大别地区不同类型岩石的超高压结晶实验及其岩石学意义

在３．５ＧＰａ、１２００℃条件下对取自大别地区的不同类型岩石进行了重结晶实验，并采用偏光显微镜、Ｘ射线粉晶衍射和电子探针对实验产物的结晶特点、矿物组成及矿物成分进行了研究。结果表明：①石榴橄缆岩的岩石成因与共生的榴辉岩相同；②榴辉岩的重结晶经历了从单斜辉石岩、石榴辉石岩到榴辉岩的过程；③超高压片麻岩主要由石榴子石、绿辉石、多硅白云母和柯石英组成，不含斜长石、黑云母和绿帘石等常规矿物；④柯石英和文石     

云开地块北缘MORB型火山岩的首次发现及其大地构造意义

最新研究发现,在云开地块北缘陆川县石窝镇-北流市清水口镇一带的"云开岩群"中出露一系列变质基性火山岩,该火山岩总体呈北东向带状分布,主要岩性为斜长角闪岩和斜长角闪片麻岩,局部岩石中还夹有少量呈透镜状、布丁状或似层状产出的石榴辉石岩、透辉石岩、透辉角闪石岩、角闪石岩、(含石榴)斜长角闪岩、角闪绿帘石岩、石榴阳起石岩、石榴斜长辉石岩等.地球化学研究表明,其为典型的大洋拉斑玄武岩(MORB型玄武岩).斜长角闪岩的锆石SHRIMP法喷发成岩年龄为(1 462±28) Ma,表明其可能代表该区消失了的古洋壳岩石,是四堡期洋壳蛇绿岩的组成部分.     

中国大陆科学钻探主孔100～2000米超高压变质岩岩相学特征与变质变形史

中国大陆科学钻探工程的实施，为超高压变质带的研究打开了新的生面，加深了对苏鲁超高压变质带的认识。从100～2000米获得的岩心的岩石学观察，得知主要岩石类型有：(1)榴辉岩及石榴辉石岩；(2)榴辉岩质片麻岩；(3)石榴橄榄岩；(4)黑云(角闪)二长片麻岩和(5)碎裂岩等。榴辉岩可分幔源和壳源两类，壳源榴辉岩在钻孔中分布较广，上部最为集中；幔源榴辉岩，包括石榴辉石岩，在空间上与超镁铁岩有密切共生关系。榴辉岩质的片麻岩是一种中酸性的超高压岩石，与壳源的榴辉岩共生，显微镜下可以追索出它们之间的结构演化关系。石榴橄榄岩以石榴单辉橄榄岩为主，是俯冲带上部地幔楔加入于俯冲板片变质而成。石榴橄榄岩中的石榴子石富镁，单斜辉石为绿辉石并常含钛斜硅镁石，说明其经过超高压变质的过程。从变质岩石的组合，面理和线理产状的差异，地震反射面和构造角砾岩带的发育，发现以1600米为界，可大致分为2个岩片。上部岩片中多金红石榴辉岩而且出现频率很高，下部岩片中多为多硅白云母榴辉岩出现频率较低。由于隆升进入中下地壳，超高压变质岩普遍发生退变质。榴辉岩的早期退变质成为具后成合晶结构的石榴角闪岩，榴辉岩质片麻岩退变质形成绿帘黑云(角闪)斜长片麻岩，变质条件为角闪岩相，它可以部分熔融或受到钾交代作用而转变为黑云角闪二长片麻岩。后期的伸展造成了局部碎裂和角砾岩带，新生矿物为绿泥石，方解石和赤铁矿、绿帘石等，属绿帘角闪岩相和绿片岩相。主孔100～2000米的超高压变质岩石组合的确定，进一步说明了巨量的地壳物质可以深俯冲进入地幔并迅速折返；超高压变质岩石记录了陆壳俯冲折返和壳幔相互作用的过程，它们是板块下覆构造和地幔动力学的信息载体。     

