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     

The nature of fluids during pegmatite development in metamorphic terrains: Evidence from Hamadan complex,Sanandaj-Sirjan metamorphic zone,Iran

In the Sanandaj-Sirjan zone of metamorphic belt of Iran, the area south of Hamadan city comprises of metamorphic rocks, granitic batholith with pegmatites and quartz veins. Alvand batholith is emplaced into metasediments of early Mesozoic age. Fluid inclusions have been studied using microthermometry to evaluate the source of fluids from which quartz veins and pegmatites formed to investigate the possible relation between host rocks of pegmatites and the fluid inclusion types. Host minerals of fluid inclusions in pegmatites are quartz, andalusite and tourmaline. Fluid inclusions can be classified into four types. Type 1 inclusions are high salinity aqueous fluids (NaCl_eq >12 wt%). Type 2 inclusions are low to moderate salinity (NaCl_eq <12 wt%) aqueous fluids. Type 3 and 4 inclusions are carbonic and mixed CO₂-H₂O fluid inclusions. The distribution of fluid inclusions indicate that type 1 and type 2 inclusions are present in the pegmatites and quartz veins respectively in the Alvand batholith. This would imply that aqueous magmatic fluids with no detectable CO₂ were present during the crystallization of these pegmatites and quartz veins. Types 3 and 4 inclusions are common in quartz veins and pegmatites in metamorphic rocks and are more abundant in the hornfelses. The distribution of the different types of fluid inclusions suggests that CO₂ fluids generated during metamorphism and metamorphic fluids might also contribute to the formation of quartz veins and pegmatites in metamorphic terrains.     

Ore Genesis and Tectonic Significance of the Xiaojiashan Tungsten Deposit,Eastern Tianshan,Xinjiang, China: Evidence from Geology,Fluid Inclusions,and Stable Isotope Geochemistry

The Xiaojiashan tungsten deposit is located about 200 km northwest of Hami City, the Eastern Tianshan orogenic belt, Xinjiang, northwestern China, and is a quartz vein‐type tungsten deposit. Combined fluid inclusion microthermometry, host rock geochemistry, and H–O isotopic compositions are used to constrain the ore genesis and tectonic setting of the Xiaojiashan tungsten deposit. The orebodies occur in granite intrusions adjacent to the metamorphic crystal tuff, which consists of the second lithological section of the first Sub‐Formation of the Dananhu Formation (D₂d ₁²). Biotite granite is the most widely distributed intrusive bodies in the Xiaojiashan tungsten deposit. Altered diorite and metamorphic crystal tuff are the main surrounding rocks. The granite belongs to peraluminous A‐type granite with high potassic calc‐alkaline series, and all rocks show light Rare Earth Element (REE)‐enriched patterns. The trace element characters suggest that crystallization differentiation might even occur in the diagenetic process. The granite belongs to postcollisional extension granite, and the rocks formed in an extensional tectonic environment, which might result from magma activity in such an extensional tectonic environment. Tungsten‐bearing quartz veins are divided into gray quartz vein and white quartz veins. Based on petrography observation, fluid inclusions in both kinds of vein quartz are mainly aqueous inclusions. Microthermometry shows that gray quartz veins have 143–354°C of T_h, and white quartz veins have 154–312°C of T_h. The laser‐Raman test shows that CO₂ is found in fluid inclusions of the tungsten‐bearing quartz veins. Quadrupole mass spectrometry reveals that fluid inclusions contain major vapor‐phase contents of CO₂, H₂O. Meanwhile, fluid inclusions contain major liquid‐phase contents of Cl^?, Na⁺. It can be speculated that the ore‐forming fluid of the Xiaojiashan tungsten deposit is characterized by an H₂O–CO₂, low salinity, and H₂O–CO₂–NaCl system. The range of hydrogen and oxygen isotope compositions indicated that the ore‐forming fluids of the tungsten deposit were mainly magmatic water. The ore‐forming age of the Xiaojiashan deposit should to be ~227 Ma. During the ore‐forming process, the magmatic water had separated from magmatic intrusions, and the ore‐bearing complex was taken to a portion where tungsten‐bearing ores could be mineralized. The magmatic fluid was mixed by meteoric water in the late stage.     

