Determining the direction of contact metamorphic fluid flow: an assessment of mineralogical and stable isotope criteria

ABSTRACT One-dimensional fluid advection-dispersion models predict differences in the patterns of mineralogical and oxygen isotope resetting during up- and down-temperature metamorphic fluid flow that may, in theory, be used to determine the fluid flow direction with respect to the palaeotemperature gradient. Under equilibrium conditions, down-temperature fluid flow is predicted to produce sharp reaction fronts that separate rocks with isobarically divariant mineral assemblages. In contrast, up-temperature fluid flow may produce extensive zones of isobarically univariant mineral assemblages without sharp reaction fronts. However, during contact metamorphism, mineral reaction rates are probably relatively slow compared with fluid velocities and distended reaction fronts may also form during down-temperature fluid flow. In addition, uncertainties in the timing of fluid flow with respect to the thermal peak of metamorphism and the increase in the variance of mineral assemblages due to solid solutions introduce uncertainties in determining fluid flow directions. Equilibrium down-temperature flow of magmatic fluids in contact aureoles is also predicted to produce sharp δ¹⁸O fronts, whereas up-temperature flow of fluids derived by metamorphic devolatilization may produce gradational δ¹⁸O vs. distance profiles. However, if fluids are channelled, significant kinematic dispersion occurs, or isotopic equilibrium is not maintained, the patterns of isotopic resetting may be difficult to interpret. The one-dimensional models provide a framework in which to study fluid-rock interaction; however, when some of the complexities inherent in fluid flow systems are taken into account, they may not uniquely distinguish between up- and down-temperature fluid flow. It is probably not possible to determine the fluid flow direction using any single criterion and a range of data is required.     

Evolution of fluid pressure and fracture propagation during contact metamorphism

Abstract Rock fracture enhances permeability and provides pathways through which fluids migrate. During contact metamorphism, fluids contained in isolated pores and fractures expand in response to temperature increases caused by the dissipation of heat from magmas. Heat transport calculations and thermomechanical properties of water-rich fluids demonstrate (1) that thermal energy is a viable mechanism to produce and maintain pore fluid pressure (P_f) in a contact metamorphic aureole; (2) that the magnitude of P_f generated is sufficient to propagate fractures during the prograde thermal history (cause hydrofracture) and enhance permeability; and (3) that P_f-driven fracture propagation is episodic with time-scales ranging from years to thousands of years. Because P_f dissipation is orders of magnitude faster than P, _f buildup, P_f oscillations and cyclical behaviour are generated as thermal heating continues. The P_f cycle amplitude depends on the initial fracture length, geometry and the rock's resistance to failure whereas the frequency of fracture depends on the rate of heating. Consequently, oscillation frequency also varies spatially with distance from the heat source. Time series of fluid pressures caused by this process suggest that cyclical fracture events are restricted to an early time period of the prograde thermal event near the intrusive contact. In the far field, however, individual fracture events have a lower frequency but continue to occur over a longer time interval. Numerous fracture cycles are possible within a single thermal event. This provides a provisional explanation for multiple generations of veins observed in outcrop. P _f cycling and oscillations may explain several petrological features. If pore fluids are trapped at various positions along a pressure cycle, the large amplitude of P_f variations for small fractures may account for different pressures recorded by fluid inclusions analysed from a single sample. P_f oscillations, during a single thermal episode, also drive chemical reactions which can produce complex mineral textures and assemblages for discontinuous reactions and/or zoning patterns for continuous reactions. These can mimic polymetamorphic or disequilibrium features. Temporal aspects of fracture propagation and permeability enhancement also constrain the likely timing of fluid flow and fluid-mineral interactions. These data suggest that fluid flow and fluid-mineral reactions are likely to be restricted to an early period in the prograde thermal history, characterized by high P_f coincident with relatively high temperatures, fracture propagation and consequent increases in permeability. This early prograde hydration event is followed by diffusional peak metamorphic reactions. This relationship is evident in the complex mineralogical textures common in some metamorphosed rocks.     

