CCSD HP-UHP变质岩中石英脉和东海水晶流体包裹体惰性气体同位素组成及其成因指示意义

中国大陆科学钻探（CCSD）岩心中存在大量石英脉和长英质脉体，关于其成因目前存在板块折返过程中的减压和流体释放以及后期流体穿插等不同成因观点。本文利用高真空气相质谱系统分析了CCSD石英脉和东海水晶流体包裹体中惰性气体同位素组成，其He-Ar、Xe和Ne同位素体系均清楚显示它们主要由地壳变质流体和大气饱和水组成，而基本不含深源地幔流体。在CCSD900～1500m深度的石英脉流体包裹体出现^40Ar／^36Ar和δ^18O同步下降，且与前人对CCSD中HP-UHP岩石的矿物原位δ^18O测定结果变化趋势相似，说明该段原岩在板块俯冲前曾与大气降水发生较为充分的水／岩反应，石英脉继承了围岩的同位素组成特征。流体包裹体惰性气体同位素组成显示CCSD中石英脉和东海水晶可能具有相似的成因，它们均主要形成于HP-UHP岩石在板块折返到地壳中的压力降低和流体释放过程。CCSD HP-UHP岩石及其石英脉中基本不合地幔流体的原因在于本区印支期快速的俯冲和折返过程，被俯冲地壳物质无法与地幔进行充分的同位素交换。此外，退变质作用及其伴生的地壳流体也进一步减弱了HP-UHP岩石中的地幔流体信息。     

超高压榴辉岩-脉体体系成因和俯冲带变质流体演化

超高压岩石-脉体体系是认识俯冲带流体性质和行为的天然实验室.通过总结大别超高压变质带3个榴辉岩（角闪岩）-脉体体系的研究成果,探讨了大陆俯冲带变质流体的溶解-结晶过程和氧逸度变化规律以及流体对轻元素硼的迁移过程.对榴辉岩-复合高压脉体的研究发现超高压流体通过溶解矿物富集溶质组分,流体随后经历3期结晶过程,分别形成绿辉石-绿帘石脉、绿帘石-石英脉和蓝晶石-绿帘石-石英脉.绿帘石La、Cr和δEu值是判断结晶次序的关键指标.对榴辉岩-角闪岩-低压脉体研究表明大陆俯冲带低压变质流体的氧逸度明显高于高压-超高压变质流体.高氧逸度条件也导致一些反常矿物（如退变金红石）的生长.对含电气石榴辉岩-脉体研究揭示变质碳酸盐岩是大陆俯冲板片中重硼同位素的重要储库,其在汇聚板块边界的脱硼作用显著影响深部硼循环.上述研究成果为理解俯冲带变质流体演化和物质循环提供重要科学依据.      

大陆板块俯冲和折返过程中的流体活动：稳定同位素证据

对大别-苏鲁造山带超高压变质岩矿物稳定同位素的系统研究发现，超高压变质过程中存在少量含水流体，但是流体的活动性很小，在不同岩相界面之间缺乏明显的流体渗透；超高压榴辉岩中的石英脉是蜂期变质后含水矿物降压分解和羟基出溶引起的流体流动结果，不是板块俯冲过程中进变质作用的产物；超高压变质岩经历了广泛的角闪岩相退变质作用，退变质流体主要来源于板块折返过程中超高压矿物中溶解经基的降压出溶。     

