Reaction-induced grain boundary cracking and anisotropic fluid flow during prograde devolatilization reactions within subduction zones

Devolatilization reactions during prograde metamorphism are a key control on the fluid distribution within subduction zones. Garnets in Mn-rich quartz schist within the Sanbagawa metamorphic belt of Japan are characterized by skeletal structures containing abundant quartz inclusions. Each quartz inclusion was angular-shaped, and showed random crystallographic orientations, suggesting that these quartz inclusions were trapped via grain boundary cracking during garnet growth. Such skeletal garnet within the quartz schist formed related to decarbonation reactions with a positive total volume change (?V _t > 0), whereas the euhedral garnet within the pelitic schists formed as a result of dehydration reaction with negative ?V _t values. Coupled hydrological–chemical–mechanical processes during metamorphic devolatilization reactions were investigated by a distinct element method (DEM) numerical simulation on a foliated rock that contained reactive minerals and non-reactive matrix minerals. Negative ?V _t reactions cause a decrease in fluid pressure and do not produce fractures within the matrix. In contrast, a fluid pressure increase by positive ?V _t reactions results in hydrofracturing of the matrix. This fracturing preferentially occurs along grain boundaries and causes episodic fluid pulses associated with the development of the fracture network. The precipitation of garnet within grain boundary fractures could explain the formation of the skeletal garnet. Our DEM model also suggests a strong influence of reaction-induced fracturing on anisotropic fluid flow, meaning that dominant fluid flow directions could easily change in response to changes in stress configuration and the magnitude of differential stress during prograde metamorphism within a subduction zone.     

Corundum + quartz and Mg-staurolite bearing granulite from the Limpopo Belt, southern Africa: Implications for a P–T path

A new occurrence of the rare corundum + quartz assemblage and magnesian staurolite has been found in a gedrite–garnet rock from the Central Zone of the Neoarchean Limpopo Belt in Zimbabwe. Poikiloblastic garnet in the sample contains numerous inclusions of corundum + quartz ± sillimanite, magnesian staurolite + sapphirine ± orthopyroxene, and sapphirine + sillimanite assemblages, as well as monophase inclusions. Corundum, often containing subhedral to rounded quartz, occurs as subhedral to euhedral inclusions in the garnet. Quartz and corundum occur in direct grain contact with no evidence of a reaction texture. The textures and Fe–Mg ratios of staurolite inclusions and the host garnet suggest a prograde dehydration reaction of St → Grt + Crn + Qtz + H₂O to give the corundum + quartz assemblage. Peak conditions of 890–930 °C at 9–10 kbar are obtained from orthopyroxene + sapphirine and garnet + staurolite assemblages. A clockwise P–T path is inferred, with peak conditions being followed by retrograde conditions of 4–6 kbar and 500–570 °C. The presence of unusually magnesian staurolite (Mg / [Fe + Mg] = 0.47–0.53) and corundum + garnet assemblages provides evidence for early high-pressure metamorphism in the Central Zone, possibly close to eclogite facies. The prograde high-pressure event followed by high- to ultrahigh-temperature metamorphism and rapid uplifting of the Limpopo Belt could have occurred as a result of Neoarchean collisional orogeny involving the Zimbabwe and Kaapvaal Cratons.     

Carbonate-silicate inclusions in garnet as evidence for a carbonate-bearing source for fluids in leucocratic granitoids associated with granulites of the Southern Marginal Zone,Limpopo Complex,South Africa