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

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

The role of the fluid phase during regional metamorphism and deformation

Evidence from rock microstructures, mass transfer and isotopic exchange indicates that substantial quantities of aqueous fluids are involved in low- and medium-grade regional metamorphism. Similar conclusions are drawn from many retrograde environments, whereas high-grade metamorphic fluids may be melt dominated. The mobile fluids play essential roles in metamorphic reactions, mass transport and deformation processes. These processes are linked by the mechanical consequences of metamorphic fluid pressures (P_f) generally being greater than or equal to the minimum principal compressive stress. Under such conditions metamorphic porosity comprises grain boundary tubules and bubbles together with continuously generated (and healed) microfractures. Deformation results in significant interconnected porosity and hence enhanced permeability. Lithologically and structurally controlled permeability variations may cause effective fluid channelling.
Simple Rayleigh-Darcy modelling of a uniformly permeable, crustal slab shows that convective instability of metamorphic fluid is expected at the permeabilities suggested for the high P_f metamorphic conditions. Complex, large-scale convective cells operating in overpressured, but capped systems may provide a satisfactory explanation for the large fluid/rock ratios and extensive mass transport demonstrated for many low- and medium-grade metamorphic environments. Such large-scale fluid circulation may have important consequences for heat transfer in and the thermal evolution of metamorphic belts.     

Fluid and enthalpy production during regional metamorphism

Models for regional metamorphism have been constructed to determine the thermal effects of reaction enthalpy and the amount of fluid generated by dehydration metamorphism. The model continental crust contains an average of 2.9 wt % water and dehydrates by a series of reactions between temperatures of 300 and 750° C. Large scale metamorphism is induced by instantaneous collision belt thickening events which double the crustal thickness to 70 km. After a 20 Ma time lag, erosion due to isostatic rebound restores the crust to its original thickness in 100 Ma. At crustal depths greater than 10 km, where most metamorphism takes place, fluid pressure is unlikely to deviate significantly from lithostatic pressure. This implies that lower crustal porosity can only be maintained if rock pores are filled by fluid. Therefore, porosities are primarily dependent on the rate of metamorphic fluid production or consumption and the crustal permeability. In the models, permeability is taken as a function of porosity; this permits estimation of both fluid fluxes and porosities during metamorphism. Metamorphic activity, as measured by net reaction enthalpy, can be categorized as endothermic or exothermic depending on whether prograde dehydration or retrograde hydration reactions predominate. The endothermic stage begins almost immediately after thickening, peaks at about 20 Ma, and ends after 40 to 55 Ma. During this period the maximum and average heat consumption by reactions are on the order 11.2·10^–14 W/cm³ and 5.9·10^–14 W/ cm³, respectively. The maximum rates of prograde isograd advance decrease from 2.4·10^–8 cm/s, for low grade reactions at 7 Ma, to 7·10^–10 cm/s, for the highest grade reaction between 45 and 58 Ma. Endothermic cooling reduces the temperature variation in the metamorphic models by less than 7% (40 K); in comparison, the retrograde exothermic heating effect is negligible. Dehydration reactions are generally poor thermal buffers, but under certain conditions reactions may control temperature over depth and time intervals on the order of 1 km and 3 Ma. The model metamorphic events reduce the hydrate water content of the crust to values between 1.0 and 0.4 wt % and produce anhydrous lower crustal granulites up to 15 km in thickness. In the first 60 Ma of metamorphism, steady state fluid fluxes in the rocks overlying prograde reaction fronts are on the order of 5·10^–11 g/cm²-s. These fluid fluxes can be accommodated by low porosities (<0.6%) and are thus essentially determined by the rate of devolitalization. The quantity of fluid which passes through the metamorphic column varies from 25000 g/cm², within 10 km of the base of the crust, to amounts as large as 240000 g/cm², in rocks initially at a depth of 30 km. Measured petrologic volumetric fluid-rock ratios generated by this fluid could be as high as 500 in a 1 m thick horizontal layer, but would decrease in inverse proportion of the thickness of the rock layer. Fluid advection causes local heating at rates of about 5.9·10^–14 W/cm³ during prograde metamorphism and does not result in significant heating. The amount of silica which can be transported by the fluids is very sensitive to both the absolute temperature and the change in the geothermal gradient with depth. However, even under optimal conditions, the amount of silica precipitated by metamorphic fluids is small (<0.1 vol %) and inadequate to explain the quartz veining observed in nature. These results are based on equilibrium models for fluid and heat transport that exclude the possibility of convective fluid recirculation. Such a model is likely to apply at depths greater than 10 km; therefore, it is concluded that large scale heat and silica transport by fluids is not extensive in the lower crust, despite large time-integrated fluid fluxes.     