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.     

Textural and isotopic development of marble assemblages during the Barrovian-style M2 metamorphic event,Naxos, Greece

A detailed petrological analysis of the marble assemblages observed within the M2 metamorphic complex on Naxos is presented. Two distinct periods of mineral growth are documented; the first is associated with prograde M2 metamorphism and the second with retrograde M2 metamorphism occurring during ductile extensional thinning of the complex. The textural and miner-alogical characteristics and the carbon and oxygen isotope compositions of each generation are described, and the P-T-X _{CO 2} conditions at which these two mineral generations were stable, and the compositions of the fluids present during metamorphism are characterised. Whereas the low variance and stable isotope compositions of prograde siliceous dolomite assemblages are consistent with internally buffered fluid evolution, the retrograde mineral generation is shown to have grown as a result of the infiltration of a water-rich fluid phase that transported silica, Al₂O₃, Na₂O and FeO into the host rocks. This observation, together with the stable isotope compositions of the retrograde calcite, and the fact that occurrences of veins of this type are limited to marbles in the highest grade areas (T>600° C) of the metamorphic complex, suggests that the fluids responsible for vein formation were generated during the crystallisation of melts as the metamorphic complex cooled from peak temperatures. The existence of this second generation of minerals has significant implications for previous studies of heat transport by fluid flow on Naxos, because many of the unusually low ¹⁸O compositions of pelites at high grades may be ascribable to the effects of interaction with retrograde M2 fluids, rather than with prograde fluids.     

Channelized fluids in subducted continental crust: constraints from δD–δ18O of quartz and fluid inclusions in quartz veins from the Chinese Continental Scientific Drilling Project

Fluid inclusions hosted by quartz veins in high-pressure to ultrahigh-pressure (HP-UHP) metamorphic rocks from the Chinese Continental Scientific Drilling (CCSD) Project main drillhole have low, varied hydrogen isotopic compositions (δD?=??97‰ to??69‰). Quartz δ¹⁸O values range from??2.5‰ to 9.6‰; fluid inclusions hosted in quartz have correspondingly low δ¹⁸O values of??11.66‰ to 0.93‰ (T _h?=?171.2～318.8°C). The low δD and δ¹⁸O isotopic data indicate that protoliths of some CCSD HP-UHP metamorphic rocks reacted with meteoric water at high latitude near the surface before being subducted to great depth. In addition, the δ¹⁸O of the quartz veins and fluid inclusions vary greatly with the drillhole depth. Lower δ¹⁸O values occur at depths of ～900–1000 m and ～2700 m, whereas higher values characterize rocks at depths of about 1770 m and 4000 m, correlating roughly with those of wall-rock minerals. Given that the peak metamorphic temperature of the Dabie-Sulu UHP metamorphic rocks was about 800°C or higher, much higher than the closure temperature of oxygen isotopes in quartz under wet conditions, such synchronous variations can be explained by re-equilibration. In contrast, δD values of fluid inclusions show a different relationship with depth. This is probably because oxygen is a major element of both fluids and silicates and is much more abundant in the quartz veins and silicate minerals than is hydrogen. The oxygen isotope composition of fluid inclusions is evidently more susceptible to late-stage re-equilibration with silicate minerals than is the hydrogen isotope composition. Therefore, different δD and δ¹⁸O patterns imply that dramatic fluid migration occurred, whereas the co-variation of oxygen isotopes in fluid inclusions, quartz veins, and wall-rock minerals can be better interpreted by re-equilibration during exhumation.

