Scaling relationships of joint and vein arrays from The Burren,Co. Clare,Ireland

We present a study of the systematics of veins and joints in Carboniferous limestones of The Burren, Ireland. Scaling relationships were established for fracture arrays mapped from low elevation aerial photographs that image fractures on numerous limestone pavements for areas up to ca 1 km². The veins and joints occur in the same sequence, but have contrasting scaling properties. The veins strike north-south and cut many beds to form vertically persistent, non-stratabound arrays. They are strongly clustered and have scale invariant geometric properties. Vein geometries suggest they grew sub-critically under relatively high differential stresses, during north-south directed Variscan compression. The joints form stratabound arrays, with regular spacings that scale with bed thickness. They show greater strike variation than the veins and have lognormal length distributions. The joints formed during uplift, under low-differential stress conditions. The contrasting scaling properties of the joints and veins are attributed to different overburden stresses at the time of formation. The veins formed at greater depths than the joints, in conditions that favoured fracture propagation across mechanical discontinuities, resulting in the development of non-stratabound scaling properties.     

Fluid inclusions in high pressure/low temperature rocks from the Calabrian Arc (Southern Italy): the burial and exhumation history of the subduction-related Diamante-Terranova unit

The Diamante-Terranova Unit (DIATU), in the Calabrian Arc of southern Italy, is part of an ophiolitic sequence involved in a high pressure/low temperature event (P=8 kbar; T =400 °C) followed by re-equilibration at greenschist facies conditions (P=3 kbar; T =300 °C). The rocks contain two types of quartz–calcite veins – an earlier generation of deformed, folded and faulted veins formed during or before subduction, and a later set of planar, undeformed veins formed during exhumation of the DIATU. The earlier folded quartz–calcite veins contain regularly shaped aqueous inclusions as well as inclusions with a highly irregular dendritic texture. The later planar veins contain only regularly shaped aqueous inclusions similar to those in the earlier veins. In both vein types, all inclusions are demonstrably secondary in origin. Regularly shaped inclusions from both vein types are low salinity (0–5 wt% NaCl). Most contain liquid and vapour and homogenize to the liquid (Th 135–180 °C), whereas others contain only liquid at room temperature. Both the two-phase and monophase inclusions occur in the same fractures and are thought to record the same trapping event, with the monophase inclusions remaining metastable liquid at room temperature. No microthermometric data could be obtained from the dendritic inclusions in the earlier folded veins. Inclusions with the highly irregular dendritic texture found in the earlier veins are similar to those produced experimentally during laboratory-induced deformation of synthetic inclusions in quartz under conditions of internal underpressure, simulating either isobaric cooling or isothermal compression. The occurrence of inclusions with the dendritic texture in the earlier folded veins, and their absence from the later planar veins, suggests that the earlier veins formed before or during subduction and were folded and faulted in a compressional environment and their contained fluid inclusions were modified to produce the dendritic texture. During later uplift of the DIATU, planar veins containing regularly shaped aqueous inclusions formed and some of the fluids forming these veins were also trapped as secondary inclusions in the earlier folded veins. The results of this study provide convincing evidence that inclusions with a highly irregular dendritic morphology represent early inclusions that have survived prograde conditions in a high pressure/low temperature metamorphic environment (but have been texturally modified). The high pressure/low temperature ‘implosion’ texture is preserved over geological time, even after being overprinted by internal overpressure conditions generated during retrograde decompression. We suggest that inclusions that have survived prograde metamorphism are common in high pressure/low temperature rocks, but are often not identified as such due to their morphology which makes their recognition difficult.     

