The Characteristics and Genetic Mechanism of Igneous Topazites in South China

Igneous topazites found recently in South China are closely associated with F-rich granites in time and space. They have a typical igneous porphyritic texture. The phenocrysts are topaz and quartz, while the groundmass consists mainly of prismatic or acicular topaz microlites and anhedral quartz. Compared with granites, topazites are rich in SiO2 (65.06%-81.12% ) and Al2O3 (13.01%-18.09%) but poor in MgO and Na2O and strongly peraluminous with A/NKC = 3.204-37.313. Geochemically, the rocks are evidently depleted in Sr and Ba, but enriched in Sn, W, Nb and F. The concentrations of Sn and partial W in the topazites are 1-2 orders of magnitude higher than the average of acid rocks. The F-riched granites related to the topazites in the area usually have much higher ISr (0.7103-0.7460) and δ18OQ‰ (9.5-14.24), but lower INdCT) (-4.8- 9.4). It suggests that the topazite is similar to S-type granites in genesis. Hydroxylated silicate melt inclusions, consisting of several grains of quartz and aqueous fluids,     

Quartz-topaz-loellingite rocks near Eldorado,Victoria

Two small dykes consisting of a quartz-topaz-loellingite rock type have recently been discovered within the aplitic phase of the Pilot Range granite, near Eldorado, in NE Victoria. Minor biotite, muscovite, chlorite, kaolinite, anatase and pharmacosiderite are associated. Apart from the loellingite, the dykes are similar mineralogically to the ‘topazites’ from New England, NSW. These were considered to be magmatic in origin, based on field relationships and high homogenization temperatures for fluid inclusions in topaz (Eadington & Nashar 1978, Contrib. Mineral. Petrol. 67, 433–438). Although experimental evidence on F-enriched ‘granitic’ systems is inconclusive, the emplacement of the Eldorado topazite dykes most likely involved both magmatic and hydrothermal components operating essentially simultaneously. The topazite melt represented a F-rich residual granitic magma, from which aqueous alkali halide-rich solutions separated during high level intrusion. Separation of these aqueous solutions was responsible for miarolitic cavities into which topaz crystals grew. F-OH equilibration calculations for coexisting topaz-biotite pairs suggest the minerals equilibrated in the presence of hydrothermal solutions of variable composition (in terms of the HF/H₂O fugacity ratio), at temperatures around 550°C. Alteration of topaz to muscovite, the precipitation of loellingite and the formation of clay and fluorite in the cavities occurred at progressively lower temperatures. The widespread alluvial topaz in the Beechworth-Eldorado area may be derived from similar quartz-topaz dykes.     

Extreme alkali bicarbonate- and carbonate-rich fluid inclusions in granite pegmatite from the Precambrian Rønne granite,Bornholm Island,Denmark

Our study of fluid and melt inclusions in quartz and feldspar from granite pegmatite from the Precambrian Rønne granite, Bornholm Island, Denmark revealed extremely alkali bicarbonate- and carbonate-rich inclusions. The solid phases (daughter crystals) are mainly nahcolite [NaHCO₃], zabuyelite [Li₂CO₃], and in rare cases potash [K₂CO₃] in addition to the volatile phases CO₂ and aqueous carbonate/bicarbonate solution. Rare melt inclusions contain nahcolite, dawsonite [NaAl(CO₃)(OH)₂], and muscovite. In addition to fluid and melt inclusions, there are primary CO₂-rich vapor inclusions, which mostly contain small nahcolite crystals. The identification of potash as a naturally occurring mineral would appear to be the first recorded instance. From the appearance of high concentrations of these carbonates and bicarbonates, we suggest that the mineral-forming media were water- and alkali carbonate-rich silicate melts or highly concentrated fluids. The coexistence of silicate melt inclusions with carbonate-rich fluid and nahcolite-rich vapor inclusions indicates a melt-melt-vapor equilibrium during the crystallization of the pegmatite. These results are supported by the results of hydrothermal diamond anvil cell experiments in the pseudoternary system H₂O–NaHCO₃–SiO₂. Additionally, we show that boundary layer effects were insignificant in the Bornholm pegmatites and are not required for the origin of primary textures in compositionally simple pegmatites at least.     

