新疆西天山高压变质带的变质矿物与变质作用演化

新疆西天山高压变质带主要由石榴石，角闪石，绿辉石，多硅白云母，钠云母，绿帘石，绿泥石，钠长石，石英，榍石和金红石等组成，石榴石主要含铁铝榴石组份，角闪石有蓝闪石，亚铁蓝闪石，青铝闪石，冻蓝闪石等类型，变质矿物组合显示高压变质带经历了由硬柱石蓝片岩相，榴辉岩相，绿帘蓝片岩相至绿片岩相的变质作用演化进程。     

澜沧江杂岩带小黑江-上允地区蓝片岩的成因及变质演化

通过对澜沧江杂岩带小黑江-上允地区蓝片岩的岩相学、地球化学、成因矿物学以及相平衡模拟的综合研究,阐述蓝片岩的原岩以及变质演化过程。地球化学分析结果显示,蓝片岩具有一致的稀土元素配分模式,具弱Eu正或负异常,稀土元素和微量元素特征与OIB相似,其原岩可能为OIB型玄武岩。详细矿物学研究表明,本区蓝片岩记录了俯冲峰期蓝片岩相变质和峰期后绿片岩相变质两个变质阶段,其矿物组合分别为蓝闪石+钠长石+多硅白云母+绿泥石+绿帘石和蓝闪石+钠长石±阳起石+绿泥石+绿帘石。通过Na_2O-Ca O-Fe O-MgO-Al_2O_3-SiO_2-H_2O-O体系相平衡计算,得到两个阶段的压力范围分别约为0.95 GPa和0.40 GPa。     

新疆西南天山哈布腾苏河高压-超高压变质带中榴辉岩和蓝片岩的岩石学及变质演化

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。     

北祁连山硬柱石蓝片岩p-T条件相平衡计算及其岩石学意义

北祁连硬柱石蓝片岩主要分布在甘肃省肃南县九个泉一带,是目前中国唯一报道的、确切地含有硬柱石的蓝片岩。文中在详细的岩石学和矿物学研究基础上,根据矿物共生组合的不同,将北祁连低温蓝片岩进一步划分为绿纤石蓝片岩、硬柱石蓝片岩和绿帘石蓝片岩。绿纤石蓝片岩的特征变质矿物组合为蓝闪石(>40%)+绿纤石(30%)+绿泥石(10%)+钠长石(8%)+石英(5%)+硬柱石(<3%)±方解石/文石(<1%)。硬柱石蓝片岩的矿物组合为蓝闪石(35%~40%)+硬柱石(35%~40%)+绿泥石(10%)+钠长石(10%)+石榴石(1%~2%)+黝帘石/斜黝帘石(<2%)+石英(<1%),副矿物有磷灰石和榍石,总含量小于2%。绿帘石蓝片岩的矿物组合为蓝闪石(30%~35%)+黝帘石/斜黝帘石/绿帘石(~30%)+绿泥石(15%)+钠长石(15%)+石榴石(2%)+石英(<2%),副矿物有金红石、磷灰石和磁铁矿,总含量小于2%。利用矿物内部一致性热力学数据和Domino/Theriak软件计算了这三种类型的蓝片岩形成的峰期温压条件,它们分别是绿纤石蓝片岩为320~350℃,0.75~0.85GPa;硬柱石蓝片岩为335~355℃,0.8~0.95GPa;绿帘石蓝片岩为345~375℃;0.75~0.85GPa。北祁连低温蓝片岩带由硬柱石蓝片岩相到绿帘石蓝片岩相的转化代表了俯冲变质过程中的递进变质过程。     

