【作者】 崔建军；

【导师】 刘晓春； 胡健民； 刘建民；

【作者基本信息】 中国地质科学院 ， 矿物学、岩石学、矿床学， 2009， 博士

【摘要】 桐柏杂岩地处扬子板块北缘,它是连接东秦岭和苏鲁-大别造山带的一个重要构造单元。尽管桐柏杂岩挟持于桐柏地区两条印支期高压变质带之间。但其内部却一直没有发现高压-超高压变质的迹象,长期以来被认为是一个前寒武纪的变质杂岩体。桐柏杂岩与两侧高压变质地体特殊的构造关系和复杂的变质演化历史使其构造性质和变质演化过程的研究倍受地质学界关注。本文从野外调研入手,主要从同位素年代学、变质岩石学和构造地质学方面对桐柏杂岩进行了综合研究,同时结合区域资料探讨了桐柏杂岩没有显示印支期高压变质的原因和燕山期造山的动力学机制。野外调研发现,桐柏杂岩由糜棱岩化花岗质岩体、变质岩包体和未变形花岗岩体三部分组成,其中糜棱岩化花岗质岩体是主体,约占变形岩石总量的75～80%。同位素年代学研究表明,桐柏杂岩中多数变质岩包体的原岩生成于新元古代,变质时代为印支期。少量副片麻岩的继承锆石形成于太古代,并在古-中元古代发生过变质作用。桐柏杂岩主体花岗质岩体侵位、变形以及伟晶岩脉侵位等一系列造山事件发生在139～131 Ma。在对桐柏杂岩进行野外地质调查的过程中,首次发现了含有石榴石的斜长角闪岩和片麻岩包体。对这些包体的变质作用研究发现,其主要矿物虽然受到晚期低温扩散的影响,但变质反应结构并不发育。变质作用温压估算结果显示,这些包体的峰期变质条件达到了高角闪岩相,其T=560～670℃,P=0.57～0.90 GPa。然而,由于在这些变质岩包体中尚未找到高压或超高压变质的证据,它们是印支期高压/超高压岩石的退变产物,还是只经历了中下地壳变质作用影响的中压岩石,尚需今后进一步的探索。构造研究发现,桐柏杂岩南北两条边界韧性剪切带具有相反的运动学指向,分别为大型右行和左行韧性剪切带,其形成时代为131 Ma。这两条韧性剪切带中的剪切面理分别朝SSW和NNE向倾斜,韧性剪切带及杂岩体内部发育的拉伸线理均稳定地朝SEE方向缓倾伏。而在杂岩体的东端发育低角度近南北向韧性剪切变形带,其糜棱面理朝SEE方向低角度缓倾斜,矿物拉伸线理也朝SEE方向低角度缓倾伏,运动学标志指示东侧地质体朝295°～310°的方向逆冲。桐柏杂岩顶部是一个低角度大型剪切带。太白顶及其以西的剪切方向指示顶部向西不对称剪切的运动学特征。因此,桐柏杂岩实际上被一个顺造山带的韧性剪切带所围限,这个韧性剪切带及杂岩体内部韧性变形运动学指示了一个早白垩世由东向西的低角度抬升过程,而这一过程直接导致了含榴辉岩高压变质地体沿着造山带的方向从地壳深部抬升到近地表。经区域综合构造分析,认为桐柏杂岩的组成和现今构造格局形成于燕山期陆内造山运动。主要动力学原因是扬子板块在燕山期陆内收缩挤压后发生了向东的伸展滑移。扬子板块滑移过程中带动了其北缘的造山带基底部分一起沿晓天-磨子潭断裂进行左行滑移。造山带基底在滑移过程中抬起了已经处于中-上地壳的高压变质地体。131 Ma之后,滑移主要沿房县-广济断裂进行,一直持续到84Ma。更多还原