The fluid regime of crystallization of water-saturated granitic and pegmatitic magmas: a physicochemical analysis

Miarolitic granite pegmatites are a unique natural object that makes it possible to study magmatic processes that lead to the formation of ore-forming media and systems. This paper summarizes modern views on phase transformations in aqueous silicate systems at parameters close to those of the transition from magmatic to hydrothermal crystallization. Comparison of phase diagrams and the results of study of pegmatite-forming media permits making conclusions about the crystallization of the water-saturated magmas of miarolitic granite pegmatites. The fluid regime of aqueous granite systems of simple composition, not enriched in fluxing components, is determined mainly by magma degassing or the supply of volatiles with flows of transmagmatic fluids. These processes cause the separation of essentially carbon dioxide or essentially hydrous fluid. During the evolution of such magmas, crystallization from silicate melt is separated in PT-space and, possibly, in time from the crystallization from aqueous or mixed carbon dioxide-aqueous super- and subcritical solutions. The evolution of chambers of water-saturated granitic and pegmatitic magma enriched in F, B, and alkali metals presupposes the formation of a heterogeneous mineral-forming medium in which crystallization occurs in the magmatic melt at high-temperature stages; as temperature decreases, crystallization can proceed in hydrous fluid, hydrosilicate, and/or hydrosaline liquids simultaneously. Hydrothermal crystallization can also take place in a heterogeneous medium consisting of aqueous solutions of different salinities and vapor or vapor-carbon dioxide gas mixture. The relationship between different fluid regimes during the evolution of volatile-saturated granitic and pegmatitic magmas determines the variety of postmagmatic rocks accompanying granite massifs.     

Fluid and magmatic processes in the formation of the Ary-Bulak ongonite massif (eastern Transbaikalia)

The paper discusses the formation conditions of the Ary-Bulak ongonite massif (eastern Transbaikalia). Studies of melt and fluid inclusions have shown that, along with crystalline phases and a silicate melt, ongonitic magma contained aqueous–saline fluids of different types, fluoride melts compositionally similar to fluorite, sellaite, cryolite, chiolite, and more complex aluminum fluorides as well as silicate melts with abnormal Cs and As contents. An ongonite melt crystallized with the participation of P–Q fluids as vapor solutions, presumably NaF-containing and slightly admixed with chlorides. We studied the properties and composition of brine inclusions from Ca- and F-rich rocks on the margin of the massif. Depending on the thermophysical properties of the host rocks and ongonite melt, the duration of its crystallization has been estimated for a magma chamber of the size and shape of the Ary-Bulak massif. Magma chamber cooling has been modeled, and the density, viscosity, and Rayleigh number of the ongonite melt have been estimated from the composition of silicate glasses in melt inclusions. These data strongly suggest intense convection in the residual magma chamber lasting for centuries. We have calculated possible fluid overpressure during the crystallization and degassing of the ongonite melt in a closed magma chamber.Calcium- and fluorine-rich aphyric and porphyritic rocks on the southwestern margin of the massif might have formed by the following mechanism. Local decompression in the magma chamber quenched an oxygen-containing calcium fluoride melt accumulated at the crystallization front, and then these rocks altered during the interaction with fluids. When penetrating the marginal zone, a P–Q magmatic fluid which coexisted with the melt in the residual chamber cooled and changed its composition and properties. This caused the fluid to boil and segregate into immiscible phases: a vapor solution and a brine extremely rich in Cl, F, K, Cs, Mn, Fe, and Al. The fluoride and silicate liquids were immiscible; the silicate melts had abnormal Cs and As contents; changes in the composition and properties of the magmatic fluids caused them to boil and produce brines. All this is evidence for complex fluid–magma interaction and heterogeneous ongonitic magma during the crystallization of the Ary-Bulak rocks. These processes were favored by the low viscosity and high mobility of the F- and water-rich ongonite melt, intense melt convection in the residual chamber, and rising fluid pressure during its degassing.     

Fluid and melt inclusion evidence for the origin of idiomorphic quartz crystals in topaz-bearing granite from the Salmi batholith, Karelia, Russia

Idiomorphic quartz crystals in topaz-bearing granite from the Salmi batholith contain primary inclusions of silicate melt and abundant mostly secondary aqueous fluid inclusions. Microthermometric measurements on melt inclusions give estimates for the granite solidus and liquidus of 640–680°C and 770–830°C, respectively. Using published solubility models for H₂O in granitic melts and the obtained solidus/liquidus temperatures from melt inclusions, the initial water concentration of the magma is deduced to have been approximately 3 wt.% and the minimum pressure about 2 kbar. At this initial stage, volatile-undersaturation conditions of magma were assumed. These results indicate that the idiomorphic quartz crystals are magmatic in origin and thus real phenocrysts. During subsolidus cooling and fracturing of the granite, several generations of aqueous fluid inclusions were trapped into the quartz phenocrysts. The H₂O inclusions have salinities and densities of 1–41 wt.% NaCl eq. and 0.53–1.18 g/cm³, respectively.     