藏北羌塘中部戈木日榴辉岩的岩石学、矿物学及变质作用pTt轨迹

羌塘中部榴辉岩位于龙木错－双湖缝合带中段,改则县古姆乡片石山地区。榴辉岩的主要矿物成分为石榴子石、绿辉石、多硅白云母、金红石、角闪石等,围岩为石榴白云母片岩和蓝片岩,石榴白云母片岩主要由石榴子石、多硅白云母和石英构成,蓝片岩由石榴子石、角闪石（含蓝闪石）、多硅白云母等构成。岩石学和矿物学研究显示,榴辉岩主要经历了3期变质作用：①峰期榴辉岩相变质作用阶段,以石榴子石、绿辉石和多硅白云母为特征,变质温度和压力分别为500℃和2.3GPa;②绿帘角闪岩相变质作用阶段,以后期形成的冻蓝闪石、镁红闪石、绿帘石、钠长石等交代早期矿物为特征;③绿片岩相变质作用阶段,以毛发状阳起石等为特征。榴辉岩的变质演化过程代表了青藏高原北部古特提斯洋俯冲消减和冈瓦纳与劳亚大陆碰撞造山的过程。     

新疆西南天山含柯石英泥质片岩的岩石学特征及变质作用p-T轨迹

新疆西南天山高压-超高压变质带主要由泥质-长英质片岩组成,其中包裹榴辉岩、蓝片岩和超基性岩等透镜体。含柯石英泥质片岩具斑状/筛状变晶结构,片状构造,矿物组合为石榴石、多硅白云母、钠长石、钠云母、蓝闪石、冻蓝闪石、石英以及少量褐帘石、金红石和榍石,柯石英包裹在石榴石变斑晶内。根据岩石学特征和相平衡模拟的结果,识别出含柯石英泥质片岩经历了3期变质演化阶段:压力峰期之前的进变质阶段(Ⅰ),由石榴石核部到含柯石英区域的环带确定,特征为温度和压力同时升高,所限定的压力峰期条件为500℃、2.9 GPa,模拟的矿物组合为石榴石+蓝闪石+硬玉+纤柱石+硬柱石+金红石+多硅白云母+柯石英,与含柯石英的事实相符;压力峰期之后的升温降压至温度峰期阶段(Ⅱ),由石榴石含柯石英区域到边部的环带确定,特征为温度升高压力降低,所限定的温度峰期条件为560℃、2.35 GPa,模拟的矿物组合为石榴石+蓝闪石+硬玉+硬柱石+金红石+多硅白云母+石英,发生连续脱水反应蓝闪石+纤柱石+硬柱石=石榴石+硬玉+水,释放出岩石中约45%的水,导致柯石英转变为石英,纤柱石消失;温度峰期之后的近等温降压阶段(Ⅲ),由晚期矿物组合钠云母+钠长石+冻蓝闪石+榍石+石英的稳定温压条件确定(495~550℃、1.0~1.15 GPa),减压抬升过程中,在约2.1 GPa处,发生脱水反应硬玉+硬柱石=蓝闪石+钠云母+水,导致硬柱石消失,钠云母出现,在约1.1~1.25 GPa处,榍石取代金红石,绿辉石消失,钠长石和冻蓝闪石出现。阶段Ⅱ强烈的连续脱水过程十分利于矿物组合的再平衡,导致绝大多数压力峰期的柯石英转变为石英,仅有极少数因包裹在刚性石榴石中而得以保存。含柯石英泥质片岩及其榴辉岩透镜体经历了完全相同的俯冲折返过程。     