大别山毛屋镁铁—超镁铁岩块的原岩与超高压变质作用

毛屋镁铁－超镁铁层状岩体由方辉橄榄岩、橄榄方辉岩、含石榴石方辉岩、石榴二辉岩、石榴透辉岩、榴辉岩、绿辉石岩等不同岩性的岩石组成的变层状堆晶岩。     

大陆碰撞造山带的两类橄榄岩——以柴北缘超高压变质带为例

论述了大陆俯冲碰撞带中地幔橄榄岩的基本特征和成岩类型,并重点讨论柴北缘超高压变质带中不同性质的橄榄岩及其成因。根据岩石学特征,我们确定柴北缘超高压带中发育有两种类型的橄榄岩:(1)石榴橄榄岩,岩石类型包括石榴二辉橄榄岩、石榴方辉橄榄岩、纯橄岩和石榴辉石岩,是大陆型俯冲带的标志性岩石。金刚石包裹体、石榴石和橄榄石的出溶结构、温压计算等均反映其来源深度大于200km。地球化学特征表明该橄榄岩的原岩是岛弧环境下高镁岩浆在地幔环境下堆晶的产物。(2)大洋蛇绿岩型地幔橄榄岩,与变质的堆晶杂岩(包括石榴辉石岩、蓝晶石榴辉岩)和具有大洋玄武岩特征的榴辉岩构成典型的蛇绿岩剖面,代表大洋岩石圈残片。这两类橄榄岩的确定对了解柴北缘超高压变质带的性质和构造演化过程有重要意义。     

徐淮地区镁铁质岩石捕虏体单斜辉石中石榴石和石英的出溶

徐淮地区早侏罗世侵入杂岩中榴辉岩,石榴辉石岩和单斜辉石岩捕虏体单斜辉石中可以观察丰富的出溶石英针和石榴石,黝帘石及角闪石的出溶叶片,榴辉岩中出溶石英针的绿辉石核部比其边部相对富含FeO和MgO,贫SiO2,Al2O3和CaO。在石榴辉石岩和单斜辉石岩捕虏体中具有出溶石榴石的单斜辉石。从靠近出溶石榴石的一侧向其核部,Al2O3,Na2O和TiO2含量降低,MgO,SiO2和CaO含量增加,单斜辉石中定向石英针的出溶表明曾经存在有超高压条件下（≥25×10^8Pa)稳定的过硅质绿辉石。单斜辉石中出溶石榴石表明温压条件的降低可能是引起出溶的一个主要原因,捕虏体中的矿物组合和岩相学特征表明它们曾经遭受了榴辉岩相和角闪岩相退化变质作用,这与因压力和温度降低引起矿物出溶的结果相吻合。     

新疆磁海铁矿床中辉石的研究与单斜辉石分类定名的多元三角图解法

一、磁海矿区单斜辉石的研究在我国新疆磁海铁矿区的辉长岩、辉绿岩、闪长岩、辉石花岗岩、石榴石辉石岩及辉石岩等不同类型的岩石中,辉石广为发育。辉石主要属单斜辉石中的透辉石—钙铁辉石系列及普通辉石系列。     

藏东南错那县麻玛沟角闪岩相石榴辉石岩成因研究

在藏东南错那县西南约20km的麻玛沟中发现了呈透镜体产出于石榴石矽线石片麻岩中的石榴辉石岩,其矿物组合为石榴石 透辉石 石英 铁角闪石 磁铁矿,并含有锆石、磷灰石等副矿物。石榴石呈他形产出,以铁铝榴石和钙铝榴石端员组分为主,贫镁铝榴石组分,并具成分环带:核部相对富铁铝榴石分子,贫镁铝榴石分子,而边部则相反。透辉石同样富铁、贫镁,Fe2 /(Fe2 Mg)比值一般大于0.6。铁角闪石呈熔蚀状被透辉石和石榴石所包裹,或产出于自形的透辉石颗粒之间,以贫钛为特征。该岩石的全岩成分以SiO2(59%～61%)、Fe2O3(全铁)(18%～19%)、CaO(12%～13%)和Al2O3(5.3%～5.4%)为主。运用石榴石-单斜辉石温压计估算出该岩石石榴石与透辉石的平衡温压条件分别为650～700℃、0.8～1.0GPa。这些特征表明该石榴辉石岩很可能系富铁的斜长角闪岩在中地壳尺度发生脱水所形成的残余。本文研究提供了一个角闪岩相石榴辉石岩的实例。     

