P–T composition and evolution of paleofluids in the Paleoproterozoic Mag Hill IOCG system,Contact Lake belt,Northwest Territories,Canada

The Echo Bay stratovolcano complex and Contact Lake Belt of the Great Bear Magmatic Zone, Northwest Territories, host a series of coalescing Paleoproterozoic hydrothermal systems that affected an area of several hundred square kilometers. They were caused by intrusion of synvolcanic diorite–monzodioritic plutons into andesitic host rocks, producing several characteristic hydrothermal assemblages. They include early and proximal albite, magnetite–actinolite–apatite, and potassic (K-feldspar) alteration, followed by more distal hematite, phyllic (quartz–sericite–pyrite), and propylitic (chlorite–epidote–carbonate±sericite±albite±quartz) alteration, and finally by late-stage polymetallic epithermal veins. These alteration types are characteristic of iron oxide copper–gold deposits, however, with distal and lower-temperature assemblages similar to porphyry Cu systems. Magnetite–actinolite–apatite alteration formed from high temperature (up to 560 °C) fluids with average salinity of 12.8 wt% NaCl equivalent. The prograde propylitic and phyllic alteration stages are associated with fluids with temperatures varying from 80 to 430 °C and a wide salinity range (0.5–45.6 wt% NaCl equivalent). Similarly, wide fluid temperature (104–450 °C) and salinity (4.2–46.1 wt% NaCl equivalent) ranges are recorded for the phyllic alteration. This was followed by Cu–Ag–U–Zn–Co–Pb sulfarsenide mineralization in late-stage epithermal veins formed at shallow depths and temperatures from 270 °C to as low as 105 °C. The polymetallic veins precipitated from high salinity (mean 30 wt% NaCl equivalent) dense fluids (1.14 g/cm³) with a vapor pressure of 3.8 bars, typical of epithermal conditions. Fluid inclusion evidence indicates that mixed fluids with evolving physicochemical properties were responsible for the formation of the alteration assemblages and mineralization at Mag Hill. An early high temperature, moderate salinity, and magmatic fluid was subsequently modified variably by boiling, mixing with cooler low-salinity meteoric water, and simple cooling. The evidence is consistent with emplacement of the source plutons and stocks into an epithermal environment within ~1 km of surface. This generated near-surface high-temperature alteration in a dynamic hydrothermal system that collapsed (telescoped) resulting in widespread evidence of boiling and epithermal mineralization superimposed on earlier stages of alteration.     

Zircon as the best mineral for P–T–time history of UHP metamorphism: A review on mineral inclusions and U–Pb SHRIMP ages of zircons from the Dabie–Sulu UHP rocks