Quartz veins in the Dabie-Sulu UHP metamorphic terrane are the product of high-Si fluids. Given that channelized fluid migration is much faster than pervasive flow, and that the veins formed through precipitation of quartz from high-Si fluids, the abundant veins indicate significant fluid mobilization and migration within this subducted continental slab. Many mineral reactions can produce high-Si fluids. For UHP metamorphic rocks, major dehydration during subduction occurred when pressure–temperature conditions exceeded the stability of lawsonite. In contrast, for low-temperature eclogites and other HP metamorphic rocks with peak metamorphic P–T conditions within the stability field of lawsonite, dehydration and associated high-Si fluid release may have occurred as hydrous minerals were destabilized at lower pressure during exhumation. Because subduction is a continuous process whereas only a minor fraction of the subducted slabs returns to the surface, dehydration during underflow is more prevalent than exhumation even in subducted continental crust, which is considerably drier than altered oceanic crust.     

苏北青龙山超高压变质榴辉岩流体包裹体特征与流体演化

根据青龙山超高压变质榴辉岩中流体包裹体的化学成分、矿物中的分布特征将岩石中的流体包裹体分为五类,即富N2包裹体、高盐度(22.4-略大于23.2wt%NaCl)的NaCl CaCl2 H2O体系流体包裹体、中高盐度(12.6-16.0wt%NaCl)的含Mg2 或Fe2 的NaCl H2O体系流体包裹体、中等盐度(6.4-10.5wt%NaCl)水溶液包裹体和低盐度(3.3-0.2wt%NaCl)的水溶液包裹体。富N2包裹体形成于超高压变质峰期阶段,高盐度的流体包裹体形成于超高压变质岩折返早期固体出溶体出溶阶段,中高盐度的流体包裹体形成于高压变质重结晶作用阶段,中等盐度的流体包裹体形成于角闪岩相变质重结晶作用阶段,低盐度的流体包裹体形成于折返晚期的绿片岩退变质作用阶段。超高压变质峰期阶段和折返早期的高盐度流体和中高盐度的流体主要来自继承原岩中的流体(如含NH4 矿物分解或片麻岩原岩中的有机质分解,名义上无水矿物中羧基水的出溶),晚期角闪岩相退变质阶段的中等盐度的流体除名义上无水矿物中羟基水的出溶外还有外来流体的加入,绿片岩相退变质作用阶段的流体主要为外来流体。     

Fluid evolution during burial and Variscan deformation in the Lower Devonian rocks of the High-Ardenne slate belt (Belgium): sources and causes of high-salinity and C–O–H–N fluids