Sealing of fluid pathways in overpressure cells: a case study from the Buntsandstein in the Lower Saxony Basin (NW Germany)

We studied veins in the Triassic Buntsandstein of the Lower Saxony Basin (NW Germany) with the aim of quantifying the evolution of in-situ stress, fluids and material transport. Different generations of veins are observed. The first generation formed in weakly consolidated rocks without a significant increase in fracture permeability and was filled syntectonically with fibrous calcite and blocky to elongate-blocky quartz. The stable isotopic signature (δ¹⁸O and δ¹³C) indicates that the calcite veins precipitated from connate water at temperatures of 55–122°C. The second vein generation was syntectonically filled with blocky anhydrite, which grew in open fractures. Fluid inclusions indicate that the anhydrite veins precipitated at a minimum temperature of 150°C from hypersaline brines. Based on δ³⁴S measurements, the source of the sulphate was found in the underlying Zechstein evaporites. The macro- and microstructures indicate that all veins were formed during subsidence and that the anhydrite veins were formed under conditions of overpressure, generated by inflation rather than non-equilibrium compaction. The large amount of fluids which are formed by the dehydrating gypsum in the underlying Zechstein and are released into the Buntsandstein during progressive burial form a likely source of overpressures and the anhydrite forming fluids.     

Borosilicate- and phengite-bearing veins from the Grenville Province of Labrador: evidence for rapid uplift

Quartzo‐feldspathic veins emplaced within a migmatite terrane near Wilson Lake in the Grenville Province of central Labrador record a metamorphic event not evident in the host rocks. The discordant veins are undeformed and have undisturbed primary igneous/hydrothermal textures. Most of the veins contain euhedral kyanite, as well as aggregates of kyanite, K‐feldspar, phlogopite and minor dumortierite which are likely pseudomorphs after primary phengite. The reconstructed phengite compositions range from 3.1 to 3.2 Si per 11 oxygen formula unit. The pseudomorph assemblage is interpreted as the product of phengite + quartz melting under H₂O‐undersaturated conditions, which brackets P–T conditions of formation to about 9–16 kbar and 775–875 °C. A parallel vein that is likely of the same generation contains the borosilicate phases, dumortierite, prismatine and grandidierite, but no kyanite. The borosilicate assemblages constrain the P–T conditions of vein crystallization to ≥10 kbar and c. 750–850 °C. Vein emplacement is constrained to T ≤ 875 °C at the same pressures, which is well within the kyanite zone. Because the host rocks and veins must have experienced the same P–T history following vein emplacement, the presence of unreacted sillimanite in the host migmatites implies insufficient time for host rock equilibration. Slow reaction rates because of anhydrous conditions are not a likely explanation given the abundance of biotite and hornblende in the host rocks. The ductility implied by the breakdown of a hydrous phase (phengite) and the production of an H₂O‐undersaturated melt in the veins contrasts with the apparently brittle behaviour of the host rocks. The absence of deformation since the time of vein emplacement, even at temperatures above 750 °C, suggests that the deep crust in this part of Labrador had a very short residence time under conditions of the kyanite zone. Rapid decompression from those conditions is consistent with quartz + phengite melting and accounts for the relatively brittle behaviour of the terrane as it was uplifted.     

Structural setting,style and timing of vein-hosted gold mineralization at the Pogo deposit,east central Alaska

