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1.
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  相似文献   

2.
南大别山朱家冲榴辉岩中产有一条岩脉。电子探针分析结果表明,朱家冲榴辉岩中较大颗粒的白云母和岩脉中的白云母均为钠云母;而榴辉岩中细小颗粒的白云母则为多硅白云母;岩脉中的蓝绿色柱状矿物为闪石(amphibole)。采用激光阶段加热技术和真空阶段击碎技术,对朱家冲冷榴辉岩体和岩脉进行了Ar-Ar定年研究。选自榴辉岩和闪石脉的两个钠云母激光加热Ar-Ar年龄均约为200Ma,选自闪石脉的闪石真空击碎流体包裹体Ar-Ar年龄也约为200Ma,代表了热液活动钠云母化发生的时间;而提取流体包裹体后的闪石矿物粉末阶段加热Ar-Ar等时线年龄约为243Ma,则可能代表了朱家冲榴辉岩角闪岩相退变质作用发生的最小年龄。  相似文献   

3.
柴北缘锡铁山一带榴辉岩的岩石学特征及其退变PT轨迹   总被引：1，自引：1，他引：0  
张聪  张立飞  张贵宾  宋述光 《岩石学报》2009,25(9):2247-2259
柴北缘锡铁山地区榴辉岩以透镜体的形式存在于花岗质片麻岩和副变质片麻岩中.根据矿物组合的不同,可以分为多硅白云母榴辉岩和角闪石榴辉岩.在多硅白云母榴辉岩中首次发现了柯石英假象.利用榴辉岩中Grt-Cpx-Phn矿物温压计.结合绿辉石中存在柯石英假象包体的现象,得到锡铁山榴辉岩的峰期温压条件为751～791℃,2.71～3.17GPa,证明了锡铁山地区与柴北缘其他地块一样,也经历了超高压变质作用.通过PT视剖面图计算了榴辉岩退变的PT轨迹具有2个阶段演化特征：即先等温降压,然后再降温降压的PT轨迹.详细的岩石学研究探讨了榴辉岩在退变过程中,各矿物的成分和结构的改变过程.石榴石在等温降压过程中成分变化不大,而在角闪石出现后,其边部镁铝榴石含量明显降低,进而形成了韭闪石+斜长石的冠状体.绿辉石在水饱和状态下经过贫硬玉化改造,而后形成了Di+Ab+Amp的后生合晶.多硅白云母分解形成白云母+黑云母及少量石英及钾长石的组合.角闪石随着温压条件的降低由钠钙质闪石逐渐向钙质闪石转化.  相似文献   

4.
北秦岭官坡地区高压—超高压榴辉岩岩相学及变质作用研究   总被引：19，自引：2，他引：19  
北秦岭官坡地区的榴辉岩及含柯石英榴辉岩产在帮岭岩群的北侧，主要由绿辉石和石榴石组成，部分石榴石和绿辉石中含柯石英包体。此外还含有退变质的多硅白云母、角闪石、黝帘石和纳长石等矿物，根据变质矿物之间的替代关系及共生组合规律，榴辉岩退变质作用可划分为四个阶段，各阶段代表性矿物组合依次为：柯石英＋绿辉石＋石榴石；石英＋绿辉石＋石榴石；多硅白云母＋绿辉石＋石榴石＋石英；角闪石＋斜长石＋白云母＋黑云母。这四个  相似文献   