Fluid inclusions in quartz veins of the High-Ardenne slate belt have preserved remnants of prograde and retrograde metamorphic fluids. These fluids were examined by petrography, microthermometry and Raman analysis to define the chemical and spatial evolution of the fluids that circulated through the metamorphic area of the High-Ardenne slate belt. The earliest fluid type was a mixed aqueous/gaseous fluid (H₂O–NaCl–CO₂–(CH₄–N₂)) occurring in growth zones and as isolated fluid inclusions in both the epizonal and anchizonal part of the metamorphic area. In the central part of the metamorphic area (epizone), in addition to this mixed aqueous/gaseous fluid, primary and isolated fluid inclusions are also filled with a purely gaseous fluid (CO₂–N₂–CH₄). During the Variscan orogeny, the chemical composition of gaseous fluids circulating through the Lower Devonian rocks in the epizonal part of the slate belt, evolved from an earlier CO₂–CH₄–N₂ composition to a later composition enriched in N₂. Finally, a late, Variscan aqueous fluid system with a H₂O–NaCl composition migrated through the Lower Devonian rocks. This latest type of fluid can be observed in and outside the epizonal metamorphic part of the High-Ardenne slate belt. The chemical composition of the fluids throughout the metamorphic area, shows a direct correlation with the metamorphic grade of the host rock. In general, the proportion of non-polar species (i.e. CO₂, CH₄, N₂) with respect to water and the proportion of non-polar species other than CO₂ increase with increasing metamorphic grade within the slate belt. In addition to this spatial evolution of the fluids, the temporal evolution of the gaseous fluids is indicative for a gradual maturation due to metamorphism in the central part of the basin. In addition to the maturity of the metamorphic fluids, the salinity of the aqueous fluids also shows a link with the metamorphic grade of the host-rock. For the earliest and latest fluid inclusions in the anchizonal part of the High-Ardenne slate belt the salinity varies respectively between 0 and 3.5 eq.wt% NaCl and between 0 and 2.7 eq.wt% NaCl, while in the epizonal part the salinity varies between 0.6 and 17 eq.wt% NaCl and between 3 and 10.6 eq.wt% for the earliest and latest aqueous fluid inclusions, respectively. Although high salinity fluids are often attributed to the original sedimentary setting, the increasing salinity of the fluids that circulated through the Lower Devonian rocks in the High-Ardenne slate belt can be directly attributed to regional metamorphism. More specifically the salinity of the primary fluid inclusions is related to hydrolysis reactions of Cl-bearing minerals during prograde metamorphism, while the salinity of the secondary fluid inclusions is rather related to hydration reactions during retrograde metamorphism. The temporal and spatial distribution of the fluids in the High-Ardenne slate belt are indicative for a closed fluid flow system present in the Lower Devonian rocks during burial and Variscan deformation, where fluids were in thermal and chemical equilibrium with the host rock. Such a closed fluid flow system is confirmed by stable isotope study of the veins and their adjacent host rock for which uniform δ¹⁸0 values of both the veins and their host rock demonstrate a rock-buffered fluid flow system.     

Silica mobility and fluid movement during metamorphism of the Connemara schists, Ireland

Quartz veins are developed in a wide range of metasediment types in the upper amphibolite facies rocks of Connemara, and attest to considerable migration of silica. Contrary to common assumptions, there is clear evidence that these veins do not primarily result from movement of fluid to regions of lower P–T down the regional geothermal gradient. Under amphibolite facies conditions, a dilute chloride fluid moving down temperature has the potential to alter 60g of plagioclase to muscovite for each gram of vein quartz precipitated, while cooling over the temperature interval from 650 to 500° C. The absence of significant metasomatic effects in the vein walls effectively precludes a simple origin from such through-flowing, externally derived fluids. The oxygen isotopic composition of matrix quartz shows considerable differences between different rock types (quartzite, pelite and marble), with a range of δ¹⁸O_SMOW from c.+ 11.5% (quartzite) to + 18.5% (marble). In each rock type, vein quartz compositions closely match those of the matrix quartz. These results demonstrate the importance of local segregation processes in the formation of veins, and suggest that fluid convection cells were not developed during metamorphism on a scale larger than the individual sedimentary formations, if at all. Both oxygen isotope data and the absence of metasomatism indicate that veins form primarily by segregation of quartz from the host lithologies, with only a relatively minor component of through flow of externally derived fluid. Veins are clearly not the major pathways of metamorphic dewatering. It is proposed that abundant veins in the predominantly pelitic Ballynakill Formation formed during peak metamorphic D3 folding because the formation was embrittled by high fluid pressures but was capped by impermeable marble. Hence the pelitic formation fractured repeatedly and the pore fluid drained through the fractures to form veins, while irreversible loss through the rest of the succession was a much less important process. In the central mountains of Connemara, rather pure, unreactive quartzites are cut by widely spaced, laterally extensive quartz veins that are axial planar to D3 folds. These veins may mark pathways whereby metamorphic fluid made its way through the massive impermeable quartzite from lower parts of the nappe pile, but here too, oxygen isotope data indicate considerable segregation of locally derived quartz, reflecting the importance of pumping of fluid between wail rocks and fractures relative to the component of through flow.     