Gradients in fluid composition across metacarbonate layers of the Wepawaug Schist, Connecticut, USA

Metamorphic index mineral zones, pressure-temperature (P-T) conditions, and CO₂-H₂O fluid compositions were determined for metacarbonate layers within the Wepawaug Schist, Connecticut, USA. Peak metamorphic conditions were attained in the Acadian orogeny and increase from ~420 °C and ~6.5 kb in the low-grade greenschist facies to ~610 °C and ~9.5 kb in the amphibolite facies. The index minerals oligoclase, biotite, calcic amphibole, and diopside formed with progressive increases in metamorphic intensity. In the upper greenschist facies and in the amphibolite facies, prograde reaction progress is greatest along the margins of metacarbonate layers in contact with surrounding schists, or in reaction selvages bordering syn-metamorphic quartz veins. New index minerals typically appear first in these more highly reacted contact and selvage zones. It has been postulated that this spatial zonation of mineral assemblages resulted from infiltration, largely by diffusion, of water-rich fluids across lithologic contacts or away from fluid conduits like fractures. In this model, the infiltrating fluids drove prograde CO₂ loss and were derived from surrounding dehydrating schists or sources external to the metasedimentary sequence. The model predicts that significant gradients in the mole fraction of CO₂ (X_CO2 X_{CO_2 } ) should have been present during metamorphism, but new estimates of fluid composition indicate that differences in X_CO2 X_{CO_2 } preserved across layers or vein selvages were very small, ~0.02 or less. However, analytical solutions to the two-dimensional advection-dispersion-reaction equation show that only small fluid composition gradients across layers or selvages are needed to drive prograde CO₂ loss by diffusion and mechanical dispersion. These gradients, although typically too small to be measured by field-based techniques, would still be large enough to dominate the effects of fluid flow and reaction along regional T and P gradients. Larger gradients in fluid composition may have existed across some layers during metamorphism, but large gradients favor rapid reaction and would, therefore, seldom be preserved in the rock record. Most of the H₂O needed to drive prograde CO₂ loss probably came from regional dehydration of surrounding metapelitic schists, although H₂O-rich diopside zone conditions may have also required an external fluid component derived from syn-metamorphic intrusions or the metavolcanic rocks that structurally underlie the Wepawaug Schist.     

Thermal metamorphism and H2O-CO2-NaCl immiscibility at Patapedia,Quebec: evidence from fluid inclusions

The methamorphic history of the Patapedia thermal zone, Gaspé, Quebec, is re-evaluated in the light of results obtained from a study of fluid inclusions contained in quartz phenocrysts of felsic dyke rocks. The thermal zone is characterised by calc-silicate bodies that have outwardly telescoping prograde metamorphic isograds and display extensive retrograde metamorphism with associated copper mineralization. Three distinct fluid inclusion types are recognized: a low to moderate salinity, high density aqueous fluid (Type I); a low density CO₂ fluid (Type II); and a high salinity, high density aqueous fluid (Type III). Fluid inclusion Types I and II predominate whereas Type III inclusions form <10% of the fluid inclusion population. All three fluid types are interpreted to have been present during prograde metamorphism. Temperatures and pressures of metamorphism estimated from fluid inclusion microthermometry and isochore calculations are 450°–500° C and 700–1000 bars, respectively. A model is proposed in which the metamorphism at Patapedia was caused by heat transferred from a low to moderate salinity fluid of partly orthomagmatic origin (Type I inclusions). During the early stages, and particularly in the deeper parts of the system, CO₂ produced by metamorphism was completely miscible in the aqueous hydrothermal fluid and locally resulted in high XCO₂ fluids. On cooling and/or migrating to higher levels these latter fluids exsolved high salinity aqueous fluids represented by the Type III inclusions. Most of the metamorphism, however, took place at temperature-pressure conditions consistent with the immiscibility of CO₂ and the hydrothermal fluid and was consequently accompanied by the release of large volumes of CO₂ vapour which is represented by Type II inclusions. The final stage of the history of the Patapedia aureole was marked by retrograde metamorphism and copper mineralization of a calcite-free calc-silicate hornfels in the presence of a low XCO₂ fluid.     

