西南天山榴辉岩的显微构造及其变形机制探讨

岩石、矿物的显微构造是岩石变形、变质等各种地质作用最直观的记录。新疆西南天山高压变质带中榴辉岩的显微构造研究,对认识其在地壳深俯冲过程中的岩石、矿物变形机制有十分重要的意义。西南天山榴辉岩的显微构造研究发现:绿辉石的动态重结晶亚晶粒化和颗粒边界迁移、云母的扭折和波状消光、方解石的机械双晶等显微变形现象,表明动态恢复作用、位错滑移和位错攀移为主的晶质塑性变形机制是榴辉岩的主要变形机制;石榴子石呈无变形刚性特征,具有变斑晶包迹结构,说明固态物质扩散迁移是榴辉岩的另一个重要变形机制;石榴子石多具有良好的晶型,说明石榴子石是在静变质状态下稳态生长而成,这与大别—苏鲁高压—超高压变质带有明显区别。因此,笔者认为位错蠕变、动态重结晶主导的晶质塑性变形和固态物质扩散迁移组合的显微构造变形现象,是西南天山高压—超高压变质榴辉岩的显微构造组合特征。这样的组合特征可能代表了洋壳深俯冲经历高压—超高压变质作用榴辉岩变形机制。     

大别山榴辉岩研究概述

榴辉岩作为典型的超高压变质岩,是研究碰撞造山带演化过程及地球深部变质作用的物质基础。该文阐述了我国大别山造山带榴辉岩的研究意义,大别山的地质背景以及榴辉岩的特征及产出特点。总结了大别山榴辉岩的同位素地球化学（包括碳、氢氧和稀有气体同位素）和稀土元素地球化学特征,并提出了一些在大别山榴辉岩的研究中尚有争论的问题,如榴辉岩原岩的成因及来源、榴辉岩形成过程中板块的俯冲深度、超高压变质过程中稀土元素的稳定性等。这些问题的解决还有赖于对大别地区的超高压变质岩做进一步的岩石地球化学及微量元素地球化学的研究。     

大别山超高压变质岩石变形构造研究

含柯石英、金刚石榴辉岩是南大别超高压变质岩石的主体,在这些榴辉岩的未退变质核心、退变质斜长角闪岩及超高压围岩中,保存有大量不同变质时期的宏观—微观变形构造．其中主要的变形构造是在单剪应力状态下,由具有顶端指向ＮＷ动向的角闪岩相变形作用Ｄ２形成的ａ型褶皱Ｆ２,以共轴样式叠加于纯剪应力状态下榴辉岩相变形作用Ｄ１形成的斜卧褶皱Ｆ１之上的二次叠加褶皱,总体形态符合Ｒａｍｓａｙ第三种干涉型．这两期变形作用揭示了超高压变质岩石在折返过程中存在两种动力学机制,它们相续作用才使得这些超高压岩石出露地表．     

柴北缘野马滩超高压地体的成因——年代学研究结果的约束

目的 探讨柴北缘野马滩超高压地体的成因.方法 利用锆石LA-ICP-MS原位微区分析技术,对超高压榴辉岩中两种类型的副片麻岩夹层进行年代学研究.结果 得到榴辉岩中两类副片麻岩夹层的变质年龄介于421～458 Ma之间,与超高压榴辉岩的变质年龄一致.结论 柴北缘野马滩超高压榴辉岩和围岩片麻岩是同一期变质事件的产物,其形成可能不应属于典型洋壳深俯冲成因,而很可能是陆壳深俯冲的结果.     

山东省蓬莱赤山新生代碱性玄武岩中榴辉岩包体的研究

在蓬莱赤山新生代碱性玄武岩中以包体形式产出的榴辉岩,由于受碱性玄武岩岩浆的热作用影响,发生了变质作用。致使榴辉岩的主要组成矿物石榴石发生了等化学组成的分解,形成尖晶石(X_(Mg)=0.51),斜长石(An=60～70)和富Al斜方辉石(X_(Mg)=0.71,Al_2O_312.6wt％)组成的集合体,达到了新的相平衡、其再平衡的温度为1200～1300℃;压力为9kbar。榴辉岩包体虽经变质作用,但没有改变它的REE丰度,它的典型的平坦型REE分配模式与碱性火山作用的富集LREE这样不相容元素的地幔区不同,它很可能是与碱性玄武岩岩浆无成因联系的随机捕虏的原始地幔物质。     

南大别石马榴辉岩多硅白云母和石英脉的年龄及其地质意义

研究发现,与大别-苏鲁超高压变质带中多硅白云母往往含过剩Ar不同,石马地区榴辉岩中多硅白云母基本不含过剩Ar.阐述了其原因,并进而探讨了超高压带中变质流体运移的特点.切穿石马地区榴辉岩及其围岩的白云母石英细脉的年龄表明,南大别超高压变质带角闪岩相退变质作用应结束在180 Ma前,而不会更晚.     