早古生代北阿尔金HP/LT混杂岩片——一个“化石”俯冲隧道

北阿尔金高压/低温(HP/LT)变质岩呈构造岩片分布在俯冲-增生杂岩中，主要由强变形的变质沉积岩(泥质片岩、钙质片岩和石英片岩)和少量呈透镜状分布在变沉积岩中的蓝片岩和榴辉岩组成，与相邻的蛇绿混杂岩呈断层接触。榴辉岩主要矿物为绿辉石、石榴子石、多硅白云母、石英、冻蓝闪石，含少量蓝闪石、绿泥石、方解石、钠长石、榍石。蓝片岩主要矿物为蓝闪石、石榴子石、碳酸盐类矿物、阳起石、绿帘石、钠云母、绿泥石、钠长石和石英，偶见多硅白云母，其中在部分蓝片岩的石榴子石中有少量硬柱石和绿辉石包体。本文对蓝片岩(样品A06-16-7)和榴辉岩(样品A03-3-5.3)开展了岩石学和相平衡模拟研究，得到它们形成的压力峰期的温压条件分别是：T=～524℃、P=～2.1GPa和T=～527℃、P=～2.2GPa,并均经历了后期蓝片岩相的退变质叠加。结合区域上已有的研究表明，北阿尔金HP/LT混杂岩片中不同类型岩石可能经历了不同的变质演化历史，反映了古俯冲隧道的不均匀性，并在俯冲隧道的较浅部混杂在一起，共同经历了蓝片岩相或蓝片岩-绿片岩过渡相条件下的透入性变形作用。     

安徽省大别山南部宿松杂岩变质作用研究

宿松杂岩的变质作用可分为３个阶段：早期、主期和晚期阶段。主期阶段的矿物组合在云母片岩中为石榴石＋多硅白云母＋石英＋磁铁矿±钠云母±绿帘石／黝帘石；在白云母钠长（二长）片岩和片麻岩中为石榴石＋多硅白云母＋钠长石＋石英＋绿帘石／黝帘石±冻蓝闪石；在石榴石钠长角闪岩中为石榴石＋冻蓝闪石＋钠长石＋黝帘石＋石英±钠云母±金红石／磁铁矿。根据多种地质温压计和变质反应可以推测主期变质条件为：Ｔ＝５２０℃～５８０℃，Ｐ＝１．２～１．４ＧＰａ，地热梯度为１２℃／ｋｍ，相当于高压过渡型。晚期阶段变质条件为：Ｔ＝４６０℃～４８０℃，Ｐ＝０．６～０．７ＧＰａ，为中压绿帘角闪岩相。宿松杂岩的变质条件介于其南部的蓝闪绿片岩相和北部的榴辉岩相之间，三者是扬子板块向华北板块之下俯冲到不同深度的产物。     

中国蓝闪片岩相的变质作用

本文论述了中国蓝闪片岩的分布、变质条件及其构造位置。中国的蓝闪片岩从中晚元古代开始,各变质期均有出现。根据矿物组合,可分为两类:第一类蓝闪片岩常含有高压矿物,如硬柱石、硬玉和文石以及蓝闪石、绿纤石、黑硬绿泥石、多硅白云母、红帘石等,属高压亚绿片岩相,称蓝闪—硬柱石片岩相,形成温度约250—350℃,压力大于500—800MPa,甚至可达1200MPa。此类蓝闪—硬柱石片岩相多代表海洋板块古消减带。第二类蓝闪片岩的常见矿物为蓝闪石、青铝闪石或镁钠闪石、黑硬绿泥石、红帘石和绿片岩相中的绿帘石、阳起石、绿泥石、白云母、有时还有黑云母、铁铝榴石和钠质辉石。形成温度约350—450℃,压力500—800MPa。此类蓝闪绿片岩相虽处于活动带,但与板块构造没有直接关系。我国西藏南部和内蒙温都尔庙属第一类,但大部分蓝闪片岩带属第二类。     

佳木斯地块南缘牡丹江地区高压变质作用：黑龙江杂岩的岩石学和地质年代学

牡丹江地区出露的黑龙江杂岩是由变质基性岩、泥质片岩、大理岩和变硅质岩系列组成的类似于蛇绿岩层序的构造混杂岩,代表了佳木斯地块西南缘碰撞增生的产物。钠长石和绿帘石变斑晶中共生的矿物组合(钠云母+蓝闪石+多硅白云母+绿帘石)的地质温压计估算结果表明,黑龙江杂岩的峰期变质的温压条件为 T=320~480℃, p=800~1600MPa,为典型的绿帘-蓝片岩相高压变质作用。黑龙江杂岩带中泥质片岩所含的多硅白云母单矿物给出的40Ar/39Ar坪年龄为164.9Ma±0.5Ma。根据白云母中K-Ar同位素体系的封闭温度(350~430℃)接近于得到的黑龙江杂岩的峰期变质温度,可以认为40Ar/39Ar坪年龄指示了这期高压变质事件的年龄,也证实了黑龙江杂岩带中普遍存在一期中侏罗世末期的变质-热事件。     