区域变质作用中的流体

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

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

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

Prograde and retrograde fluid flow during contact metamorphism of siliceous carbonate rocks from the Ballachulish aureole,Scotland

 Siliceous dolomites and limestones contain abundant retrograde minerals produced by hydration-carbonation reactions as the aureole cooled. Marbles that contained periclase at the peak of metamorphism bear secondary brucite, dolomite, and serpentine; forsterite-dolomite marbles have retrograde tremolite and serpentine; wollastonite limestones contain secondary calcite and quartz; and wollastonite-free limestones have retrograde tremolite. Secondary tremolite never appears in marbles where brucite has replaced periclase or in wollastonite-bearing limestones. A model for infiltration of siliceous carbonates by CO₂-H₂O fluid that assumes (a) vertical upwardly-directed flow, (b) fluid flux proportional to cooling rate, and (c) flow and reaction under conditions of local equilibrium between peak temperatures and ≈400 °C, reproduces the modes of altered carbonate rocks, observed reaction textures, and the incompatibility between tremolite and brucite and between tremolite and wollastonite. Except for samples from a dolomite xenolith, retrograde time-integrated flux recorded by reaction progress is on the order of 1000 mol fluid/cm² rock. Local focusing of flow near the contact is indicated by samples from the xenolith that record values an order of magnitude greater. Formation of periclase, forsterite, and wollastonite at the peak of metamorphism also required infiltration with prograde time-integrated flux approximately 100–1000 mol/cm². The comparatively small values of prograde and retrograde time-integrated flux are consistent with lack of stable isotope alteration of the carbonates and with the success of conductive thermal models in reproducing peak metamorphic temperatures recorded by mineral equilibria. Although isobaric univariant assemblages are ubiquitous in the carbonates, most formed during retrograde metamorphism. Isobaric univariant assemblages observed in metacarbonates from contact aureoles may not record physical conditions at the peak of metamorphism as is commonly assumed. Received: 19 September 1995 / Accepted: 14 March 1996     

Compositionally zoned Cl-rich amphiboles from North Dabie Shan, China: monitor of high-pressure metamorphic fluid/rock interaction processes

Amphiboles containing up to 4.2 wt.% Cl are found in felsic granulites from Yanzihe within the North Dabie area of the Dabie–Sulu ultrahigh- and high-pressure metamorphic belt in eastern China. Most amphibole grains show considerable zonations with Cl contents ranging from 0 to 4.2 wt.%. Based on their textural features, amphiboles can be divided into four generations: (1) amphibole occurring as inclusions in orthopyroxene (Am-in) with Cl contents around 3.5 wt.%; (2) amphibole forming cores of grains in the matrix (AM-I) with Cl contents between 3.0 and 4.2 wt.%; (3) amphibole with Cl contents of 0.2 to 2.5 wt.% (Am-II) occurring as hydrothermally altered parts of the original amphibole; (4) Cl-free amphibole (Am-III) usually developed at the outermost rim of the grain. Major and rare earth elements show significant variations for Am-I, Am-II and Am-III.

Different generations of amphiboles are related to different metamorphic stages of the granulite in Yanzihe, and provide a monitor for fluid/rock interactions and P–T evolution during the high-pressure metamorphism of Dabie Shan. Pressure and temperature estimates suggest that Am-in was formed during prograde metamorphism of 10 kbar and 700–800 °C; Am-I was formed under peak metamorphic conditions (20 kbar, 800–960 °C), whereas Am-II and Am-III were formed during retrograde metamorphic stages (560–770 °C and 5–7 kbar, and 520–670 °C and <5 kbar, respectively). In contrast to most previous studies, in which the earliest amphiboles to form are typically Cl-poor and later amphiboles become progressively Cl-rich, we show that the earliest amphiboles in the investigated rock are Cl-rich and later formed amphiboles are Cl-poor. The present study also demonstrates that the fluid system of the granulites in North Dabie Shan did not evolve in a simple way: while it behaved as a closed system during prograde and peak metamorphism, after the metamorphic peak it probably acted as an open system in which new fluids were introduced. The varying magnitude of Cl contents in amphiboles stresses the very local fluid control during metamorphism.     

Mechanical controls on fluid flow during regional metamorphism: some numerical models