We present a study of carbonate-bearing polyphase inclusions in garnets from leucocratic granitoids intruding metapelitic granulites of the Southern Marginal Zone (SMZ) of the Neoarchean Limpopo high-grade complex, South Africa, during the post-peak stage (2710–2650 Ma; U-Pb ages for zircons and monazites). Ternary feldspar thermometry suggests that the granitoid magma cooled from temperatures 800–900 °C at a pressure of ca. 6.5 kbar. Abundant CO₂ fluid inclusions in quartz and T-X_CO2 phase equilibria modeling via PERPLE_X imply action of an essentially carbonic fluid in the granitoids. Cores of almandine-rich garnet grains from the granitoids contain polyphase carbonate-bearing inclusions with a distinct negative crystal shape. The major carbonate in the inclusions is a strongly zoned magnesite-siderite variety, whereas pyrophyllite is the predominant silicate phase. Raman spectra of unexposed inclusions revealed a presence of CO₂, as well as CH₄ and H₂O. The carbonate-bearing inclusions coexist with larger polyphase inclusions composed of biotite, quartz, K-feldspar, plagioclase, sillimanite, which are interpreted as relics of granitic melts. Modeling the mineral assemblage preserved within the carbonate-bearing inclusions shows that their present mineral and chemical compositions are a product of interaction of the trapped aqueous‑carbonic fluid with host garnet during cooling below 400 °C. Despite strong modifications, the inclusions bear evidence for initial saturation of the fluid with Mg‑carbonate. This is taken as an evidence for an origin of the fluids by devolatilization of the Mg-rich carbonate-bearing ultrabasic greenstone rocks of the Kaapvaal Craton that were buried under the SMZ. Being generated at temperatures between 650 and 700 °C, the fluid subsequently participated in anatexis and coexisted with the granite magma during exhumation and interaction of the SMZ granulites with cratonic rocks.     

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.     

高压-超高压变质岩石中不同成因的石榴石

石榴石是高压-超高压变质岩石中最重要的变质矿物之一,是研究俯冲带深部变质和熔融过程的理想研究对象.通过对俯冲带内不同条件下形成的石榴石进行详细研究,确定了岩浆成因、变质成因和转熔成因石榴石.岩浆石榴石是岩浆熔体在冷却过程中结晶形成,成分主要为锰铝榴石-铁铝榴石,通常含有石英、长石、磷灰石等晶体包裹体.变质石榴石是在亚固相条件下通过变质反应形成,包裹体为参与变质反应的矿物组合;进变质生长的石榴石通常显示核部到边部锰铝榴石降低的特征.转熔石榴石是在超固相条件下通过转熔反应形成,通常含有晶体包裹体,其中既有从转熔熔体结晶的矿物包裹体,也有转熔反应残留的矿物包裹体.对超高压变质岩石中转熔石榴石的识别,可以为深俯冲陆壳岩石的部分熔融提供重要的岩石学证据,是大陆俯冲带部分熔融研究的重要进展之一.      

Fluid Inclusions and Hydrogen,Oxygen, Sulfur Isotopes of Nuri Cu‐W‐Mo Deposit in the Southern Gangdese,Tibet