5.
拉萨地块东部松多(超)高压榴辉岩记录了古特提斯洋俯冲及折返过程。松多榴辉岩带已发现松多、新达多、白朗和吉朗4个榴辉岩出露区,它们的峰期温压条件及变质p-T轨迹的研究对揭示拉萨地块古特提斯时期的俯冲及折返过程有重要意义。松多榴辉岩带东段吉朗榴辉岩的主要矿物为石榴子石、绿辉石、多硅白云母、角闪石、金红石、绿帘石、石英以及退变形成的后成合晶结构(透辉石+角闪石+斜长石)和少量的黑云母。石榴子石具有含丰富矿物包裹体的"脏"核和极少包裹体的"净"边,具有典型的进变质成分环带特征,从核部到边部镁铝榴石组分升高,锰铝榴石和钙铝榴石组分降低。石榴子石边部发育窄的角闪石+斜长石(An=28)组成的冠状体,表明石榴子石边部发生了后期角闪岩相退变质作用。通过变质相平衡模拟计算得到石榴子石以及多硅白云母记录的峰期温压条件为563℃、2. 4 GPa。结合岩相学特征,确定吉朗榴辉岩经历了4期变质演化阶段:(1)进变质阶段以石榴子石核部及其包裹体为代表性矿物组合;(2)峰期变质阶段矿物组合为石榴子石边部、绿辉石、多硅白云母、蓝闪石、硬柱石、金红石和石英;(3)早期退变质阶段以硬柱石分解产生绿帘石为特征;(4)晚期退变质阶段以绿辉石发育后成合晶和石榴子石生长冠状体为特征。认为吉朗榴辉岩为典型的低温高压榴辉岩,经历了顺时针p-T演化轨迹,折返过程为近等温降压过程。与松多带内其他(超)高压岩石相比,吉朗榴辉岩峰期温压条件较低,其围岩为变石英岩,区别于区内其他(超)高压榴辉岩的石榴子石白云母片岩及蛇纹岩围岩。推测吉朗榴辉岩来自于俯冲带浅部,由俯冲隧道中低密度沉积物裹挟折返。  相似文献   

6.
大别山碧溪岭榴辉岩中有三种含水矿物:多硅白云母、角闪石和黑云母,它们分别是超高压(UHP)阶段(即柯石英榴辉岩相阶段)或者石英榴辉岩相阶段、退变质后成合晶阶段和角闪岩相退变质阶段的产物,本文利用离子探针技术对它们进行了氢同位素和硼同位素的分析。三种矿物内部的同位素组成都是均一的,多硅白云母的δD为-105‰±9‰,δ~(11)B为-25.9‰±2.0‰;角闪石的δD为-100‰±9‰,δ~(11)B为-24.4‰±0.9‰;黑云母的δD为-65‰±4‰,δ~(11)B为-19.3‰±1.3‰。多硅白云母和角闪石的氢-硼同位素组成在误差范围内是相同的,而和黑云母则有明显的差别,这表明,从UHP阶段或者石英榴辉岩相阶段到随后的后成合晶阶段,变质流体是内部缓冲的,而在角闪岩相变质阶段,则有了外来流体的加入,这个流体是相对富集D和~(11)B的。碧溪岭榴辉岩矿物相对于其地壳原岩表现出低δ~(11)B的特征,说明俯冲过程中板块经历了强烈的脱硼。  相似文献   

7.
澜沧谦迈地区位于西南三江昌宁-孟连结合带中段。新近在该地区发现榴辉岩,与双江县勐库地区退变榴辉岩一样,谦迈地区榴辉岩也产于湾河蛇绿混杂岩带内,岩石新鲜,后期退变质作用改造弱,岩石学信息保留完整。主要矿物成分为石榴子石、绿辉石、多硅白云母、金红石、角闪石、绿帘石、石英等,直接围岩为白云(钠长)片岩和斜长角闪岩,白云(钠长)片岩主要由多硅白云母、石英和长石构成。根据岩石学和矿物学特征将谦迈地区榴辉岩划分为4个变质阶段,其中峰期矿物组合为石榴子石+绿辉石+多硅白云母+硬柱石+蓝闪石+金红石+石英。初步研究表明,峰期榴辉岩相变质温度和压力分别为600℃和2.5GPa。新发现的榴辉岩为研究三江地区特提斯构造演化提供了关键性资料,该榴辉岩变质演化p-T轨迹的研究对探讨古特提斯洋的俯冲-造山过程具有重要意义。  相似文献   