The Sabzevar blueschists of the North-Central Iranian micro-continent as remnants of the Neotethys-related oceanic crust subduction

The Sabzevar ophiolites mark the Neotethys suture in east-north-central Iran. The Sabzevar metamorphic rocks, as part of the Cretaceous Sabzevar ophiolitic complex, consist of blueschist, amphibolite and greenschist. The Sabzevar blueschists contain sodic amphibole, epidote, phengite, calcite ± omphacite ± quartz. The epidote amphibolite is composed of sodic-calcic amphibole, epidote, albite, phengite, quartz ± omphacite, ilmenite and titanite. The greenschist contains chlorite, plagioclase and pyrite, as main minerals. Thermobarometry of a blueschist yields a pressure of 13–15.5 kbar at temperatures of 420–500 °C. Peak metamorphic temperature/depth ratios were low (~12 °C/km), consistent with metamorphism in a subduction zone. The presence of epidote in the blueschist shows that the rocks were metamorphosed entirely within the epidote stability field. Amphibole schist samples experienced pressures of 5–7 kbar and temperatures between 450 and 550 °C. The presence of chlorite, actinolite, biotite and titanite indicate greenschist facies metamorphism. Chlorite, albite and biotite replacing garnet or glaucophane suggests temperatures of >300 °C for greenschist facies. The formation of high-pressure metamorphic rocks is related to north-east-dipping subduction of the Neotethys oceanic crust and subsequent closure during lower Eocene between the Central Iranian Micro-continent and Eurasia (North Iran).     

Iranshahr blueschist: subduction of the inner Makran oceanic crust

The Makran accretionary prism in SE Iran and SW Pakistan is one of the most extensive subduction accretions on Earth. It is characterized by intense folding, thrust faulting and dislocation of the Cenozoic units that consist of sedimentary, igneous and metamorphic rocks. Rock units forming the northern Makran ophiolites are amalgamated as a mélange. Metamorphic rocks, including greenschist, amphibolite and blueschist, resulted from metamorphism of mafic rocks and serpentinites. In spite of the geodynamic significance of blueschist in this area, it has been rarely studied. Peak metamorphic phases of the northern Makran mafic blueschist in the Iranshahr area are glaucophane, phengite, quartz±omphacite+epidote. Post peak minerals are chlorite, albite and calcic amphibole. Blueschist facies metasedimentary rocks contain garnet, phengite, albite and epidote in the matrix and as inclusions in glaucophane. The calculated P–T pseudosection for a representative metabasic glaucophane schist yields peak pressure and temperature of 11.5–15 kbar at 400–510 °C. These rocks experienced retrograde metamorphism from blueschist to greenschist facies (350–450 °C and 7–8 kbar) during exhumation. A back arc basin was formed due to northward subduction of Neotethys under Eurasia (Lut block). Exhumation of the high‐pressure metamorphic rocks in northern Makran occurred contemporarily with subduction. Several reverse faults played an important role in exhumation of the ophiolitic and HP‐LT rocks. The presence of serpentinite shows the possible role of a serpentinite diapir for exhumation of the blueschist. A tectonic model is proposed here for metamorphism and exhumation of oceanic crust and accretionary sedimentary rocks of the Makran area. Vast accretion of subducted materials caused southward migration of the shore.     

黑龙江依兰地区黑硬绿泥石片岩岩石学与P-T条件研究

依兰地区黑龙江杂岩中黑硬绿泥石片岩矿物组合为黑硬绿泥+绿泥石+多硅白云母+钠长石+透闪石+钾长石±黑云母.岩石矿物学研究表明黑硬绿泥石片岩形成于250～400℃,6～9kbar的变质条件下.黄褐色黑硬绿泥石呈束状、放射状集合体.黑硬绿泥石片岩形成于佳木斯地块向西与松嫩地块俯冲拼贴过程中蓝片岩变质作用的后期,压力稍有降低而温度略有升高的变质变形环境,是蓝片岩向绿片岩转变的蓝闪绿片岩相变质条件下的产物.     