大别山北麓尖晶石橄榄岩中石榴辉石岩包体的成因矿物学信息

大别山北坡霍山饶拔寨等地的超基性岩中含有石榴辉石岩的包体。石榴辉石岩为草绿色致密块状 ,呈分米级的块体出现于蛇纹石化强烈的橄榄岩中。运用成因矿物学的方法 ,研究对比了石榴辉石岩的主要矿物组成石榴子石 ( Prp2 5— 3 5 )和钠质普通辉石 ( Jd1 0— 2 5 )等。岩石结构显示退变质作用有两期 :榴辉岩相退变形成的麻粒岩相矿物组合明显地被角闪岩相所切割。石榴辉石岩的寄主岩是尖晶石橄榄岩类 ,包括尖晶石方辉橄榄岩和尖晶石二辉橄榄岩。由于强烈的蛇纹石化 ,残余的橄榄石 ( Fo92— 93 )仅占 5%～ 4 0 % ,斜方辉石富镁 ( En87— 93)并有解理弯曲等韧性变形现象。采用 Ellisand Green的石榴子石单斜辉石 Fe-Mg交换平衡温度计 ,可计算出石榴辉石岩的 Fe-Mg分配系数 ( KD)为 4 .0 6～ 5.2 8。变质温度 t=84 1～ 94 3℃ ,估算压力 p=1 .5GPa,可以推测该橄榄岩体是从深度约 60 km的地幔 ,固态侵位于下地壳 ,而后与之一起隆升到地表。显然 ,此种石榴辉石岩应属 Coleman所划分的 A型榴辉岩 ,它与地幔岩浆作用有密切关系。石榴辉石岩和橄榄岩的岩石化学特征和稀土配分形式 ,说明它们的化学性质相当于地幔部分熔融所形成的玄武岩熔体及其残留体。在侧重探讨石榴辉石岩及其有关岩石中主要造岩矿物的成因矿物?     

东海榴辉岩的早期退变质—后成合晶的形成、特点及地质意义

在苏北东海地区出露一种由富石榴子石基质与绿辉石脉组成的榴辉岩。这种榴辉岩有些经历过很强的后榴辉岩阶段变形作用，有些则没有。不论变形与否，其早期退变质均表现为绿辉石脉中沿绿辉石边界发育的细脉状透辉石＋钠长石后成合晶。后成合晶的形态及成分变化特点显示：弱变形榴辉岩的退变质作用是在榴辉岩折返到一定深度后，绿辉石颗粒间产生微裂隙，富SiO2流体进入并与绿辉石反应，形成后成合晶；而在强变形榴辉岩中，伸展性构造变形使绿辉石颗粒间形成微裂隙，富SiO2流体沿此进入，与绿辉石反应形成后成合晶。不同原因引起的变形为流体进入榴辉岩内部提供通道。榴辉岩的早期退变质发生在绿辉石脆－塑性变形发生转换的温压条件下，反映了榴辉岩折返过程中榴辉岩体与花岗质围岩间的物质交换。     

An experimental investigation of sub-solidus assemblages formed at high pressure in high-alumina basalt,kyanite eclogite and grosspydite compositions

The sub-solidus fields of crystallization of a spectrum of synthetic aluminous basic compositions (high-alumina basalt, anorthite-enriched high-alumina basalt, kyanite eclogite, grosspydite and gabbroic anorthosite) have been investigated at pressures of up to 36 kb. At low pressures the assemblages are characterized by abundant plagioclase, clinopyroxene and possibly minor olivine and orthopyroxene. These correspond to natural gabbroic and pyroxene granulite assemblages. As pressure is increased garnet appears and increases gradually in amount at the expense of other ferromagnesian minerals and plagioclase, until finally at pressures of >23 kb at 1,100° C, plagioclase disappears and high pressure clinopyroxene+garnet+kyanite±quartz assemblages equivalent to eclogite are obtained. In the eclogite stability field, with further rise in pressure, the ratio ga/cpx and the grossular content of the garnet increase.In the high-alumina basalt composition the transitional garnet granulite assemblage (clinopyroxene+plagioclase+garnet±quartz) is spread over a pressure interval of 11 kb at 1,100° C. This is a greater interval than observed for other basalt compositions and is important in considering the hypothesis that the Mohorovicic Discontinuity is a phase change from basalt to eclogite. It indicates that the change in V _p would be spread over a significant depth range, and no sharp seismic velocity discontinuity could result.The first experimental synthesis of kyanite eclogite from both high-alumina basalt and kyanite eclogite compositions has been obtained, as well as synthesis of unusual grossular-clinopyroxene-kyanite assemblages (grosspydite) from grosspydite and gabbroic anorthosite compositions. The pressures needed to synthesize these assemblages are somewhat greater than the pressures needed to synthesize eclogite from basic compositions of lower alumina content at the same temperature. Experimental confirmation of the observation that there is a direct relation between Gross/Alm + Py ratio of garnet and the Jd/Di ratio of co-existing pyroxene in grosspydite and kyanite eclogite assemblages found in kimberlite pipes has also been obtained.     