Gold-bearing veins within the Liese zone of the Pogo deposit display a two-stage evolutionary history that records temporal variation in kinematics, fluid chemistry and temperature. Several stacked shallow northwest-dipping shear veins are developed at Pogo, and collectively comprise the Liese Zone. Veins consist of: (1) early, narrow biotite-bearing shear veins; (2) white quartz veins with pyrite-arsenopyrite bands, referred to as main stage quartz veins, that have sericite-Fe-Mg carbonate alteration envelopes and which exploit the early shear veins; and (3) extension veins that form as steeper offshoots from the main stage veins. The presence and orientation of oblique fabrics developed in the older biotite-bearing shear veins are indicative of top-to-the-south displacement under ductile to semi-brittle conditions at higher temperatures. In contrast, the orientation of the extension veins and local sigmoidal shapes indicate a component of top-to-the-northwest normal displacement on the main stage veins in their present orientation, and brittle to semi-brittle conditions of formation. Dolomite-sericite alteration surrounding main stage veins may represent late to post-mineral hydrothermal fluid exploitation of vein margins during ongoing normal displacement along vein systems. All types of veining overprint 107–106 Ma, post-metamorphic granitic dykes. Molybdenite in main stage quartz assemblages has returned Re-Os ages of 104.2±1.1 Ma, significantly older than 96 to 91 Ma ⁴⁰Ar/³⁹Ar ages obtained from vein alteration assemblages that may reflect thermal resetting during post-mineral fault related hydrothermal activity, magmatism and/or retrograde cooling of the lithologic sequence. Unlike typical mesothermal shear vein hosted gold systems, Pogo is temporally and tectonically separated from metamorphic deformation events, and has a comparable kinematic and geometric architecture to Cretaceous plutonic gold deposits in the region. We interpret the deposit to have formed during a regional Cretaceous extensional event during multi-stage exploitation of extensional fault surfaces by hydrothermal fluid from a cooling magmatic source.Editorial handling: S.G. Hagemann     

Subduction channel fluid-rock interaction:Indications from rutile-quartz veins within eclogite from the Yuka terrane,North Qaidam orogen

High-pressure(HP)or ultrahigh-pressure(UHP)rutile-quartz veins that form at mantle depths due to fluid-rock interaction can be used to trace the properties and behavior of natural fluids in subduction zones.To explore the fluid flow and the associated element mobility during deep subduction and exhumation of the continental crust,we investigated the major and trace elements of Ti-rich minerals.Additionally,U–Pb dating,trace element contents,and Lu–Hf isotopic composition of zircon grains in the UHP eclogite and associated rutile-quartz veins were examined in the North Qaidam UHP metamorphic belt,Yuka terrane.The zircon grains in the rutile-quartz veins have unzoned or weak oscillatory zonings,and show low Th/U ratios,steep chondrite-normalized patterns of heavy rare earth elements(HREEs),and insignificant negative Eu anomalies,indicating their growth in metamorphic fluids.These zircon grains formed in 431
3 Ma,which is consistent with the 432
2 Ma age of the host eclogite.As for the zircons in the rutile-quartz veins,they showed steep HREE patterns on one hand,and were different from the zircons present in the host eclogite on the other.This demonstrates that their formation might have been related to the breakdown of the early stage of garnet,which corresponds to the abundance of fluids during the early exhumation stage.The core-rim profile analyses of rutile recorded a two-stage rutile growth across a large rutile grain;the rutile core has higher Nb,Ta,W,and Zr contents and lower Nb/Ta ratios than the rim,indicating that the rutile domains grew in different metamorphic fluids from the core towards the rim.The significant enrichment of high field strength elements(HFSEs)in the rutile core suggests that the peak fluids have high solubility and transportation capacity of these HFSEs.Furthermore,variations in the Nb vs.Cr trends in rutile indicate a connection of rutile to mafic protolith.The zircon grains from both the rutile-quartz veins and the host eclogite have similar Hf isotopic compositions,indicating that the vein-forming fluids are internally derived from the host eclogite.These fluids accumulated in the subduction channel and were triggered by local dehydration of the deeply subducted eclogite during the early exhumation conditions.     

Ore formation at the Kupol epithermal gold-silver deposit in northeastern Russia deduced from fluid inclusion study

The Kupol epithermal gold-silver deposit-the largest of this type of mineralization in northeastern Russia-is situated in the outer zone of the Okhotsk-Chukotka volcanic belt. The results of thermobarogeochemical study of fluid inclusions in quartz from ore veins at the Kupol deposit are compared with the data on the Dvoinoi and Arykvaam deposits. The study of aqueous extracts from fluid inclusions revealed that the chemical compositions of ore-forming fluids at the Dvoinoi and Kupol deposits are similar in most elements. The only substantial difference is that fluids from the Kupol deposit are considerably enriched in sulfate, as is characteristic of the alunite-subtype of epithermal high-sulfidation mineralization. The salinity of aqueous solutions filling inclusions in amethyst and quartz from ore veins at the Kupol and Dvoinoi deposits is two-three times higher than the salinity of fluid inclusions from the barren veins at the Arykvaam occurrence. The data obtained support the hypothesis put forward earlier that fumaroles and solfataras played a part in ore deposition at the Kupol deposit.     