中国大陆科学钻探（CCSD）HP-UHP变质岩中石英脉流体包裹体δD-δ^18O同位素组成及其意义

流体包裹体δD-δ^18O同位素测定显示CCSD石英脉具有较稳定的氢同位素（8D=-97‰～-69‰）和相对较低且变化较大的氧同位素组成，其矿物δ^18O为-1．9‰-9．6‰，相应流体的δ^18O为-11．66‰～0．93‰，说明其变质岩围岩在板块俯冲前曾在地表与大气降水发生过程度不同的水／岩反应，而石英脉继承了其各自寄主变质岩的δ^18O组成；在CCSD纵向上，石英脉的δ^18O同位素组成出现“∑”型变化，分别在900m～1500m和2700m出现极低值，而在1770m和4000m出现高正值，说明CCSD变质岩原岩在俯冲前与大气降水间的水／岩反应受到局部侵入的岩浆岩带来的高温和构造空间的控制；CCSD中石英脉δ^18O在纵向上的变化基本同步于其寄主围岩变质矿物的δ^18O组成变化，说明石英脉与其他变质矿物一样．也经历了HP，甚至UHP变质，但其主体应形成于板块折返过程中HP-UHP岩石的减压重结晶及退变质；CCSD石英脉、东海地表石英脉或水晶矿的δD-δ^18O同位素值分布的不均一性，说明HP-UHIP岩石在板块折返及其后退变质中释放出的流体活动范围有限，没有经历大规模的流动或迁移。东海水晶的流体包裹体δD-δ^18O组成与CCSD石英脉相似，显示它们的成因基本一致，主要形成于晚三叠世板块折返过程。     

CCSD HP-UHP变质岩中石英脉微量元素和稀土元素地球化学

利用等离子质谱(ICP-MS)测定了CCSD HP-UHP变质岩中石英脉及东海水晶的微量元素和稀土元素组成,结果显示其中微量元素总体含量很低,Bb/Sr、Nb/Ta和Zr/Hf变化很大,说明Rb和Sr、Nb和Ta、Zr和Hf之间发生了不同程度的分异,但变质岩未曾发生过广泛强烈的水岩反应.部分样品中Rb和Cs含量特别低,显示CCSD中部分石英脉可能曾经历过超高压变质过程.CCSD石英脉和东海水晶均具有LREE富集而HREE明显亏损的特征,显示其来源主要为壳源.CCSD石英脉中正负Ce异常同时出现,显示板块俯冲前,超高压变质岩的原岩曾在地表较氧化的环境下经历过强烈的风化作用.对比研究显示,CCSD不同围岩中石英脉、地表石英脉和东海水晶的微量元素N-MORB标准化模式和BEE球粒陨石标准化配分曲线,与其各自围岩具有一定的对应相似关系,表明大多数石英脉(水晶)来自于围岩,同时显示板块俯冲和折返所释放的流体活动范围较小,没有发生大规模的迁移.     

东秦岭商丹缝合带沉积岩系内石英脉及其中流体包裹体成因初探

商丹缝合带沉积岩系是商丹缝合带的组成部分 ,主要由变质杂砂岩、泥砂质岩、火山碎屑岩、大理岩等组成。该岩系经历多次构造热事件 ,变质、变形强烈。整套岩系中发育大量不同期次的石英脉 ,本文试通过石英脉及其中的流体包裹体研究 ,讨论流体与构造作用的关系。根据石英脉特征及其与围岩组构的关系 ,可将其划分为 4期 :第 1期石英脉 (Ｑｚ1) :产于丹凤蛇绿岩中的早期石英脉。石英脉的形成和变形与南秦岭板块由南向北的俯冲作用有关。第 2期石英脉(Ｑｚ2 ) :商丹沉积岩系中与糜棱面理或片理平行的石英脉。其变形与本区左行剪切有关。第 3期石…     

伏牛山构造带变质流体成分特征及构造意义

伏牛山构造带由多条韧性剪切带组成,剪切带中广泛发育同构造变质流体形成的脉体。脉体矿物成分以石英为主,石英脉中石英晶体内发育大量的流体包裹体和气液相包裹体。包裹体成分以含中高盐度水溶液、低CO2、低δ18O~δD以及少量有机质为特征,包裹体的均一温度为1706~3378℃,白云母压力计计算出的压力为027~087 GPa。整个伏牛山构造带变质流体成分和同位素均继承了原岩海底火山喷发的地化特征。伏牛山构造带变质程度以绿片岩相为主,压力较大,具有俯冲带的特性。包裹体的特点均反映了宽坪岩块和二郎坪岩块依次向华北板块下俯冲、汇聚时产生了强变形,弱变质的构造作用。     