8.
珍珠云母,这种二八面体层状硅酸盐矿物,以前只知它们常见于富铝矿床中70年代中后期,方知它们是低至中级变质岩中并不少见的一类标准变质岩矿物,可是至今在我国的变质岩石学研究中却很少报道。我们研究的珍珠云母首先见于河南信阳地区,它是我国变质岩中的首例原生珍珠云母。信阳地区的珍珠云母产于时代有争议的信阳群龟小组变余糜棱岩中。珍珠云母在变余糜棱岩中呈鱼形残斑定向分布,并有复杂褶皱发育。和珍珠云母同期共生的矿物还有多硅白云母,十字石,石榴石和石英。前三者也多为变形残斑,石墨则为珍珠云母和白色云母中的包体,石  相似文献   

9.
对西南天山哈布腾苏河一带出露的典型榴辉岩和蓝片岩进行了详细的岩相学、矿物化学和温压条件综合研究。榴辉岩可分为蓝闪石榴辉岩、钠云母榴辉岩、绿帘石榴辉岩和蓝闪石榴角闪岩(退变榴辉岩)4类,蓝片岩可分为含蓝闪石石榴白云母钠长片岩、石榴白云母蓝闪片岩和石榴白云母蓝闪石英片岩3类。新鲜榴辉岩主要矿物组合为石榴石+绿辉石+钠云母+绿帘石,退变榴辉岩则为石榴石+蓝闪石+角闪石;蓝片岩主要矿物组合为石榴石+蓝闪石+多硅白云母+钠云母+钠长石+石英。榴辉岩和蓝片岩中石榴石变斑晶均保存进变质生长环带,从核部到边部XMn和XFe降低,XMg和XCa升高,指示了升温进变质的演化过程。根据榴辉岩矿物共生组合、石榴石内部包体组合分布特征及传统地质温压计估算结果,确定榴辉岩经历了4阶段的变质演化:早期硬柱石蓝片岩相进变质阶段、峰期榴辉岩相变质阶段(t=543～579℃,p=1.5～1.6 GPa)、峰后绿帘蓝片岩相退变质阶段(t=～450℃,p1.0GPa)和晚期蓝闪绿片岩相退变质阶段(t400℃,p0.5 GPa)。利用p-T视剖面图计算的榴辉岩、蓝片岩峰期变质温压条件与传统地质温压计估算结果十分相近,其中榴辉岩的峰期变质条件t=520～550℃,p=1.7～1.9 GPa;蓝片岩峰期变质条件t=520～620℃,p=1.7～2.3 GPa。本文估算的榴辉岩峰期变质压力条件与前人根据柯石英的发现而认为研究区部分榴辉岩及其围岩曾经历超高压变质作用的认识明显相悖,原因可能如下:①后期退变质作用引起研究区榴辉岩全岩成分、矿物化学成分的调整,在采用Grt-Cpx-Phe温压计和以全岩成分为基础的相平衡模拟方法估算峰期温压条件时受到影响,从而使估算峰期压力条件普遍偏低;②西南天山的榴辉岩可能并非全都经历了超高压变质作用,高压、超高压榴辉岩可能分别代表了不同变基性岩块在不同俯冲深度变质的产物。  相似文献   

10.
大别山西段含蓝闪石-蓝晶石榴辉岩的相平衡研究   总被引：3，自引：2，他引：3  
目前对于大别山西段超高压榴辉岩仍存在一些不清楚的问题和模糊的认识,如蓝闪石和蓝晶石组合的稳定范围,峰期温压条件和矿物组合,以及早期退变质过程的矿物演化和流体作用。本文对取自大别山西段新县高压-超高压榴辉岩单元内不同地点的超高压榴辉岩样品进行了详细的岩石学和矿物学研究,在此基础上使用相平衡定量分析方法的 PT 视剖面图对它们进行了正演模拟计算,结果表明:含蓝闪石和蓝晶石榴辉岩处于相对低温或低压的蓝闪石榴辉岩和相对高温高压的蓝晶石榴辉岩的过渡区,其稳定的温压范围大致为温度590～700℃,压力1.7～3.3GPa,而且压力大于2.5GPa 时温度范围很窄,为600～640℃。由石榴石边缘成分和 PT 视剖面图确定的榴辉岩峰期温压条件为压力2.85～2.95GPa 和温度625～630℃,峰期矿物组合为石榴石 绿辉石 蓝闪石 蓝晶石 硬柱石 柯石英±多硅白云母。峰期之后,榴辉岩经历了快速近等温降压(ITD)的早期高压退变质作用,这是一个非平衡过程,所发生的主要变化如下:柯石英→石英,硬柱石→黝帘石 蓝晶石,在相对富镁岩石中出现滑石,当水含量较高时可以出现钠云母,蓝闪石在原来基础上有一定量的生长,并且绿辉石和多硅白云母很可能只部分地发生了成分变化,而石榴石几乎未发生改变。这样形成了目前观察到的矿物组成为石榴石 绿辉石 蓝闪石 蓝晶石 黝帘石/绿帘石 石英±多硅白云母±钠云母±滑石,它代表了 UHP 榴辉岩在早期高压退变质阶段结束时所具有的矿物组成,这一阶段结束时的温压条件大致为2.0～2.2GPa 和600～630℃;早期高压退变质阶段是脱水过程,流体是内部缓冲的。  相似文献   

