拉萨地体南北两侧碰撞后岩浆作用的岩浆起源和岩石成因

发布时间：2018-01-16 19:01

本文关键词：拉萨地体南北两侧碰撞后岩浆作用的岩浆起源和岩石成因　出处：《中国地质大学(北京)》2016年博士论文　论文类型：学位论文

【摘要】：拉萨地体南北两侧的拉萨-羌塘弧-陆或弧-弧碰撞带和印度-欧亚陆-陆碰撞带均发育大量同碰撞到碰撞后岩浆岩,是研究碰撞后岩浆作用成因和机制、碰撞带大陆地壳生长的理想地区。已有研究主要集中在区域性年代学框架和构造背景的探讨,对这些岩浆活动的岩浆源区和岩浆作用过程还缺乏精细的刻画。本论文选取了这两条碰撞带典型的碰撞后岩浆活动为对象,对其岩浆起源和岩石成因开展了详细研究。在拉萨-羌塘碰撞带发现区域性角度不整合面之上的约90 Ma富镁火山岩,除1件样品外,多数富镁火山岩样品均不具有埃达克岩的地球化学特征,它们具有负的?Nd(t)值(?3.2~?1.7)和低的初始(87Sr/86Sr)i值(0.7054~0.7065),显示正的锆石?Hf(t)值(+5.6~+8.7)和全岩?Hf(t)值(+3.8~+7.0),指示富镁安山岩可能来自于拆沉镁铁质下地壳(包括跨越中部和北部拉萨地块的地壳底部)的部分熔融,该熔体随后经历了熔体与软流圈地幔橄榄岩的相互作用和浅部岩浆房高分异岩浆组分的改造,角闪石的分离结晶导致了安山岩向英安岩的成分转变。结合野外地质观察、同期火山岩的年代学和地球化学数据,指示卓嘎普富镁火山岩和北部拉萨地体的同期岩浆作用,很可能是拉萨-羌塘碰撞之后增厚岩石圈拆沉作用的结果。在印度-欧亚碰撞带冈底斯岩基首次发现同时侵位的约43 Ma中等程度分异和高分异的花岗岩,中等程度分异的花岗岩具有变化的分异指数(DI=84~93),低的重稀土元素(HREEs)和Y含量;高分异花岗岩样品以高的Si O2含量(75~78 wt.%)和分异指数(DI=95~97)以及更加明显的Ba、Sr、P和Ti元素的负异常为特征。这两组样品的锆石?Hf(t)值和全岩Nd Hf Pb同位素组成相似,指示两组来源于一个共同的以石榴子石作为残留矿物相的源区。中等程度分异的样品可能来自南部拉萨地体新生下地壳中的含石榴子石角闪岩(而不是榴辉岩)的部分熔融,并且混入了来自古老印度大陆和(或者)中部拉萨地体古老基底的富集组分,高分异花岗岩则是进一步分离结晶(斜长石、钾长石、黑云母、磷灰石、榍石等)的产物。结合区域研究,提出两者很可能是印度-欧亚碰撞后新特提斯洋壳板片断离的后续岩浆响应。它们低的HREEs和Y含量以及高分异花岗岩的出现均指示了冈底斯地壳在约43 Ma已经增厚了。通过对青藏高原大量已知不同成因类型花岗岩锆石微量元素的分析,发现I型花岗岩以低的Pb丰度、高的(Nb/Pb)N比值为特征,明显不同于S型花岗岩锆石,而A型花岗岩类的这些参数的变化范围介于I型和S型花岗岩类之间。这些差异对利用锆石微量元素鉴别碰撞后花岗岩的成因类型提供了可能。
[Abstract]:The Lhasa-Qiangtang arc-land or arc-arc collision zone and the India-Eurasian continental collision zone on both sides of Lhasa-Qiangtang arc-arc collision zone on both sides of Lhasa terrane have developed a large number of post-collision magmatic rocks, which is the origin and mechanism of post-collision magmatism. An ideal area for continental crustal growth in collision zones. Previous studies have focused on the regional chronological framework and tectonic background. The magmatic source region and magmatism process of these magmatic activities are not well described. In this paper, the typical post-collision magmatic activity of these two collision zones is selected as the object. The magma origin and petrogenesis are studied in detail. In the Lhasa-Qiangtang collision zone, about 90 Ma mafic volcanic rocks, except one sample, were found on the regional unconformable surface. Most mafic volcanic samples do not have the geochemical characteristics of the adakite, and they are negative? Ndt) value? 3.2? 1. 7) and a low initial value of 87 Sr / 86 Sr ~ (2 +) = 0. 7054 ~ 0. 7065, indicating positive zircon? Value (5.6 ~ 8.7) and whole rock? The HfT values (3.8 ~ 7.0) indicate that the mafic-rich andesite may have originated from partial melting of the lower crust (including the bottom of the crust across the central and northern Lhasa massif). The melt then experienced the interaction between the melt and the mantle peridotite in the asthenosphere and the transformation of the high content heterogeneous magma components in the shallow magma chamber. The separation and crystallization of amphibole resulted in the composition transformation from andesite to dolomite. Combined with field geological observations, the geochronology and geochemistry data of volcanic rocks in the same period were obtained. It indicates the contemporaneous magmatism of the Zhuogapu mafic volcanic rocks and the Lhasa terrane in the north. It is very likely that it is the result of thickening lithosphere delamination after the Lhasa-Qiangtang collision. In the India-Eurasian collision zone, the Gangdis rock base was the first to be emplaced at about 43 Ma with moderate differentiation and high differentiation. The moderately differentiated granites have a variable differentiation index (DI _ (84) ~ (93)), low heavy REE _ (es) and Y contents; The samples of high grade isomorphic granites are characterized by high Sio _ 2 content (75 ~ 78wt.) and differentiation index (~ (95 ~ (97))), as well as more obvious Ba~ (+) Sr. The negative anomalies of P and Ti elements are characterized by zircon from these two groups of samples. The values of HF ~ (t) are similar to the isotopic compositions of ND ~ (+) HF ~ (+) Pb in the whole rock. The results indicate that the two groups are derived from a common source with pomegranate as the residual mineral facies. The moderately differentiated samples may come from the eclogite amphibolite (rather than eclogite) from the Cenozoic lower crust of the Lhasa terrane in southern Lhasa. Partial melting. And mixed with the enrichment components from the ancient basement of the ancient Indian continent and / or central Lhasa terrane, the high-grade isomorphic granite is further separated and crystallized (plagioclase, potassium feldspar, biotite, apatite). The product of sphene, etc. It is suggested that both of them may be the subsequent magmatic response to the partial separation of the NeoTethys oceanic crust after the India-Eurasian collision. Their low HREEs and Y contents and the presence of high-grade isomorphic granites all indicate that the Gangdis crust is located in about 4. 4% of the earth's crust. 3. Ma has been thickened. Based on the analysis of zircon trace elements of a large number of known granites of different genetic types in the Qinghai-Xizang Plateau. It is found that the I-type granite is characterized by low Pb abundance and high NB / PbN ratio, which is obviously different from the zircon of S-type granite. The variation range of these parameters of A-type granitoids is between I-type and S-type granitoids. These differences make it possible to identify the genetic types of post-collision granites by zircon trace elements.
【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】：2016
【分类号】：P588.1

【参考文献】