Zircon is the best mineral to record the complex evolution history of ultrahigh-pressure (UHP) metamorphic rocks as mineralogical and geochemical tracers of UHP metamorphism are almost obliterated in matrix assemblages resulted from subsequent retrogression during exhumation. Zircons from Dabie–Sulu UHP rocks, including outcrop and core samples from drill holes ranging from 432 to 5158 m in depth contain abundant mineral inclusions of protolith, prograde, peak (UHP) and retrograde minerals in different domains; these minute inclusions were identified by laser Raman spectroscopy and/or electronic microprobe analysis. Systematic studies on inclusions in zircons from previous and present studies indicate that the Dabie–Sulu UHP terrane extends for >2000 km, is about 50 km wide, and has at least 10 km thick, probably the largest UHP terrane recognized in the world thus far. The internal structure of zircon revealed by cathodoluminescence (CL) imaging displays a distinct zonation, which comprises an inherited (magmatic or detrital) core, prograde, peak (UHP), and outmost retrograde domains, each with distinctive mineral inclusion assemblages. Low-pressure, igneous mineral inclusions are common in the inherited (magmatic or detrital) zircon cores. In contrast, quartz eclogite-facies inclusion assemblages occur in prograde domains, coesite eclogite-facies inclusion assemblages are preserved in UHP domains, and amphibolite-facies inclusion assemblages are enclosed in outmost retrograde rims. Parageneses and compositions of inclusion minerals preserved in distinct zircon domains were used to constrain the metamorphic P–T path of many Dabie–Sulu UHP rocks. The results indicate that Neoproterozoic supracrustal rocks together with minor mafic-ultramafic rocks were subjected to a prograde subduction-zone metamorphism at 570–690 °C and 1.7–2.1 GPa, and UHP metamorphism at 750–850 °C and 3.4–4.0 GPa, following by rapid decompression to amphibolite-facies retrograde metamorphism at 550–650 °C and 0.7–1.05 GPa. Sensitive high-resolution ion microprobe (SHRIMP) U–Pb spot analyses of the zoned zircons show four discrete and meaningful ages of the Dabie–Sulu metamorphic evolution: (1) Neoproterozoic protolith ages (800–750 Ma); (2) 246–244 Ma for early-stage quartz eclogite-facies prograde metamorphism; (3) 235–225 Ma for UHP metamorphism; and (4) 215–208 Ma for late-stage amphibolite-facies retrogression. This indicates that Neoproterozoic voluminous igneous protoliths of orthogneiss in response to the breakup of Rodinia supercontinent, together with various sedimentary rocks, and minor mafic-ultramafic intrusive and extrusive rocks, were subjected to coeval Triassic subduction to mantle depths and exhumation during the collision between the South China Block and North China Block. The estimated subduction and exhumation rates for the Dabie–Sulu UHP terrane would be up to 4.7–9.3 km Myr^?1 and 5.0–11.3 km Myr^?1, respectively. The zonal distribution of mineral inclusions and the preservation of index UHP minerals such as coesite imply that zircon is the best mineral container for each metamorphic stage, particular for supracrustal rocks as their metamorphic evolution and UHP evidence have been almost or completely obliterated. Similar conclusions have been documented elsewhere for other UHP terranes.     

Natural and experimental constraints on formation of the continental crust based on niobium–tantalum fractionation

Fractionation between Nb and Ta, elements generally regarded as geochemical ‘identical twins’, is a key to deciphering the formation of the continental crust (CC). Here we show that Nb/Ta of rutile grains in eclogitic rocks from the Chinese Continental Scientific Drilling (CCSD) project are remarkably heterogeneous but overall subchondritic at core depths of 100–700 m, and are less variable and mainly suprachondritic at core depths of 700–3025 m, indicating clear Nb/Ta fractionation across a subducted slab. To understand the potential mechanism of Nb/Ta fractionation within the subducted plate, we analysed by laser ablation ICPMS a thermal migration experiment in which a wet andesite was placed in a large thermal gradient (300°C/cm with ends ranging from 950–350°C) at 0.5Gpa. Results show that Nb, Ta and Ti, driven by the thermal gradient, preferentially migrate by diffusion through supercritical fluids into the cooler end of the experiment (at 650–350°C). Due to contrasting Nb and Ta thermal migration patterns, dramatic fractionation between Nb, Ta, and Ti took place in the cooler end. Experimental results are consistent with the measured Nb, Ta in rutile from CCSD drillhole samples. We consider that major fractionation between Nb, Ta must occur before rutile appears, most likely during the prograde blueschist to amphibole–eclogite transformation, when Ti is also mobile. Before rutile appears, partitioning between Ti‐rich dominant minerals such as amphiboles and fluids in the hotter region where dehydration preferentially occurs, produces Nb–Ta–Ti‐rich fluids with subchondritic Nb/Ta, and dehydration residues with suprachondritic Nb/Ta. Meanwhile, owing to evolution of the thermal gradient within the subducting slab, thermal migration of Nb, Ta, and Ti in aqueous fluids result in Nb, Ta, and Ti enrichment in the cooler region and depletion in the hotter region. As a result of high‐pressure metamorphism, hydrous rutile‐rich eclogites with overall subchondritic Nb/Ta form in the cooler region, whereas relatively anhydrous rutile‐poor eclogites with suprachondritic Nb/Ta form in the hotter region. Subsequently, partial melting of hydrous rutile‐rich eclogites with initial subchondritic Nb/Ta at deeper levels transfers overall subchondritic Nb/Ta coupled with Nb, Ta, and Ti depletion characteristics to the CC, leaving dry rutile‐poor eclogites with suprachondritic Nb/Ta and rutile‐rich residual eclogites with overall, heterogeneous subchondritic Nb/Ta as a complementary reservoir to the CC.     