11.
Eclogites of the Dabie Region: Retrograde Metamorphismand Fluid Evolution   总被引：4，自引：0，他引：4  
Based upon fluid effects, retrograde metamorphism of eclogites in the Dabie region can be divided into the fluid-poor, fluid-bearing and fluid-rich stages. The fluid-poor stage is marked by polymorphic inversion, recrystallization and exsolution of solid solutions, and is thought to represent eclogite-facies retrograde environments. The fluid-bearing stage is likely to have occurred at the late stage of ecologite-facies diaphthorosis and is represented by kyanite porphyroblasts, rutile, and sodic pyroxene in association with high-pressure hydrous minerals such as phengite and zoisite (clinozoisite) without significant amount of hydrous minerals such as amphibole, epidote and biotite. The fluid-rich stage might have commenced concomitantly with lower amphibolite-facies diaphthoresis and persisted all the way towards the near-surface environment. The product of this stage is characterized by plentiful hydrous and volatile-bearing phases.The dissemination-type rutile mineralizations in eclogites might have  相似文献   

12.
Zhang Zeming  Xu Zhiqin  Xu Huifen 《Lithos》2000,52(1-4):35-50
The 558-m-deep ZK703 drillhole located near Donghai in the southern part of the Sulu ultrahigh-pressure metamorphic belt, eastern China, penetrates alternating layers of eclogites, gneisses, jadeite quartzites, garnet peridotites, phengite–quartz schists, and kyanite quartzites. The preservation of ultrahigh-pressure metamorphic minerals and their relics, together with the contact relationship and protolith types of the various rocks indicates that these are metamorphic supracrustal rocks and mafic-ultramafic rock assemblages that have experienced in-situ ultrahigh-pressure metamorphism. The eclogites can be divided into five types based on accessory minerals: rutile eclogite, phengite eclogite, kyanite–phengite eclogite, quartz eclogite, and common eclogite with rare minor minerals. Rutile eclogite forms a thick layer in the drillhole that contains sufficient rutile for potential mining. Two retrograde assemblages are observed in the eclogites: the first stage is characterized by the formation of sodic plagioclase+amphibole symplectite or symplectitic coronas after omphacite and garnet, plagioclase+biotite after garnet or phengite, and plagioclase coronas after kyanite; the second stage involved total replacement of omphacite and garnet by amphibole+albite+epidote+quartz. Peak metamorphic P–T conditions of the eclogites were around 32 to 40 kbar and 720°C to 880°C. The retrograde P–T path of the eclogites is characterized by rapidly decreasing pressure with slightly decreasing temperature. Micro-textures and compositional variations in symplectitic minerals suggest that the decompression breakdown of ultrahigh-pressure minerals is a domainal equilibrium reaction or disequilibrium reaction. The composition of the original minerals and the diffusion rate of elements involved in these reactions controlled the symplectitic mineral compositions.  相似文献   