The control of fluid flow by plastic deformation during metamorphism is critical to our understanding of metamorphic processes. Various geological observations and field studies demonstrate the consequences of fluid flow control by deformation, so that the concept appears to be accepted, at least for small-scale PUBLIC (for example faults and vein PUBLIC). However, the concept appears to be less well recognized at regional scales. Considered here are examples of simple, conceptual models based on fully coupled mechanical–fluid flow concepts; they include deformation of a section of fluid-saturated crust containing a block or a layer of material of different properties from its surrounds. In particular, rheological and permeability contrasts between rock types during deformation associated with regional metamorphism are sufficient to control the form of fluid flow over the range of a few kilometres. Low contrasts and small strains allow pervasive fluid flow, whereas greater contrasts and increasing strains cause focusing of the flow. Such focusing is generally associated with localization of the deformation, especially for a strongly dilatant elastic–plastic material. However, a rate of fluid flow much greater than the rate of deformation may result in pervasive flow, although for most models pervasive flow is difficult to attain over regional distances. Furthermore, lateral and downward fluid flow may occur, demonstrated here by simple models for folding and for deformation of regions containing plutons. Therefore, such modelling may be used as a means of testing the various hypotheses concerning the volumes of fluid predicted to have passed through some rock volumes. Numerical models of the future will become increasingly complex and powerful, allowing greater coupling of thermal, mechanical, chemical and fluid flow effects, and based more on the physical processes involved. Combined field and laboratory studies will provide correspondingly greater understanding and will permit the determination of the timing of fluid flow and structural controls on fluid flow patterns.     

Timing and nature of fluid flow and alteration during Mesoproterozoic shear zone formation, Olary Domain, South Australia

The development of shear zones at mid‐crustal levels in the Proterozoic Willyama Supergroup was synchronous with widespread fluid flow resulting in albitization and calcsilicate alteration. Monazite dating of shear zone fabrics reveal that they formed at 1582 ± 22 Ma, at the end of the Olarian D3 deformational event and immediately prior to the emplacement of regional S‐type granites. Two stages of fluid flow are identified in the area: first an albitizing event which involved the addition of Na and loss of Si, K and Fe; and a second phase of calcsilicate alteration with additions of Ca, Fe, Mg and Si and removal of Na. Fluid fluxes calculated for albitization and calcsilicate alteration were 5.56 × 10⁹ to 1.02 × 10¹⁰ mol m^?2 and 2.57 × 10⁸–5.20 × 10⁹ mol m^?2 respectively. These fluxes are consistent with estimates for fluid flow through mid‐crustal shear zones in other terranes. The fluids associated with shearing and alteration are calculated to have δ¹⁸O and δD values ranging between +8 and +11‰, and ?33 and ?42‰, respectively, and ?Nd values between ?2.24 and ?8.11. Our results indicate that fluids were derived from metamorphic dehydration of the Willyama Supergroup metasediments. Fluid generation occurred during prograde metamorphism of deeper crustal rocks at or near peak pressure conditions. Shear zones acted as conduits for major crustal fluid flow to shallow levels where peak metamorphic conditions had been attained earlier leading to the apparent ‘retrograde’ fluid‐flow event. Thus, the peak metamorphism conditions at upper and lower crustal levels were achieved at differing times, prior to regional granite formation, during the same orogenic cycle leading to the formation of retrograde mineral assemblages during shearing.     

Calc-silicate reactions and bedding-controlled isotopic exchange in the Notch Peak aureole, Utah: implications for differential fluid fluxes with metamorphic grade

Fluid compositions and bedding‐scale patterns of fluid flow during contact metamorphism of the Weeks Formation in the Notch Peak aureole, Utah, were determined from mineralogy and stable isotope compositions. The Weeks Formation contains calc‐silicate and nearly pure carbonate layers that are interbedded on centimetre to decimetre scales. The prograde metamorphic sequence is characterized by the appearance of phlogopite, diopside, and wollastonite. By accounting for the solution properties of Fe, it is shown that the tremolite stability field was very narrow and perhaps absent in the prograde sequence. Unshifted oxygen and carbon isotopic ratios in calcite and silicate minerals at all grades, except above the wollastonite isograd, show that there was little to no infiltration of disequilibrium fluids. The fluid composition is poorly constrained, but X(CO₂)_fluid must have been >0.1, as indicated by the absence of talc, and has probably increased with progress of decarbonation reactions. The occurrence of scapolite and oxidation of graphite in calc‐silicate beds of the upper diopside zone provide the first evidence for limited infiltration of external aqueous fluids. Significantly larger amounts of aqueous fluid infiltrated the wollastonite zone. The aqueous fluids are recorded by the presence of vesuvianite, large decreases in δ¹⁸O values of silicate minerals from c. 16‰ in the diopside zone to c. 10‰ in the wollastonite zone, and extensive oxidation of graphite. The carbonate beds interacted with the fluids only along margins where graphite was destroyed, calcite coarsened, and isotopic ratios shifted. The wollastonite isograd represents a boundary between a high aqueous fluid‐flux region on its higher‐grade side and a low fluid‐flux region on its lower‐grade side. Preferential flow of aqueous fluids within the wollastonite zone was promoted by permeability created by the wollastonite‐forming reaction and the natural tendency of fluids to flow upward and down‐temperature near the intrusion‐wall rock contact.     

