祁连造山带高压—超高压变质岩地球化学研究

发布时间：2018-04-25 21:31

  本文选题：柴北缘 + 北祁连　； 参考：《中国科学技术大学》2016年博士论文

【摘要】：对板块俯冲变质带中榴辉岩的研究可以揭示高压／超高压变质作用和俯冲之前区域构造的信息。榴辉岩可以在大洋俯冲带或者大陆俯冲带形成,两类榴辉岩都具有不同的来源和复杂的地球化学组成。由于陆壳俯冲是由洋壳俯冲牵引的,在大陆俯冲变质带洋壳榴辉岩可能和陆壳岩石共存,记录了从洋壳俯冲到陆壳俯冲的构造转换。位于青藏高原东北缘的祁连造山带是一个由其北部的北祁连洋壳高压变质带和南部的柴北缘陆壳超高压变质带组成的复合型造山带。本文对北祁连和柴北缘榴辉岩分别进行了全岩地球化学和锆石学研究,结果揭示了洋壳和陆壳榴辉岩原岩的多样性,并利用地球化学手段区分了柴北缘变质带的洋壳和陆壳榴辉岩,同时也对大陆超高压变质带从洋壳俯冲到陆壳俯冲的构造演化提供了制约。另外,对柴北缘锡铁山地区长英质脉体及其围岩的岩石学和地球化学研究则系统探讨了不同岩性陆壳岩石深熔作用的熔融结构、时限、机制和地球化学演化等。对北祁连低温高压洋壳榴辉岩和基性蓝片岩进行了全岩地球化学、单矿物O同位素和锆石学的综合研究。结果表明,这些洋壳榴辉岩和蓝片岩的原岩是弧后盆地基性岩,而非通常认为的洋中脊玄武岩(MORB)型基性岩。这些变基性岩具有不均一的主量和微量元素组成,在微量元素图解上它们从类似于岛弧玄武岩(IAB)、MORB变化到类似于洋岛玄武岩(0IB)。这种变化多样的成分特征和弧后盆地中不同阶段发育的玄武岩类似。变基性岩成分的差别是从原岩继承来的,全岩Nd同位素组成表明,弧后盆地地幔源区中存在不同比例沉积物衍生熔体。矿物之间的O同位素分馏多处于不平衡状态。石榴石的O同位素组成表明,全岩的δ18O值高于或低于正常地幔值,这是继承自不同温度下海水热液蚀变修正的弧后盆地基性岩。残留的岩浆锆石保留振荡环带,具有高的Th和U含量、高的Th/U比值,陡峭的重稀土配分形式并伴有显著的Eu负异常,U-Pb定年揭示变基性岩的原岩年龄为496-486Ma。相比于残留岩浆锆石,变质锆石含有绿辉石、金红石和多硅白云母等矿物包裹体,具有低的Th、U和HREE等含量,低的Th/U比值,U-Pb定年给出榴辉岩相变质年龄为463±10 Ma。锆石具有亏损的Hf同位素组成,其Hf模式年龄稍微或显著老于原岩年龄,表明变基性岩原岩源区中有地壳组分加入。多数锆石的δ18O值都不同于正常地幔值,并且不同样品中锆石δδ180值高低与石榴石δ18O值保持一致,表明在变质过程中流体活动是内部缓冲的。总之,具有不同成分的变质基性岩是来自早古生代弧后盆地的不同玄武质岩石,弧后盆地在形成后转变为洋壳俯冲带并发生了榴辉岩相变质作用。为了限定柴北缘榴辉岩的原岩性质和变质演化,我们对其进行了详细的全岩主微量元素、Sr-Nd同位素、矿物O同位素,以及锆石U-Pb年龄、微量元素、矿物包裹体和O-Hf同位素研究。结果被用来区分洋壳和陆壳榴辉岩、制约造山带从洋壳俯冲到陆壳俯冲的构造转换。暗色CL锆石区域具有高的Th和U等微量元素含量,高的Th/U比值,陡峭的重稀土配分形式并伴有显著的Eu负异常,表明它们是经历过不同程度变质改造的残留原岩岩浆锆石。对这些残留原岩锆石的分析揭示了两类榴辉岩：第一类榴辉岩具有～830 Ma的原岩年龄和850--1100 Ma的Hf模式年龄,而第二类榴辉岩具有-500 Ma的原岩年龄和500～650 Ma的Hf模式年龄。占主体的第一类榴辉岩具有类似于来自不同地幔源区的大陆裂谷玄武岩的地球化学特征,如过渡型微量元素组成、大体正的εNd(t)值和正常地幔的δ18O值。这类榴辉岩是由新元古代罗迪尼亚超大陆裂解过程中产生的基性岩变质而来的陆壳榴辉岩。第二类榴辉岩仅有少量分布.它们具有从类似于IAB变化到类似于MORB和OIB的微量元素配分形式,正的εNN(t)值和低的δ18O值。这类榴辉岩是由经历过海底高温热液蚀变的弧后盆地辉长岩转变而来的洋壳榴辉岩。相比于残留原岩锆石,浅色CL锆石区域含有石榴石、绿辉石和金红石等矿物包裹体,具有低的Th和U等微量元素含量,低的Th/U比值,平坦的重稀土配分形式并没有Eu的负异常。因此,这些锆石是在榴辉岩相变质条件下生长的变质锆石;然而不论采样地点和原岩性质,这些变质锆石给出了一致的433-440 Ma榴辉岩相变质年龄。对两个片麻岩中锆石的分析也得到了427～439 Ma类似的榴辉岩相变质年龄,而且其中一个片麻岩的锆石中还发现了柯石英包裹体。洋壳榴辉岩和陆壳榴辉岩及片麻岩相同的榴辉岩相变质年龄表明,洋壳榴辉岩可能原先位于洋陆转换带附近,因此与陆壳岩石具有相同的俯冲带变质演化。总之,洋壳榴辉岩和陆壳榴辉岩的共存记录了柴北缘超高压变质带从洋壳俯冲到陆壳俯冲的构造转换。另外,柴北缘和北祁连洋壳变基性岩具有相似的原岩年龄和弧后盆地基性岩化学成分,表明北祁连和柴北缘可能代表了一个从洋壳俯冲演化到陆壳俯冲的复合造山带,它们与祁连地块一起构成祁连造山带。对柴北缘锡铁山地区的长英质脉体及其围岩片麻岩和变基性岩进行了综合的岩石学、年代学和地球化学研究,结果揭示了深俯冲陆壳折返时的部分熔融。岩相学证据表明,片麻岩和变基性岩都发生了部分熔融。混合岩化片麻岩中保留了熔体存在的显微结构,如具有不同轮廓的花岗质矿物集合体,矿物边界具有小的二面角的矿物颗粒以及具有岩浆习性的长石颗粒等。片麻岩和退变榴辉岩中的长英质脉体都显示良好的熔体结晶结构,表明它们是深熔熔体结晶的产物。脉体和围岩的Sr-Nd同位素分析表明,片麻岩中脉体结晶自片麻岩深熔产生的熔体,而变基性岩中脉体的物质来源既有变基性岩的贡献也有片麻岩的贡献。在An-Ab-Or图解上,片麻岩中的脉体具有与类似条件下产生的实验熔体相似的化学成分。另外,片麻岩中的脉体具有与片麻岩大体平行的微量元素配分型式,但是它们的微量元素含量较片麻岩低并且具有轻微的Sr和Eu的正异常。片麻岩中脉体的地球化学特征受到部分熔融过程中特定矿物分解和残留的控制。变基性岩中的长英质脉体富集石英或者斜长石,几乎不含钾长石。它们具有很低的微量元素含量,而且富长石的脉体具有显著的Eu和Sr的正异常。考虑到一些脉体具有堆晶结构,推测这些脉体是从不同来源的深熔熔体中结晶的产物。但是,所有的脉体都具有一些共同的微量元素配分型式,如相对富集LILE和LREE并相对亏损HFSE和HREE。脉体和混合岩化片麻岩中的深熔锆石给出了～420 Ma的年龄,小于附近的榴辉岩相变质年龄,因此部分熔融发生在折返过程中。结合岩相学观察、前人P-T轨迹和实验岩石学制约,推断片麻岩经历了白云母脱水熔融而变基性岩的部分熔融是在外来流体参与下进行的。不同成分的超高压变质岩在折返初期都经历了部分熔融,产生了具有不同微量元素特征但具有相似配分型式的深熔熔体。
[Abstract]:The study of Eclogite in the plate subduction metamorphic belt can reveal the information of high pressure / ultrahigh pressure metamorphism and regional tectonics before subduction. Eclogite can be formed in the oceanic subduction zone or the continental subduction zone. The two types of eclogites have different sources and complex geochemical formation. In the continental subduction metamorphic belt, the eclogite may coexist with the continental crust rocks, and the tectonic transition