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

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

Ushia - a new genus from Kamyshin Paleocene flora

Ushia kamyschinensis (Goeppert) n. gen. and n. comb. is established as a monotypic genus for leaves peculiar to the Paleocene of the U. S. S. R. The specimen designated as the holotype is that described by Goeppert in 1845 as Phyllites kamyschinensis from the Paleocene (Saratov Stage) found at Mt. Ushi near Kamyshin. The leaf, although very polymorphous, is ovate-oval to ovateprolate in outline with a broadly rounded to rounded cuneate base, generally somewhat asymmetrical. The leaf margin is entire below the basal secondary veins, crenate to crenulate above. The veins are coarse and prominent. Secondary veins alternate, 8-12 (14) in number, straight or slightly arcuate. Tertiary veins are almost perpendicular to secondary veins, mostly unbranched. Lowest pair of secondary veins branch unilaterally outward, the branchlets forming loops along the base of the leaf. This latter character differentiates the leaf from other members of the Fagaceae, the family to which Ushia most probably should be assigned. -- F. M. Hueber.     

Geometry and genesis of feeder zones of massive sulphide deposits: constraints from the Rio Tinto ore deposit (Spain)

Vein-related data have been collected around the giant Rio Tinto orebody in southern Spain within the root zones of the massive sulphide deposits. Here, we report the main results of this study, concerning the geometry of the stockwork and the conditions of formation. Although field and thin-section studies have shown that a wide range of vein configurations exist, from micro cracks (fluid-inclusion planes) to large paleo-flow channels, two groups seem to dominate. The first corresponds to small, constricted micro cracks and capillary-flow channels, now mainly filled with quartz, whereas the veins of the second group have large widths, are continuous over several meters and are filled with quartz and sulphides. Most are tension veins and only very few (<0.1%) show evidence of shearing. The pyrite-dominated variety (i.e., pyrite?>?quartz) tends to post-date the quartz-dominated veins (quartz?>?pyrite). The vein-thickness and -spacing distribution is modal rather than logarithmic, and their densities are not fractal, but are characterized by a Poisson distribution. From the immediate sub-surface zone to more than 100?m below the base of the massive sulphide deposits, most hydrothermal quartz-sulphide stockwork veins are sub-parallel to the base of the massive sulphide deposit. The assumption that the base of this deposit corresponds to a paleo-horizontal plane, implies that most veins were sub-horizontal. This is particularly evident for small veins, but the larger ones can be strongly oblique to the base of the deposit. The hydrothermal fluids that generated the massive sulphide deposits and underlying stockworks, were very saline and probably underwent sub- or super-critical phase separation in the root zones of the system. This phase separation was the probable mechanism producing the periodic over-pressures of at least 20 MPa that were necessary to generate the sub-horizontal veins of the stockworks.     

Modelling the heat pulses generated on a fault plane during coseismic slip: Inferences from the pseudotachylites of the Copanello cliffs (Calabria, Italy)