Geochemical evolution of halogen-enriched granite magmas and mineralizing fluids of the Zinnwald tin-tungsten mining district,Erzgebirge, Germany

We remelted and analyzed crystallized silicate melt inclusions in quartz from a porphyritic albite-zinnwaldite microgranite dike to determine the composition of highly evolved, shallowly intruded, Li- and F-rich granitic magma and to investigate the role of crystal fractionation and aqueous fluid exsolution in causing the extreme extent of magma differentiation. This dike is intimately associated with tin- and tungsten-mineralized granites of Zinnwald, Erzgebirge, Germany. Prior research on Zinnwald granite geochemistry was limited by the effects of strong and pervasive greisenization and alkali-feldspar metasomatism of the rocks. These melt inclusions, however, provide important new constraints on magmatic and mineralizing processes in Zinnwald magmas.The mildly peraluminous granitic melt inclusions are strongly depleted in CAFEMIC constituents (e.g., CaO, FeO, MgO, TiO₂), highly enriched in lithophile trace elements, and highly but variably enriched in F and Cl. The melt inclusions contain up to several thousand ppm Cl and nearly 3 wt% F, on average; several inclusions contain more than 5 wt% F. The melt inclusions are geochemically similar to the corresponding whole-rock sample, except that the former contain much more F and less CaO, FeO, Zr, Nb, Sr, and Ba. The Sr and Ba abundances are very low implying the melt inclusions represent magma that was more evolved than that represented by the bulk rock. Relationships involving melt constituents reflect increasing lithophile-element and halogen abundances in residual melt with progressive magma differentiation. Modeling demonstrates that differentiation was dominated by crystal fractionation involving quartz and feldspar and significant quantities of topaz and F-rich zinnwaldite. The computed abundances of the latter phases greatly exceed their abundances in the rocks, suggesting that the residual melt was separated physically from phenocrysts during magma movement and evolution.Interactions of aqueous fluids with silicate melt were also critical to magma evolution. To better understand the role of halogen-charged, aqueous fluids in magmatic differentiation and in subsequent mineralization and metasomatism of the Zinnwald granites, Cl-partitioning experiments were conducted with a F-enriched silicate melt and aqueous fluids at 2,000 bar (200 MPa). The results of the experimentally determined partition coefficients for Cl and F, the compositions of fluid inclusions in quartz and other phenocrysts, and associated geochemical modeling point to an important role of magmatic-hydrothermal fluids in influencing magma geochemistry and evolution. The exsolution of halogen-charged fluids from the Li- and F-enriched Zinnwald granitic magma modified the Cl, alkali, and F contents of the residual melt, and may have also sequestered Li, Sn, and W from the melt. Many of these fluids contained strongly elevated F concentrations that were equivalent to or greater than their Cl abundances. The exsolution of F-, Cl-, Li-, ± W- and Sn-bearing hydrothermal fluids from Zinnwald granite magmas was important in effecting the greisenizing and alkali-feldspathizing metasomatism of the granites and the concomitant mineralization.Editorial Handling: B. Lehmann     

A melt and fluid inclusion assemblage in beryl from pegmatite in the Orlovka amazonite granite, East Transbaikalia, Russia: implications for pegmatite-forming melt systems

Beryl crystals from the stockscheider pegmatite in the apical portion of the Li-F granite of the Orlovka Massif in the Khangilay complex, a tantalum deposit, contain an assemblage of melt and fluid inclusions containing two different and mutually immiscible silicate melts, plus an aqueous CO₂-rich supercritical fluid. Pure H₂O and CO₂ inclusions are subordinate. Using the terminology of Thomas R, Webster JD, Heinrich W. Contrib Mineral Petrol 139:394–401 (2000) the melt inclusions can be classified as (i) water-poor type-A and (ii) water-rich type-B inclusions. Generally the primary trapped melt droplets have crystallized to several different mineral phases plus a vapor bubble. However, type-B melt inclusions which are not crystallized also occur, and at room temperature they contain four different phases: a silicate glass, a water-rich solution, and liquid and gaseous CO₂. The primary fluid inclusions represent an aqueous CO₂-rich supercritical fluid which contained elemental sulfur. Such fluids are extremely corrosive and reactive and were supersaturated with respect to Ta and Zn. From the phase compositions and relations we can show that the primary mineral-forming, volatile-rich melt had an extremely low density and viscosity and that melt-melt-fluid immiscibility was characteristic during the crystallization of beryl. The coexistence of different primary inclusion types in single growth zones underlines the existence of at least three mutually immiscible phases in the melt in which the large beryl crystals formed. Moreover, we show that the inclusions do not represent an anomalous boundary layer.     