13.
Hydrous high-pressure veins formed during dehydration of eclogites in two paleo-subduction zones (Trescolmen locality in the Adula nappe, central Alps and Münchberg Gneiss Massif, Variscan fold belt, Germany) constrain the major and trace element composition of solutes in fluids liberated during dehydration of eclogites. Similar initial isotopic compositions of veins and host eclogites at the time of metamorphism indicate that the fluids were derived predominantly from the host rocks. Quartz, kyanite, paragonite, phengite, zoisite and omphacite are the dominant minerals in the veins. The major element compositions of the veins are in agreement with experimental evidence indicating that the composition of solutes in such fluids is dominated by SiO2 and Al2O3. Relative to N-MORB, the veins show enrichments of Cs, Rb, Ba, Pb, and K, comparable or slightly lower abundances of Sr, U, and Th, and very low abundances of Nd, Sm, Zr, Nb, Ti and Y. The differential fractionation of highly incompatible elements such as K, U and Th in the veins, as well as the presence of hydrous minerals in the eclogites rule out partial melting as a cause for vein formation. These results confirm previous suggestions that fluids derived from subducted basalt may have low abundances of high field strength elements, rare earth elements and Y. Variable vein-eclogite enrichment factors of incompatible alkalis and to a lesser extent Pb appear to reflect mineralogical controls (phengite, epidote-group minerals) on partitioning of these elements during dehydration of eclogite in subduction zones. However, abundance variations of incompatible elements in minerals from eclogites suggest that the composition of fluids released from eclogites at temperatures <700°C may not reflect true equilibrium partitioning during dehydration. Simple models for the trace elements U and Th indicate the relative importance of the basaltic and sedimentary portions of subducted oceanic crust in producing the characteristic chemical signatures of these elements in convergent plate margin volcanism.  相似文献   

14.
White mica from the Liassic black shales and slates in Central Switzerland was analysed by transmission electron microscopy (TEM) and electron microprobe to determine its textural and compositional evolution during very low-grade prograde metamorphism. Samples were studied from the diagenetic zone, anchizone and epizone (T ≈100°–450 °C). Phyllosilicate minerals analysed include illite/smectite (I/S), phengite, muscovite, brammallite, paragonite, margarite and glauconite. Textural evolution primarily is towards larger, more defect-free grains with compositions that approach those of their respective end-members. The smectite-to-illite transformation reduced the amounts of the exchange components SiK?1Al?1, MgSiAl?2, and Fe3+Al?1. These trends continue to a lesser degree in the anchizone and epizone. Correlations between the proportion of smectite in I/S and the composition of I/S indicate that smectite layers may contain a high layer charge. Illite in I/S bears a compositional resemblance to macrocrystalline phengite in some samples, but is different in others. Paragonite first appears in the upper diagenetic zone or lower anchizone as an interlayer-deficient brammallite, and it may be mixed with muscovite on the nanometre scale. Owing to the small calculated structure factor for paragonite-muscovite superstructures, conventional X-ray powder diffraction cannot distinguish between mixed-layer structures and a homogeneous compositionally intermediate solid solutions. However, indirect TEM evidence shows that irregularly shaped domains of Na- and K-rich mica exist below 10 nm. Subsequent coarsening of domains at higher grades produced discrete paragonite grains at the margins of muscovite crystals or in laths parallel to the basal plane of the host muscovite. Margarite appears in the epizone and follows a textural evolution similar to paragonite in that mixtures of margarite, paragonite, and muscovite may initially occur on the nanometre scale. However, no evidence of interlayer-poor margarite has been found.  相似文献   

15.
The northern part of the Cycladic island of Sifnos (Greece)is formed by a coherent sequence of interlayered acid and basicmetavolcanic rocks and metasediments, which underwent a high-pressureblueschist facies metamorphism during the Eocene. The metabasicrocks, including eclogites, blueschists, and actinolite-bearingrocks, are discussed in terms of their mineral assemblages,and bulk-rock and mineral chemistries. Metamorphic conditionsof 470 ? 30 ?C and 15 ? 3 kb are indicated by garnet-omphacitegeothermometry and by the development of deerite in meta-ironstonesand jadeite +quartz in meta-acidites.Mineral textures and systematicelement distributions between coexisting minerals suggest attainmentof chemical equilibrium. A new projection from garnet, epidote,quartz and vapour onto the NaAlO2-Al2O3-CaMgO2 plane is usedto illustrate equilibrium phase relations between omphacite,glaucophane, actinolite, paragonite, and chloritoid. It is demonstratedthat eclogites, blueschists, and actinolite-bearing metabasitesrepresent different bulk-rock compositions that recrystallizedunder the same fluid pressure and temperature conditions. Eclogitescontaining hydrous minerals such as glaucophane, actinolite,phengite, or paragonite in equilibrium with garnet and omphacitecan occur together with blueschists in high-pressure terraneswithout indicating different metamorphic conditions.  相似文献   