A pseudotachylite vein network crosscutting late Hercynian foliated tonalites can be observed along the Copanello cliffs (Calabria, Southern Italy). Pseudotachylites formed during the Oligocene–Miocene at intermediate crustal levels (ca. 10 km). They show variable thickness ranging from few mm up to 10 cm, as observed in injection veins branching from the fault plane. Microscopic observations indicate that pseudotachylite matrix mainly consists of plagioclase (An₄₆–An₅₈) and biotite microlites. Rounded clasts of quartz, plagioclase or of plagioclase–quartz lithic fragments are disseminated in the matrix. Intergranular, flow and spherulitic textures are commonly observed. Microstructural features are consistent with rapid crystallisation from melt. EDS analyses of rare and tiny glass veins indicated a trachyandesite or An₅₀ plagioclase melt composition.The conditions for pseudotachylite formation were reproduced by an analytical model taking into account the heat released by friction along a horizontal fault plane during a seismic event. The model is based on a three-stage rupture history that includes nucleation, propagation and stopping. In addition, by means of a numerical approach, the model reproduces cooling that follows the stopping stage.According to previous studies, the thermal perturbation induced by fault displacement is very intense. In fact, temperatures exceeding the tonalite and even An₅₀ plagioclase liquidus (1470 °C) are reproduced by small amount of slip (≤ 6 cm) in suprahydrostatic regime. On the other hand, the thermal perturbation is strongly localised and of short duration. Peak temperatures abruptly decrease at a short distance from the fault plane (typically in few millimetres). In these conditions a thin film of melt can be produced. Therefore, the presence of cm-scale pseudotachylite veins can be only explained assuming an efficient and fast melt migration towards dilatant sites, such as pull-apart structures and injections veins. Results of the model may be useful to predict the thermal disturbance produced by earthquakes of low intensity.     

Albite vein formation during exhumation of high-pressure terranes: a case study from alpine Corsica

The formation of late‐stage veins can yield valuable information about the movement and composition of fluids during uplift and exhumation of high‐pressure terranes. Albite veins are especially suited to this purpose because they are ubiquitously associated with the greenschist facies overprint in high‐pressure rocks. Albite veins in retrogressed metabasic rocks from high‐pressure ophiolitic units of Alpine Corsica (France) are nearly monomineralic, and have distinct alteration haloes composed of actinolite + epidote + chlorite + albite. Estimated P–T conditions of albite vein formation are 478 ± 31 °C and 0.37 ± 0.14 GPa. The P–T estimates and petrographic constraints indicate that the albite veins formed after the regional greenschist facies retrogression, in response to continued decompression and exhumation of the terrane. Stable isotope geochemistry of the albite veins, their associated alteration haloes and unaltered hostrocks indicates that the vein‐forming fluid was derived from the ophiolite units and probably from the metabasalts within each ophiolite slice. That the vein‐forming fluid was locally derived means that a viable source of fluid to form the veins was retained in the rocks during high‐pressure metamorphism, indicating that the rocks did not completely dehydrate. This conclusion is supported by the observation of abundant lawsonite at the highest metamorphic grades. Fluids were liberated during retrogression via decompression dehydration reactions such as those that break down hydrous high‐pressure minerals like lawsonite. Albite precipitation into veins is sensitive to the solubility and speciation of Al, which is more pressure sensitive than other factors which might influence albite vein formation such as silica saturation or Na:K fluid ratios. Hydraulic fracturing in response to fluid generation during decompression was probably the main mechanism of vein formation. The associated pressure decrease with fracturing and fluid decompression may also have been sufficient to change the solubility of Al and drive albite precipitation in fracture systems.     

Geochemistry and petrogenesis of a peralkaline granite complex from the Midian Mountains, Saudi Arabia

A zoned intrusion with a biotite granodiorite core and arfvedsonite granite rim represents the source magma for an albitised granite plug near its eastern margin and radioactive siliceous veins along its western margin. A study of selected REE and trace elements of samples from this complex reveals that the albitised granite plug has at least a tenfold enrichment in Zr, Hf, Nb, Ta, Y, Th, U and Sr, and a greatly enhanced heavy/light REE ratio compared with the peralkaline granite. The siliceous veins have even stronger enrichment of these trace elements, but a heavy/light REE ratio and negative eu anomaly similar to the peralkaline granite. It is suggested that the veins were formed from acidic volatile activity and the plug from a combination of highly fractionated magma and co-existing alkaline volatile phase. The granodiorite core intrudes the peralkaline granite and has similar trace element geochemistry. The peralkaline granite is probably derived from the partial melting of the lower crust in the presence of halide-rich volatiles, and the granodiorite from further partial melting under volatile-free conditions.     