苏鲁-大别超高压岩石中锆石SHRIMP U-Pb定年研究——综述和最新进展

如何建立苏鲁-大别超高压岩石深俯冲-超高压-快速折返过程连续而完整的P-T-t轨迹及精细的年代谱系,是目前地学界研究的热点。而变质锆石是否记录深俯冲石英榴辉岩相进变质阶段的年代学信息和超高压峰期变质时代的准确归属,是目前苏鲁-大别超高压变质带需要深入研究的核心问题。本文在对前人同位素年代学方面所取得的成果进行系统总结的基础上。采用锆石中矿物包体激光拉曼和电子探针测试、锆石阴极发光图像成因分析以及SHRIMP U-Pb定年等综合研究手段,确定苏鲁-大别地体榴辉岩及其强退变质围岩在深俯冲-构造折返过程中主要经历了四个阶段的变质演化:深俯冲石英榴辉岩相进变质(Ⅰ)、超高压峰期变质(Ⅱ)、构造折返初期石英榴辉岩相退变质(Ⅲ)和构造折返晚期角闪岩相退变质(Ⅳ)。研究发现,扬子板块(中)新元古代巨量的陆壳物质在早三叠纪(246～244Ma)俯冲到华北板块之下约65km的深处。发生了石英榴辉岩相进变质,相应的变质温压条件为T=542～693℃,P=1.7～2.02GPa。这些高压石英榴辉岩相岩石在中-新三叠纪继续向下俯冲,在235～225Ma期间,俯冲的深度至少达到了170km的地幔深处,并发生了峰期柯石英榴辉岩相超高压变质,相应的变质温压条件为T=722～866℃,P>5.5GPa。苏鲁-大别超高压地体自石英榴辉岩相进变质阶段到超高压峰期变质阶段的俯冲速率为7.0km/Myr。这些超高压岩石在219～216Ma期间,发生了第一次构造抬升至75km的深处,并经历了石英榴辉岩相退变质作用的改造,退变质温压条件为T=730～780℃,P=1.7～2.6GPa。这些退变质岩石在212～205Ma期间,又经历了第二次抬升至25km中-下地壳深处,并叠加了角闪岩相退变质作用,该阶段变质温压条件为T=610～710℃,P=0.7～1.2GPa。苏鲁-大别超高压地体两次构造抬升的速率大致相同,为5.6km/Myr。该项成果不仅确定了苏鲁-大别榴辉岩及其强退变质岩石深俯冲过程石英榴辉岩相进变质-超高压峰期变质、构造折返过程石英榴辉岩相-角闪岩相退变质连续而完整的变质演化P-T-t轨迹及精细的年代谱系,而且对于重新建立苏鲁-大别巨量陆壳物质快速超深俯冲-快速折返的动力学模式有着重要的科学意义。     

超高压变质岩中柯石英→石英相变研究的进展

柯石英是石英的高压同质多相变体，是超高压变质作用的产物，它是识别超高压变质作用的重要标志矿物。柯石英的发现表明了近地表的岩石随板块运动曾被俯冲至地下大于90km的深度并经受了超高压变质作用。研究柯石英→石英相变过程中的微结构变化，对进一步研究超高压变质作用的机理及超高压变质岩折返过程的动力学机制具有重要意义。本文简述了近年来有关超高压变质岩中柯石英→石英相变研究的一些进展。     

木兰山低温高压微变质体中SiO_2非晶相的特征和意义

木兰山低温高压变质岩中的石英脉及其围岩的矿物组成、变质结构和界面特征的光学显微镜和电子探针研究表明,该石英脉为同构造重结晶脉,经历过低温高压变质作用,其变质温度压力条件与围岩矿物的形成条件相同。透射电镜亚微米纳米尺度分析表明,该石英脉中存在一种已部分转变成α-石英纳米晶的SiO2非晶相,且观察到类似于柯石英假象的胀裂现象。并认为SiO2非晶相的形成及其变化特征可能是大别山高压超高压变质带南缘蓝片岩的一次俯冲和快速折返过程的超显微证据。     