Segregation of magmatic fluids and their potential in the mobilization of platinum-group elements in the South Kawishiwi Intrusion,Duluth Complex,Minnesota — Evidence from petrography,apatite geochemistry and coexisting fluid and melt inclusions

Pegmatitic and other felsic rock pockets and dike-like intrusions are abundant in the South Kawishiwi Intrusion of the Duluth Complex, including the basal, Cu–Ni–PGE mineralized units. These occurrences are found as pockets, pods or as veins and contain abundant accessory apatite and quartz. Quartz hosts primary fluid inclusions as well as silicate melt inclusions. Combined microthermometry and Raman spectroscopy helped to determine the bulk composition of primary fluid inclusions that are CO₂-rich (95 mol%) and contain small amounts of H₂O (4.5 mol%), CH₄ (0.4 mol%) and trace N₂, respectively. This combined technique also made it possible to measure total homogenization temperatures of the inclusions (Th_tot = ~ 225 ± 10 °C), otherwise not detectable during microthermometry. Silicate melt inclusions have been quenched to produce homogeneous glasses corresponding to the original melt. Composition of the entrapped melt is granitoid, peraluminous and is very poor in mafic components. We interpret the melt as a product of partial melting of the footwall rocks due to the contact effect of the South Kawishiwi Intrusion. The presence of CO₂ in the vapor bubbles of the quenched melt inclusions and petrographic evidence suggest that the fluid and melt inclusion assemblages are coeval. The composition of the fluid and melt phase implies that the fluid originates from the mafic magma of the South Kawishiwi Intrusion and the fluid and melt phases coexisted as a heterogeneous melt–fluid system until entrapment of the inclusions.Coexistence of primary fluid and melt inclusions makes it possible to calculate a minimum entrapment pressure (~ 1.7 kbar) and thus estimate formation depth (~ 5.8 km) for the inclusions. Chlorine is suggested to behave compatibly in the silicate melt phase in the fluid–melt system represented by the inclusions, indicated by the high (up to 0.3%) Cl-concentrations of the silicate melt and CO₂-rich nature of the fluid.Apatite halogen-contents provide further details on the behavior of Cl. Apatite in pegmatitic pockets often has elevated Cl-concentrations compared to troctolitic rocks, suggesting enrichment of Cl with progressive crystallization. Systematic trends of Cl-loss at some differentiated melt pockets suggest that in some places Cl exsolved into a fluid phase and migrated away from its source. The segregation of Cl from the melt is probably inhibited by the presence of CO₂-rich fluids until the last stages of crystallization, increasing the potential for the development of late-stage saline brines.Platinum-group minerals are often present in microcracks in silicate minerals, in late-stage differentiated sulfide veinlets and in association with chlorapatite, indicating the potential role of Cl-bearing fluids in the final distribution of PGEs.     

Magmatic-Hydrothermal Evolution in the “Bottoms” of Porphyry Copper Systems: Evidence from Silicate Melt and Aqueous Fluid Inclusions in Granitoid Intrusions in the Gyeongsang Basin,South Korea

The Gyeongsang Basin of southeastern Korea contains numerous Cretaceous-early Tertiary (120–40 Ma) granitoid intrusions formed at a convergent plate boundary. The geotectonic setting is similar to that associated with porphyry-type mineralization elsewhere in the Circumpacific region. However, erosion has removed higher-level economic mineralization and exposed deeper levels of the granitoids, representing the poorly mineralized “bottoms” of porphyry copper systems. The intrusions of the Gyeongsang Basin thus provide a unique opportunity to advance our understanding of magmatic-hydrothermal evolution in the roots of porphyry-type systems, below the level of economic mineralization.