区域变质作用中的流体

区域变质条件下流体的流动有 4种标志 :( 1)细脉 ;( 2 )岩石学 ;( 3 )稳定同位素 ;( 4 )常量元素的交代作用。不同级别的区域变质作用中 ,流体影响着岩石的变质反应和变形 ;在高级变质的情况下甚至有熔体出现。在超高压变质条件下 ,流体量比地壳范围区域变质要少得多 ,从大别山超高压变质带的资料可知 ,流体的演化有明显的阶段性 ,局部曾发现熔融包裹体。水流体的介入 ,引起岩石的退变质和元素地球化学变异 ,是超高压变质岩抬升、进入中下地壳的产物。新近的实验岩石学成果说明 ,多硅白云母、角闪石等含羟基的矿物 ,在俯冲达 10 0km以下依然稳定 ,而一些花岗岩体系在超高压的条件下产生的超临界流体 ,乃是花岗岩、片麻岩只能部分保留超高压矿物组合的原因。     

Distinct zircon U–Pb and O–Hf–Nd–Sr isotopic behaviour during fluid flow in UHP metamorphic rocks: evidence from metamorphic veins and their host eclogite in the Sulu Orogen,China

Fluid plays a key role in metamorphism and magmatism in subduction zones. Veins in high‐pressure (HP) to ultrahigh‐pressure (UHP) rocks are the products of fluid‐rock interaction, and can thus provide important constraints on fluid processes in subduction zones. This contribution is an integrated study of zircon U–Pb and O–Hf, as well as whole‐rock Nd–Sr isotopic compositions for a quartz vein, a complex vein, and their host eclogite in the Sulu UHP terrane to decipher the timing and source of fluid flow under HP‐UHP metamorphic conditions. The inherited magmatic zircon cores from the host eclogite constrain the protolith age at c. 750 Ma. Their variable ε_Hf(t) values from ?1.11 to 2.54 and low δ¹⁸O values of 0.32–3.40‰ reflect a protolith that formed in a rift setting due to the breakup of the supercontinent Rodinia. The hydrothermal zircon from the quartz and the complex veins shows euhedral shapes, relatively flat HREE pattern, slight or no negative Eu anomaly, low ¹⁷⁶Lu/¹⁷⁷Hf ratios, and low formation temperatures of 660–690 °C, indicating they precipitated from fluids under HP eclogite facies conditions. This zircon yielded similar U–Pb ages of 217 ± 2 and 213 ± 3 Ma within analytical uncertainty, recording the timing of fluid flow during the exhumation of the UHP rock. It is inferred that the fluids might be of internal origin based on the homogeneity of δ¹⁸O values of the hydrothermal zircon from the quartz (?2.41 ± 0.13‰) and complex veins (?2.35 ± 0.12‰), and the metamorphic grown zircon of the host eclogite (?2.23 ± 0.16‰). The similar ε_Nd(t) values of the whole rocks also support such a point. Zircon O and whole‐rock Nd isotopic compositions are therefore useful to identify the source of fluid, for they are major and trace components in minerals involved in metamorphic reactions during HP‐UHP conditions. On the other hand, the hydrothermal zircon from the veins and the metamorphic zircon from the host eclogite exhibit variable ε_Hf(t) values. Model calculation suggests that the Hf was derived from the breakdown of major rock‐forming minerals and recycling of the inherited magmatic zircon. The variable whole‐rock initial ⁸⁷Sr/⁸⁶Sr ratios might be caused by subsequent retrograde metamorphism after the formation of the veins.     

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.     