超高压变质作用过程中的流体——来自苏鲁超高压变质岩岩石学、氧同位素和流体包裹体研究的限定

苏鲁造山带超高压变质岩岩石学、氧同位素、流体包裹体和名义上无水矿物的研究表明，流体-岩石相互作用在大陆地壳的俯冲与折返过程中起到多重的重要作用，并形成了复杂的流体演化过程：（1）大陆表壳岩通过与高纬度大气降水的交换作用被广泛水化，并获得了异常低的氧同位素成分；（2）在水化陆壳物质的俯冲过程中发生了一系列的进变质脱水反应，所释放的流体主要结合进了高压、超高压含水矿物和名义上无水超高压矿物；（3）在超高压变质过程中，以水为主的变质流体通过选择性的吸收使其盐度逐渐升高，并在峰期出现高密度、高盐度的H2O或CO2-H2O流体。有机质的分解反应在局部形成了以CO2、N2、CH4或它们的混合物为主要成分的变质流体；（4）名义上无水超高压矿物的结构水出溶是早期退变质流体的主要来源，并在局部富集形成了高压变质脉体；（5）透入性的中、低盐度水流体活动使超高压变质岩通过一系列的水化反应转变成角闪岩相变质岩；（6）沿韧性剪切带和脆性破碎带的强烈水流体活动为绿片岩相退变质作用和低压石英脉的形成提供了变质流体；（7）可变盐度的H2O或CO2-H2O流体是整个超高压变质岩形成与折返过程中的主要流体，但局部的流体．岩石相互作用形成了非极性的变质流体。     

庐山星子群变质流体的包裹体研究

庐山星子群沉积变质岩中发育平行区域片理的石英脉和长英质脉体,这些脉体的石英晶体内富含原生的流体包裹体,包括低盐度的含液体ＣＯ２包裹体、液体包裹体、纯ＣＯ２包裹体和高盐度含子矿物包裹体。它们与中生代伟晶岩脉体中包裹体在均一温度、盐度和ＣＯ２密度等方面的明显差异和变质脉体中含液体ＣＯ２包裹体的等容线位置,表明变质脉体石英中的流体包裹体是在变质作用期间被捕获的部分变质流体,进一步证实了脉体是与变质作用同     

Fluid generation, vein formation and the degree of fluid–rock interaction during decompression of high-pressure terranes: the Schistes Lustrés, Alpine Corsica, France

Centimetre‐ to decimetre‐wide quartz+calcite veins in schistes lustrés from Alpine Corsica were formed during exhumation at 30–40 Ma following blueschist facies metamorphism. The δ¹⁸O and δ¹³C values of the veins overlap those of the host schistes lustrés, and the δ¹⁸O values of the veins are much higher than those of other rocks on Corsica. These data suggest that the vein‐forming fluids were derived from the schistes lustrés. Fluids were probably generated by reactions that broke down carpholite, lawsonite, chlorite and white mica at 300–350 °C during decompression between c. 1400 and 800 MPa. However, the δ¹⁸O values of the veins are locally several per mil higher than expected given those of their host rocks. The magnitude of oxygen isotope disequilibrium between the veins and the host rock is inversely proportional to the δ¹⁸O value of the host rock. Additionally, calcite in some schists is in isotopic equilibrium with calcite in adjacent veins, but not with the silicate fraction of the schists. Locally, the schists are calcite bearing only within 1–20 cm of the veins. The vein‐forming fluids may have been preferentially derived from calcite‐bearing, high‐δ¹⁸O rocks that are common within the schistes lustrés and that locally contain abundant (>15%) veins. If the fluids were unable to completely isotopically equilibrate with the rocks, due to relatively rapid flow at moderate temperatures or being confined to fractures, they could form veins with higher δ¹⁸O values than those of the surrounding rocks. Alteration of the host rocks was probably inhibited by isolation of the fluid in ‘quartz‐armoured’ veins. Overall, the veins represent a metre‐ to hectometre‐scale fluid‐flow system confined to within the schistes lustrés unit, with little input from external sources. This fluid‐flow system is one of several that operated in the western Alps during exhumation following high‐pressure metamorphism.     