The physical and chemical environment during crystallization of the magmas has been characterized through studies of silicate melt and aqueous fluid inclusions in the granitoids. Two different types of silicate melt inclusions were recognized based on occurrence and room-temperature appearance. Type-I inclusions contain one or more crystalline phases and vapor; type-II inclusions consist of a cluster of small crystals, partially devitrified glass, and vapor. Petrographic and Raman analyses indicate that most silicate melt inclusions contain muscovite daughter crystals. Some also contain feldspar. Solidus temperatures of type-I inclusions in quartz phenocrysts range from ≈630to 650°C, whereas solidus temperatures of type-I and type-II inclusions in vug quartz are slightly higher (640–670°C). Liquidus temperatures span a much wider range compared to solidus temperatures, with maximum liquidus temperatures of melts in phenocrysts being slightly higher (≤930°C) than those in vug quartz (≤910°C).

Three types of aqueous inclusions were observed based on occurrence and room temperature phase proportions. Type-A inclusions are liquid rich and low to moderate in salinity; type-B inclusions are vapor rich and low in salinity; type-C inclusions are liquid rich and contain a halite daughter mineral. Some type- A inclusions with a salinity of approximately 25 wt% NaCl equivalent are spatially associated with silicate melt inclusions in phenocrysts, where they occur as three-dimensional clusters of tiny inclusions surrounding the silicate melt inclusion. Type-A inclusions also occur along fractures in quartz phenocrysts. Non-fracture-controlled type-C inclusions are rare in phenocryst quartz, but are common in vug quartz, where they are associated with silicate melt inclusions. Type-C inclusions that coexist with silicate melt inclusions generally homogenize by halite dissolution after the vapor bubble and show a wide range in salinity, from about 30 to >60 wt% NaCl equivalent. Coexisting halite-bearing (Type-C) and vapor-rich (Type-B) inclusions in phenocryst quartz suggest local immiscibility in the late-or post-magmatic fluid.

Pressure-temperature conditions during the final stages of magmatic-hydrothermal activity associated with the granitoid intrusions of the Gyeongsang Basin were approximately 630° to 670° C and 1.9 to 2.5 kbars. These results suggest that the granitoids do not contain economic porphyry coppertype mineralization because the magmas crystallized at high pressures (relative to typical porphyry copper magmas) and did not become saturated in water until a relatively late stage in the crystallization history. Failure to reach water saturation resulted in most of the copper in the original melt being sequestered as a trace component in earlier-crystallizing silicate and sulfide phases to produce anomalous but subeconomic copper grades. Furthermore, owing to the depth of emplacement, less energy was available to fracture the rocks when water did exsolve from the magma, and the pressure remained too high for aqueous fluid immiscibility to be an important metal-concentrating or depositing mechanism. Geological, petrographic, and geochemical characteristics suggest that the granitoid rocks of the Gyeongsang Basin represent ethroot zones of porphyry-type systems, and any higher-grade mineralization that may have been present higher in the system has since been removed by erosion.     

江西凤凰岽含锡黄英岩脉特征及成因

江西会昌县岩背火山盆地密坑山含黄玉花岗岩体的北侧外接触带，产有数条含锡黄英岩脉。据野外产状、岩石结构构造等表明岩脉属岩浆成因。稀土元素研究表明，凤凰岽黄英岩ＲＥＥ含量极低，∑ＲＥＥ及配分曲线与脉侧黄玉石英化蚀变岩有着明显的区别。富氟花岗岩浆结晶晚期衍生的富Ｓｉ，Ａｌ，Ｆ含少量Ｋ，Ｎａ的残余熔浆沿断裂上侵因压力降低产生二次沸腾，使Ｋ，Ｎａ分离进入含水相，这是富Ｓｉ，Ａｌ，Ｆ；极贫Ｋ，Ｎａ的黄英岩形成的决定因素。     

Compositions of magmas and carbonate–silicate liquid immiscibility in the Vulture alkaline igneous complex, Italy

This paper presents a study of melt and fluid inclusions in minerals of an olivine-leucite phonolitic nephelinite bomb from the Monticchio Lake Formation, Vulture. The rock contains 50 vol.% clinopyroxene, 12% leucite, 10% alkali feldspars, 8% hauyne/sodalite, 7.5% nepheline, 4.5% apatite, 3.2% olivine, 2% opaques, 2.6% plagioclase, and < 1% amphibole. We distinguished three generations of clinopyroxene differing in composition and morphology. All the phenocrysts bear primary and secondary melt and fluid inclusions, which recorded successive stages of melt evolution. The most primitive melts were found in the most magnesian olivine and the earliest clinopyroxene phenocrysts. The melts are near primary mantle liquids and are rich in Ca, Mg and incompatible and volatile elements. Thermometric experiments with the melt inclusions suggested that melt crystallization began at temperatures of about 1200 °C. Because of the partial leakage of all primary fluid inclusions, the pressure of crystallization is constrained only to minimum of 3.5 kbar. Combined silicate–carbonate melt inclusions were found in apatite phenocrysts. They are indicative of carbonate–silicate liquid immiscibility, which occurred during magma evolution. Large hydrous secondary melt inclusions were found in olivine and clinopyroxene. The inclusions in the phenocrysts recorded an open-system magma evolution during its rise towards the surface including crystallization, degassing, oxidation, and liquid immiscibility processes.     