Olivine‐bearing symplectites in fractured garnet from eclogite,Moldanubian Zone (Bohemian Massif) – a short‐lived,granulite facies event

Recent petrological studies on high‐pressure (HP)–ultrahigh‐pressure (UHP) metamorphic rocks in the Moldanubian Zone, mainly utilizing compositional zoning and solid phase inclusions in garnet from a variety of lithologies, have established a prograde history involving subduction and subsequent granulite facies metamorphism during the Variscan Orogeny. Two temporally separate metamorphic events are developed rather than a single P–T loop for the HP–UHP metamorphism and amphibolite–granulite facies overprint in the Moldanubian Zone. Here further evidence is presented that the granulite facies metamorphism occurred after the HP–UHP rocks had been exhumed to different levels of the middle or upper crust. A medium‐temperature eclogite that is part of a series of tectonic blocks and lenses within migmatites contains a well‐preserved eclogite facies assemblage with omphacite and prograde zoned garnet. Omphacite is partly replaced by a symplectite of diopside + plagioclase + amphibole. Garnet and omphacite equilibria and pseudosection calculations indicate that the HP metamorphism occurred at relatively low temperature conditions of ~600 °C at 2.0–2.2 GPa. The striking feature of the rocks is the presence of garnet porphyroblasts with veins filled by a granulite facies assemblage of olivine, spinel and Ca‐rich plagioclase. These minerals occur as a symplectite forming symmetric zones, a central zone rich in olivine that is separated from the host garnet by two marginal zones consisting of plagioclase with small amounts of spinel. Mineral textures in the veins show that they were first filled mostly by calcic amphibole, which was later transformed into granulite facies assemblages. The olivine‐spinel equilibria and pseudosection calculations indicate temperatures of ~850–900 °C at pressure below 0.7 GPa. The preservation of eclogite facies assemblages implies that the granulite facies overprint was a short‐lived process. The new results point to a geodynamic model where HP–UHP rocks are exhumed to amphibolite facies conditions with subsequent granulite facies heating by mantle‐derived magma in the middle and upper crust.     

胶东威海地区榴辉岩退变质的地球动力学信息

威海地区榴辉岩退变质过程表现为三个阶段：第一阶段,原生绿辉石分解形成钠质单斜辉石＋斜长石合晶体;第二阶段,原生石榴石及钢质单斜辉石＋斜长石合晶体,周边出现角闪石＋斜长石　状反应边;第三阶段,石榴石及钢质单斜辉石消失。其后榴辉岩相退变质的ｐ－ｔ演化轨迹是压力相对温度快速降低的顺时针形式,反映本区榴辉岩折返的地球动力学过程可能是在经历了快速上升的构造侵位同时,晚元古宙巨量花岗岩浆可将相辉岩块体携带到上部地壳。     

P–T evolution of kyanite eclogite from the Pirin Mountains (SW Bulgaria): implications for the Rhodope UHP Metamorphic Complex