柴达木盆地北缘高压—超高压变质带榴辉(闪)岩中钛载体的比对研究及地质意义

为研究柴达木盆地北缘高压—超高压变质带榴辉(闪)岩中钛矿物的赋存状态及载体之间的差异,在前人研究的基础上,通过2016年地表查证,从岩相学、矿物学、岩石地球化学、人工重砂、选矿实验等方面进行了比对研究。结果表明:鱼卡和丁叉叉南坡榴辉岩中的钛载体主要为金红石,而胜利口榴闪岩中钛载体主要为钛铁矿和榍石,进一步证实了榴辉岩在退变阶段和钛成矿作用中金红石转化为钛铁矿的理论观点。研究成果为柴达木盆地北缘高压—超高压变质带内寻找榴辉岩型金红石矿奠定了基础。     

大别山地区岩石高压弹性波速及其对岩石圈组成和壳-幔循环的限制*

在室温和等静水压下测定了大别超高压变质带榴辉岩和麻粒岩至1GPa压力下的P波和S波速度.研究表明,超高压榴辉岩具有高密度(3.3～3.6g·cm^-3)、高波速和弱的各向异性(1.4%～2.6%),麻粒岩呈现了低的密度(2.8～3.1g·cm^-3)和波速、高的Poisson比(0.28～0.29),高压榴辉岩具有最强的各向异性(6.1%～8.4%).岩石波速同深地震测深剖面对比表明,大别造山带下地壳可能存在榴辉岩,但是数量应很少,而上地幔具有同超高压榴辉岩或者蛇纹石化/含水纯橄岩相似的地震波速度.推测地壳物质以榴辉岩的形式,通过拆沉作用进入地幔,少部分榴辉岩折返回地壳.     

部分变生褐帘石的谱学、成因与工艺性质

本文用电子探针分析、X射线衍射、红外光谱及穆斯堡尔谱研究了其稀土矿床中部分变生的褐帘石,褐帘石的穆斯堡尔谱表明Fe~(2+)/Fe~(3+)>1;Fe~(2+)和Fe~(3+)在M(1)和M(3)位置中的无序度分别为0.49和0.61,大于未变生的褐帘石;说明该褐帘石在温度较高,较还原的条件下形成。褐帘石晶格中的U,Th等放射性元素使其部分变生。用浮选剂C_(16-18)硫化羧酸不能有效地浮选部分变生的褐帘石,但其中Th,U的放射性衰变与氧化作用使稀土元素从晶格中释放出来,形成风化壳型的离子吸附稀土矿床,用盐水等电解质溶液即可提取80％以上的稀土元素。     

新疆西天山长阿吾子蛇绿混杂岩中与榴辉岩伴生的异剥钙榴岩的发现及其地质意义

在西天山长阿吾子蛇绿岩中的蛇纹石化超基性岩中,首次发现了由原榴辉岩相岩石经异剥钙榴岩化作用形成的异剥钙榴岩.该异剥钙榴岩主要由钙铝榴石、透辉石、葡萄石、绿泥石和少量榴辉岩相变质矿物绿辉石和铁镁铝榴石等组成.详细的岩石学研究表明,该异剥钙榴岩是在超基性岩二次蛇纹岩化过程中由榴辉岩相岩石经异剥钙榴岩化作用形成.异剥钙榴岩化过程开始于370～410℃/650～850MPa;大规模的异剥钙榴岩化出现在200～350℃/400～600MPa的温压条件.在P-T图上由榴辉岩相到异剥钙榴岩构成了明显的退变轨迹,这表明西天山长阿吾子异剥钙榴岩是由原榴辉岩相变质岩石抬升、回返过程中发生异剥钙榴岩化作用形成的.异剥钙榴岩并不只限于洋底变质的构造环境中,也可以形成于俯冲带中.     

Eclogites in the interior of the Tibetan Plateau and their geodynamic implications