P–T evolution of meta-peridotite in the Penjwin ophiolite, northeastern Iraq

The Penjwin meta-peridotite rock represents one of the five main metamorphosed ultramafic bodies in Kurdistan region, Northwest Zagros Thrust Zone. It underwent at least two successively low-retrograde metamorphic events with one progressive one which all modified the original mineralogy and texture of primary dunite and harzburgite. The primary upper mantle mineral assemblage olivine?+?orthopyroxene?+?chromian spinel is replaced by olivine?+?tremolite–actiolite?+?anthopylite?+?talc?+?ferichromite?+?Cr-chlorite assemblage of amphibolite facies. The further retrograde metamorphic amphibolite facies assemblage is replaced by lizardite–chrysotile?+?Cr-chlorite?+?syn-serpentinization Cr-magnetite of lower greenschist facies. Later at the main Zagros thrust fault, low greenschist facies underwent progressive metamorphism due to the local effect of shear stress as a result of the exhumation and obduction of Penjwin ophiolite suite over Merga Red bed series during Tertiary. Lizardite–chrysotile transformed to antigorite and producing antigorite?+?carbonate?+?syn-serpentinization Cr-magnetite?+?Cr-chlorite assemblage of upper greenschist facies. Chromian spinel is concentrically zoned as a result of multi-stages retrogressive metamorphic events, in which the Cr # (Cr/(Cr?+?Al)) increases from core to rim (0.5 to 1). Three zones can be identified from core to rim: The core is primary Al-rich and mantled by ferrichromite of amphibolite facies. The most outer zone of chromian spinel grains is represented by syn-serpentinization Cr-magnetite of greenschist facies.     

Tectonic evolution of a composite collision orogen: An overview on the Qinling–Tongbai–Hong'an–Dabie–Sulu orogenic belt in central China

The formation of collisional orogens is a prominent feature in convergent plate margins. It is generally a complex process involving multistage tectonism of compression and extension due to continental subduction and collision. The Paleozoic convergence between the South China Block (SCB) and the North China Block (NCB) is associated with a series of tectonic processes such as oceanic subduction, terrane accretion and continental collision, resulting in the Qinling–Tongbai–Hong'an–Dabie–Sulu orogenic belt. While the arc–continent collision orogeny is significant during the Paleozoic in the Qinling–Tongbai–Hong'an orogens of central China, the continent–continent collision orogeny is prominent during the early Mesozoic in the Dabie–Sulu orogens of east-central China. This article presents an overview of regional geology, geochronology and geochemistry for the composite orogenic belt. The Qinling–Tongbai–Hong'an orogens exhibit the early Paleozoic HP–UHP metamorphism, the Carboniferous HP metamorphism and the Paleozoic arc-type magmatism, but the three tectonothermal events are absent in the Dabie–Sulu orogens. The Triassic UHP metamorphism is prominent in the Dabie–Sulu orogens, but it is absent in the Qinling–Tongbai orogens. The Hong'an orogen records both the HP and UHP metamorphism of Triassic age, and collided continental margins contain both the juvenile and ancient crustal rocks. So do in the Qinling and Tongbai orogens. In contrast, only ancient crustal rocks were involved in the UHP metamorphism in the Dabie–Sulu orogenic belt, without involvement of the juvenile arc crust. On the other hand, the deformed and low-grade metamorphosed accretionary wedge was developed on the passive continental margin during subduction in the late Permian to early Triassic along the northern margin of the Dabie–Sulu orogenic belt, and it was developed on the passive oceanic margin during subduction in the early Paleozoic along the northern margin of the Qinling orogen.Three episodes of arc–continent collision are suggested to occur during the Paleozoic continental convergence between the SCB and NCB. The first episode of arc–continent collision is caused by northward subduction of the North Qinling unit beneath the Erlangping unit, resulting in UHP metamorphism at ca. 480–490 Ma and the accretion of the North Qinling unit to the NCB. The second episode of arc–continent collision is caused by northward subduction of the Prototethyan oceanic crust beneath an Andes-type continental arc, leading to granulite-facies metamorphism at ca. 420–430 Ma and the accretion of the Shangdan arc terrane to the NCB and reworking of the North Qinling, Erlangping and Kuanping units. The third episode of arc–continent collision is caused by northward subduction of the Paleotethyan oceanic crust, resulting in the HP eclogite-facies metamorphism at ca. 310 Ma in the Hong'an orogen and low-P metamorphism in the Qinling–Tongbai orogens as well as crustal accretion to the NCB. The closure of backarc basins is also associated with the arc–continent collision processes, with the possible cause for granulite-facies metamorphism. The massive continental subduction of the SCB beneath the NCB took place in the Triassic with the final continent–continent collision and UHP metamorphism at ca. 225–240 Ma. Therefore, the Qinling–Tongbai–Hong'an–Dabie–Sulu orogenic belt records the development of plate tectonics from oceanic subduction and arc-type magmatism to arc–continent and continent–continent collision.     