Genesis of apatite-fluorite rock in the Burpala pluton

Burpala is a unique peralkaline pluton known to the world. Alkaline pegmatites of the pluton contain about 70 rare-metal minerals. A new scheme of rock crystallization is offered: shonkinite → nepheline syenite → alkali syenite → quartz syenite → vein rocks: mariupolite, rare-metal pegmatite, apatite-fluorite, and alkali granite. Investigation of fluid inclusions in fluorite from the apatite-fluorite rocks established the high temperatures (520–560°C) of homogenization of multiphase salt inclusions. Fluids from inclusions are dominated by hydrocarbonates and chlorides as anions and sodium and calcium as cations; microelements include strontium, barium, boron, iron, manganese, lithium, rubidium, and cesium, i.e., components characteristic of magmatogenic fluids. These rocks are analogous to foskorites of carbonatite complexes in the high calcium content, but calcite is replaced with fluorite along with other foskorite minerals such as apatite, magnetite, mica, and pyroxene.     

花岗岩类矿床成矿流体形成过程的原位观测实验

花岗岩浆液态不混溶作用和饱和H₂O花岗岩浆的热液出溶作用是花岗岩类矿床成矿流体形成的重要机制。利用最新式热液金刚石压腔,开展了成矿流体形成机制的原位观测实验。在岩浆热液出溶过程的实验中,初始样品为各类硅酸盐和纯H₂O或LiCl水溶液,在H₂O饱和状态中,硅酸盐熔体珠不断分异出富H₂O的流体。花岗岩浆液态不混溶实验的初始样品为NaAlSi₃O₈-LiAlSiO₄-SiO₂-LiCl-H₂O。在硅酸盐完全重熔后的降温过程中,硅酸盐熔体珠分离出富H₂O熔体相和贫H₂O熔体相,压力的突然降低促进了相分离的发生。研究表明：岩浆热液的出溶作用发生在H₂O饱和的条件下,是岩浆的“第二次”沸腾作用,对花岗岩型稀有金属矿床的形成具有重要意义;花岗岩浆液态不混溶产生的富H₂O熔体易于结晶出粗大晶体,暗示岩浆液态不混溶作用可能是一些花岗伟晶岩形成的主要机制。两类成矿流体形成机制实验条件的差异表明,Li是花岗岩浆发生不混溶作用的重要因素。在今后的研究中,应把热液金刚石压腔的原位观测与微束分析技术结合,在高温高压状态下分析成矿元素的迁移和富集规律。     

硅酸盐-盐（硫酸盐）流体不混溶：蒙古南部Mushugai-Khuduk含火成碳酸岩杂岩体矿物中的熔体包裹体研究

Crystalline and melt inclusions were studied in garnet,diopside,potassium feldspar,and sphene from the garnet syenite porphyry of the carbonatite-bearing complex Mushugai-Khuduk,southern Mongolia.Phlogopite,clinopyroxene,albite,potassium feldspar,spheric,wollastonite,magnetite,Ca and Sr sulfates,fluorite,and apatite were identified among the crystalline inclusions. The melt inclusions were homogenized at 1010～1080℃and analyzed on an electron microprobe.Silicate,salt,and combined silicate- salt melt inclusions were found.Silicate melts show considerable variations in SiO_2 concentration(56 to 66wt% ),high Na_2O K_2O (up to 17wt% ),and elevated Zr,F,and C1 contents.In terms of bulk rock chemistry,the silicate melts are alkali syenites.During thermometric experiments,salt melt inclusions quenched into homogeneous glasses of predominantly sulfate compositions containing no more than 1.3wt% SiO_2.These melts are enriched in alkalis,Ba,Sr,P,F,and C1.The investigation of the silicate and salt melt inclusions in minerals of the garnet syenite porphyries indicate that these rocks were formed under influence of the processes of crystallization differentiation and magma separation into immiscible silicate and salt(sulfate)liquids.     