Low-thermal-gradient Hercynian metamorphism in the eastern Pyrenees

Abstract Fluids with compositions in the system CO₂-H₂O-NaCl were trapped in quartz veins enclosed in low-grade metamorphic rocks (chlorite zone) on the southern flank of the Canigó Massif, eastern Pyrenees. The veins, which also contain arsenopyrite crystals, were formed contemporaneously with the main Hercynian foliation and metamorphism. Volumetric properties of the fluid and the results of arsenopyrite geothermometry suggest P-T trapping conditions of 4.6–6 kbar and 450–530° C. This implies that an episode of metamorphism with an average geothermal gradient of 25° C km⁻¹ occurred during the main deformation event. This episode preceded the low- P /high- T metamorphism described around domes and to date considered as characteristic of the Hercynian orogeny in the Pyrenees.     

北大别的高温超高压变质作用与多阶段折返

近期的变质岩石学、地球化学及同位素年代学研究表明,北大别整体经历了高温超高压变质作用和多阶段折返历史,因而表现为广泛发育的多期减压结构和极少保留早期的超高压变质记录。北大别榴辉岩以高温变质作用以及折返期间因麻粒岩相和角闪岩相退变质变质作用而形成的多期后成合晶为显著特征。石榴子石中伴有放射状胀裂纹的单晶和多晶石英包体指示早期柯石英的转变结果,这已被锆石中发现的柯石英残晶所证实。结合北大别北部榴辉岩和片麻岩中发现的金刚石等超高压证据以及三叠纪变质记录,由此证明北大别整体经历过深俯冲和印支期超高压变质作用。北大别榴辉岩的多阶段高温条件主要来自石榴子石-绿辉石矿物对温度计、单斜辉石中紫苏辉石+石英针状矿物出熔体以及金红石中较高的Zr含量和变质锆石中较高的Ti含量等得出的温度证据。此外,多期后成合晶以及石榴子石和单斜辉石等矿物中成分分带的存在,证明该区榴辉岩经历了一个多阶段、快速折返过程;而不同变质阶段的温度、压力和形成时代,却反映该区榴辉岩经历了长时间的高温变质演化与缓慢冷却过程。长时间的高温变质作用与缓慢冷却过程也许正是北大别长期难以发现柯石英和有关超高压记录的重要原因。因此,这些成果为大别山三个不同超高压岩片的差异折返模型的建立提供了新的证据。     

西藏邦布石英脉型金矿床的成因: 流体包裹体及氢-氧同位素证据

西藏邦布石英脉型金矿床是产于印度-亚洲板块陆-陆造山主碰撞汇聚环境下、与大洋俯冲无关的新型造山型金矿床。该矿床位于雅鲁藏布江缝合带南侧朗杰学增生楔的东段南缘,矿体受区域内EW向金地-鲁农复向斜和错古-折木朗壳型脆-韧性剪切带及其次级构造的控制。金矿化主要与石英脉密切相关,并包裹于脉内细粒/粗粒硫化物中。矿区内主要分布有3期石英脉:成矿前钩状石英脉、成矿期石英大脉和成矿后陡立状石英脉。文章对3期石英脉流体包裹体形态、形成温度、密度及H-O同位素等方面进行了详细的对比研究,试图查明成矿流体来源以及金的沉淀机制等问题。研究表明,钩状石英脉内包裹体主要为液相(L)包裹体,成分主要为H₂O溶液,其流体可能为早期区域变质的产物;石英大脉内包裹体主要为含CO₂气液(VL)两相包裹体,体积较大,成分主要为H₂O+CO₂+CH₄±N₂,成矿流体为深源变质流体,并与变质地层中的有机质发生强烈反应;陡立状石英脉内包裹体主要为气液两相包裹体,体积较小,其主要成分为H₂O+CO₂,流体主要与后期区域变质事件有关,为成矿后变质作用的产物。邦布金矿的主要成矿流体源自深部变质流体,流体不混溶作用可能是导致金矿沉淀的主要原因。     