Lithospheric structure and crust–mantle decoupling in the southeast edge of the Tibetan Plateau

Convergence between the Indian plate and the Eurasian plate has resulted in the uplift of the Tibetan Plateau, and understanding the associated dynamical processes requires investigation of the structures of the crust and the lithosphere of the Tibetan Plateau. Yunnan is located in the southwest edge of the plateau and adjacent to Myanmar to the west. Previous observations have confirmed that there is a sharp transition in mantle anisotropy in this area, as well as clockwise rotations of the surface velocity, surface strain, and fault orientation. We use S receiver functions from 54 permanent broad-band stations to investigate the structures of the crust and the lithosphere beneath Yunnan. The depth of the Moho is found to range from 36 to 40 km beneath southern Yunnan and from 55 to 60 km beneath northwestern Yunnan, with a dramatic variation across latitude 25–26°N. The depth of the lithosphere–asthenosphere boundary (LAB) ranges from 180 km to less than 70 km, also varying abruptly across latitude 25–26°N, which is consistent with the sudden change of the fast S-wave direction (from NW–SE to E–W across 26–28°N). In the north of the transition belt, the lithosphere is driven by asthenospheric flow from Tibet, and the crust and the upper mantle are mechanically coupled and moving southward. Because the northeastward movement of the crust in the Burma micro-plate is absorbed by the right-lateral Sagaing Fault, the crust in Yunnan keeps the original southward movement. However, in the south of the transition belt, the northeastward mantle flow from Myanmar and the southward mantle flow from Tibet interact and evolve into an eastward flow (by momentum conservation) as shown by the structure of the LAB. This resulting mantle flow has a direction different from that of the crustal movement. It is concluded that the Sagaing Fault causes the west boundary condition of the crust to be different from that of the lithospheric mantle, thus leading to crust–mantle decoupling in Yunnan.     

Petrology,geochemistry and metamorphic evolution of Lancang Group in the Changning-Menglian complex belt and its implications on the tectonic evolution of the Paleo-TethysEI北大核心CSCD

Lancang Group within the Changning-Menglian complex belt in the Sanjiang area, Yunnan Province involves many kinds of meta-sediments, including staurolite-kyanite-bearing garnet-mica schist, garnet-mica schist, chloritoid-white mica schist and chlorite-glaucophane-albite schist. Detailed petrographic observation, mineral chemistry analysis and phase equilibrium modelling have shown that these meta-sediments preserve distinctly metamorphic evolutions. The staurolite-kyanite-bearing garnet-mica schist records the decompression and cooling histories related to retrograded metamorphic processes from middle-temperature eclogite facies to amphibolite facies with a peak mineral assemblage of garnet + kyanite + phengite + jadeite formed at the P-T condition of about 19 similar to 30kbar and 600 similar to 750 degrees C. For the garnet-mica schist, the peak metamorphic mineral association constrained by X-Prp and X-Grs in garnet, and Si content in phengite includes garnet + phengite + omphacite + lawsonite + paragonite and the related P-T condition is around 17 similar to 19. 5kbar and 430 similar to 475 degrees C . The chloritoid-white mica schist is characterized by the mineral assemblage of chloritoid + phengite + paragonite + chlorite whereas the peak mineral assemblage includes phengite + paragonite + carpholite. The peak P-T condition defined by Si content in phengite is limited in the range of 17 similar to 19kbar and 300 similar to 330 degrees C. Both garnet-mica schist and chloritoid-white mica schist consistently record heating and decompression processes from lawsonite-blueschist facies to epidote-blueschist facies. Metamorphic reactions and mineralogy of chlorite-glaucophane-albite schist roughly give the P-T condition of 9 similar to 11kbar and 430 similar to 520 degrees C. Studies on the geochemistry of Lancang Group reveal that these meta-sediments show the geochemistry affinity to the continental arc, active continental margin and upper crust sediments. The protoliths are mainly mud rock and sandstone with low maturity and a little of mafic-intermediate volcanic rock. The sediment sources are predominantly intermediate-acid magmatic rocks with old sedimentary contamination to different degree. Tectonic discrimination diagrams show that meta-sediments in the Lancang Group are mainly derived from the continental island arc or active continental margin tectonic setting. Combined with the metamorphism and geochemistry characteristics of these rocks in the Changning-Menglian complex belt, it is therefore inferred that the meta-sediments of Lancang Group display various metamorphic evolutions. Lancang Group are considered to have experienced multi-phase/stage and complex tectonic evolution histories.     

