关于大别山榴辉岩相变质作用

大别山产出的榴辉岩相岩石榴橄榄、榴辉岩,榴云片岩,榴辉片麻岩,榴玉英岩和榴辉大理岩等不同系列,它们均分布于花岗质片麻岩中。矿物共生序列研究表明,榴辉岩相岩石经历了从绿帘角闪岩相,柯石英榴辉岩相,角闪榴辉岩相,绿帘角闪岩相到绿片岩相的演化过程。     

大别山超高压榴辉岩带榴辉岩的特征和变质作用

大别山超高压柯石英榴辉岩带中多处产出含柯石英榴辉岩,岩石中广泛分布柯石英假象。它们产在片麻岩和大理岩中,并有不同的特征矿物组合。与鄂北等地高压型榴辉岩比较,超高压(柯石英)榴辉岩的绿辉石富硬玉组分,而石榴石为一般的富铁铝榴石贫镁铝榴石的石榴石。钙质角闪石和钠钙质冻蓝闪石是超高压(柯石英)榴辉岩中主要的次生角闪石类型。榴辉岩的原岩是陆内拉张过程中形成的广义拉斑玄武岩,它们在中朝一扬子大陆板块汇聚造山的俯冲大地构造背景中,发生渐进变质。变质时地热增温率极低。造山运动后期,榴辉岩随构造运动抬升又经历两阶段迭加变质。     

大别山榴辉岩的退变质过程与流体作用

根据矿物成分及其结构演变,将大别山榴辉岩的退变质过程分为三个阶段:贫流体阶段、弱流体阶段和富流体阶段。贫流体阶段发生于榴辉岩相环境中,其主要作用是柯石英-石英和文石- 方解石等同质多象转变、石榴子石和绿辉石等的重结晶,以及绿辉石中硬玉和钠长石的固溶体出溶等。弱流体阶段发生于榴辉岩相退变质的晚期,含水矿物闪石、绿帘石和云母,以及钠长石等低压矿物大量形成之前,其标志是蓝晶石变斑晶和金红石脉的形成,以及浸染状金红石的富集成矿。富流体阶段始于低级角闪岩相退变质环境,并可能一直持续到近地表处。该阶段以出现大量含水和挥发份的矿物(如闪石、绿帘石、多硅白云母、钠云母、黑云母、磷灰石和碳酸盐等)为特征。围绕石榴子石和绿辉石的闪石次变边、闪     

大别山东部榴辉岩同位素体系和变质作用的关系

大别山东部榴辉岩同位素地质年代学研究反映出三方面的问题:(1)Sm-Nd两点等时线的不确定性;(2)部分高压-超高压变质年龄实际是片麻岩的分析结果;(3)首批发表的"C型榴辉岩"Sm-Nd年龄,测定岩石不是榴辉岩.大别山东部榴辉岩年龄反映的是加里东期高压-超高压变质的年代学信息.进一步研究应着重于榴辉岩锆石成因及Sm-Nd体系适用性.岩相学是研究榴辉岩锆石成因的重要方法.     

大别山朱家冲高压榴辉岩变质演化特征分析

研究表明大别山朱家冲榴辉岩属于典型的高压榴辉岩,其具有复杂的变质演化历史。根据岩相学、矿物成份化学和热力学分析表明,该类榴辉岩记录了6期变质过程,具有“顺时针”的P?T演化轨迹。由Ⅰ?阶段、Ⅱ?阶段至Ⅲ?阶段显示了近于等温增压进变质特征,并在 Ⅲ?阶段压力值最高达到P＝2.53 GPa; Ⅳ?阶段和Ⅴ?阶段则表现为增温降压退变过程,温度在 Ⅴ?阶段最高达到T＝627 ℃; 到Ⅵ?阶段则为降温降压过程。该过程暗示了高压板片在俯冲和折返过程中可能处于一个非匀速的状态。推测朱家冲榴辉岩后期增温退变的过程可能是源于受扰动的地温线恢复、超高压榴辉岩退变过程的“散热”以及该类榴辉岩对温度变化极为敏感所致。     