Metamorphic and post-metamorphic fluid flow in the low-grade rocks of the Harlech Dome, north Wales

A combination of fluid inclusion, stable isotope and geochemical techniques has been used to study the nature of fluids present and their behaviour during Caledonian low-grade metamorphism of the Harlech Dome, north Wales. Fluid inclusion studies show that in most of the metasedimentary sequence the peak metamorphic fluid was an aqueous Na–K–Cl brine but in the graphitic Clogau Formation and in parts of the overlying Maentwrog Formation immiscible H₂O-rich and CH₄-rich fluids coexisted. Late-stage inclusions are of calcium-rich brine and a dilute aqueous fluid. The chemical composition of chlorite in metamorphic veins and rocks varies between different formations and quartz-oxygen isotopic compositions show considerable variation between different units. Both of these features are taken to indicate that there was little or no pervasive movement of fluid between different units at the peak of metamorphism. After the metamorphic peak there was focused flow of fluid upward through the sequence along fractures, in response to end-Caledonian uplift and unloading. Where the migrating fluid crossed the graphitic shales, interaction between the fluid and the shales gave rise to the formation of the auriferous veins of the Dolgellau Gold Belt. Subsequent to this mineralizing event there was widespread development of ¹⁸O-enriched calcites and micas. In the case of vein minerals it is possible that these crystallized directly from late-stage fluids at lower temperature than the quartz in the same veins. Alternatively, the original vein minerals may have re-equilibrated with later ¹⁸O-enriched or cooler fluid. In the case of muscovites in the rock matrix it is proposed that the isotopically heavy compositions are the result of re-equilibration of initially light grains with an introduced fluid, requiring considerable influx of fluid. This event may relate to either of two late-stage fluids observed as inclusions.     

Metamorphism in Archaean greenstone belts: calculated fluid compositions and implications for gold mineralization

An inescapable consequence of the metamorphism of greenstone belt sequences is the release of a large volume of metamorphic fluid of low salinity with chemical characteristics controlled by the mineral assemblages involved in the devolatilization reactions. For mafic and ultramafic sequences, the composition of fluids released at upper greenschist to lower amphibolite facies conditions for the necessary relatively hot geotherm corresponds to those inferred for greenstone gold deposits (X_CO2= 0.2–0.3). This result follows from the calculation of mineral equilibria in the model system CaO–MgO–FeO–Al₂O₃–SiO₂–H₂O–CO₂, using a new, expanded, internally consistent dataset. Greenstone metamorphism cannot have involved much crustal over-thickening, because very shallow levels of greenstone belts are preserved. Such orogeny can be accounted for if compressive deformation of the crust is accompanied by thinning of the mantle lithosphere. In this case, the observed metamorphism, which was contemporaneous with deformation, is of the low-P high-T type. For this type of metamorphism, the metamorphic peak should have occurred earlier at deeper levels in the crust; i.e. the piezothermal array should be of the ‘deeper-earlier’type. However, at shallow crustal levels, the piezothermal array is likely to have been of ‘deeper-later’type, as a consequence of erosion. Thus, while the lower crust reached maximum temperatures, and partially melted to produce the observed granites, mid-crustal levels were releasing fluids prograde into shallow crustal levels that were already retrograde. We propose that these fluids are responsible for the gold mineralization. Thus, the contemporaneity of igneous activity and gold mineralization is a natural consequence of the thermal evolution, and does not mean that the mineralization has to be a consequence of igneous processes. Upward migration of metamorphic fluid, via appropriate structurally controlled pathways, will bring the fluid into contact with mineral assemblages that have equilibrated with a fluid with significantly lower X_CO2. These assemblages are therefore grossly out of equilibrium with the fluid. In the case of infiltrated metabasic rocks, intense carbonation and sulphidation is predicted. If, as seems reasonable, gold is mobilized by the fluid generated by devolatilization, then the combination of processes proposed, most of which are an inevitable consequence of the metamorphism, leads to the formation of greenstone gold deposits predominantly from metamorphic fluids.     