Metamorphic evolution of the Maud Belt: P–T–t path for high-grade gneisses in Gjelsvikfjella,Dronning Maud Land,East Antarctica

A metamorphic petrological study, in conjunction with recent precise geochronometric data, revealed a complex P–T–t path for high-grade gneisses in a hitherto poorly understood sector of the Mesoproterozoic Maud Belt in East Antarctica. The Maud Belt is an extensive high-grade, polydeformed, metamorphic belt, which records two significant tectono-thermal episodes, once towards the end of the Mesoproterozoic and again towards the late Neoproterozoic/Cambrian. In contrast to previous models, most of the metamorphic mineral assemblages are related to a Pan-African tectono-thermal overprint, with only very few relics of late Mesoproterozoic granulite-facies mineral assemblages (M₁) left in strain-protected domains. Petrological and mineral chemical evidence indicates a clockwise P–T–t path for the Pan-African orogeny. Peak metamorphic (M_2b) conditions recorded by most rocks in the area (T = 709–785 °C and P = 7.0–9.5 kbar) during the Pan-African orogeny were attained subsequent to decompression from probably eclogite-facies metamorphic conditions (M_2a).The new data acquired in this study, together with recent geochronological and geochemical data, permit the development of a geodynamic model for the Maud Belt that involves volcanic arc formation during the late Mesoproterozoic followed by extension at 1100 Ma and subsequent high-grade tectono-thermal reworking once during continent–continent collision at the end of the Mesoproterozoic (M₁; 1090–1030 Ma) and again during the Pan-African orogeny (M_2a, M_2b) between 565 and 530 Ma. Post-peak metamorphic K-metasomatism under amphibolite-facies conditions (M_2c) followed and is ascribed to post-orogenic bimodal magmatism between 500 and 480 Ma.     

Fore‐arc evolution in the Tasman Geosyncline: The origin of the southeast Australian continental crust

A new general model describing the extended evolution of fore‐arc terrains is used to analyse the evolution of the southern Tasman Geosyncline and the concomitant growth and kratonisation of the continental crust of southeast Australia during the Palaeozoic. The southern Tasman Geosyncline comprises ten arc terrains (here defined), most of which are east‐facing, and several features formed by crustal extension. Each arc terrain consists of several strato‐tectonic units: a volcanic arc, subduction complex and fore‐arc sequence formed during subduction; and an overlying post‐arc sequence which post‐dates subduction and is composed of flysch, neritic sediments or subaerial volcanics.

When these materials attained a thickness of c. 20 km their internal heat‐balance caused partial melting of the subduction complex and the hydrated oceanic lithosphere trapped beneath it, to yield S‐ and I‐type granitic magma. The magma rose, inducing pervasive deformation of each arc terrain and emplacement of granitoid plutons at high levels in the evolving crust. Transitional basins then developed in many terrains on top of their volcanic arcs or the thinner parts of the buried accretionary prisms. After deformation of the transitional sequences, platform cover accumulated, marking the completion of kratonisation.

Analysis of each arc terrain in terms of the above units leads to a predicted ‘stratigraphy’ for the continental crust of southeast Australia. The crust is complexly layered, with lateral discontinuities reflecting the boundaries of arc terrains which were successively accreted, principally back‐arc to fore‐arc, during crustal development.     