The Nuri Cu‐W‐Mo deposit is located in the southern subzone of the Cenozoic Gangdese Cu‐Mo metallogenic belt. The intrusive rocks exposed in the Nuri ore district consist of quartz diorite, granodiorite, monzogranite, granite porphyry, quartz diorite porphyrite and granodiorite porphyry, all of which intrude in the Cretaceous strata of the Bima Group. Owing to the intense metasomatism and hydrothermal alteration, carbonate rocks of the Bima Group form stratiform skarn and hornfels. The mineralization at the Nuri deposit is dominated by skarn, quartz vein and porphyry type. Ore minerals are chalcopyrite, pyrite, molybdenite, scheelite, bornite and tetrahedrite, etc. The oxidized orebodies contain malachite and covellite on the surface. The mineralization of the Nuri deposit is divided into skarn stage, retrograde stage, oxide stage, quartz‐polymetallic sulfide stage and quartz‐carbonate stage. Detailed petrographic observation on the fluid inclusions in garnet, scheelite and quartz from the different stages shows that there are four types of primary fluid inclusions: two‐phase aqueous inclusions, daughter mineral‐bearing multiphase inclusions, CO₂‐rich inclusions and single‐phase inclusions. The homogenization temperature of the fluid inclusions are 280°C–386°C (skarn stage), 200°C–340°C (oxide stage), 140°C–375°C (quartz‐polymetallic sulfide stage) and 160°C–280°C (quartz‐carbonate stage), showing a temperature decreasing trend from the skarn stage to the quartz‐carbonate stage. The salinity of the corresponding stages are 2.9%–49.7 wt% (NaCl) equiv., 2.1%–7.2 wt% (NaCl) equiv._, 2.6%–55.8 wt% (NaCl) equiv. and 1.2%–15.3 wt% (NaCl) equiv., respectively. The analyses of CO₂‐rich inclusions suggest that the ore‐forming pressures are 22.1 M Pa–50.4 M Pa, corresponding to the depth of 0.9 km–2.2 km. The Laser Raman spectrum of the inclusions shows the fluid compositions are dominated in H₂O, with some CO₂ and very little CH₄, N₂, etc. δD values of garnet are between ?114.4‰ and ?108.7‰ and δ¹⁸O_H2O between 5.9‰ and 6.7‰; δD of scheelite range from ?103.2‰ to ?101.29‰ and δ¹⁸O_H2O values between 2.17‰ and 4.09‰; δD of quartz between ?110.2‰ and ?92.5‰ and δ¹⁸O_H2O between ?3.5‰ and 4.3‰. The results indicate that the fluid came from a deep magmatic hydrothermal system, and the proportion of meteoric water increased during the migration of original fluid. The δ³⁴S values of sulfides, concentrated in a rage between ?0.32‰ to 2.5‰, show that the sulfur has a homogeneous source with characteristics of magmatic sulfur. The characters of fluid inclusions, combined with hydrogen‐oxygen and sulfur isotopes data, show that the ore‐forming fluids of the Nuri deposit formed by a relatively high temperature, high salinity fluid originated from magma, which mixed with low temperature, low salinity meteoric water during the evolution. The fluid flow through wall carbonate rocks resulted in the formation of layered skarn and generated CO₂ or other gases. During the reaction, the ore‐forming fluid boiled and produced fractures when the pressure exceeded the overburden pressure. Themeteoric water mixed with the ore‐forming fluid along the fractures. The boiling changed the pressure and temperature, oxygen fugacity, physical and chemical conditions of the whole mineralization system. The escape of CO₂ from the fluid by boiling resulted in scheelite precipitation. The fluid mixing and boiling reduced the solubility of metal sulfides and led the precipitation of chalcopyrite, molybdenite, pyrite and other sulfide.     

Dehydration melting of ultrahigh‐pressure eclogite in the Dabie orogen: evidence from multiphase solid inclusions in garnet

Several types of multiphase solid (MS) inclusions are identified in garnet from ultrahigh‐pressure (UHP) eclogite in the Dabie orogen. The mineralogy of MS inclusions ranges from pure K‐feldspar to pure quartz, with predominance of intermediate types consisting of K‐feldspar + quartz ± silicate (plagioclase or epidote) ± barite. The typical MS inclusions are usually surrounded with radial cracks in the host garnet, similar to where garnet contains relict coesite. Barite aggregates display significant heterogeneity in major element composition, with total contents of only 57–73% and highly variable SiO₂ contents of 0.32–25.85% that are positively correlated with BaO and SO₃ contents. The occurrence of MS inclusions provides petrographic evidence for partial melting in the UHP metamorphic rock. The occurrence of barite aggregates with variably high SiO₂ contents suggests the coexistence of aqueous fluid with hydrous melt under HP eclogite facies conditions. Thus, local dehydration melting is inferred to take place inside the UHP metamorphic slice during continental collision. This is ascribed to phengite breakdown during ‘hot’ exhumation of the deeply subducted continental crust. As a consequence, the aqueous fluid is internally buffered in chemical composition and its local sink is a basic trigger to the partial melting during the continental subduction‐zone metamorphism.     

A corundum–quartz assemblage from the Eastern Ghats Granulite Belt,India: evidence for high P–T metamorphism?

Corundum+quartz-bearing assemblages occur in small lenses in granulite facies metapelites in Rayagada, north-central part of the Eastern Ghats Granulite Belt, India. Corundum porphyroblasts and quartz coexist with porphyroblastic almandine-rich garnet, hercynite spinel, ilmenite and magnetite. Corundum and quartz are separated by sillimanite or a composite corona consisting of sillimanite and garnet, whereas corundum shows sharp grain boundaries with spinel, ilmenite and magnetite. Porphyroblastic corundum contains prismatic sillimanite inclusions in which irregularly shaped quartz is enclosed. Two distinct reactions are inferred from the textural features: corundum+quartz=sillimanite and spinel+quartz=garnet+sillimanite. From the petrographical features, we infer that corundum–quartz–garnet–spinel was the peak metamorphic assemblage. Although large uncertainties exist regarding the positions of the respective reactions in P–T  space, from several published experimental results and theoretical calculations a peak metamorphic condition of 12  kbar and 1100  °C is estimated as the lower stability limit of the corundum–quartz assemblage. Decompression from the peak P–T  condition to c .  9  kbar, 950  °C is inferred.     