【Abstract】 The Tongbai complex, one of important geological unites on the northern margin of the Yangtze craton, is situated at the junction between the Qinling and Dabie-Sulu orogens. Although the complex occurs in between two Indosinian high-pressure (HP) metamorphic belts in the Tongbai area, no evidence indicates that it has undergone Indosinian HP metamorphism. Thus, this complex has long been thought to be a Precambrian metamorphic terrane. Owing to the lack of petrological and geochronological data, the nature and metamorphic process of the Tongbai complex remain unclear. Therefore, the issue concerning formation process and special structural relationship between the Tongbai complex and HP metamorphic belts have a strong appeal to geologists. In this paper, we present original results of our field investigation, and geochronological, petrological and structural studies on the Tongbai complx. Coupled with the available regional data, the geodynamic mechanism of the Tongbai complx during the Indosinian to Yanshanian was discussed.The detailed investigation in the field reveals that the Tongbai complex mainly consists of mylonitic granitic bodies, undiformed granitic bodies and metamorphic enclaves. The mylonitic granites are the main part of the Tongbai complex, which forms 75-80% volume of the deformed rocks. U-Pb zircon dating indicates that the protoliths of the most metamorphic enclaves in the mylonitic granites were emplaced during the Neoproterozoic, whereas metamorphism took placed during the Indosinian. Detrital zircons from metasedimentary rocks came from Archean igneous intrusives and Paleoproterozoic metamorphic rocks, respectively. Furthermore, the emplacement and deformation of mylonitic granites and emplacement of pegmatitic dykes related to orogenic events occurred at 139-131 Ma.Garnet-bearing amphibolite and gneiss enclaves arc found at first in the Tongbai complex. The reaction textures have not been observed in such metamorphic enclaves, although some garnets have been affected by the late diffusion. Thermobarometry suggests that the peak metamorphism of the rocks reaches upper amphibolite facies conditions of 560-670℃and 0.57-0.90 GPa. Since no HP/UHP evidence has been found in the metamorphic enclaves, whether these rocks are retrograde products of the HP/UHP rocks, or are only medium-pressure rocks metamorphosed at the mid to lower crustal levels, remains poorly constrained.On the two sides of the Tongbai complex, there are two eclogite-bearing HP metamorphic belts stretching from NWW to SEE. Two ductile shear zones, which formed at 131 Ma, occur alone the southern and northern boundaries of the complex. The strike-slip directions of the shear zones are opposite; the southern boundary is a dextral ductile shear zone, and the northern boundary is a sinistral ductile one. The mylonitic foliations dip to NNE in the northern shear zone, and to SSW in the southern shear zone. In the ductile shear zones as well as the inner part of the Tongbai complex, the stretching lineations invariably dip toward SEE. On the eastern end of the complex, a south-north stretching thrust ductile shear zone occurs, in which low-angle mylonitic foliations dip toward SEE and lineations on the foliations also plunge toward SEE. These dynamic features indicate that the eastern metamorphic terrane thrusted onto the Tongbai complex with a direction of about 295°-310°. Therefore, the Tongbai complex is actually surrounded by a huge ductile shear zone streching alone the orogen. The deformation characteristics in the shear zone and the inner part of the Tongbai complex suggest a low-angle exhumation for the HP metamorphic terrane from the east to the west along the orogen during the early Cretaceous, which led to the HP metamorphic rocks being exhumed to the Earth’s surface from the crustal levels.On the basis of tectonic analysis, we proposed that the main components and the present tectonic framework of the Tongbai complex were formed during the Yanshanian intraplate orogeny. The major geodynamic mechanism that caused the intraplate orogeny is the eastward movement of the Yangtze block after the Yanshanian intraplate compressional deformation. The eastward movement of the Yangtze block resulted in the eastward extrution of the orogenic basement along the Xiaotian-Mozitan fault. At the sametime. HP metamorphic terranes that have been exhumed to the middle-upper crustal levels during the Indosinian were uplifted by the eastward movement of the orogenic basement. After 131 Ma, the eastward movement of the Yangtze block mainly occurred alone the Fangxian-Guangji fault and continued to 84 Ma.更多还原