Petrogenesis of Fe–Ti oxides in amphibole-rich veins from the Lherz orogenic peridotite (Northeastern Pyrénées, France)

Accessory, homogeneous ilmenite and rutile are important oxide phases in amphibole-rich high-pressure cumulate veins which crosscut the Lherz orogenic lherzolite massif. Those veins crystallized from alkaline melts at P = 1.2–1.5 GPa within the uppermost lithospheric mantle. Transitional basalts contaminated by peridotitic wall-rocks and then uncontaminated alkali basalts (basanites) reused the same vein conduits. Petrographic observations give evidence that Fe–Ti oxide saturation depends on the silica contents of each parental melt. The water-poor silica-rich transitional melts that generated websterites and plagioclase-rich clinopyroxenites reached early Ti-oxide saturation (1,200°C; 1.5 GPa). Rutile is as abundant as ilmenite. It is enriched with Nb–Zr–Hf by a factor of 10–100 relative to either amphibole or ilmenite. The amphibole pyroxenites and hornblendites crystallized from basanites reached late Fe–Ti oxide saturation after precipitation of amphibole, with ilmenite crystallizing along with phlogopite in the latter. The Lherz ilmenites are devoid of exsolution and contain very little trivalent iron. This compositional feature indicates more reducing crystallization conditions than usually inferred for alkali lavas and their megacrysts (FMQ ± 1). The veins incompletely equilibrated for redox conditions with their wall-rock peridotites which record more oxidizing conditions (FMQ ± 1). The veins also exchanged magnesium and chromium, as suggested by Cr-bearing, Mg-rich ilmenite (up to 44 mol% MgTiO₃₎ in veins less than 3–4 cm thick. Mg-rich ilmenite megacrysts occurring in alkali basalts could be actually xenocrysts from veins similar in thickness to those occurring at the Lherz massif, although crystallized from more oxidized magmas.     

Ca-rich majorite derived from high-temperature melt and thermally stressed hornblende in shock veins of crustal rocks from the Ries impact crater (Germany)

Shock veins up to 1.1 mm thick were found within non-porous lithic clasts from suevite breccia of the Nördlinger Ries impact structure. These veins were studied by optical microscopy in transmitted and reflected light and by scanning electron microscopy. In shocked amphibolites, two types of Ca-rich majorite occur within and adjacent to the veins. The first type crystallized from shock-induced melts within the veins. Si contents of these majorites suggest dynamic pressure of ~15–17 GPa, implying minimum temperatures in the range of ~2,150–2,230°C. The second type of majorite was formed adjacent to the shock veins within pargasitic hornblende. This majorite contains significant amounts of H₂O (0.7–0.9 wt%). Based on the textural setting, the shrinkage cracks and the chemical compositions of both phases, a solid-state mechanism is deduced for the hornblende to majorite phase transition. Both genetic types of Ca-rich majorite are described for the first time from a terrestrial impact crater. Along with stishovite, majorite constitutes the second silicate mineral displaying sixfold coordination of Si at Ries. Using micro-Raman spectroscopy, jadeite + coesite and jadeite + grossular were identified within local melt glasses of alkali feldspar and plagioclase composition, respectively. Stishovite aggregates, produced by solid-state reaction, along with shock-induced high-pressure melt glasses of almandine composition were also detected in shock veins of a garnet-cordierite-sillimanite restite. The quenched, homogeneous almandine glasses point to melting temperatures of more than ~2,500°C for the veins. Our findings demonstrate that terrestrial shock veins can give valuable information on shock-induced mineral transformations and transient high pressures of host rocks during a natural impact event.     