Quartz deformation mechanisms during Barrovian metamorphism: Implications from crystallographic orientation of different generations of quartz in pelites

The behaviour of quartz during metamorphism is studied based on two case studies from the Barrovian terrains of Sulitjelma in arctic Scandinavia and Loch Tay in the Central Highlands Dalradian of Scotland. Both terrains preserve evidence for metamorphism in pelites involving nucleation and growth of garnet at different times in the deformation history. Data are presented on the size, shape and crystallographic orientation of quartz preserved as inclusions in garnet and as grains in the surrounding matrix. While quartz-grains remain small and dispersed between mica grains, deformation appears to be dominated by grain-boundary sliding accommodated by dissolution–precipitation. At amphibolite facies, textural coarsening occurs by dissolution of small quartz grains and growth of larger quartz grains, coupled with segregation of quartz from mica. As a result, quartz deforms by dislocation creep, developing crystallographic preferred orientations (CPO) consistent with both coaxial and non-coaxial strain. Quartz CPOs with <0001> axes lying parallel to foliation and stretching direction are commonly developed, and best explained by mechanical rotation of inequant (detrital?) quartz grains. There is no evidence for selective entrapment of quartz inclusions in garnet on the basis of quartz crystallographic orientation.     

内蒙古红岭铅锌矿床成矿流体地球化学特征及矿床成因

红岭铅锌矿是内蒙古东南部的大型代表性矿床之一.目前,对该矿床成矿流体地球化学特征、性质及演化问题尚缺乏系统研究.对其展开了系统的流体包裹体研究.结果表明,矿区矽卡岩期Ⅰ阶段石榴石中发育含NaCl子矿物三相(SL)、气相-富气相(LV)及气液两相(VL)3种类型的原生流体包裹体,Ⅱ阶段中石英颗粒主要发育LV和VL两种类型原生流体包裹体,测温结果表明矽卡岩期成矿流体属中-高温、高盐度的不均匀NaCl-H₂O体系热液,在成矿过程中发生过沸腾作用而导致铅、锌、铜等有用元素沉淀富集.石英-硫化物期Ⅲ→Ⅵ阶段中矿物均主要发育较单一的VL型包裹体,其中Ⅲ阶段热液均一温度较矽卡岩期明显降低,而盐度没有明显变化;Ⅳ阶段成矿流体均一温度明显增高、盐度明显降低,反映了有新的高温、低盐度体系热液的加入;而Ⅴ→Ⅵ阶段成矿流体均一温度及盐度逐渐降低,体现了一种不断与外来天水混合的演变趋势;整体上看,石英-硫化物期流体为简单的中-低温、低盐度NaCl-H₂O体系热液.流体包裹体C、H、O同位素研究表明,红岭矿床矽卡岩期Ⅱ阶段成矿流体以岩浆水为主;石英-硫化物期成矿流体源自大气降水与岩浆水的混合流体,晚阶段逐渐演化为以大气降水为主.矿床S、Pb同位素研究表明,区内成矿物质具深源特点.      

Composite carbonate and silicate multiphase solid inclusions in metamorphic garnet from ultrahigh‐P eclogite in the Dabie orogen