Genesis of mineralized cavities (Miaroles) in granitic pegmatites and granites

Analysis the development of large fluid segregations in a flux of small fluid bubbles during the degassing of granitic (pegmatitic) melts indicates that the velocity of the buoyant ascent of fluid bubbles depends on their sizes, the viscosity and density of the melts, and the duration of melt flow. Possible variants of the primary and secondary boiling of magma are discussed depending on the P-T conditions and concentrations of H₂O, F, B, and other components dissolved in the magma. The possible density ranges of the fluid phases are considered, along with the viscosity and density of granitic (pegmatitic) melts, velocities of the buoyant ascent of fluid bubbles in them, and the processes of their coalescence and accumulation in the temperature range of 650–850°C. Provisional evaluates are obtained for the duration of melt crystallization and the development of intrusive massifs and dikes of granites and syngenetic intragranite and epigenetic (intruded into the host rocks) granite pegmatites. Simulation data and geological observations suggest that large fluid segregations were formed already in the magma chambers in which the heterogeneous granite (pegmatitic) magma was derived, before its emplacement into the host rocks. These generation regions could be magma chamber areas within granite intrusions, in which melts enriched in volatiles were accumulated and then degassed with the release of fluid phases of various composition and density. The crystallization of fluid-rich melts under favorable conditions gives rise to granites with miarolitic structures. The emplacement of heterogeneous pegmatitic magma (which consists of immiscible silicate melts and large fluid segregations) into the host rocks results in that these segregations (would-be miaroles) occur in any part of the pegmatite-hosting chamber. This explains why miaroles of significantly different composition and with broadly varying proportions of their filling minerals may occur in various parts of pegmatite veins or their swells, as well as near contacts with the host rocks.     

A Quantitative Study of Magmatic Inclusions in the Plutonic Ejecta of Ascension Island

Plutonic blocks in the volcanic and pyroclastic rocks of AscensionIsland contain a variety of silicate melt and fluid inclusions,mainly in the intermediate (monzonite-syenite) and granite types.The silicate melt inclusions consist of trapped granitic meltwith a small contraction bubble. The fluid inclusions are: (i)highly saline (40 to > 67 wt. per cent NaCI) aqueous inclusions;(ii) low density CO₂ + H₂O inclusions, predominantly vapourand (iii) high density approximately pure H₂O inclusions. Mixedsilicate melt-saline aqueous fluid inclusions also occur showingthat the original magma became saturated with respect to salineaqueous fluid and a magmatic origin is thus indicated for thesaline fluid inclusions. Microprobe analysis of the silicatemelt inclusions suggests that the original magma may have containedgreater than 5 wt. per cent H₂O before saturation was reachedand homogenization temperatures show the trapping of the silicatemelt took place between 715-790?C. Separated amphiboles fromthe granites have D values which are significantly differentfrom the whole rock D values of the dry but otherwise chemicallysimilar obsidians. Sheppard & Harris (1985) interpret thisdifference as reflecting a sea water source for the water inthe gramtes. Qualitative and quantitative study of the fluidinclusions (extending the earlier work of Roedder & Coombs,1967) suggests that at least one episode of fluid immiscibility(into aqueous liquid+vapour, i.e. ‘boiling’) occurred.This resulted in a loss of H₂O via the vapour and increasedthe salinity of the remaining liquid. The initial fluids belongedto the system H₂O-CO₂-NaCl-KCl and the CO₂ was strongly partitionedinto the vapour phase during fluid unmixing, hence X_co2 decreasedas the fluid evolved. A suite of gabbroic blocks from Dark SlopeCrater contain pure CO fluid inclusions of just higher thancritical density. Assuming a trapping temperature of 1000–1200?Cthe pressure must have been > 18 kb–28 kb or 7-11 kmin depth, i.e. well below the base of the Ascension Cone. Thepure CO₂ inclusions in this suite of gabbros and the fluidsin the H₂O-CO₂NaCl-KCl system would seem to require entirelydifferent origins and it may be significant that these gabbrosshow a different isotopic Pb signature than do the volcanicsand other blocks.     

A Preliminary Research on Skarns of Magmatic Origin

ＩＮＴＲＯＤＵＣＴＩＯＮＴｈｅｍｉｄｄｌｅａｎｄｌｏｗｅｒｒｅａｃｈｅｓｏｆＣｈａｎｇｊｉａｎｇＲｉｖｅｒａｒｅｏｎｅｏｆｔｈｅｍａｉｎｒｅｇｉｏｎｓｉｎＣｈｉｎａｗｈｉｃｈｉｓｃｈａｒａｃｔｅｒｉｓｔｉｃｏｆｔｈｅｗｉｄｅｓｐｒｅａｄｄｉｓｔｒｉｂ...     