Fluid generation and evolution during exhumation of deeply subducted UHP continental crust: Petrogenesis of composite granite–quartz veins in the Sulu belt,China

Composite granite–quartz veins occur in retrogressed ultrahigh pressure (UHP) eclogite enclosed in gneiss at General's Hill in the central Sulu belt, eastern China. The granite in the veins has a high‐pressure (HP) mineral assemblage of dominantly quartz+phengite+allanite/epidote+garnet that yields pressures of 2.5–2.1 GPa (Si‐in‐phengite barometry) and temperatures of 850–780°C (Ti‐in‐zircon thermometry) at 2.5 GPa (~20°C lower at 2.1 GPa). Zircon overgrowths on inherited cores and new grains of zircon from both components of the composite veins crystallized at c. 221 Ma. This age overlaps the timing of HP retrograde recrystallization dated at 225–215 Ma from multiple localities in the Sulu belt, consistent with the HP conditions retrieved from the granite. The ε_Hf(t) values of new zircon from both components of the composite veins and the Sr–Nd isotope compositions of the granite consistently lie between values for gneiss and eclogite, whereas δ¹⁸O values of new zircon are similar in the veins and the crustal rocks. These data are consistent with zircon growth from a blended fluid generated internally within the gneiss and the eclogite, without any ingress of fluid from an external source. However, at the peak metamorphic pressure, which could have reached 7 GPa, the rocks were likely fluid absent. During initial exhumation under UHP conditions, exsolution of H₂O from nominally anhydrous minerals generated a grain boundary supercritical fluid in both gneiss and eclogite. As exhumation progressed, the volume of fluid increased allowing it to migrate by diffusing porous flow from grain boundaries into channels and drain from the dominant gneiss through the subordinate eclogite. This produced a blended fluid intermediate in its isotope composition between the two end‐members, as recorded by the composite veins. During exhumation from UHP (coesite) eclogite to HP (quartz) eclogite facies conditions, the supercritical fluid evolved by dissolution of the silicate mineral matrix, becoming increasingly solute‐rich, more ‘granitic’ and more viscous until it became trapped. As crystallization began by diffusive loss of H₂O to the host eclogite concomitant with ongoing exhumation of the crust, the trapped supercritical fluid intersected the solvus for the granite–H₂O system, allowing phase separation and formation of the composite granite–quartz veins. Subsequently, during the transition from HP eclogite to amphibolite facies conditions, minor phengite breakdown melting is recorded in both the granite and the gneiss by K‐feldspar+plagioclase+biotite aggregates located around phengite and by K‐feldspar veinlets along grain boundaries. Phase equilibria modelling of the granite indicates that this late‐stage melting records P–T conditions towards the end of the exhumation, with the subsolidus assemblage yielding 0.7–1.1 GPa at <670°C. Thus, the composite granite–quartz veins represent a rare example of a natural system recording how the fluid phase evolved during exhumation of continental crust. The successive availability of different fluid phases attending retrograde metamorphism from UHP eclogite to amphibolite facies conditions will affect the transport of trace elements through the continental crust and the role of these fluids as metasomatic agents interacting with the mantle wedge in the subduction channel.     