Eclogites have been recently reported in the interior of the Tibetan Plateau, including in the central Qiangtang metamorphic belt, in the Basu metamorphic massif of the eastern Bangong-Nujiang suture zone, and at Songdo and Pengco in the eastern Lhasa terrane. Some typical ultrahigh-pressure （UHP） metamorphic phenomena, e.g., garnet exsolution from clinopyroxene, were documented in the Basu and Pengco eclogites. The UHP metamorphism in the interior of the Tibetan Plateau marked by these eclogites generally took place in the Early Mesozoic. Along with exhumation of these eclogites, （post-） collision-related magmatism extensively occurred around the central Qiangtang belt, the eastern Bangong-Nujiang suture zone, and the eastern Lhasa terrane. The occurrence of these Early Mesozoic eclogites manifests an out-of-sequence evolution of the Tethys, and they could be a product of diachronous collision between the eastern Qiangtang terrane and the irregular continental margin of the united western Qiangtang-Lhasa plate, along the linked eastern Bangong-Nujiang-central Qiangtang zone. The collision-related magmatic rocks could have been originated from lithospheric thickening, melting, or detachment due to the collision. The presence of UHP metamorphic rocks in central Qiangtang and Basu implies likely continental deep-subduction, and the denudation of these two metamorphic zones could have served as the source of the Triassic turbidites in the Songpan-Garze complex and the Jurassic turbidites in the western Bangong-Nujiang zone, respectively. However, studies of the eclogites in the interior of the Tibetan Plateau just began, and many principal aspects still remain to be explored, such as their distributions, typical lithologies and minerals, temperature-pressure conditions, timing of formation and exhumation, protoliths and tectonic setting, and relationship with the evolution of the Tethys and large-scale basins in Tibet.     

Heterogeneity of water in UHP eclogites from Bixiling in Dabieshan： Evidence fromgarnet

Garnets in ultrahigh pressure (UHP) eclogites from Bixiling in Dabieshan were investigated by Fourier transform infrared spectrometer (FTIR). The results demonstrate that all garnets contain structural water which occurs as hydroxyl (OH), with contents ranging from 164 to 2034 ppm (H2O wt.) and mostly higher than 500 ppm. Like omphacite which is another major OH-rich mineral in eclogites, garnet is an important carrier that can recycle the surface water into deep mantles. Heterogeneity of water in garnets exists not only among different samples of the same outcrop (～150 m), but also among different crystals of the same sample (～1 cm). This indicates that the mobility of fluids during UHP metamorphism is very limited (possibly on centimeter scales), and that both subduction and exhumation processes of UHP rocks are very fast.     

Discovery of ultrahighpressure magnesite-bearing garnet lherzolite (>3.8 GPa) in the Altyn Tagh,Northwest China

Magnesite-bearing garnet lherzolite from the Altyn Tagh, associated with garnet pyroxenite, and garnet-bearing felsic gneiss, crops out as lenses in Proterozoic gneiss about 100 km east of the Jianggelasayi River. The garnet lherzolite, together with the eclogite in western Jianggelasayi, composes a high-pressure to ultrahigh- pressure metamorphic belt in the southern margin of the Altyn Tagh. Parageneses of minerals from magnesite-bearing garnet lherzolite indicate that the rock evolved a multi-stage metamorphism. The peak-stage metamorphism produces an assemblage of Grt+Ol+Opx+Cpx±Mgs, in which Al₂O₃ content of Opx is very low (0.30 — 0.66wt%). The calculated P-T condition of the peak stage is 3.8—5.1GPa and 880—970℃, some exsolution rods of clinopyroxene and rutile occur in the Grt and magnesite is rimed by dolomite and orthopyroxene, all implying that the peak stage was a UHP metamorphic process. Together with regional geological studies, isotopic dating and the discovery of coesite inclusions in zircon separates from felsic gneiss from the northern margin of the Qaidam Basin, the presence of a very long UHP metamorphic belt in northwest China is suggested.     

Evidence for UHP metamorphism of eclogites from the Altun Mountains

Ultrahigh pressure (UHP) metamorphism refers to metamorphism that has occurred at pressures for the stability of coesite. The polycrystalline quartz inclusions showing the characteristic texture within garnets of eclogites indicates the pre-existence of coesites under the peak metamorphic condition. The unusual exsolution textures in ompacites and apatites, and the pressure estimations of phengite-bearing eciogites have been taken to provide further proof of eclogite formation under the UHP conditions.Combined with the fact that coesites have been observed in country rocks of eclogites in North Qaidam Mountains, another UHP metamorphic belt cut by the large-scale strikeslip fault in the AItun-North Qaidam area of China is confirmed.``     

Fluid inclusions evidence for differential exhumation of ultrahigh pressure metamorphic rocks in the Sulu terrane

Since the discovery of coesite and diamond inclusions in eclogites from the Dabie-Sulu orogen, east-central China[1―3], this largest ultrahigh pressure (UHP) metamor- phic terrane in the world has attracted extensive scientific interests. A number of hydrous minerals such as zoisite, phengite, magnesite and talc have been found in the UHP rocks, showing that fluids have played an important role in this type of extreme metamorphic evolution[4―8]. Sev-eral techniques have been applied to th…     

Water contents and hydrogen isotopes in nominally anhydrous minerals from UHP metamorphic rocks in the Dabie-Sulu orogenic belt