Conditions of crystallization of olivine shonkinites in the Inagli massif (Central Aldan)

The olivine shonkinites localized among dunites and alkali gabbroids in the northern part of the alkaline ultrabasic Inagli massif (northwestern part of Central Aldan) have been studied. The obtained data on the chemical and trace-element compositions of the rocks and minerals and the results of melt inclusion study showed that the olivine shonkinites crystallized from alkaline basanite melt enriched in Cl, S, CO2, and trace elements. Clinopyroxene crystallized at 1180-1200 °C from a homogeneous silicate-salt melt, which was probably separated into immiscible silicate and carbonate-salt fractions with temperature decreasing. The composition of the silicate fraction evolved from alkaline basanite to alkaline trachyte. The carbonate-salt fraction had an alkaline carbonate composition and was enriched in S and Cl. The same trend of evolution of clinopyroxene-hosted melts and the igneous rocks of the Inagli massif suggests that the alkali gabbroids, melanocratic alkali syenites, and pulaskites formed from the same magma, which had a near-alkaline basanite composition during its crystallization differentiation. The geochemical studies showed that the olivine shonkinites and glasses of homogenized melt inclusions in clinopyroxene grains have similar contents of trace elements, one or two orders of magnitude higher than those in the primitive mantle. The high contents of LILE (K, Rb, and Sr) and LREE in the olivine shoshonites and homogenized inclusions suggest the enriched mantle source, and the negative anomalies of HFSE and Ti are a specific feature of igneous rocks formed with the participation of crustal material. The slight depletion in HREE relative to LREE and the high (La/Yb)_n ratios in the rocks and inclusion glasses (10.0-11.4 and 4.7-6.2, respectively) suggest the presence of garnet in the mantle source.     

Geochemistry and fluid evolution of the peralkaline rare-metal granite, Gabal Gharib, Eastern Desert of Egypt

The Neoproterozoic pluton of Gabal Gharib granite Eastern Desert of Egypt is intruded in subduction-related calc-alkaline granitic rocks of granodiorite to adamellite composition. A zone of metasomatized granite was developed along the contacts at the expense of the calc-alkaline granite. The granite of Gabal Gharib is hypersolvus, composed mainly of orthoclase-microperthite, quartz, and interstitial arfvedsonite. Fluorite, zircon, ilmenite, allanite, and astrophyllite are the main accessories. Pegmatite pods as well as miarolitic cavities (mineral-lined cavities) are common and ranging in size from a few millimeters to 50?cm. Rare-metal minerals such as columbite, cassiterite, and fluorite have been identified from the miarolitic cavities. Geochemical studies revealed that Gabal Gharib granite is a highly fractionated granite, homogeneous in composition, with high contents of SiO₂, and alkalis, high Ga/Al, and Fe/Mg ratios, and low concentrations of Al, Mg, and CaO relative to granodiorite?Cadamellite country rocks. Gabal Gharib granite is metaluminous to peralkaline with ASI (0.94?C1.07). Trace element characteristics of Gabal Gharib granite include abundances of Rb, Nb, Ta, Sn, Th, U, Y, Ga, Zn, rare earth elements (REEs, except Eu), and F, and depletion in Sr, and Ba relative to granodiorite?Cadamellite country rocks. It has the geochemical characteristic of anorogenic A-type granite. The uniform trends of differentiation, normal REE distribution patterns, and low calculated tetrad effects of REE (<0.2) indicate that the effect of post-magmatic subsolidus processes were minimal in the studied granite. Fluid inclusions were studied in quartz crystals from Gabal Gharib granite, quartz pods, and metasomatized granite. The study revealed the presence of high-temperature (480?C550°C), high-salinity (19.45?C39.13?wt.% NaCl eq.) primary inclusions in both metasomatized and rare-metal granites coexisting with melt inclusions and medium-temperature (350?C450°C), medium-salinity (10?C16?wt.% NaCl esq.) aqueous inclusions coexisting hydrocarbon-bearing inclusions. Hydrocarbon is represented by magmatic CH4 in Gabal Gharib granite, while heavier aliphatic compounds may be present in quartz pods. Melt inclusions with temperatures of homogenization >600°C were also reported. Petrographic, geochemical, and fluid inclusion studies constrain that the peralkaline anorogenic granite of Gabal Gharib was derived from highly evolved magma probably originated by fractional crystallization of mantle source.