16.
Glaucophane‐bearing ultrahigh pressure (UHP) eclogites from the western Dabieshan terrane consist of garnet, omphacite, glaucophane, kyanite, epidote, phengite, quartz/coesite and rutile with or without talc and paragonite. Some garnet porphyroblasts exhibit a core–mantle zoning profile with slight increase in pyrope content and minor or slight decrease in grossular and a mantle–rim zoning profile characterized by a pronounced increase in pyrope and rapid decrease in grossular. Omphacite is usually zoned with a core–rim decrease in j(o) [=Na/(Ca + Na)]. Glaucophane occurs as porphyroblasts in some samples and contains inclusions of garnet, omphacite and epidote. Pseudosections calculated in the NCKMnFMASHO system for five representative samples, combined with petrographic observations suggest that the UHP eclogites record four stages of metamorphism. (i) The prograde stage, on the basis of modelling of garnet zoning and inclusions in garnet, involves P–T vectors dominated by heating with a slight increase in pressure, suggesting an early slow subduction process, and P–T vectors dominated by a pronounced increase in pressure and slight heating, pointing to a late fast subduction process. The prograde metamorphism is predominated by dehydration of glaucophane and, to a lesser extent, chlorite, epidote and paragonite, releasing ～27 wt% water that was bound in the hydrous minerals. (ii) The peak stage is represented by garnet rim compositions with maximum pyrope and minimum grossular contents, and P–T conditions of 28.2–31.8 kbar and 605–613 °C, with the modelled peak‐stage mineral assemblage mostly involving garnet + omphacite + lawsonite + talc + phengite + coesite ± glaucophane ± kyanite. (iii) The early decompression stage is characterized by dehydration of lawsonite, releasing ～70–90 wt% water bound in the peak mineral assemblages, which results in the growth of glaucophane, j(o) decrease in omphacite and formation of epidote. And, (iv) The late retrograde stage is characterized by the mineral assemblage of hornblendic amphibole + epidote + albite/oligoclase + quartz developed in the margins or strongly foliated domains of eclogite blocks due to fluid infiltration at P–T conditions of 5–10 kbar and 500–580 °C. The proposed metamorphic stages for the UHP eclogites are consistent with the petrological observations, but considerably different from those presented in the previous studies.  相似文献   

17.
Reactions producing Al‐rich index minerals in the south‐eastern part of the Lepontine Dome (Central Alps, Switzerland) are investigated using mineral distribution maps, microstructural observations and equilibrium phase diagrams. The apparent staurolite mineral zone boundary corresponds to the paragonite breakdown reaction Pg + Grt + Qtz = Pl + Al2O3 + W. Equilibrium phase diagrams show that most natural metapelites do not contain staurolite or alumosilicates as long as univalent cations are predominantly accommodated in white mica. For a wide range of metapelitic compositions the paragonite breakdown releases sufficient Al for the formation of these minerals. Rare occurrences of staurolite and kyanite, north of the formerly mapped mineral zone boundaries, coexist with paragonite and are restricted to extremely Al‐rich bulk compositions. The stable branch of the kyanite‐forming paragonite breakdown reaction above 660 °C yields an additional mapable isograd. The second set of Al‐releasing reactions is biotite‐producing phengite breakdown. However, these reactions are less suitable to produce well defined reaction isograds in the field as they are more continuous and their progress is strongly dependent on bulk composition. Well developed fibrolite in metapelites does not appear until staurolite starts to breakdown. We conclude that amphibolite facies conditions in the study area were attained by decompression, without substantial heating at low pressures.  相似文献   