四川里伍黑牛洞铜矿床成矿流体特征及矿床成因研究

黑牛洞铜矿床经历了早期韧性变形-变质作用和成矿期的韧-脆性变形作用。早期韧性变形-变质作用过程中围岩可能分异出含CO2流体,并形成顺片理发育的磁黄铁矿、黄铜矿等矿化。成矿期为伴随江浪穹窿隆升的韧-脆性变形期,含矿石英脉中流体包裹体主要以富液相流体包裹体为主,次为纯气相流体包裹体。流体包裹体成分测试结果显示,富液相流体包裹体主要成分为水,含少量CO2和甲烷等烃类碳质流体;纯气相流体包裹体主要为甲烷,表明成矿流体为富水含炭质流体。流体包裹体氢、氧同位素测试结果表明,黑牛洞矿床含矿流体中的水主要来源于围岩中的变质水。燕山期花岗岩侵位,江浪穹窿隆升,韧-脆性变形形成系列滑脱断层。在此减压、降温过程中,成矿流体被活化并聚集、填充到滑脱断层内。     

CO2-Brine immiscibility at high temperatures,evidence from calcareous metasedimentary rocks

Study of fluid inclusions in quartz segregations and in the rock matrix of a calcareous psammite and a carbonate schist suggests that brines containing 23–24 weight percent salt (NaCl equivalent) are immiscible with CO₂ at the metamorphic conditions of approximately 600° and 6.5 Kb. The presence of a high temperature solvus between saline brine and CO₂ is supported by other fluid inclusion studies as well as experimental measurements from the literature. As saline brines are common in metamorphic and hydrothermal systems, CO₂-brine immiscibility should play an important role in petrogenesis. The fluid inclusions preserved in the quartz segregations probably represent the fluids generated by prograde metamorphic reactions, whereas the compositions of the fluids trapped in the rock matrix quartz suggest they have reequilibrated with the matrix minerals during incipient retrograde reactions. The isochores from the densest inclusions observed in this study pass close to the inferred peak metamorphic conditions; other isochores suggest an episode of deformation and recrystallization at 275° C and 1.4 Kb. Using the density information preserved in all the inclusions, a convex-downward uplift path on a P-T diagram is inferred for these rocks.     

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 record of rock exhumation across the brittle–ductile transition during formation of a Metamorphic Core Complex (Naxos Island,Cyclades, Greece)

Fluid inclusions trapped in quartz veins hosted by a leucogneiss from the southern part of the Naxos Metamorphic Core Complex (Attic‐Cycladic‐Massif, Greece) were studied to determine the evolution of the fluid record of metamorphic rocks during their exhumation across the ductile/brittle transition. Three sets of quartz veins (V‐M2, V‐BD & V‐B) are distinguished. The V‐M2 and V‐BD are totally or, respectively, partially transposed into the foliation of the leucogneiss. They formed by hydrofracturing alternating with ductile deformation accommodated by crystal‐plastic deformation. The V‐B is discordant to the foliation and formed by fracturing during exhumation without subsequent ductile transposition. Fluids trapped during crystal–plastic deformation comprise two very distinct fluid types, namely a CO₂‐rich fluid and a high‐salinity brine, that are interpreted to represent immiscible fluids generated from metamorphic reactions and the crystallization of magmas respectively. They were initially trapped at ～625 °C and 400 MPa and then remobilized during subsequent ductile deformation resulting in various degrees of mixing of the two end‐members with later trapping conditions of ～350 °C and 140 MPa. In contrast, brittle microcracks contain aqueous fluids trapped at 250 °C and 80 MPa. All veins display a similar δ¹³C pointing to carbon that was trapped at depth and then preserved in the fluid inclusions throughout the exhumation history. In contrast, the δD signature is marked by a drastic difference between (i) V‐M2 and V‐BD veins that are dominated by carbonic, aqueous‐carbonic and high‐salinity fluids of metamorphic and magmatic origin characterized by δD between ?56‰ and ?66‰, and (ii) V‐B veins that are dominated by aqueous fluids of meteoric origin characterized by δD between ?40‰ and ?46‰. The retrograde P–T pathway implies that the brittle/ductile transition separates two structurally, chemically and thermally distinct fluid reservoirs, namely (i) the ductile crust into which fluids originating from crystallizing magmas and fluids in equilibrium with metamorphic rocks circulate through a geothermal gradient of 30 °C km^?1 at lithostatic pressure, and (ii) the brittle upper crust through which meteoric fluids percolate through a high geothermal gradient of 55 °C km^?1 at hydrostatic pressure.     