SOME TRENDS IN THE STUDY OF CARBONATE SEDIMENTS IN THE FEDERAL REPUBLIC OF GERMANY

A stratigraphically coherent blueschist terrane near Aksu in northwestern China is unconformably overlain by unmetamorphosed sedimentary rocks of Sinian age (～600 to 800 Ma). The pre-Sinian metamorphic rocks, termed the Aksu Group, were derived from shales, sandstones, basaltic volcanic rocks, and minor cherty sediments. They have undergone multi-stage deformation and transitional blueschist/greenschist-facies metamorphism, and consist of strongly foliated chlorite-stilpnomelane-graphite schist, stilpnomelane-phengite psammitic schist, greenschist, blueschist, and minor quartzite, metachert, and meta-ironstone. Metamorphic minerals of basaltic blueschists include crossitic amphibole, epidote, chlorite, albite, quartz, and actinolite. Mineral parageneses and compositions of sodic amphibole suggest blueschist facies recrystallization at about 4 to 6 kbar and 300 to 400° C. Many thin diabasic dikes cut the Aksu Group; they are characterized by high alkali, TiO₂, and P₂O₅ contents and possess geochemical characteristics of within-plate basalts; some of these diabasic rocks contain sodic clinopyroxene and amphibole as primary phases and have minor pumpellyite, albite, epidote, chlorite, and calcite as the prehnite/pumpellyite-facies metamorphic assemblage. This prehnite/pumpellyite-facies overprint did not affect the host rocks of the blueschist-facies lithologies.

K-Ar and Rb-Sr ages of phengite and whole rocks from pelitic schists are ～690 to 728 Ma, and a ⁴⁰Ar/³⁹Ar age of crossite from the blueschist is 754 Ma. The basal conglomerate of the overlying Sinian to Eocambrian sedimentary succession contains clasts of both the blueschist and cross-cutting dike rocks, clearly demonstrating that conditions required for blueschist-facies metamorphism were attained and ceased at least 700 Ma. The northward-increasing metamorphic grade of the small blueschist terrane may reflect northward subduction of an accretionary complex beyond the northern edge of the Tarim craton. Abundant subparallel diabasic dikes indicate a subsequent period of Pre-Sinian rifting and diabasic intrusion along the northern margin of Tarim; a Sinian siliciclastic and carbonate sequence was deposited unconformably atop the Aksu Group and associated diabase dikes.     

Alpine metamorphism of Hercynian hornblende granodiorite beneath the blueschist facies schistes lustrés nappe of NE Corsica

Abstract The Hercynian granitic basement which forms the Tenda Massif in NE Corsica represents part of the leading edge of the European Plate during middle-to-late Cretaceous (Eoalpine) high P metamorphism. The metamorphism of this basement, induced by the overthrusting of a blueschist facies (schistes lustrés) nappe, was confined to a major ductile shear zone (c. 1000m thick) within which deformation increases upwards towards the overlying nappe. Metamorphism within the basement mostly records lower blueschist facies conditions (crossite + epidote) except near the base of the shear zone where the greenschist facies assemblage albite + actinolitic amphibole has developed instead of crossite. Study of the primary mafic phase breakdown reactions within hornblende granodiorite reveals the following metamorphic zonation. Zone 1: biotite to chlorite. Towards zone 2: biotite to phengite. Zone 2: Hornblende to actinolitic Ca-amphibole + albite + sphene, and biotite to actinolitic Ca-amphibole + albite + phengite + Ti-ore + epidote. Zone 3: Hornblende to crossite + low Ti-biotite + phengite + sphene, and biotite to crossite + low Ti-biotite + phengite + Ti-ore + sphene ± epidote. P-T conditions at the base of the shear zone are estimated to have been 390-490°C at 600-900 M Pa (6-9kbar) and the Corsican basement is therefore deduced to have been buried to 20-30 km during metamorphism. This relatively shallow metamorphism contrasts with some other areas in the Western Alps where the Eoalpine event apparently buried the European continental crust to depths of 80 km or more. As there is no evidence for a long history of blueschist facies metamorphism prior to the involvement of the European continent, it is deduced that the Eoalpine blueschists were produced during the collision of the Insubric plate with Europe, rather than during Tethyan intraoceanic subduction. Coherent blueschist terrains such as the schistes lustres probably record buovant feature collision and obduction tectonics rather than any preceding oceanic subduction.     