Composite multiphase solid (MS) inclusions composed of carbonate and silicate minerals have been found for the first time in metamorphic garnet from ultrahigh‐P eclogite from the Dabie orogen. These inclusions are morphologically euhedral to subhedral, and some show relatively regular shapes approaching the negative crystal shape of the host garnet. Radial fractures often occur in garnet hosting the inclusions. The inclusions are primarily composed of variable proportions of carbonate and silicate minerals such as calcite, quartz, K‐feldspar and plagioclase, with occasional occurrences of magnetite, zircon and barite. They are categorized into two groups based on the proportions of carbonate and silicate phases. Group I is carbonate‐dominated with variable proportions of silicate minerals, whereas Group II is silicate‐dominated with small proportions of carbonates. Trace element analysis by LA‐ICPMS for the two groups of MS inclusions yields remarkable differences. Group I inclusions exhibit remarkably lower REE contents than Group II inclusions, with significant LREE enrichment and large fractionation between LREE and HREE in the chondrite‐normalized REE diagram. In contrast, Group II inclusions show rather flat REE patterns with insignificant fractionation between LREE and HREE. In the primitive mantle‐normalized spidergram, Group I inclusions exhibit positive anomalies of Zr and Hf, whereas Group II inclusions show negative anomalies of Zr and Hf. Nevertheless, both groups exhibit positive anomalies of Ba, U, Pb and Sr, but negative anomalies of Nb and Ta, resembling the composition of common continental crust. Group I inclusions have higher Ba and U contents than Group II inclusions. Combined with petrological observations, the two groups of MS inclusions are interpreted as having crystallized from composite silicate and carbonate melts during continental subduction‐zone metamorphism. The differences in trace element composition between the two groups are primarily attributed to the proportions of carbonate and silicate phases in the MS inclusions. The silicate melts were derived from the breakdown of hydrous minerals such as paragonite and phengite, whereas the occurrence of carbonate melts indicates involvement of carbonate minerals in the partial melting and thus has great bearing on recycling of supracrustal carbon into the mantle. The coexistence of silicate and carbonate melts in the eclogitic garnet provides insights into the nature of hydrous melts in the subduction factory.     

Fluid-induced high-temperature metasomatism at Rundv?gshetta in the Lützow-Holm Complex,East Antarctica: Implications for the role of brine during granulite formation

We report new petrological, phase equilibria modeling, and fluid inclusion data for pelitic and mafic granulites from Rundv?gshetta in the highest-grade region of the Neoproterozoic Lützow-Holm Complex(LHC),East Antarctica, and provide unequivocal evidence for fluid-rock interaction and high-temperature metasomatism in the presence of brine fluid. The studied locality is composed dominantly of well-foliated pelitic granulite(K-feldspar+quartz+sillimanite+garnet+ilmenite) with foliation-parallel bands and/or layers of mafic granulite(plagioclase+orthopyroxene+garnet+ilmenite+quartz+biotite). The boundary between the two lithologies is defined by thin(about 1 -20 cm in thick) garnet-rich layers with a common mineral assemblage of garnet+plagioclase+quartz+ilmenite+biotite ? orthopyroxene. Systematic increase of grossular and decrease of pyrope contents in garnet as well as decreasing Mg/(Fe+Mg) ratio of biotite from the pelitic granulite to garnet-rich rock and mafic granulite suggest that the garnet-rich layer was formed by metasomatic interaction between the two granulite lithologies. Phase equilibria modeling in the system NCKFMASHTO demonstrates that the metasomatism took place at 850 -860℃, which is slightly lower than the peak metamorphism of this region, and the modal abundance of garnet is the highest along the metapeliteemetabasite boundary(up to 40%), which is consistent with the field and thin section observations. The occurrence of brine(7.0 -10.9 wt.% Na Cleqfor ice melting or 25.1 -25.5 wt.% NaC leqfor hydrohalite melting) fluid inclusions as a primary phase trapped within plagioclase in the garnet-rich layer and the occurrence of Cl-rich biotite(Cl = 0.22 -0.60 wt.%) in the metasomatic rock compared to that in pelitic(0.15 -0.24 wt.%) and mafic(0.06-0.13 wt.%) granulites suggest infiltration of brine fluid could have given rise to the high-temperature metasomatism. The fluid might have been derived from external sources possibly related to the formation of major suture zones formed during the Gondwana amalgamation.     