Fluid inclusions in coesite-bearing eclogites and jadeite quartzite at Shuanghe, Dabie Shan (China)

Fluid inclusions in coesite‐bearing eclogites and jadeite quartzite at Shuanghe in Dabie Shan, East‐central China, have preserved remnants of early, prograde and/or peak metamorphic fluids, reset during post‐UHP (ultrahigh‐pressure) metamorphic uplift. Inclusions occur in several minerals (e.g. omphacite & epidote), notably as isolated, primary inclusions in quartz included in various host minerals. Two major fluid types have been identified: non‐polar fluid species (N₂ or CO₂) and aqueous, the latter is by far the most predominant. Aqueous fluids cover a wide range of salinity, from halite‐bearing brines to low salinity fluids. For non‐polar fluids, few N₂ inclusions occur in undeformed eclogite, whereas a number of CO₂‐rich inclusions have been found in microshear zones of eclogite or jadeite quartzite in close proximity to marble occurrences. The primary character of N₂ and high‐salinity aqueous inclusions indicates that they are remnants from UHP metamorphic fluids and for some there is the distinct possibility that they are ultimately derived from pre‐metamorphic fluids. This conclusion is supported by the preservation, in some samples, of microdomains containing synchronous inclusions of variable salinities, which tend to relate to the chemical composition of the host crystal. Carbonic fluids may be derived from neighbouring rocks, notably marble and carbonate‐bearing metasediments, during post‐metamorphic uplift. During post‐UHP exhumation, a limited decrease of the fluid density has occurred, with formation of new sets of fluid inclusions. Fluid movements, however, remained exceedingly limited, at the scale of the enclosing crystal.     

Andalusite-bearing veins at Vedrette di Ries (eastern Alps, Italy): fluid phase composition based on fluid inclusions

Abstract Andalusite-bearing veins formed during contact metamorphism in the aureole of the Vedrette di Ries tonalite. In the veins, quartz crystals that are completely armoured by andalusite or that occur in strain shadow areas contain three generations of fluid inclusions: low-salinity H₂O-CO₂-CH₄ mixtures with CH₄/(CO₂+ CH₄) ± 0.35 (type A); low-salinity aqueous fluids (type B); H₂O-free, CO₂-CH₄ fluids with the same carbonic speciation as A (type C). Carbonic types A and C typically have a dark appearance, which is attributed to graphite coatings on inclusion walls. Microstructural analysis of the host quartz and calculated densities indicate that type A inclusions were likely trapped during vein formation. These inclusions underwent strain-assisted re-equilibration during cooling that resulted in density increases without change of composition. After the rocks had cooled below about 350 ° C, type C inclusions appear to have formed from one of the immiscible fractions after unmixing of the H₂O-CO₂-CH₄ fluid mixtures. Aqueous type B inclusions, apparently trapped between 225 and 350 ° C, could represent an independent fluid, or could be the H₂O-rich fraction of unmixed type A fluids. Taking account of the uncertainties, the composition and density of the complex type A inclusion fluids are in good agreement with the properties of primary fluids calculated from the petrological data. The fluid inclusion data support the model of vein formation by hydrofracturing as a result of dehydration of graphitic metapelites. These new results also demonstrate the importance of considering strain in the interpretation of metamorphic fluid inclusions.     