Reappraising the P–T evolution of the Rogaland–Vest Agder Sector,southwestern Norway

The Rogaland-Vest Agder Sector of southwestern Norway comprises high-grade metamorphic rocks intruded by voluminous plutonic bodies that include the ~1000 km~2 Rogaland Igneous Complex(RIC).New petrographic observations and thermodynamic phase equilibria modelling of three metapelitic samples collected at various distances(30 km,10 km and ~ 10 m) from one of the main bodies of RIC anorthosite were undertaken to assess two alternative P-T-t models for the metamorphic evolution of the area.The results are consistent with a revised two-phase evolution.Regional metamorphism followed a clockwise P-T path reaching peak conditions of ~ 850-950 ℃ and ~7-8 kbar at ~1035 Ma followed by high-temperature decompression to ~5 kbar at ~950 Ma,and resulted in extensive anatexis and melt loss to produce highly residual rocks.Subsequent emplacement of the RIC at ~930 Ma caused regional-scale contact metamorphism that affected country rocks 10 km or more from their contact with the anorthosite.This thermal overprint is expressed in the sample proximal to the anorthosite by replacement of sillimanite by coarse intergrowths of cordierite plus spinel and growth of a second generation of garnet,and in the intermediate(10 km) sample by replacement of sapphirine by coarse intergrowths of cordierite,spinel and biotite.The formation of late biotite in the intermediate sample may suggest the rocks retained small quantities of melt produced by regional metamorphism and remained at temperatures above the solidus for up to 100 Ma.Our results are more consistent with an accretionary rather than a collisional model for the Sveconorwegian Orogen.     

Polyphase Deformation of the Weihai-Rongcheng UHP Unit Rocks, NE Sulu： Insights into the Tectonic Evolution of the Dabie-Sulu UHP and HP Belts, China

Different scales of structural data reveal a complex deformation history of ultrahigh- pressure （UHP） rocks exposed in the Weihai-Rongcbeng area, NE Sulu （northern Jiangsu-eastern Shandong）, eastern China. Excluding pre-UHP deformations, at least five major sequential deformational stages （D1-Ds） are recognized. The first deformation （DO produced a weak foliation and lineation in massive eclogites. The foliated eclogite with a dominant foliation containing a stretching and mineral lineation was developed during the I）2 deformation. Both the D1 and D2 deformations occurred under UHP metamorphic conditions, and are well preserved in the eclogite bodies. D3 structures which developed shortly after the formation of granulite/amphibolite facies symplectites are characterized by imbricated associations marked by a regional, steeply dipping foliation, compositional layering, eclogite boudinage, isoclinal folds and reverse ductile shear zones. The D3 deformation was accompanied by decompressional partial melting. A regional, gently dipping amphibolite facies foliation and stretching lineation, low-angle detachments, and dome- and arc-shaped structures formed during the D4 deformation stage dominate to some degree the map pattern of the Weihai-Rongcbeng UHP domain. The last stage of deformation （Ds） gave rise to the final exhumation of the UHP rocks. Ds is characterized by development of brittle-dominated high-angle faulting associated with emplacement of large volmnes of undeformed granite plutons and dykes dated at 134-100 Ma. The deformational and metamorphic sequence followed by the UHP rocks in the Weihai-Rongcheng area is similar to that studied in the entire Dabie-Sulu UHP and HP metamorphic belts from microscopic to mapping scale. Based on structural data, combined with available petrographic, metamorphic and geochronological data, a speculative tectonic evolutionary model for the Dabie-Sulu UHP and IIP belts is proposed, involving continental subduction/collision between the Sino-Korean and Yangtze cratons and subsequent polyphase exhumation histories of the UHP and IIP metamorphic rocks.     

Study of the Behavior of Uranium,Niobium, and Tantalum in the Granite Melt–Fluoride Fluid System at 800–950°C, 2300 Bar

Geology of Ore Deposits - Experimental studies were carried out on the solubility of uranium, niobium, and tantalum in acidic melts of Li–F granites and predominantly fluoride fluids at...     