P–T evolution of glaucophane–omphacite bearing HP–LT rocks in the western Tianshan Orogen, NW China:new evidence for 'Alpine-type' tectonics

The late Palaeozoic western Tianshan high‐pressure /low‐temperature belt extends for about 200 km along the south‐central Tianshan suture zone and is composed mainly of blueschist, eclogite and epidote amphibolite/greenschist facies rocks. P–T conditions of mafic garnet omphacite and garnet–omphacite blueschist, which are interlayered with eclogite, were investigated in order to establish an exhumation path for these high‐pressure rocks. Maximum pressure conditions are represented by the assemblage garnet–omphacite–paragonite–phengite–glaucophane–quartz–rutile. Estimated maximum pressures range between 18 and 21 kbar at temperatures between 490 and 570 °C. Decompression caused the destabilization of omphacite, garnet and glaucophane to albite, Ca‐amphibole and chlorite. The post‐eclogite facies metamorphic conditions between 9 and 14 kbar at 480–570 °C suggest an almost isothermal decompression from eclogite to epidote–amphibolite facies conditions. Prograde growth zoning and mineral inclusions in garnet as well as post‐eclogite facies conditions are evidence for a clockwise P–T path. Analysis of phase diagrams constrains the P–T path to more or less isothermal cooling which is well corroborated by the results of geothermobarometry and mineral textures. This implies that the high‐pressure rocks from the western Tianshan Orogen formed in a tectonic regime similar to ‘Alpine‐type’ tectonics. This contradicts previous models which favour ‘Franciscan‐type’ tectonics for the southern Tianshan high‐pressure rocks.     

Glaucophane-bearing assemblage overprinted by greenschist-facies metamorphism in the Variscan Kaczawa complex, Sudetes, Poland

An Early Palaeozoic (Ordovician ?) metamudstone sequence near Wojcieszow, Kaczawa Mts, Western Sudetes, Poland, contains numerous metabasite sills, up to 50 m thick. These subvolcanic rocks are of within-plate alkali basalt type. Primary igneous phases in the metabasites, clinopyroxene (salite) and kaersutite, are veined and partly replaced by complex metamorphic mineral assemblages. Particularly, the kaersutite is corroded and rimmed by zoned sodic, sodic–calcic and calcic amphiboles. The matrix is composed of actinolite, pycnochlorite, albite (An ≤ 0.5%), epidote (Ps 27–33), titanite, calcite, opaques and, occasionally, biotite, phengite and stilpnomelane. The sodic amphiboles are glaucophane to crossite in composition with Na^B from 1.9 to 1.6. They are rimmed successively by sodic–calcic and calcic amphiboles with compositions ranging from magnesioferri-winchite to actinolite. No compositions between Na^B= 0.92 and Na^B= 1.56 have been ascertained. The textures may be interpreted as representing a greenschist facies overprint on an earlier blueschist (or blueschist–greenschist transitional) assemblage. The presence of glaucophane and no traces of a jadeitic pyroxene + quartz association indicate pressures between 6 and 12 kbar during the high-pressure episode. Temperature is difficult to assess in this metamorphic event. The replacement of glaucophane by actinolite + chlorite + albite, with associated epidote, allows restriction of the upper pressure limit of the greenschist recrystallization to <8 kbar, between 350 and 450°C. The mineral assemblage representing the greenschist episode suggests the P–T conditions of the high-pressure part of the chlorite or lower biotite zone. The latest metamorphic recrystallization, under the greenschist facies, may have taken place in the Viséan.     