Fluid inclusion constraints on the uplift history of the metamorphosed massive sulphide deposits at Ducktown, Tennessee

Abstract Standard petrographic, microthermometric and Raman spectroscopic analyses of fluid inclusions from the metamorphosed massive sulphide deposits at Ducktown, Tennessee, indicate that fluids with a wide range of compositions in the C–O–H–N–S–salt system were involved in the syn- to post-metamorphic history of these deposits. Primary fluid inclusions from peak metamorphic clinopyroxene contain low-salinity, H₂O–CH₄ fluids and calcite, quartz and pyrrhotite daughter crystals. Many of these inclusions exhibit morphologies resembling those produced in laboratory experiments in which confining pressures significantly exceed the internal pressures of the inclusions. Secondary inclusions in metamorphic quartz from veins, pods, and host matrix record a complex uplift history involving a variety of fluids in the C–O–H–N–salt system. Early fluids were generated by local devolatilization reactions while later fluids were derived externally. Isochores calculated for secondary inclusions in addition to the chronology of trapping and morphological features of primary and secondary fluid inclusions suggest an uplift path which was concave toward the temperature axis over the P–T range 6–3 kbar and 550–225° C. Immiscible H₂O–CH₄–N₂–NaCl fluids were trapped under lithostatic to hydrostatic pressure conditions at 3–0.5 kbar and 215 ± 20° C. Entrapment occurred during Alleghanian thrusting, and the fluids may have been derived by tectonically driven expulsion of pore fluids and thermal maturation of organic material in lower-plate sedimentary rocks which are thought to underlie the deposits. Episodic fracturing and concomitant pressure decreases in upper-plate rocks, which host the ore bodies, would have allowed these fluids to move upward and become immiscible. Post-Alleghanian uplift appears to have been temperature-convex. Uplift rates of 0.10–0.05 mm year^?1 from middle Ordovician to middle Silurian – late Devonian, and 0.07–0.12 mm year^?1 from middle Silurian – late Devonian to late Permian are suggested by our uplift path and available geochronological data.     

Remnants of premetamorphic fluid and oxygen isotopic signatures in eclogites and garnet clinopyroxenite from the Dabie-Sulu terranes, eastern China

A combined oxygen‐isotope and fluid‐inclusion study has been carried out on high‐ and ultrahigh‐pressure metamorphic (HP/UHPM) eclogites and garnet clinopyroxenite from the Dabie‐Sulu terranes in eastern China. Coesite‐bearing eclogites/garnet clinopyroxenite and quartz eclogites have a wide range in whole‐rock δ¹⁸O_VSMOW, from 0 to 11‰. The high‐T oxygen‐isotope fractionations preserved between quartz and garnet preclude significant retrograde isotope exchange during exhumation, and the wide range in whole‐rock oxygen‐isotope composition is thought to be a presubduction signature of the precursors. Aqueous fluids with variable salinities and gas species (N₂‐, CO₂‐, or CH₄‐rich), are trapped as primary inclusions in garnet, omphacite and epidote, and in quartz blebs enclosed within eclogitic minerals. In high‐δ¹⁸O HP/UHPM rocks from Hujialin and Shima, high‐salinity brine and/or N₂ inclusions occur in garnet porphyroblasts, which also contain inclusions of coesite, Cl‐rich blue amphibole and dolomite. In contrast, in low‐δ¹⁸O eclogites from Qinglongshan and Huangzhen, the Cl concentrations in amphibole are very low, < 0.2 wt.%, and low‐salinity aqueous inclusions occur in quartz inclusions in epidote porphyroblasts and in epidote cores. These low‐salinity fluid inclusions are believed to be remnants of meteoric water, although the fluid composition was modified during pre‐ and syn‐peak HP/UHPM. Eclogites at Houshuichegou and Hetang contain CH₄‐rich fluid inclusions, coexisting with high‐salinity brine inclusions. Methane was probably formed under the influence of CO₂‐rich aqueous fluids during serpentinisation of mantle‐derived peridotites prior to or during plate subduction. Remnants of premetamorphic low‐ to high‐salinity aqueous fluid with minor N₂ and/or other gas species preserved in the Dabie‐Sulu HP/UHPM eclogites and garnet clinopyroxenite indicate a great diversity of initial fluid composition in the precursors, implying very limited fluid–rock interaction during syn‐ and post‐peak HP/UHPM.