之中。叶巴组火山岩主要包括同期喷发的玄武岩、玄武安山岩、英安岩和流纹岩,构成双峰式火山岩组合,喷发年龄为183-174Ma。近同期喷发的桑日群火山岩包括玄武岩、玄武安山岩和安山岩。基性火山岩(包括叶巴组和桑日群的玄武岩、玄武安山岩)具有富集大离子亲石元素、轻稀土元素,亏损高场强元素,和亏损的Sr-Nd-Hf同位素特征,暗示了岩石源于俯冲物质交代的地幔楔部分熔融作用。中酸性火山岩(叶巴组英安岩、流纹岩,桑日群安山岩)与基性火山岩之间无成因联系,其中酸性火山岩为新生下地壳与变质硬砂岩混合源区在高温低压条件下部分熔融成因;中性火山岩具有镁安山岩的特征,是板片熔体与上覆地幔楔橄榄岩的反应的产物。综合已有资料对比结果显示拉萨地体南缘早侏罗世玄武岩应是新特提斯洋北向俯冲体系构造背景的弧后岩浆作用产物。拉萨地体南缘白垩系沉积地层主要形成于大陆边缘弧的弧后盆地环境之中。下白垩统楚木龙组的沉积时限约为121-112Ma,物源多来自中北部拉萨地体物质的再循环;上白垩统设兴组沉积时限约为88-70Ma,物源多数来自拉萨地体南缘弧岩浆岩的风化剥蚀。碎屑沉积岩的岩石地球化学数据分析再现了白垩纪时期拉萨地体南缘的构造环境,早白垩世拉萨地体总体呈现北高南低的低缓地势,气候湿热;晚白垩世地体南缘发生了快速的地壳隆升和剥蚀。结合区域资料,本文首次提出了拉萨地体南缘从三叠纪新特提斯洋开始俯冲至古近纪印度-欧亚大陆碰撞之间经历了多期“弧盆系统”的构造演化和拉萨地体南缘地壳生长的“弧后增生”模型。拉萨地体南缘在三叠纪发育基本无酸性岩的不成熟岛弧体系,到侏罗纪弧盆系统有中酸性岩浆岩出现,直至白垩纪岩浆活动大爆发发育各种类型岩浆岩。在地壳伸展阶段,幔源岩浆不断注入弧后区域并引发地壳的部分熔融,形成普遍具有高温低压成因的酸性岩类;随后的挤压阶段新生组分被深埋入地下,免受俯冲带侵蚀;而俯冲板片的后撤又引发了新一期的弧后伸展作用。在多阶段伸展、挤压、伸展的构造体制转换下,新生地壳组分在弧后区域被不断增生至拉萨地体南缘,最终的新特提斯洋盆闭合、印度欧亚大陆碰撞,使得这些新生组分得以保留至今。
[Abstract]:The Lhasa ground body is the southernmost tectonic unit of the Eurasian continent and one of the important components of the Qinghai Tibet Plateau. To study the tectonic evolution of the pre collision in Lhasa, it is not only better to understand the collisions between India and Eurasia, but also provide important basic data for the mineralization of the Qinghai Tibet plateau. There has been a lot of controversy, and there is no sufficient understanding of the proliferation mechanism of the new crust in the southern margin of Lhasa. Under this background, this paper carried out a comprehensive chronological and geochemical study of the early Jurassic and Cretaceous sedimentary rocks in the southern edge of the Lhasa geo body, which is intended to be the Mesozoic tectonic and crustal growth mechanism of the Lhasa geo body. The early Jurassic volcanic rocks in the southern margin of the Lhasa terrain are mainly distributed in the Ye Ba group and the sangday group. The volcanic rocks of the leaf bar group mainly include basalt, basaltic andesite, angianite and rhyolite, which constitute the Shuangfeng type volcanic rock assemblage, and the eruption age of the volcanic rocks of the sangsun group, which is 183-174Ma. near the same period of eruption, includes basalt. Rock, basaltic andesite and andesite. Basic volcanic rocks (including basalt and basaltic andesite of the leaf bar group and Sansun group, basaltic andesite) are rich in large ion stone elements, light rare earth elements, loss of high field and strong elements, and loss of Sr-Nd-Hf isotopes, suggesting that the rocks originated from the partial melting of the mantle wedge in the subduction of the subduction material. There is no genetic connection between the rock (EANA group, rhyolite, andesite) and basic volcanic rocks, and the acid volcanic rocks are the cause of partial melting of the nascent lower crust and the metamorphic hard sandstone under the condition of high temperature and low pressure, and the neutral volcanic rocks are characterized by magnesium andesite, which is the reaction between the sheet melt and the overlying mantle wedge peridotite. The results of the comprehensive data show that the early Jurassic basalt in the southern margin of the Lhasa terrain should be the product of the post arc magmatism in the tectonic setting of the North subduction system of the new Tethys ocean. The Cretaceous sedimentary strata in the southern margin of the Lhasa terrestrial body are mainly formed in the back arc basin environment of the continental margin arc. The limit is about 121-112Ma, and the source is mostly from the recirculation of the Lhasa ground substance in the middle and northern part of the middle part of the upper Cretaceous. The sedimentary time of the upper Cretaceous setting up group is about 88-70Ma. Most of the source of the material comes from the weathering and erosion of the arc magma of the southern margin of the Lhasa ground. The analysis of the rock geochemical data of the clastic sedimentary rocks reproduces the tectonic setting of the southern margin of the Cretaceous period of Lhasa, early white The Lhasa ground body in the Cretaceous generally presents low North High South low low terrain, the climate is damp and hot, the late Cretaceous Earth Body south margin of rapid crustal uplift and denudation. Combined with regional data, this paper first proposed that the southern margin of the Lhasa ground body began to subduction from the Triassic new Tethys ocean to the Paleogene India Eurasian collision. The tectonic evolution of the system and the "post arc proliferation" model of the crustal growth in the southern margin of Lhasa, the immature island arc system of the basic acidity of the Triassic in the southern margin of the Lhasa ground body, the emergence of intermediate acid magmatic rocks in the Jurassic arc basin, and the large eruption and development of various types of magmatic rocks in the Cretaceous magmatism. The mantle derived magma continuously injected into the post arc region and triggered partial melting of the crust, forming an acid rock generally having high temperature and low pressure. The subsequent extrusion stage was deeply buried under the subduction zone from the subduction zone, while the withdrawal of subduction plates triggered a new period of post arc extension. Under the transformation of structural system, the new crustal components in the back arc region are constantly proliferated to the southern edge of the Lhasa earth body, and the final new Tethys ocean basin is closed and the India Eurasian continent collide, which makes these new groups so far as retained.
【学位授予单位】：中国地质大学(北京)
【学位级别】：博士
【学位授予年份】：2017
【分类号】：P548;P534.5

【相似文献】