Triassic blueschists and eclogites from northwest Turkey: vestiges of the Paleo-Tethyan subduction

Triassic eclogite and blueschist facies rocks occur as a thrust sheet, 25-km long and over 2-km thick, in an Eocene fold-and-thrust belt in northwest Turkey along the zmir–Ankara suture. The thrust sheet consists mainly of metabasites with minor marble, phyllite and metachert, and rare lenses of serpentinite. The common blueschist facies mineral assemblage in the metabasites is sodic amphibole+epidote+albite+chlorite+phengite±garnet. Sodic amphibole commonly shows replacement by barroisite, and there is continuous petrographic transition from blueschist–metabasites to barroisite-bearing epidote–amphibolites. Eclogite with the mineral assemblage of garnet+sodic pyroxene+sodic–calcic amphibole+epidote is found only in one locality. P–T conditions of the epidote–blueschist facies metamorphism are estimated as 450±50 °C and 11±2 kbar. The blueschist formation was followed by a decrease in pressure and increase in temperature, leading to the development of barroisite-bearing epidote–amphibolites. Phengite, sodic amphibole and barroisite Ar/Ar ages from three metabasic rocks range between 215 and 205 Ma, and indicate Late Triassic high-pressure metamorphism. The Triassic blueschists in northwest Turkey constitute part of a much larger allochthonous tectonic unit of Triassic mafic volcanic rocks. They probably represent the upper layers of a Triassic oceanic plateau, which was accreted to the Laurasian margin during the latest Triassic. The close spatial association of the Triassic and Cretaceous blueschists along the zmir–Ankara suture suggests that the suture represents a long-lived plate boundary of Late Palaeozoic to early Tertiary age.     

P–T path of high-pressure/low-temperature rocks and tectonic implications in the western Tianshan Mountains, NW China

The Chinese western Tianshan high-pressure/low-temperature (HP–LT) metamorphic belt, which extends for about 200 km along the South Central Tianshan suture zone, is composed of mainly metabasic blueschists, eclogites and greenschist facies rocks. The metabasic blueschists occur as small discrete blocks, lenses, bands, laminae or thick beds in meta-sedimentary greenschist facies country rocks. Eclogites are intercalated within blueschist layers as lenses, laminae, thick beds or large massive blocks (up to 2 km² in plan view). Metabasic blueschists consist of mainly garnet, sodic amphibole, phengite, paragonite, clinozoisite, epidote, chlorite, albite, accessory titanite and ilmenite. Eclogites are predominantly composed of garnet, omphacite, sodic–calcic amphibole, clinozoisite, phengite, paragonite, quartz with accessory minerals such as rutile, titanite, ilmenite, calcite and apatite. Garnet in eclogite has a composition of 53–79 mol% almandine, 8.5–30 mol% grossular, 5–24 mol% pyrope and 0.6–13 mol% spessartine. Garnet in blueschists shows similar composition. Sodic amphiboles include glaucophane, ferro-glaucophane and crossite, whereas the sodic–calcic amphiboles mainly comprise barroisite and winchite. The jadeite content of omphacite varies from 35–54 mol%. Peak eclogite facies temperatures are estimated as 480–580 °C for a pressure range of 14–21 kbar. The conditions of pre-peak, epidote–blueschist facies metamorphism are estimated to be 350–450 °C and 8–12 kbar. All rock types have experienced a clockwise P–T path through pre-peak lawsonite/epidote-blueschist to eclogite facies conditions. The retrograde part of the P–T path is represented by the transition of epidote-blueschist to greenschist facies conditions. The P–T path indicates that the high-pressure rocks formed in a B-type subduction zone along the northern margin of the Palaeozoic South Tianshan ocean between the Tarim and Yili-central Tianshan plates.