A new occurrence of kyanite eclogite in the Pirin Mountains of southwestern Bulgaria within the rocks belonging to the Obidim Unit of the Rhodope Metamorphic Complex is presented. This eclogite provides important information about the peak–pressure conditions despite strong thermal overprint at low pressure. Textural relationships, phase equilibrium modelling and conventional geothermobarometry were used to constrain the metamorphic evolution. Garnet porphyroblasts with inclusions of omphacite (up to 43 mol.% Jd), phengite (up to 3.5 Si p.f.u.), kyanite, polycrystalline quartz, pargasitic amphibole, zoisite and rutile in the Mg‐rich cores (X_Mg = 0.44–0.46) record a prograde increase in P–T conditions from ～2.5 GPa and 650 °C to ～3 GPa and 700–750 °C. Maximum pressure values fall within the stability field of coesite. During exhumation, the peak–pressure assemblage garnet + omphacite + phengite + kyanite was variably overprinted by a lower pressure one forming symplectitic textures, such as diopside + plagioclase after omphacite and biotite + plagioclase after phengite. The development of spinel (X_Mg = 0.4–0.45) + corundum + anorthite assemblage in the kyanite‐bearing domains at ～1.1 GPa and 800–850 °C suggests a thermal overprint in the high‐pressure granulite facies stability field. This thermal event was followed by cooling at ～0.8 GPa under amphibolite facies conditions; retrograde kelyphite texture involving plagioclase and amphibole was developed around garnet. Our results add to the already existing evidence for ultra high pressure (UHP) metamorphism in the Upper Allochthon of the Rhodope Metamorphic Complex as in the Kimi Unit and show that it is more widespread than previously known. Published age data and field structural relations suggest that the Obidim Unit represents Variscan continental crust involved into the Alpine nappe edifice of the Rhodopes and that eclogite facies metamorphism was Palaeozoic, in contrast to the Kimi Unit where age determinations suggest a Jurassic or Cretaceous age for UHP metamorphism. This implies that UHP metamorphism in the Upper Allochthon of the Rhodopes may have occurred twice, during Alpine and pre‐Alpine orogenic events, and that two independent HP/UHP provinces of different age overlap in this area.     

Integrating X‐ray mapping and microtomography of garnet with thermobarometry to define the P–T evolution of the (near) UHP Międzygórze eclogite,Sudetes, SW Poland

Detailed X‐ray compositional mapping and microtomography have revealed the complex zoning and growth history of garnet in a kyanite‐bearing eclogite. The garnet occurs as clusters of coalesced grains with cores revealing slightly higher Ca and lower Mg than the rims forming the coalescence zones between the grains. Core regions of the garnet host inclusions of omphacite with the highest jadeite, and phengite with the highest Si, similar to values in the cores of omphacite and phengite located in the matrix. Therefore, the core compositions of garnet, omphacite, and phengite have been chosen for the peak pressure estimate. Coupled conventional thermobarometry, average P–T, and phase equilibrium modelling in the NCKFMMnASHT system yields P–T conditions of 26–30 kbar at 800–930°C. Although coesite is not preserved, these P–T conditions partially overlap the coesite stability field, suggesting near ultra‐high–pressure (UHP) conditions during the formation of this eclogite. Therefore, the peak pressure assemblage is suggested to have been garnet–omphacite–kyanite–phengite–coesite/quartz–rutile. Additional lines of evidence for the possible UHP origin of the Mi?dzygórze eclogite are the presence of rod‐shaped inclusions of quartz parallel to the c‐axis in omphacite as well as relatively high values of Ca‐Tschermak and Ca‐Eskola components. Late zoisite, rare diopside–plagioclase symplectites rimming omphacite, and minor phlogopite–plagioclase symplectites replacing phengite formed during retrogression together with later amphibole. These retrograde assemblages lack minerals typical of granulite facies, which suggests simultaneous decompression and cooling during exhumation before the crustal‐scale folding that was responsible for final exhumation of the eclogite.     

High-pressure eclogites in northern Jiangsu – southern Shandong province, eastern China