from subduction to continental subduction is recorded. The Qilian orogenic belt, located in the northeastern margin of the Qinghai Tibet Plateau, is a complex orogenic belt composed of the northern Qilian oceanic crust high pressure metamorphic belt and the Southern Chai Bei marginal continental crust ultrahigh pressure metamorphic belt. In this paper, the total rock geochemistry and zircon study of the Eclogite in northern Qilian and the northern Qaidam area were carried out respectively. The results revealed the diversity of the oceanic and crustal eclogites, and used geochemical methods to distinguish the oceanic crust and eclogite from the northern margin of the Qaidam metamorphic belt and the subduction of the continental ultrahigh pressure metamorphic belt from the oceanic crust to the continental subduction. In addition, the petrology and geochemistry of the long angled veins and its surrounding rock in the tin Tieshan area of the northern margin of the Qaidam area were studied, and the melting structure, time limit, mechanism and geochemical evolution of different lithologic continental crust rocks were discussed. The eclogite and the basic blue schist of the low temperature and high pressure oceanic crust in North Qilian were carried out. A comprehensive study of all rock geochemistry, single mineral O isotopes and zirconium studies. The results show that the original rocks of these oceanic eclogites and blue schist rocks are the base rocks of the back arc basin, but not the commonly considered basalt (MORB) base rocks of the mid ocean ridge. These metamorphic rocks are composed of inhomogeneous principal and trace elements, and they are diagrammed in trace elements. From similar to island arc basalts (IAB), MORB changes to similar Yu Yang island basalt (0IB). This variety of components is similar to the basalts developed at different stages in the back arc basin. The variation of basic rock components is inherited from the original rock, the whole rock Nd isotope composition table shows that there are different proportions in the mantle source area of the back arc basin. The O isotopic fractionation between minerals is mostly in unbalanced state. The O isotopic composition of garnet indicates that the delta 18O value of the whole rock is higher or lower than the normal mantle value. This is the base rock of the back arc basin that inherited the alteration of the hydrothermal alteration under different temperatures. The residual magmatic zircon retained the oscillating belt, which has a high Th and U content. High Th/U ratio, steep rare-earth distribution form and significant Eu negative anomaly. U-Pb dating shows that the original rock age of the metamorphic rocks is 496-486Ma. compared to the residual magma zircon. The metamorphic zircon contains mineral inclusions such as omroxene, Jin Hongshi and polysilicon muscovite, with low Th, U and HREE content, low Th/U ratio, U-Pb dating. The metamorphic age of the eclogite facies is 463 + 10 Ma. zircon with a loss of Hf isotopes, and its Hf mode age is slightly or significantly older than the original rock age, indicating that there are crustal components in the basic lithic source area. The delta 18O values of most of the zircon are different from that of the normal mantle, and the values of the delta delta 180 of zircon in different samples and the