Fluid evolution of the late Archaean Rämepuro gold deposit in the Ilomantsi greenstone belt in eastern Finland

The late Archaean (ca. 2.7 Ga) Ilomantsi greenstone belt hosts a large number of small mesozonal gold occurrences. The Rämepuro deposit is spatially related to a feldspar porphyry dyke which has intruded the contact between metagreywackes and intermediate metavolcanic rocks. It consists of gold-bearing quartz-tourmaline-sulphide veins located mainly within an intensively altered and sheared zone about 20-30 m in width. Two types of fluid inclusions were distinguished in the quartz veins: (1) H₂O-CH₄ (Й equiv. wt% NaCl), and (2) H₂O-CO₂ (⢬ equiv. wt% NaCl). The two compositionally different fluid-inclusion types occur in separate veins. The compositions of decrepitate residues indicate that the type 1 fluid inclusions consist predominantly of Na, S, Ca, Cl, and in lesser proportions of K and Fe. The residues of the type 2 fluid inclusions are dominated by Na, Ca, Cl and minor K. For the type 1 and type 2 fluid inclusions, microthermometric experiments indicate average homogenisation temperatures of 310-350 and 220-250 °C respectively. Both types of fluid inclusions contain 1-3 anisotropic solids (Ca/Mg carbonate?, nahcolite?, tourmaline?). Sulphide daughter minerals occur only in the type 1 fluid inclusions. This fluid type is also often associated with short trails of sulphide inclusions which suggest mobilisation and redistribution of ore constituents. Some type 1 fluid inclusions display distinct morphological features ("implosion textures") related to post-trapping re-equilibration. The presence of the fluid-inclusion re-equilibration textures in some auriferous quartz veins and their absence in other gold-bearing quartz veins suggest an age difference between them. This conclusion is also supported by the compositionally different fluid regimes associated with the different quartz veins, indicating a change from reducing (CH₄) to a more oxidising (CO₂) fluid environment. The fluid-inclusion evidence suggests two separate gold mineralisation events, one related to late Archaean regional metamorphism, the other to Palaeoproterozoic thrusting and metamorphism.     

Sources of Paleozoic granitic rocks and isotopic heterogeneity of the continental crust of the Aktau–Dzhungar microcontinent,Central Kazakhstan

Several generations of Paleozoic granitic rocks are studied with Sm–Nd isotopic methods in the northwestern part of the Aktau–Dzhungar microcontinent of Central Kazakhstan (Atasu–Mointy divide). The initial Nd isotopic composition of the granitic rocks varies in a relatively narrow range from–0.1 to–3.5ε; the Nd model ages are also similar (1.11–1.46 Ga). These results indicate that the crustal source of all the Paleozoic granitic rocks of the region had similar composition and, probably, age. It is shown that the t_Nd(DM) values of the Paleozoic granites reflect different proportions between ancient and juvenile material in the crustal source.     

Melt–Fluid and Fluid–Fluid Immiscibility in a Na2SO4–SiO2–H2O System and Implications for the Formation of Rare Earth Deposits

Liquid–liquid immiscibility has crucial influences on geological processes, such as magma degassing and formation of ore deposits. Sulfate, as an important component, associates with many kinds of deposits. Two types of immiscibility, including (i) fluid–melt immiscibility between an aqueous solution and a sulfate melt, and (ii) fluid–fluid immiscibility between two aqueous fluids with different sulfate concentrations, have been identified for sulfate–water systems. In this study, we investigated the immiscibility behaviors of a sulfate- and quartz-saturated Na2SO4–SiO2–H2O system at elevated temperature, to explore the phase relationships involving both types of immiscibility. The fluid–melt immiscibility appeared first when the Na2SO4–SiO2–H2O sample was heated to ~270°C, and then fluid–fluid immiscibility emerged while the sample was further heated to ~450°C. At this stage, the coexistence of one water-saturated sulfate melt and two aqueous fluids with distinct sulfate concentrations was observed. The three immiscible phases remain stable over a wide pressure–temperature range, and the appearance temperature of the fluid–fluid immiscibility increases with the increased pressure. Considering that sulfate components occur extensively in carbonatite-related deposits, the fluid–fluid immiscibility can result in significant sulfate fractionation and provides implications for understanding the formation of carbonatite-related rare earth deposits.