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Formation of the Tibetan Metamarphic Domes

西藏变质穹顶的形成

基本信息

批准号：
13573011
负责人：
WALLIS R・Simon
金额：
$ 8.58万
依托单位：
Nagoya University
依托单位国家：
日本
项目类别：
Grant-in-Aid for Scientific Research (B)
财政年份：
2001
资助国家：
日本
起止时间：
2001 至 2004
项目状态：
已结题

项目摘要

A series of metamorphic domes that occur in the boundary domain between the Himalayas and Tibet expose rocks originally formed in the mid crust before being brought to the Earth's surface and they offer a window into processes occurring in otherwise inaccessible regions. These domes potentially contain important information about how the structures of the Himalayas and the deep crust of Tibet are linked. However, they have been very little studied. We aimed to help fill this gap in our knowledge by carrying out field based and laboratory studies of the dome regions. We also carried out a number of related studies important for understanding the processes occurring deep in the crust in other related regions. The most important results directly related to the southern Tibet metamorphic domes are as follows.Definition of Domes The metamorphic domes are broadly elliptical domains of metamorphic and igneous rocks that occur in southern Tibet. The domes have a granite core surrounded by roug … More hly concentric Barrovian type metamorphism. The main deformation can be divided into two stages, D1 and D2. D1 is associated with crustal thickening and top-to-the-south sense of shear. D2 is associated with crustal thinning and top-to-the-north sense of shear. D2 can be correlated with the major normal fault the Southern Tibetan Detachment(STD) that lies along the northern margin of the Himalayas. The spatial association between the granites and the deformation and metamorphism suggests these features are all related.Age of the Domes The metamorphic domes are generally thought to be 20-10 Ma. In contrast, the central granites are considered to be around 500 Ma and unrelated to the Himalayan orogeny. This preconception has prevented consideration of models that link granite intrusion with the deformation and metamorphism of the domes. However, in the Malashan dome we show the granitic core formed around 18 Ma. This granite, therefore, intruded during the Himalayan orogen. The large number of similarities between the domes suggests that all of them contain similar young granites at their cores. Our proposal means that it is now possible to examine models that link granite intrusion with the formation of the domes.Dome Formation and Himalayan Tectonics We suggest the domes formed directly as a result of intrusion of granitic magma. Associated contact metamorphism produced the concentric metamorphic zones around the central grantie bodies. In addition, the thermal weakening of the rocks caused a change from crustal thickening during D1 to crustal thinning related to D2. The driving force for the extensional deformation in a compressive tectonic setting is the generated by the action of gravity on the surface slope of the Himalayan front. Less

出现在喜马拉雅山和西藏交界处的一系列变质穹顶暴露了最初在中地壳形成的岩石，然后被带到地球表面，它们提供了一个窗口，可以看到在其他难以接近的地区发生的过程。这些圆顶可能包含有关喜马拉雅山脉和西藏深部地壳结构如何联系的重要信息。然而，人们对它们的研究很少。我们的目标是通过对穹顶地区进行实地和实验室研究，帮助填补我们知识上的这一空白。我们还进行了一些相关研究，这些研究对于了解其他相关区域地壳深处发生的过程很重要。与藏南变质穹隆直接相关的最重要结果如下：穹隆的定义变质穹隆是产于藏南的变质岩和火成岩的宽阔椭圆形区域。圆顶有一个花岗岩核心，周围环绕着粗糙的…更多的HLY同心巴罗型变质作用。主要变形可分为两个阶段，即D1期和D2期。D_1与地壳增厚和自上而下的剪切感有关。D2与地壳减薄和自上而北的剪切感有关。D2可以与位于喜马拉雅北缘的主要正断层藏南支队(STD)进行对比。花岗岩与变形变质作用的空间组合表明，这些特征都是相关的，穹隆和变质穹隆的年龄通常被认为是20-10 Ma。相反，中央花岗岩的年龄约为500 Ma，与喜马拉雅造山作用无关。这种先入为主的观念阻碍了将花岗岩侵入与穹顶的变形和变质联系起来的模型的考虑。然而，在马拉山穹隆中，我们看到花岗岩核形成于18 Ma左右。因此，这块花岗岩是在喜马拉雅造山带侵入的。穹顶之间的大量相似之处表明，它们的核心都含有相似的年轻花岗岩。我们的建议意味着现在有可能检验花岗岩侵入与国内穹顶形成和喜马拉雅构造形成之间的联系模式。我们认为穹顶直接形成于花岗岩岩浆侵入的结果。伴生接触变质作用在中央花岗岩体周形成同心变质带。此外，岩石的热弱化导致了从D1期的地壳增厚到D2期的地壳减薄的变化。挤压构造背景下伸展变形的驱动力是重力对喜马拉雅前缘地表斜坡的作用。较少