Abstract Eclogites are distributed for more than 500 km along a major tectonic boundary between the Sino-Korean and Yangtze cratons in central and eastern China. These eclogites usually have high-P assemblages including omphacite + kyanite and/or coesite (or its pseudomorph), and form a high-P eclogite terrane. They occur as isolated lenses or blocks 10 cm to 300 m long in gneisses (Type I), serpentinized garnet peridotites (Type II) and marbles (Type III). Type I eclogites were formed by prograde metamorphism, and their primary metamorphic mineral assemblage consists mainly of garnet [pyrope (Prp) = 15–40 mol%], omphacite [jadeite (Jd) = 34–64 mol%], pargasitic amphibole, kyanite, phengitic muscovite, zoisite, an SiO₂ phase, apatite, rutile and zircon. Type II eclogites characteristically contain no SiO₂ phase, and are divided into prograde eclogites and mantle-derived eclogites. The prograde eclogites of Type II are petrographically similar to Type I eclogites. The mantle-derived eclogites have high MgO/(FeO + Fe₂O₃) and Cr₂O₃ compositions in bulk rock and minerals, and consist mainly of pyrope-rich garnet (Prp = 48–60 mol%), sodic augite (Jd = 10–27 mol%) and rutile. Type III eclogites have an unusual mineral assemblage of grossular-rich (Grs = 57 mol%) garnet + omphacite (Jd = 30–34 mol%) + pargasite + rutile. Pargasitic and taramitic amphiboles, calcic plagioclase (An₆₈), epidote, zoisite, K-feldspar and paragonite occur as inclusions in garnet and omphacite in the prograde eclogites. This suggests that the prograde eclogites were formed by recrystallization of epidote amphibolite and/or amphibolite facies rocks with near-isothermal compression reflecting crustal thickening during continent–continent collision of late Proterozoic age. Equilibrium conditions of the prograde eclogites range from P > 26 kbar and T= 500–750°C in the western part to P > 28 kbar and T= 810–880°C in the eastern part of the high-P eclogite terrane. The prograde eclogites in the eastern part are considered to have been derived from a deeper position than those in the western part. Subsequent reactions, manifested by (1) narrow rims of sodic plagioclase or paragonite on kyanite and (2) symplectites between omphacite and quartz are interpreted as an effect of near-isothermal decompression during the retrograde stage. The conditions at which symplectites re-equilibrated tend to increase from west (P < 10 kbar and T < 580°C) to east (P > 9 kbar and T > 680°C). Equilibrium temperatures of Type II mantle-derived eclogites and Type III eclogite are 730–750°C and 680°C, respectively.     

Petrology of ultrahigh-pressure eclogites from the ZK703 drillhole in the Donghai, eastern China

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.     

The Effects of Disequilibrium and Deformation on the Mineralogical Evolution of Quartz Diorite During Metamorphism in the Eclogite Facies

In the Sesia Zone, Western Alps, a large volume of orthogneissformed as a result of eclogite fades metamorphism and deformationof quartz diorite during early Alpine underthrusting and subduction.Rare lenses of undeformed metaquartz diorite, preserved withinthe orthogneiss, represent an early stage in the evolution ofthis latter rock type. The metamorphic and microstructural evolutionof the orthogneiss in the eclogite fades has been reconstructedfrom studies of gradational contacts between undeformed andstrongly deformed rocks. High pressure transformations of the original igneous plagioclase+ biotite + quartz assemblage to jadeitic pyroxene (Jd_{0.95 –0.85}+ zoisite + quartz + garnet + 2 muscovites developed prior todeformation. Slow intergranular diffusion resulted in a stateof disequilibrium between small textural domains in the metaquartzdiorite. The compositions of the phases of the undeformed metaquartzdiorite do not reflect the bulk rock composition, but were controlledby their position relative to reactant phases. The jadeiticpyroxenes, for example, formed in localized domains which originallyconsisted of sodic plagioclase whereas omphacite was the equilibriumpyroxene for the bulk rock composition. Mineralogical changes which occurred during subsequent deformationof the metaquartz diorite are interpreted as resulting froma progressive enlargement of equilibrium domains and the partialequilibration of mineral compositions to the bulk rock compositionrather than from changes in pressure and temperature. Initiallyduring high-strain deformation, fine-grained aggregates of jadeiticpyroxene + quartz + zoisite (originally pseudomorphing plagioclase)are inferred to have deformed by a mechanism of grain boundarysliding accommodated by diffusive mass transfer. Muscovite andgarnet compositions homogenized during the deformation but dueto slow intracrystalline diffusion, pyroxene compositions (Jd_0.95–0.80) remained metastable. The coarsening of pyroxeneeventually terminated deformation by grain boundary slidingand this mineral subsequently deformed by intracrystalline plastidty.This latter process was accompanied by and perhaps catalyseda change in pyroxene composition from metastable jadeite towardsomphacite by a reaction involving the resorption of garnet andthe nucleation and growth of paragonite. The resulting orthogneissconsists of quartz + omphadte + garnet + phengite + paragonite+ zoisite. The rock is characterized by a broad range of pyroxenecompositions (Jd_{0.8 –0.5}) due to the incomplete equilibrationof this mineral to the bulk rock composition and a lack of Fe-Mgexchange equilibrium between pyroxene and garnet. However, incontrast to the undeformed metaquartz diorite, there are noobvious textural indications of disequilibrium between phasesin the orthogneiss     