Fluid inclusions in granulites, granulitized eclogites and garnet clinopyroxenites from the Dabie–Sulu terranes, eastern China

Minor granulites (believed to be pre-Triassic), surrounded by abundant amphibolite-facies orthogneiss, occur in the same region as the well-documented Triassic high- and ultrahigh-pressure (HP and UHP) eclogites in the Dabie–Sulu terranes, eastern China. Moreover, some eclogites and garnet clinopyroxenites have been metamorphosed at granulite- to amphibolite-facies conditions during exhumation. Granulitized HP eclogites/garnet clinopyroxenites at Huangweihe and Baizhangyan record estimated eclogite-facies metamorphic conditions of 775–805 °C and ≥15 kbar, followed by granulite- to amphibolite-facies overprint of ca. 750–800 °C and 6–11 kbar. The presence of (Na, Ca, Ba, Sr)-feldspars in garnet and omphacite corresponds to amphibolite-facies conditions. Metamorphic mineral assemblages and P–T estimates for felsic granulite at Huangtuling and mafic granulite at Huilanshan indicate peak conditions of 850 °C and 12 kbar for the granulite-facies metamorphism and 700 °C and 6 kbar for amphibolite-facies retrograde metamorphism. Cordierite–orthopyroxene and ferropargasite–plagioclase coronas and symplectites around garnet record a strong, rapid decompression, possibly contemporaneous with the uplift of neighbouring HP/UHP eclogites.

Carbonic fluid (CO₂-rich) inclusions are predominant in both HP granulites and granulitized HP/UHP eclogites/garnet clinopyroxenites. They have low densities, having been reset during decompression. Minor amounts of CH₄ and/or N₂ as well as carbonate are present. In the granulitized HP/UHP eclogites/garnet clinopyroxenites, early fluids are high-salinity brines with minor N₂, whereas low-salinity fluids formed during retrogression. Syn-granulite-facies carbonic fluid inclusions occur either in quartz rods in clinopyroxene (granulitized HP garnet clinopyxeronite) or in quartz blebs in garnet and quartz matrices (UHP eclogite). For HP granulites, a limited number of primary CO₂ and mixed H₂O–CO₂(liquid) inclusions have also been observed in undeformed quartz inclusions within garnet, orthopyroxene, and plagioclase which contain abundant, low-density CO₂±carbonate inclusions. It is suggested that the primary fluid in the HP granulites was high-density CO₂, mixed with a significant quantity of water. The water was consumed by retrograde metamorphic mineral reactions and may also have been responsible for metasomatic reactions (“giant myrmekites”) occurring at quartz–feldspar boundaries. Compared with the UHP eclogites in this region, the granulites were exhumed in the presence of massive, externally derived carbonic fluids and subsequently limited low-salinity aqueous fluids, probably derived from the surrounding gneisses.     

Hydrothermal Quartz Vein Formation, Revealed by Coupled SEM-CL Imaging and Fluid Inclusion Microthermometry: Shuteen Complex, South Gobi, Mongolia

Abstract. Scanning electron microscopy-cathodoluminescence (SEM-CL) imaging of vein quartz in the Cu-mineralised, Shuteen Complex (South Gobi, Mongolia) has revealed a complex history of crystal growth, dissolution and microfracture healing, associated with several hydrothermal events that could not be detected using other observational techniques (e.g. transmitted/reflected light microscopy, back-scattered electron imaging, or secondary electron imaging).
The quartz initially grew as CL-bright/grey crystals in a 345±30C liquid reservoir, as inferred by the analysis of primary liquid fluid inclusions (average Th of 343C; 6.6∼7.7 wt% NaCl_eq). Quartz precipitation occurred at the edge of the crystals as reservoir fluids cooled to 260±25C, as indicated by micron-scale CL-dark/CL-bright quartz growth bands containing abundant fluid inclusions (with an average T_h values of 261C). Pressure fluctuations were the likely cause of dissolution, as SEM-CL imaging reveals the quartz have corroded or rounded crystal edges, and precipitation of later quartz into open space. SEM-CL imaging shows the quartz contains healed microfractures that trapped low salinity fluids (3.9 wt% NaC1_eq) with Th values of 173±15C.
SEM-CL imaging provides a means of deciphering the thermal and chemical evolution of the fossil Shuteen hydrothermal system, and the nature of hydrothermal quartz vein-forming processes, by facilitating the correlation of distinct fluid inclusion populations and their relative chronology, with specific hydrothermal events.     