Synchronous peak Barrovian metamorphism driven by syn-orogenic magmatism and fluid flow in southern Connecticut, USA

Recent work in Barrovian metamorphic terranes has found that rocks experience peak metamorphic temperatures across several grades at similar times. This result is inconsistent with most geodynamic models of crustal over‐thickening and conductive heating, wherein rocks which reach different metamorphic grades generally reach peak temperatures at different times. Instead, the presence of additional sources of heat and/or focusing mechanisms for heat transport, such as magmatic intrusions and/or advection by metamorphic fluids, may have contributed to the contemporaneous development of several different metamorphic zones. Here, we test the hypothesis of temporally focussed heating for the Wepawaug Schist, a Barrovian terrane in Connecticut, USA, using Sm–Nd ages of prograde garnet growth and U–Pb zircon crystallization ages of associated igneous rocks. Peak temperature in the biotite–garnet zone was dated (via Sm–Nd on garnet) at 378.9 ± 1.6 Ma (2σ), whereas peak temperature in the highest grade staurolite–kyanite zone was dated (via Sm–Nd on garnet rims) at 379.9 ± 6.8 Ma (2σ). These garnet ages suggest that peak metamorphism was pene‐contemporaneous (within error) across these metamorphic grades. Ion microprobe U–Pb ages for zircon from igneous rocks hosted by the metapelites also indicate a period of syn‐metamorphic peak igneous activity at 380.6 ± 4.7 Ma (2σ), indistinguishable from the peak ages recorded by garnet. A 388.6 ± 2.1 Ma (2σ) garnet core age from the staurolite–kyanite zone indicates an earlier episode of growth (coincident with ages from texturally early zircon and a previously published monazite age) along the prograde regional metamorphic T–t path. The timing of peak metamorphism and igneous activity, as well as the occurrence of extensive syn‐metamorphic quartz vein systems and pegmatites, best supports the hypothesis that advective heating driven by magmas and fluids focussed major mineral growth into two distinct episodes: the first at c. 389 Ma, and the second, corresponding to the regionally synchronous peak metamorphism, at c. 380 Ma.     

Zircon growth during subduction and exhumation metamorphism of mafic rocks,Aktyuz terrain,North Tiahshan,Kyrgyzstan

The interpretation of whether a dated metamorphic zircon generation grew during the prograde, peak or retrograde stage of a metamorphic cycle is critical to geological interpretation. This study documents a case at Aktyuz metamorphic terrain, in the southern of Kokchetav‐North Tianshan belt, involving progressive metamorphic recrystallization of mafic rock to eclogite and associated behavior of zircon. Zircons in eclogites are mainly fine grains (5 to 20 μm), and preferentially concentrated with rutile/ilmenite. They also occur as individual grains or clusters in amphibole coronas of garnet. A few larger grains commonly preserve inherited cores and evidence of dissolution and metamorphic outgrowths. Zircon grains separated from amphibolites show inherited zircons with typically magmatic feature, although this become progressively blurred in response to resorption and recrystallization. Mineral inclusions represent epidote‐amphibolite facies in the prograde metamorphism, and the embayed boundary between recrystallized domains and inherited zircons suggest fluid/melt participation. The metamorphic domains are mainly simple overgrowth around the inherited cores or recrystallization domains. The absence of peak metamorphic mineral inclusions and steep pattern of MREE‐HREE indicate no sufficient garnet formed before the metamorphic zircon overgrowth. A tiny rim with homogeneously bright CL image can be distinguished in most zircons. Amphibole inclusions have similar compositions to those in the coronas of garnets, suggesting a retrograde metamorphic origin. The inherited zircon crystallized at 880‐730 Ma, revealing similar age range to the gneiss in Aktyuz area, whereas metamorphic zircon dates prograde metamorphism at 497.9 ±1.4 Ma. In this case, the bulk Zr budget in rocks will become locked into Zr‐bearing minerals during the mafic magma intrusion, when the inherited zircon melting and resorption. The texture shows that metamorphic zircon grew both in the prograde and retrograde stage, and Zr‐bearing magmatic minerals and rutile/ilmenite are by far the main source of Zr for the two stages, respectively.     