garnet Delta 18O The value remains consistent, indicating that fluid activity is internal buffer during the process of metamorphism. In a word, the metamorphic base rocks with different components are different basaltic rocks from the early Paleozoic back arc basin. After formation, the back arc basin changed into the oceanic subduction zone and the eclogite phase transformation of the eclogite occurred. In order to limit the original lithology of the Eclogite in the northern margin of the Qaidam Basin Qualitative and metamorphic evolution, we have carried out a detailed total rock main trace elements, Sr-Nd isotopes, mineral O isotopes, and zircon U-Pb age, trace elements, mineral inclusions and O-Hf isotopes. The results were used to distinguish Oceanic and crustal eclogites from the tectonic transformation of the orogenic belt from the oceanic subduction to the continental subduction. The dark CL zircon The region has a high content of trace elements such as Th and U, high Th/U ratio, steep rare-earth distribution and significant negative Eu abnormality, indicating that they are remnant primary magmatic zircon from varying degrees of metamorphism. The analysis of these residual zircon reveals two types of eclogite: the first type of eclogite has 830 Ma. The age of the original rock and the Hf pattern age of 850--1100 Ma, and the second type eclogite with -500 Ma of the original rock age and the Hf mode age of 500~650 Ma. The first type of eclogite, which is the main body, is similar to the geochemical characteristics of the continental rift Xuan Wuyan from different mantle sources, such as the composition of the transitional trace elements, and the gross positive e Nd (T) values. The delta 18O value of the normal mantle. This type of eclogite is a terrestrial eclogite from the metamorphic base rocks of the Neoproterozoic ultralonian supercontinent. The second types of eclogites have only a small number of eclogites. They have the distribution form of the trace elements similar to IAB to MORB and OIB, the positive value of the NN (T) and the low delta 18O value. The eclogite is the oceanic eclogite that has changed from the gabbro in the back arc basin of the seabed high temperature hydrothermal alteration. Compared with the residual zircon, the light color CL zircon region contains the mineral inclusions of garnet, omyroxene and Jin Hongshi, with low content of trace elements such as Th and U, low Th/U ratio, and flat rare-earth distribution form. There is no negative anomaly of Eu. Therefore, these zircons are metamorphic zircons grown under the metamorphic condition of the eclogite facies. However, the metamorphic zircon gives the consistent 433-440 Ma eclogite facies metamorphic age regardless of the sampling sites and the original rock properties. The analysis of zircon in two gneiss has also obtained 427~439 Ma similar eclogite phase transition years. The COO quartz inclusions are also found in one of the gneiss zircon. The same eclogite metamorphic age of the pomegranate Hui Yan and Lu Keliu Hui Yan and the gneiss indicates that the pomegranate Hui Yan may have been originally located near the oceanic transition zone, so that the continental crust rocks have the same subduction zone metamorphic evolution. The coexistence of the eclogite records the tectonic transition from subduction to