Geochronological and petrological constraints on Palaeoproterozoic granulite facies metamorphism in southeastern margin of the North China Craton

In the southeastern margin of the North China Craton, high-pressure (HP) granulite facies meta-basic rocks exposed as bands or lenses in the Precambrian metamorphic basement (e.g. Bengbu) and as xenoliths in Mesozoic intrusions (e.g. Jiagou) are characterized by the assemblage garnet + clinopyroxene + plagioclase + quartz + rutile ± Ti-rich hornblende. Cathodoluminescence imaging and mineral inclusions reveal that most zircon from the three dated samples displays distinct core-mantle-rim structures. The cores show typical igneous zircon characteristics and give ages of 2.5–2.4 Ga, thus dating the protolith of the metabasites. The mantles formed at granulite facies conditions as evidenced by inclusions of the HP granulite mineral assemblage garnet + clinopyroxene + rutile + plagioclase + quartz ± hornblende and Ti-rich biotite and yield ages of 1839 ± 31, 1811 ± 19 and 1800 ± 15 Ma. An inclusion-free rim yields an age of 176 ± 2 Ma with the lower Th/U ratio of 0.02. The geochronological and preliminary petrological data of this study suggest that the lower crust beneath the southeastern margin of the North China Craton formed at 2.5–2.4 Ga and underwent HP granulite facies metamorphism at c. 1.8 Ga. This HT-HP metamorphic event may be ascribed to large-scale crustal heating and thickening related to mantle-derived magma underplating at the base of the lower crust, as evidenced by widespread extension, rifting and related mafic magma emplacement in the North China Craton during this period. The age of 176 ± 2 Ma most likely records the late amphibolite facies retrogression occurring during exhumation.     

Early Palaeozoic North Qaidam UHP metamorphic belt on the north-eastern Tibetan plateau and a paired subduction model

ABSTRACT An Early Palaeozoic UHP metamorphic belt was recently discovered in the North Qaidam Mountains in the north-eastern part of the Tibet Plateau. The belt contains abundant eclogite with lesser amounts of garnet peridotite and gneiss. The paragneiss contains abundant zircon with coesite inclusions whereas the eclogite has garnet with coesite pseudomorphs and omphacite with exsolved quartz. The calculated peak metamorphic conditions for the eclogite are T  = 730 °C and P  = 2.8 GPa. The garnet peridotites are magnesium-rich with Mg#s [100 × Mg/(Mg + Fe)] up to 92. Olivine in the peridotites is highly magnesian (Fo > 91.5) and TiO₂-poor and is typical of low- T garnet peridotites. Calculated peak metamorphic conditions for the peridotites are T  = 837 °C and P  = 2.5 GPa. U–Pb and Sm–Nd isotope dating indicate that continental subduction started during the early Ordovician (∼ 495 Ma). The North Qaidam UHP, together with the North Qilian HP belt about 300 km to the north, constitute a pair of genetically related early Palaeozoic subduction zones along the northern margin of the Tibet Plateau.     

辽宁五龙金矿床地质特征及成矿流体地球化学性质

对五龙金矿床含金石英脉中发育的流体包裹体进行了系统的岩相学、显微测温及单个包裹体激光拉曼光谱成分分析,结果表明:含金石英脉中主要发育CO₂±CH₄、H₂O-CO₂±CH₄及气液两相等3种类型的原生流体包裹体;H₂O-CO₂±CH₄包裹体均一温度为287.8～382.5℃,盐度(w（NaCl）)为0.42%～4.87%;气液两相包裹体均一温度为198.5～338.4℃,w（NaCl）为2.24%～6.88%;包裹体气相成分以CO₂、CH₄为主,且含量变化较大。综合分析认为,形成五龙金矿含金石英脉的流体系来源于岩浆的中温、低盐度含CO₂流体,在其运移汇集过程中经与围岩反应导致CH₄不断加入,而最终演化成为富含CO₂、CH₄等挥发分的含矿热液。     

Morphology of intergranular pores and wetting angles in pelitic schists studied by transmission electron microscopy

In pelitic schists composed mainly of quartz and albite grains, the morphology of intergranular pores, which were filled with water, was studied by transmission electron microscopy (TEM). Although some pores are defined by crystallographic planes (F-face), most of their form has an ideal shape determined by interface tensions between grains and fluid. High-resolution TEM observations demonstrate that pore-free regions at grain boundaries are tight even at the nanometer scale, showing that the wetting angle is larger than 0° in this rock. The pore distribution in two-grain junctions can be compared to a "necklace microstructure" developed by instability of a fluid film along the boundary induced by microcracking. Wetting angles for pores located at grain edges of quartz and albite decrease in the order albite/albite, quartz/quartz, and quartz/albite. The quartz/quartz wetting angle in a calcite-free sample is smaller than that in a calcite-containing sample. This angle also changes due to grain misorientation. Our results confirm that solid-solid and solid-fluid interfacial energies control the geometry of intergranular fluid in natural rocks.     