18.
大别山榴辉岩退变质多硅白云母及地压计可用性讨论   总被引：3，自引：0，他引：3  
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Phel、榴辉岩相退变制裁晚阶段的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Phel，其Si和Fe^2 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i和Fe^2 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、Fe^2 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。  相似文献   

19.
White mica (phengite and paragonite) K–Ar ages of eclogite-facies Sanbagawa metamorphic rocks (15 eclogitic rocks and eight associated pelitic schists) from four different localities yielded ages of 84–89 Ma (Seba, central Shikoku), 78–80 Ma (Nishi-Iratsu, central Shikoku), 123 and 136 Ma (Gongen, central Shikoku), and 82–88 Ma (Kotsu/Bizan, eastern Shikoku). With the exception of a quartz-rich kyanite-bearing eclogite from Gongen, white mica ages overlap with the previously known range of phengite K–Ar ages of pelitic schists of the Sanbagawa metamorphic belt and can be distinguished from those of the Shimanto metamorphic belt. The similarity of K–Ar ages between the eclogites and surrounding pelitic schists supports a geological setting wherein the eclogites experienced intense ductile deformation with pelitic schists during exhumation. In contrast, phengite extracted from the Gongen eclogite, which is less overprinted by a ductile shear deformation during exhumation, yielded significantly older ages. Given that the Gongen eclogite is enclosed by the Higashi-Akaishi meta-peridotite body, these K–Ar ages are attributed to excess 40Ar gained during an interaction between the eclogite and host meta-peridotite with mantle-derived noble gas (very high 40Ar/36Ar ratio) at eclogite-facies depth. Fluid exchange between deep-subducted sediments and mantle material might have enhanced the gain of mantle-derived extreme 40Ar in the meta-sediment. Although dynamic recrystallization of white mica can reset the Ar isotope system, limited-argon-depletion due to lesser degrees of ductile shear deformation of the Gongen eclogite might have prevented complete release of the trapped excess argon from phengites. This observation supports a model of deformation-controlled K–Ar closure temperature.  相似文献   

20.
The (ultra‐) high pressure eclogites from Sumdo area, recorded the subduction and exhumation process of the Paleo‐Tethys oceanic crust. Previous studies showed that there are significant differences in temperature and pressure conditions of the eclogites in four regions, e.g. Sumdo, Xindaduo, Bailang and Jilang. The cause of this differences remains unclear. Studying the peak metamorphic conditions and P‐T path of Sumdo eclogite is of great significance to reveal the subduction and exhumation mechanism of Paleo‐Tethys ocean. In this paper, we choose the Jilang eclogite as an example, which has a mineral assemblage of garnet, omphacite, phengite, hornblende, rutile, epidote, quartz and symplectit (diopside + amphibole + plagioclase), and minor biotite. Garnet has a “dirty” core with abundant mineral inclusions and a “clear” rim with less mineral inclusions, showing typical growth zoning. From the core to the rim, Prp content in garnet increasing while Grs content decreasing. P‐T pseudosection calculated with Domino constrained peak P‐T conditions of Jilang eclogite as 563°C, 2.4 GPa. Combined with petrographical observation, four stages of metamorphism have been recognized: (1) early stage prograde metamorphism represent by the core of garnet and mineral inclusions therein; (2) peak metamorphism represent by the rim of garnet, omphacite, phengite, glaucophane, rutile and quartz; (3) first stage of retrograde metamorphism characterized by decomposition of lawsonite to zoisite; (4) second stage of retrograde metamorphism characterized by symplectites surrounding omphacite and cornona rimmed garnet. Jilang eclogite shows a clockwise P‐T path, and near isothermal decompression during exhumation. It differs from eclogites in other area, which are hosted by garnet‐bearing mica schists or serpentinites. Jilang eclogites are enclosed in metamorphic quartzites, with relatively low P‐T conditions. We infer that the Jilang eclogite was derived from the shallow part of the subduction zone, and was exhumated by low density materials in the subduction channel.  相似文献   

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