Eclogite facies relics and a multistage breakdown in metabasites of the KTB pilot hole,NE Bavaria: implications for the Variscan tectonometamorphic evolution of the NW Bohemian Massif

Complex reaction textures in coronitic metagabbros and retrograded eclogites of the KTB pilot and an adjacent drilling provide evidence for a multistage metamorphic history in the Variscan basement of the NW Bohemian Massif. The eclogites show complete metamorphic recrystallization leaving no textural or mineral relics of their igneous precursors. In contrast, textural relics of the igneous protolith are still preserved in the metagabbros where the metamorphic overprint under high pressure conditions achieved only partial replacement of the initial assemblage plagioclase + augite + amphibole (+olivine or orthopyroxene?) + ilmenite to form the eclogite facies assemblage garnet + omphacite + kyanite + zoisite + quartz+rutile. The garnets in the metagabbros occur in the typical ‘necklace’ fashion at the borders between the original plagioclase and mafic phase domains. In the same rocks, omphacite formed by a topotactic reaction mechanism replacing igneous augite as well as in smaller grains at the margins of the texturally igneous clinopyroxene where it occurs without fixed orientation with respect to the relict phase. Both eclogites and metagabbros show a partial breakdown under high pressure granulite (transitional to high pressure amphibolite) facies conditions during which omphacite broke down to vermicular symplectites of diopside + plagioclase. A later pervasive medium pressure metamorphism under amphibolite facies conditions led to the development of assemblages dominated by hornblende + plagioclase+titanite: phases prevailing in the overwhelming majority of the surrounding metabasites. Subsequent vein-associated retrogression produced minerals typical of the greenschist to zeolite facies. All metamorphic stages may be represented in a single thin section but although the overall reaction sequence is apparent, the obvious disequilibrium in the rocks makes the use of conventional geothermobarometry difficult. However, calculations made by assuming an approach to domainal equilibrium show that both the eclogite facies and early breakdown occurred above 10 kb. As the metamorphic unit hosting these particular metabasites is generally characterized by pressures below 10 kb these results have important implications for understanding the tectonometamorphic evolution of the region. The relationship between the studied rocks and other units in the NW Bohemian Massif exhibiting a multistage metamorphic evolution is discussed and possible tectonic models evaluated.     

Prograde and retrograde history of eclogites from the Eastern Blue Ridge, North Carolina, USA

Abstract The prograde metamorphism of eclogites is typically obscured by chemical equilibration at peak conditions and by partial requilibration during retrograde metamorphism. Eclogites from the Eastern Blue Ridge of North Carolina retain evidence of their prograde path in the form of inclusions preserved in garnet. These eclogites, from the vicinity of Bakersville, North Carolina, USA are primarily comprised of garnet–clinopyroxene–rutile–hornblende–plagioclase–quartz. Quartz, clinopyroxene, hornblende, rutile, epidote, titanite and biotite are found as inclusions in garnet cores. Included hornblende and clinopyroxene are chemically distinct from their matrix counterparts. Thermobarometry of inclusion sets from different garnets record different conditions. Inclusions of clinozoisite, titanite, rutile and quartz (clinozoisite + titanite = grossular + rutile + quartz + H₂O) yield pressures (6–10 kbar, 400–600 °C and 8–12 kbar 450–680 °C) at or below the minimum peak conditions from matrix phases (10–13 kbar at 600–800 °C). Inclusions of hornblende, biotite and quartz give higher pressures (13–16 kbar and 630–660 °C). Early matrix pyroxene is partially or fully broken down to a diopside–plagioclase symplectite, and both garnet and pyroxene are rimmed with plagioclase and hornblende. Hypersthene is found as a minor phase in some diopside + plagioclase symplectites, which suggests retrogression through the granulite facies. Two‐pyroxene thermometry of this assemblage gives a temperature of c. 750 °C. Pairing the most Mg‐rich garnet composition with the assemblage plagioclase–diopside–hypersthene–quartz gives pressures of 14–16 kbar at this temperature. The hornblende–plagioclase–garnet rim–quartz assemblage yields 9–12 kbar and 500–550 °C. The combined P–T data show a clockwise loop from the amphibolite to eclogite to granulite facies, all of which are overprinted by a texturally late amphibolite facies assemblage. This loop provides an unusually complete P–T history of an eclogite, recording events during and following subduction and continental collision in the early Palaeozoic.