Petrological and zircon evidence for anatexis of UHP quartzite during continental collision in the Sulu orogen

Petrological evidence is provided for anatexis of ultrahigh‐pressure (UHP) metamorphic quartzite in the Sulu orogen. Some feldspar grains exhibit elongated, highly cuspate shapes or occur as interstitial, cuspate phases constituting interconnected networks along grain boundaries. Elongated veinlets composed of plagioclase + quartz ± K‐feldspar also occur in grain boundaries. These features provide compelling evidence for anatexis of the UHP quartzite. Zircon grains from impure quartzite are all metamorphic growth with highly irregular shape. They contain inclusions of coesite, jadeite, rutile and lower pressure minerals, including multiphase solid inclusions that are composed of two or more phases of muscovite, quartz, K‐feldspar and plagioclase. All zircon grains exhibit steep REE patterns, similar U–Pb ages and Hf isotope compositions with a weighted mean of 218 ± 2 Ma. Most grains have similar δ¹⁸O values of ?0.6 to 0.1‰, but a few fall in the range ?5.2 to ?4.3‰. Thus, these grains would have grown from anatectic melts at various pressures. Zircon O isotope differences indicate that anatectic melts were derived from different sources with contrasting O isotopes, but similar Hf isotopes, that is, one from the quartzite itself and the other probably from the country‐rock granitic gneiss. Zircon grains from pure quartzite contain relict magmatic cores and significant metamorphic overgrowths. Domains that contain eclogite facies minerals exhibit flat HREE patterns, no Eu anomalies and concordant U–Pb ages of c. 220 Ma. Similar U–Pb ages are also obtained for domains that contain lower pressure minerals and exhibit steep REE patterns and marked negative Eu anomalies. These observations indicate that zircon records subsolidus overgrowth at eclogite facies conditions but suprasolidus growth at lower pressures. Zircon enclosed by garnet gave consistent U–Pb ages of c. 214 Ma. Such garnet is interpreted as a peritectic product of the anatectic reaction that involves felsic minerals and possibly amphibole and titanite. The REE patterns of epidote and titanite also record multistage growth and metasomatism by anatectic melts. Therefore, the anatexis of UHP metamorphic rocks is evident during continental collision in the Triassic.     

Geology,Fluid Inclusions,and Isotopic Geochemistry of the Jinman Sediment‐Hosted Copper Deposit in the Lanping Basin,China

The Jinman Cu polymetallic deposit is located within Middle Jurassic sandstone and slate units in the Lanping Basin of southwestern China. The Cu mineralization occurs mainly as sulfide‐bearing quartz–carbonate veins in faults and fractures, controlled by a Cenozoic thrust–nappe system. A detailed study of fluid inclusions from the Jinman deposit distinguishes three types of fluid inclusions in syn‐ore quartz and post‐ore calcite: aqueous water (type A), CO₂–H₂O (type B), and CO₂‐dominated (type C) fluid inclusions. The homogenization temperatures of CO₂–H₂O inclusions vary from 208°C to 329°C, with corresponding salinities from 0.6 to 4.6 wt.% NaCl equivalent. The homogenization temperatures of the aqueous fluid inclusions mainly range from 164°C to 249°C, with salinities from 7.2 to 20.2 wt.% NaCl equivalent. These characteristics of fluid inclusions are significantly different from those of basinal mineralization systems, but similar to those of orogenic or magmatic mineralization systems. The H and O isotope compositions suggest that the ore‐forming fluid is predominantly derived from magmatic water, with the participation of basinal brine. The δ³⁴S values are widely variable between ?9.7 ‰ and 9.7 ‰, with a mode distribution around zero, which may be interpreted by the variation in physico‐chemical conditions or by compositional variation of the sources. The mixing of a deeply sourced CO₂‐rich fluid with basinal brine was the key mechanism responsible for the mineralization of the Jinman deposit.