Subduction zone intermediate-depth seismicity: Insights from the structural analysis of Alpine high-pressure ophiolite-hosted pseudotachylyte (Corsica,France)

Pseudotachylyte in the Cima di Gratera ophiolite, Alpine Corsica, is distributed in the peridotite unit and in the overlying metagabbro unit and was formed under blueschist to eclogite metamorphic facies conditions, corresponding to a 60–90 km depth range. Peridotite pseudotachylyte is clustered in fault zones either beneath the tectonic contact with overlying metagabbros or at short distance from it. Fault zones are either parallel to the contact or make an angle of 55° to it. Displacement sense criteria associated with fault veins indicate top-to-the-west or top-to-the-northwest reverse senses. Cataclasite flanking most veins was formed before or coevally with frictional melting and likely mechanically weakened the peridotite, facilitating subsequent seismic rupture. In the basal part of the metagabbro unit, post-mylonitization pseudotachylyte can be distinguished from pre-mylonitization pseudotachylyte formed earlier. In the equant metagabbro above the mylonitic sole, only one episode of pseudotachylyte formation can be identified. Kinematics associated with metagabbro pseudotachylyte remain unknown. The geometry and kinematics of the pseudotachylyte veins from the peridotite unit and to a lesser extent from the metagabbro unit are similar to modern seismic ruptures of the upper parts of the Wadati-Benioff zones such as in the Pacific plate beneath NE Japan.     

Nb–Ta fractionation induced by fluid‐rock interaction in subduction‐zones: constraints from UHP eclogite‐ and vein‐hosted rutile from the Dabie orogen,Central‐Eastern China

Niobium and Ta concentrations in ultrahigh‐pressure (UHP) eclogites and rutile from these eclogites and associated high pressure (HP) veins were used to study the behaviour of Nb–Ta during dehydration and fluid‐rock interaction. Samples were collected through a ～2 km profile at the Bixiling complex in the Dabie orogenic belt, Central‐Eastern China. All but one eclogite away from veins (EAVs) display nearly constant Nb/Ta ratios ranging from 16.1 to 19.2, with an average of 16.9 ± 0.8 (2 SE), similar to that of their gabbroic protolith from the Yangtze Block. Nb/Ta ratios of rutile from the EAVs range from 12.7 to 25.3 among different individual grains, with the average values close to those of the corresponding bulk rocks. These observations show that Nb and Ta were not significantly fractionated by prograde metamorphism up to eclogite facies when no significant fluid‐rock interaction occurs. In contrast, Nb/Ta ratios of rutile from eclogites close to veins (ECVs) are highly variable from 17.8 to 49.8, which are systematically higher (by up to 17) than those of rutile from the veins. These observations demonstrate that Nb and Ta were mobilized and fractionated during localized fluid flow and intensive fluid‐rock interaction. This is strongly supported by Nb/Ta zoning patterns in single rutile grains revealed by in situ LA‐ICP‐MS analysis. Ratios of Nb/Ta in the ECV‐hosted rutile decrease gradually from cores towards rims, whereas those in the EAV‐hosted rutile are nearly invariable. Furthermore, the vein rutile shows Nb/Ta zoning patterns that are complementary to those in rutile from their immediate hosts (ECVs), suggesting an internal origin for the vein‐forming fluids. The Nb/Ta ratios of such fluids evolved from low values at the early stage of subduction to higher values at later supercritical conditions with increased temperature and pressure. Quantitative modelling was conducted to constrain the compositional evolution of metamorphic fluids during dehydration and fluid‐rock interaction focusing on Nb–Ta distribution. The modelling results based on our proposed multistage fluid phase evolution path can essentially reproduce the natural observations reported in the present study.     