Windows of metamorphic sulfur liberation in the crust: Implications for gold deposit genesis

Understanding the source of metamorphic sulfur is critical to clarifying the complete cycle of ore genesis, from source to sink, for several mineral deposit types. In this study, a mass balance approach and the thermodynamic computer programs Thermocalc and PerpleX were used to constrain the P-T range of pyrite breakdown to pyrrhotite (which liberates sulfur) in common metamorphic lithologies. The results suggest that most of the continental crust’s metamorphic sulfur is liberated in a relatively narrow temperature-pressure window corresponding to the terminal breakdown of chlorite at moderate to low pressures. This is because pyrite stability is controlled partly by temperature and pressure, and partly by the amount of H₂O present. During prograde metamorphism from the greenschist to the amphibolite facies, metamorphic H₂O is produced primarily through chlorite breakdown in mafic to pelitic bulk compositions. As temperature increases, more sulfur is required from pyrite to maintain equilibrium proportions of H₂O, H₂S and SO₂ in the fluid, and in addition, progressively more sulfur is required at lower pressures. At low temperatures, little sulfur is required by metamorphic fluid released during initial chlorite breakdown, whereas at higher temperatures coinciding with the terminal breakdown of chlorite, not only is more fluid present, but the fluid’s sulfur requirement has also increased dramatically. In this way, metamorphic dehydration drives pyrite breakdown and generation of sulfur-rich hydrothermal fluids at mesothermal conditions. Beyond the chlorite stability field there is minimal metamorphic fluid production, except at low pressures and high temperatures where muscovite can break down without causing melting; conditions that are a long way from typical crustal geotherms. However, deformation also plays a key role in pyrite breakdown. Without deformation, small amounts of fluid in chemical communication with individual pyrite grains will quickly acquire equilibrium concentrations of the sulfur species and minimal pyrite breakdown is necessary. Whereas during deformation, there may be a continuous fluid flux past pyrite grains, promoting ongoing sulfur liberation. In this way, periods of deformation may be the major sulfur-liberating episodes during a metamorphic cycle. Since hydrothermal fluids are inherently buoyant and consequently tend to migrate upwards and towards cooler temperatures through the crust, these results imply that orogenic gold deposits are most likely to form at lower-amphibolite to prehnite-pumpellyite facies conditions, and unlikely to form at higher temperatures. The pressure constraint on metamorphic sulfur liberation implies that tectonic settings that allow prograde metamorphism to follow low pressure P-T-t paths in an occasionally compressional or transpressional environment are necessary. Settings that promote extensive injection of felsic magma into a mid-crust that contains a significant proportion of pyritic carbonaceous metasediment are shown to be ideal for orogenic gold deposit genesis. Inverted back-arc basins are interpreted to be the most favourable of these.     

Fluid/rock interaction and mass transfer in continental subduction zones: constraints from trace elements and isotopes (Li,B, O,Sr, Nd,Pb) in UHP rocks from the Chinese Continental Scientific Drilling Program,Sulu, East China

In order to better understand the role of fluids during subduction and subsequent exhumation, we have investigated whole-rock and mineral chemistry (major and trace elements) and Li, B as well as O, Sr, Nd, Pb isotopes on selected continuous drill-core profiles through contrasting lithological boundaries from the Chinese Continental Scientific Drilling Program (CCSD) in Sulu, China. Four carefully selected sample sets have been chosen to investigate geochemical changes as a result of fluid mobilization during dehydration, peak metamorphism, and exhumation of deeply subducted continental crust. Our data reveal that while O and Sr-Nd-Pb isotopic compositions remain more or less unchanged, significant Li and/or B isotope fractionations occur between different lithologies that are in close contact during various metamorphic stages. Samples that are supposed to represent prograde dehydration as indicated by veins formed at high pressures (HP) are characterized by element patterns of highly fluid-mobile elements in the veins that are complementary to those of the host eclogite. A second sample set represents a UHP metamorphic crustal eclogite that is separated from a garnet peridotite by a thin transitional interface. Garnet peridotite and eclogite are characterized by a >10% difference in MgO, which, together with the presence of abundant hydroxyl-bearing minerals and compositionally different clinopyroxene grains demonstrate that both rocks have been derived from different sources that have been tectonically juxtaposed during subduction, and that hydrous silicate-rich fluids have been added from the subducting slab to the mantle. Two additional sample sets, comprising retrograde amphibolite and relatively fresh eclogite, demonstrate that besides external fluids, internal fluids can be responsible for the formation of amphibolite. Li and B concentrations and isotopic compositions point to losses and isotopic fractionation during progressive dehydration. On the other hand, fluids with isotopically heavier Li and B are added during retrogression. On a small scale, mantle-derived rocks may be significantly metasomatized by fluids derived from the subducted slab. Our study indicates that during high-grade metamorphism, Li and B may show different patterns of enrichment and of isotopic fractionation.