continental subduction in the UHP metamorphic belt of the northern margin of the Qaidam Basin. In addition, the metamorphic base rocks of the northern margin of the Qaidam and Northern Qilian oceanic crust have similar proto rock age and the chemical composition of the base rocks in the back arc basin, indicating that the northern Qilian and the Qaidam basin may represent a subduction from the oceanic crust to Lu Kefu. The compound orogenic belt, which is formed by the Qilian massif, constitutes the Qilian orogenic belt together with the geo petrology, geochronology and geochemistry of the long angled veins and its surrounding rock gneiss and metamorphic rocks in the tin Tieshan area of the northern Qaidam border. The results reveal the partial melting of the reentry of the deep subducted continental crust. Petrographic evidence indicates that gneiss Partial melting has occurred in the metamorphic rocks. The microstructures of the melts are retained in the mixed rock gneiss, such as the granitic mineral aggregates with different outlines, mineral particles with small dihedral angles and feldspar particles with magma habits, and the feldspar veins in gneiss and degenerative eclogites all show The good crystalline structure of the melt indicates that they are the products of the crystallization of the deep melt melt. The Sr-Nd isotope analysis of the veins and surrounding rocks indicates that the veins in the gneiss are crystallized from the melts produced by the gneiss, and the material source of the veins in the metamorphic rocks has both the contribution of the metamorphic rock and the contribution of the gneiss. In the An-Ab-Or diagram, the gneiss The vein body is similar to the experimental melts produced under similar conditions. In addition, the veins in the gneiss have a distribution pattern of trace elements that are generally parallel to the gneiss, but their trace elements are lower than the gneiss and have a slight abnormal Sr and Eu. The geochemical characteristics of the veins in the gneiss are subject to the Department. The control of the decomposition and residue of specific minerals during the melting process. The feldspar veins in the metamorphic rocks are enriched with quartz or plagioclase, almost without potassium feldspar. They have a very low trace element content, and the rich feldspar veins have significant positive anomalies of Eu and Sr. The crystallization of the melts from different sources. However, all the veins have some common trace element distribution patterns, such as the relative enrichment of LILE and LREE, and the relative loss of HFSE and HREE. veins and the deep melting zircons in the mixed lithic gneiss. The age of ~ 420 Ma is less than the metamorphic age of the surrounding eclogite facies. The partial melting occurred during the process of exhumation. In combination with petrographic observation, the predecessors' P-T trajectory and experimental petrology have concluded that the gneiss experienced the dehydration and melting of the muscovite and the partial melting of the metamorphic rocks was carried out under the participation of foreign fluids. Deep melt melts with similar trace elements but similar distribution patterns.

【学位授予单位】：中国科学技术大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：P588.3
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本文编号：1803044