大别山榴辉岩退变质多硅白云母及地

大别山榴辉岩中的多硅白云母可以分成三个世代形成于榴辉岩相进变质阶段的原生Phe1、榴辉岩相退变质晚阶段的Phe2和角闪岩相至绿片岩相退变质阶段的Phe3.电子探针测得,超高压下形成的Phe1,其Si和Fe2++Mg值出乎意料地低于低压下形成的Phe3,这与多硅白云母地压计的基本原理,即多硅白云母的Si和Fe2++Mg含量与其形成压力成正比这一原理产生了矛盾.产生这种矛盾的可能原因是,多硅白云母的成分除了取决于压力因素以外,还取决于主岩的铝-硅比值、Fe2++Mg含量、氧逸度的变化、流体的存在等因素.因此,利用多硅白云母的硅含量来探讨退变质过程压力时必须加倍小心.     

安徽省大别山东段江岭地区榴辉岩及其围岩的变质作用研究

安徽省大别山东段江岭地区的南大别碰撞杂岩主要由榴辉岩、云母片岩－变粒岩及片麻岩系组成,这三套岩石具有不同的变质演化阶段和Ｐ－Ｔ轨迹：榴辉岩有５个变质阶段,早期出现超高压型、高压型、晚期为中压型;云母片岩－变粒岩有４个变质阶段,早期出现高压型,晚期为中压型;片麻岩系仅有一期中压绿帘角闪岩相变质。三套岩石变质历史的差异,反映其所经历的构造过程不同,三者在构造作用下混杂到一起,构成碰撞混杂岩。     

大别山地区柯石英榴辉岩变质作用的P-T-t轨迹及其地球动力学意义

本文对大别山地区的柯石英榴辉岩进行了详细全面的岩相学和矿物化学研究,确定了柯石英榴辉岩的变质条件,识别出了榴辉岩化前的矿物包体及其角闪岩化、绿帘角闪岩化和绿片岩化3期退变质作用,进而再塑了一个呈顺时针方向旋转的复杂P-T-t轨迹。由此讨论了柯石英榴辉岩的形成机制和地球动力学意义。     

Transformation of garnet epidote amphibolite to eclogite, western Dabie Mountains, China

Omphacite and garnet coronas around amphibole occur in amphibolites in the Hong'an area, western Dabie Mountains, China. These amphibolites consist of an epidote–amphibolite facies assemblage of amphibole, garnet, albite, clinozoisite, paragonite, ilmenite and quartz, which is incompletely overprinted by an eclogite facies assemblage of garnet, omphacite and rutile. Coronas around amphibole can be divided into three types: an omphacite corona; a garnet–omphacite–rutile corona; and, a garnet–omphacite corona with less rutile. Chemographic analysis for local reaction domains in combination with petrographical observations show that reactions Amp + Ab + Pg = Omp +Czo + Qtz + H₂O, and Amp + Ab = Omp ± Czo + Qtz + H₂O may lead to the development of omphacite coronas. The garnet–omphacite–rutile corona was formed from the reaction Amp + Ab + Czo + Ilm ± Qtz = Omp + Grt + Rt + H₂O. In garnet–omphacite coronas, the garnet corona grew during an early stage of epidote amphibolite facies metamorphism, whereas omphacite probably formed by the reactions forming the omphacite corona during the eclogite facies stage. It is estimated that these reactions occurred at 0.8–1.4 GPa and 480–610 °C using the garnet–clinopyroxene thermometer and omphacite barometer in the presence of albite.     