Production of carbonic fluids during metamorphism of graphitic pelites in a collisional orogen—An assessment from fluid inclusions

Fluid inclusions in quartz veins within Proterozoic metamorphic rocks in the Black Hills, South Dakota, were examined by microthermometry and Raman spectroscopy to assess the evolution of fluid compositions during regional metamorphism of organic-rich shales and late-orogenic magmatism, both of which were related to the collision of the Wyoming and Superior crustal blocks. Fluid inclusions occur in veins that began to be generated before or during regional compression and metamorphism that reached at least garnet-grade conditions, and in veins within the aureole of the Harney Peak Granite (HPG), where temperatures reached second-sillimanite grade conditions. Early veins in the schists have undergone recrystallization during heating and deformation that modified the composition of early CH₄ or CO₂ and N₂-dominated inclusions. These fluids were apparently trapped under conditions of immiscibility with a saline aqueous fluid phase. They are interpreted to represent components generated during maturation of organic matter and dehydration of phyllosilicates during incipient metamorphism at reducing fO₂ conditions. Most inclusions in the quartz veins are, however, secondary CO₂-bearing. They imply a transition to higher fO₂ conditions with increasing temperature of regional metamorphism. The fO₂ conditions may have been controlled by the mineral assemblage in the host metapelites. The prevalence of bimodal distributions of trapped CO₂-N₂ and aqueous endmembers in the biotite and garnet zones also suggests that two immiscible fluid phases existed during the regional metamorphism.In the aureole of the HPG, graphite was evidently consumed by influx of magmatic fluids. CO₂-H₂O fluid inclusions dominate, but they have significantly less N₂ than inclusions at lower metamorphic grades. All inclusions define secondary trails in mostly unstrained quartz. The bimodality of inclusion compositions is not as well defined as at lower grades, with many inclusions containing intermediate CO₂-H₂O compositions. This suggests that a single fluid phase existed at the high temperatures in the granite aureole, but then unmixed during cooling. A set of late quartz veins with graphitized and tourmalinized selvages in the granite aureole contains CH₄-bearing inclusions with little N₂. The existence of CH₄ in these inclusions is attributed to complexing of magmatic B with hydroxyl anions taken from the CO₂-H₂O fluid phase, effectively causing reduction in fO₂ and promoting precipitation of graphite.     

Incipient metamorphism and deformation in the Variscides of SW Dyfed, Wales: first steps towards isotopic equilibrium

Incipient metamorphism accompanying thrusting, folding and cleavage development has been investigated in a varied sequence of Palaeozoic sediments near the Variscan front in SW Dyfed, Wales. The aim was to evaluate a critical stage in the progression from heterogeneous sediment, whose detrital phases are neither in equilibrium with one another, nor with pore fluids, through indurated sedimentary rock to metamorphic rock comprising newly formed crystals that equilibrated with one another as they grew. Quartz veins are widely developed in the area, especially in the more psammitic lithologies, while finer grained rocks became cleaved during tectonic deformation. Mineralogical constraints and fluid inclusion measurements suggest maximum temperatures around 200-310d? C (slightly higher in the Marloes-Musselwick Thrust Sheet than in other parts of the structural succession) at depths of the order of 6-13 km. Quartz veins yield distinctly heavier oxygen isotopic compositions than detrital quartz grains in the adjacent wall rocks, although care must be taken in interpreting the data because slivers of detrital grains may become incorporated into veins, while matrix detrital grains may incorporate veinlets or rims of newly formed quartz. It is concluded that vein quartz grew in isotopic equilibrium with a fluid phase whose isotopic composition was primarily controlled by exchange with phyllosilicates, not detrital quartz grains. Vein and matrix quartzes from the Marloes-Musselwick Thrust Sheet are distinctly lighter (δ¹⁸O_veins=+14 to +18% and δ¹⁸O_matrix=+11 to +14%) than those from other thrust sheets (δ¹⁸O =+17 to +20% and +14 to +17%, respectively). We conclude that vein quartz and phyllosilicate grains in cleavage domains probably attained equilibrium with a locally buffered pore fluid at the peak of metamorphism, but many relict grains of different chemical and isotopic composition remained elsewhere in the rock. Local fluid migration along veins and through cleavage lamellae facilitated the attainment of equilibrium, but there is little evidence for large-scale infiltration of externally derived fluids. With further metamorphism the quartz in these rocks would attain an isotopic composition intermediate between that of the heavy vein material and light detritus which coexist here.