A continuous flow method, by a combination of thermal conversion elemental analyzer （TC/EA） with isotope ratio mass spec- trometry （MS）, was developed to determine both H isotope composition and H2O concentration of ultrahigh-pressure （UHP） metamorphic rocks in the Dabie-Sulu orogenic belt. By using the developed step-heating technique, we have studied H2O concen- tration and H isotope composition of the different forms of water （structural OH and molecular H2O） in garnet. The quantitative measurements of HzO concentration and H isotope composition of minerals in UHP metamorphic rocks from several typical out- crops indicate that the gneisses can release more amounts of water than the eclogites during exhumation of the deeply subducted continental crust. Therefore, by decompression dehydration at the contact between eclogite and gneiss, the released water could flow from the gneiss to the eclogite and result in significant hydration of the eclogite adjacent to the gneiss. The measured maxi- mum water contents of minerals in eclogites indicate that garnet and omphacite have the maximum water solubilities of 2500 and 3500 ppm, respectively, under the peak UHP metamorphic conditions.     

Coesite inclusions in zircon from gneisses identified by laser Raman microspectrometer in ultrahigh pressure zone of Dabie Mountains, China

The mineral inclusions in zircon from gneisses in ultra-high pressure (UHP) zone of the Dabie Mountains were identified by using a laser Raman microspectrometer. Coesite occurs as inclusions in zircons from all types of gneiss. Other important minerals, such as jadeite, omphacite, aragonite, barite, and anhydrite were also found as inclusion minerals. These discoveries indicate that (ⅰ) gneissic country rocks had metamorphosed at the same time as the enclosed eclogites; and (ⅱ) SO₄^-2-bearing fluids were present in the UHP metamorphic process, which is manifested by occurrence of barite and anhydrite coexisting with coesite.     

Carbon isotope anomaly in marbles associated with eclogites from the Dabie Mountains

This is a report of carbon isotope anomaly in marbles associated with the UHP eclogites from the Dabie Mountains. The results place constraints on the degree of crust-mantle interaction during the UHP metamorphism.     

Partial melting of deeply subducted continental crust and the formation of quartzofeldspathic polyphase inclusions in the Sulu UHP eclogites

Two types of quartzofeldspathic inclusions hosted by omphacite and garnet were identified in the Sulu UHP eclogites. The first consists of albite, quartz, and various amounts of K-feldspar. In contrast, the second consists predominantly of K-feldspar and quartz without any albite. The presence of quartzofeldspathic inclusions within the UHP mafic eclogites indicates that partial melting occurred in deeply subducted continental crust via mica dehydration melting reactions at an early stage of rapid exhumation. Such a melting event generated hydrous Na-K-Al-Si melts. These melts infiltrated into the mafic eclogite and were captured by recrystallizing garnet or omphacite, which together followed by dehydration and crystallization to form feldspar-bearing polyphase inclusions. Formation of silicate melts within the deeply subducted continental slab not only provides an excellent medium to transport both mobile （LILE） and immobile （HFSE） elements over a large distance, but also induces effective changes in the physical properties of the UHP slab. This process could be a major factor that enhances rapid exhumation of a deeply subducted continental slab.     

New finding of micro-diamonds in eclogites from Dabie-Sulu region in central-eastern China

Micro-diamonds were only found ten years ago in eclogite associated with marble at Xindian in the Dabie Mountains. This paper reports our new finding of micro-diamonds not only in eclogites at Maobei in the Sulu region and at Xindian and Laoyoufang in the south part of the Dabie Mountains (South Dabie), but also in eclogites at Baizhangya and Huangweihe in the northern part of the Dabie Mountains (North Dabie) that has usually been considered not to experience ultrahigh pressure metamorphism.Except the micro-diamond at Huangweihe that was found from the artificial heavy sands of zircons used for isotopic dating, the micro-diamonds from other localities were identified in thin sections of the eclogites. Besides a few interstitial grains, most of the micro-diamond grains in thin sections occur as inclusion in garnet. Three crystals of microdiamond at Maobei in the Sulu region are sized in 120, 60 and 30 lira, respectively. Crystal forms look like octahedron and the composite of octahedron and hexahedron. The largest micro-diamond crystal comes from Xindian, which is measured to be 180 lira in diameter with distinct zonal structure and inclusions. The zonal structure occurs as an inclined octahedron inside rounded by an incomplete hexagonal girdle. A smaller micro-diamond inclusion occurs inside the central octahedron, and a larger graphite inclusion is within the outer zone. The Laoyoufang micro-diamond is partially retrograded to graphite. Micro-diamond from the Baizhangya eclogite in the ultramafic rock belt of North Dabie is an aggregate of 70 lira X 90 lira in size. All the micro-diamonds are confirmed by the Raman spectrum analysis. The occurrence of the micro-diamonds from the eclogites in the ultramafic rock belt of North Dabie demonstrates that this region was also subjected to ultrahigh pressure metamorphism as well as the South Dabie did.