Geochemical constraints of the eclogite and granulite facies metamorphism as recognized in the Raobazhai complex from North Dabie Shan,China

A combined study of major and trace elements, fluid inclusions and oxygen isotopes has been carried out on garnet pyroxenite from the Raobazhai complex in the North Dabie Terrane (NDT). Well‐preserved compositional zoning with Na decreasing and Ca and Mg increasing from the core to rim of pyroxene in the garnet pyroxenite indicates eclogite facies metamorphism at the peak metamorphic stage and subsequent granulite facies metamorphism during uplift. A P–T path with substantial heating (from c. 750 to 900 °C) after the maximum pressure reveals a different uplift history compared with most other eclogites in the South Dabie Terrane (SDT). Fluid inclusion data can be correlated with the metamorphic grade: the fluid regime during the peak metamorphism (eclogite facies) was dominated by N₂‐bearing NaCl‐rich solutions, whereas it changed into CO₂‐dominated fluids during the granulite facies retrograde metamorphism. At a late retrograde metamorphic stage, probably after amphibolite facies metamorphism, some external low‐salinity fluids were involved. In situ UV‐laser oxygen isotope analysis was undertaken on a 7 mm garnet, and impure pyroxene, amphibole and plagioclase. The nearly homogeneous oxygen isotopic composition (δ¹⁸O_VSMOW = c. 6.7‰) in the garnet porphyroblast indicates closed fluid system conditions during garnet growth. However, isotopic fractionations between retrograde phases (amphibole and plagioclase) and garnet show an oxygen isotopic disequilibrium, indicating retrograde fluid–rock interactions. Unusual MORB‐like rare earth element (REE) patterns for whole rock of the garnet pyroxenite contrast with most ultra‐high‐pressure (UHP) eclogites in the Dabie‐Sulu area. However, the age‐corrected initial ε_Nd(t) is ? 2.9, which indicates that the protolith of the garnet pyroxenite was derived from an enriched mantle rather than from a MORB source. Combined with the present data of oxygen isotopic compositions and the characteristic N₂ content in the fluid inclusions, we suggest that the protolith of the garnet pyroxenite from Raobazhai formed in an enriched mantle fragment, which has been exposed to the surface prior to the Triassic metamorphism.     

大别山太湖—马庙断裂带两侧变质地体的增温变质作用

大别山榴辉岩地体以太湖 马庙断裂带为南界。该断裂带近东西走向, 向南倾斜。通过对石榴石中 Ｍｇ 含量由核部向边缘增高的生长环带构造的分析和详细的 Ｐ Ｔ 轨迹的计算表明, 断裂带北侧的榴辉岩和南侧的角闪岩及角闪片麻岩均经历了降压增温变质过程。热榴辉岩由地幔深处的折返和地温梯度的升高为可能的热源。     

安徽大别山北部榴辉岩的分布及主要特征

安徽大别山北部新发现的榴辉岩主要产于花岗质片麻岩及基性—超基性岩中 ,与大别山南部的榴辉岩在产出围岩、矿物组成及地球化学特征等方面均十分相似。现今榴辉岩南多北少的分布 ,主要是在中生代热隆构造体制下剧烈差异抬升剥蚀 ,中生代花岗岩大面积侵位所致。     

U—Pb Dating of Marble Assoicated With Eclogite from the Dabie Mountains,East China

Marble is associated with ultrahigh pressure eclogite in the Dabie Mountains, East China. U-Pb isotope data for the marble define a²³⁸U-²⁰⁶Pb isochron age of 435 ± 45 Ma with an initial²⁰⁶Pb/²⁰⁴Pb value of 18.075 ± 0.006. The age of ∼435 Ma is interpreted to represent the time of limestone diagenesis rather than the time of metamorphic resetting accompanied by the formation of the ultrahigh pressure eclogites at Triassic. The paleontologic and stratigraphic studies also favor a local excursion in the carbon isotopic composition of the latest Ordovician water within a sedimentary basin between the Sino-Korean and Yangtze cratons. Because the latest Ordovician limestone was involved in the eclogite formation, the continental collision between the Yangtze and Sino-Korean plates would postdate Ordovician. This project was financially supported by the National Natural Science Foundation of China and the Chinese Academy of Sciences.     