湘东桐木山云英岩型锡矿床锡石及共生石英中流体包裹体研究

桐木山云英岩型锡矿床是湘东锡田锡多金属矿田中一个典型矿床,在详尽的野外考察、矿石结构观察以及流体包裹体岩相学研究的基础上,采用流体包裹体组合的研究方法,利用冷热台、激光拉曼等测试手段,对矿床中锡石中流体包裹体进行直接测定,同时开展与锡石共生的石英及切割矿体的后期石英脉石英中流体包裹体对比研究。结果显示,锡石中流体包裹体的组分、均一温度、盐度与共生的石英存在明显差异,表明锡石与石英形成的P-T-X条件不同。切割矿体的石英脉为成岩成矿后流体作用的产物。与锡石形成相关的流体为中高温、中低盐度的Na Cl-H_2O流体体系,与石英形成相关的流体为复杂的含碳流体。流体体系的冷却作用及流体与围岩的反应可能是导致锡石沉淀的主要因素,与石英形成相关的流体在演化过程中则经历了明显的流体不混溶作用。     

Microcrystalline textures resulting from rapid crystallization in a pseudotachylite melt in a meta-anorthosite

The very fine-grained (1 m) polygonal microcrystalline texture occurring in the groundmass of pseudotachylite veins in the Harris meta-anorthosite and its variation with position are described and an origin by crystallization from a melt at very large undercooling suggested. The intrusive nature of the veins is shown by their geometry and internal structures. Clasts, which are almost always only plagioclase, are generally concentrated towards the centres of veins. Flow of tens of millimetres can account for this concentration in millimetre-thick veins as a result of the Bagnold effect. The veins are generally thin (5 m to 5 mm or more), are frequently zoned and always contain transparent granules of high relief (probably Al-rich pyroxene) and opaque granules of magnetite up to a few micrometres in size. The granules are either uniformly distributed in microcrystalline textures or concentrated locally giving cellular textures. In some veins, spherulitic or bow-tie textures occur. The coarsest textures are found in the centres of the thickest veins. The groundmass of the pseudotachylite is never completely isotropic but consists of a mosaic of transparent plagioclase crystals decreasing in size from the centres of thick veins to less than 1 m in thin veins or in the margins of thicker veins. This fine microcrystalline texture was studied by both scanning and transmission electron microscopy and consists of polyhedral crystals of regular size in the range 0.2–1.5 m, which show little sign of deformation. The local composition of the pseudotachylites varies little from the average compositions of the rocks in optically homogeneous veins, the variation being within the compositional space defined by the minerals of the host rock. This shows that homogenization of the pseudotachylite has occurred. In cellular veinsdifferentiation has occurred as the compositions of the cell centres lie outside those of the minerals of the host rock. This was produced by segregation of the granules, pyroxene being absent from the host rock. The plagioclase in the pseudotachylite is more disordered than that in the host rock. All the microtextures described are absent from the associated cataclasites and cannot be due to recrystallization of a fine-grained and intensely strained rock powder. The physical state on and after intrusion was that of a melt and injection was followed by crystallization. The melt was produced by more or less total fusion of the host rock minerals at shallow depth by heat produced during local faulting and perhaps during crack propagation. The fine microcrystalline texture very closely resembles that produced during hypercooling of molten metals and alloys. It thus possibly formed not by devitrification but from a melt at much greater degrees of undercooling than the spherulitic and bow-tie textures.     

“Crack seal” vein geometry in eclogitic rocks

Abstract

Fluid-deposited veins occur in eclogite facies rocks in the Mont Viso meta-ophiolitic complex, Western Alps. These veins contain omphacite, garnet, rutile and apatite implying that veining took place at near to peak metamorphic conditions. Two types of omphacitic vein structure are described, (a) Cross-fibre veins which are charactenzed by omphacite fibres which long axis is parallel to the stretching direction of the country rocks. The fibres are cut accross by linear arrays of solid/fluid inclusions oriented normal to the long axes of the fibres, which are interpreted as crack-seal phenomena. (b) Massive, coarse-grained randomly oriented concentric and sector zoned omphacite with garnet, rutile and apatite in " hydrostatic " veins suggests nucleation in open fractures during eclogite facies metamorphic conditions.

Formation of syn-kinematic veins under conditions of at least P > 12 kbar and 500 < T < 550 °C indicates that fracturing and solution transfer processes occurred during subduction event.