Early Tertiary eclogite facies metamorphism in the Monviso Ophiolite

Although crucial for the construction of tectonic models of the Alps the timing of high pressure metamorphism is still poorly determined and controversial. It is likely to vary from one tectonic unit to another depending on when each became involved in subduction. This in turn relates to palaeogeographic position with respect to the active ocean basin. Well defined, reliable geochronological data are too few to test this hypothesis. This paper extends the database by determining Sm–Nd mineral isochrons on two samples from the Monviso Ophiolite in the Piemonte Zone, carefully selected to minimize the problems of Sm–Nd dating of eclogites encountered elsewhere in the Alps. The dated samples have eclogite facies mineral assemblages typical of the Lago Superiore unit of the ophiolite; mineral compositions are similar to previously reported samples and indicate pressures of around 2  GPa and temperatures of 400–500  °C. Sm–Nd isochron ages of 60±12 and 62±9  Ma are defined by garnet and clinopyroxene, while the Rb–Sr age on phengite which is part of the high- P assemblage is 40±1  Ma. The new data fit an emerging pattern of ages in which high- P metamorphism in the oceanic realm is Early Tertiary, with slightly older ages in the overlying Sesia Zone and younger, Oligocene ages in the underlying internal basement massifs which only became involved in subduction when closure of the Piemont ocean was complete.     

大别山北部榴辉岩的退变质特征及其地质意义

研究了大别山北部榴辉岩的变质岩岩石学。结果表明，该区榴辉岩相变质作用可分为早期(超高压)和晚期(高压)两个阶段，并在折返过程中形成了一系列特征性的退变质显微构造。其中，退变质结构主要包括：(1)由于压力降低而出溶形成的一些定向针状或叶片状矿物包裹体，如钠质单斜辉石中石英及石榴子石中的金红石、单斜辉石和磷灰石等；(2)冠状体或后成合晶，特别是石榴子石外围发育两期(“双层”)后成合晶；(3)反应边或退变边，如绿辉石的透辉石退变边、透辉石的角闪石退变边和金红石的钛铁矿退变边等。这些退变质结构为本区榴辉岩高级变质岩的快速折返过程和抬升历史提供了强有力的岩石学依据；石榴子石中针状矿物出溶体进一步证明研究区榴辉岩早期经历了超高压变质作用，峰期变质压力应大干4．0GPa，甚至可能达到5～7GPa或更高。     

大别山碧溪岭未经历超高压变质的片麻状花岗岩

关于大别山超高压变质带中与榴辉岩密切伴生的花岗片麻岩是否也经历了超高压变质作用一直是个争议的问题。笔者最近在碧溪岭片麻状花岗岩中首次发现了可靠的岩相学和矿物学证据 ,证明该岩石只遭受到绿片岩相变质作用的改造 ,而未曾记录超高压变质矿物组合。根据片麻状花岗岩与围岩接触部位强烈糜棱岩化的发育 ,认为超高压岩石在抬升到地壳范围时与以花岗质岩石为主构成的扬子陆壳发生了大规模的构造并置。     

大别超高压变质过程中流体作用的微量元素效应及其地球化学意义

对大别地区角闪岩相－麻粒岩相和榴辉岩相岩石进行较系统的地球化学研究,探讨了超高压俯冲变质及折返过程中元素的再分配行为,并就其对变质过程中的部分熔融作用和流体交代作用的指示进行了分析。研究表明,大别榴辉岩原岩在其俯冲过程中发生了部分熔融作用,并在其早期脱水和后期退变质作用过程中与变质流体相之间产生了交换。脱水作用使榴辉岩Ｎｂ／Ｔａ比值升高,但后期的退变质作用使榴辉岩与其寄主围岩间的Ｎｂ／Ｔａ比值相对