Allochemical retrograde metamorphism in shear zones: an example in metapelites, Virginia, USA

Abstract Metapelites in the Altavista area, southwest Virginia Piedmont, USA, underwent allochemical hydrothermal retrograde metamorphism in synmetamorphic shear zones. The metapelites of the Evington Group were metamorphosed in a prograde sequence of chlorite, staurolite, and sillimanite zones. Garnet–biotite geothermometry and phase relations support eastward increasing metamorphic grade, ranging from 570° C in the staurolite zone to 650° C in the sillimanite zone at c. 5.8 kbar. Sillimanite-zone rocks later underwent progressive retrogression around shear zones which acted as fluid conduits. Retrograde assemblages are successively zoned around the shear zones with staurolite-, chloritoid- and kyanite-bearing assemblages. The shear zones commonly contain kyanite or tourmaline veins. Applicable phase equilibria indicate that retrogression occurred during isobaric cooling through c. 200–270° C. Rock compositional changes with retrogression occurred in steps: SiO₂ was gained in the early stages of the retrogression but lost in the late stages; Al₂O₃, K₂O, and H₂O were increasingly gained through the sequence; CaO was increasingly lost. Addition of H₂O and decreasing temperatures resulted in new ferromagnesian minerals (staurolite, chloritoid, chlorite) and changes in H₂O, SiO₂, Al₂O₃, K₂O, and CaO contents produced muscovite and sodic plagioclase. Subsequent to prograde metamorphism, deeply derived fluids migrated upwards along shear zones, providing fluid and energy for the retrograde reactions. The sheared rocks underwent fluid infiltration with fluid fluxes of 1.8 × 10⁷–4.3 × 10⁷ cm³/cm² corresponding to minimum estimated fluid-to-rock ratios of 7.5–21 as a function of position within the shear zone. Fluid flow was from high to low temperature early and low to high temperature later in the retrogression.     

Partial melting and retrogression during exhumation of high-grade metapelites,the Tatra Mountains,Western Carpathians

Partial melting and retrogression have been recognized in high-grade metapelites of the Tatra Mountains, Western Carpathians (Slovakia) related to exhumation during Variscan orogeny. Reaction textures and phase equilibria define a clockwise P-T path. The prograde metamorphism from ca 600 °C and 9–10 kbar to >700 °C at 11–12 kbar resulted in muscovite dehydration-melting in the kyanite stability field. Further heating at decreasing pressure led to the dehydration-melting of biotite at >750 °C in the sillimanite stability field. This was followed by nearly isothermal decompression down to 4–5 kbar, producing cordierite and some additional melt. Later nearly isobaric cooling led to melt crystallization and sub-solidus retrogression. CO₂-N₂ fluids (5–30 mol. % N₂) were generated at pressures <6 kbar by interaction between the melt-derived water and graphite at oxidizing conditions.     

Thermal granulite-facies metamorphism with diffuse retrogression in Archaean orthogneisses, Fiskefjord, southern West Greenland

Around Fiskefjord, southern West Greenland, Archaean amphibolite-facies, granulite-facies and retrograde orthogneisses occur in lithological and structural continuity with each other. The granulite-facies rocks here—and elsewhere in West Greenland—are surrounded by extensive areas of retrograde gneisses. Both the prograde and retrograde metamorphism took place in a major event of continental crust formation c. 3000 Ma ago, which gave rise to granulite-facies conditions in part of the rock complex exposed today. In the Fiskefjord area distributions of major and trace elements, as well as strontium and lead isotopes, show that the fades transformations were accompanied by pronounced metasomatism, and mineral chemistry indicates that the hydrous retrograde metamorphism took place under amphibolite-facies conditions and was gradual and incomplete. The metamorphic and metasomatic processes in the Fiskefjord area are believed to have been controlled by heat from continuous intracrustal injection of large masses of tonalitic magma, which caused gradual dehydration and partial melting, followed by liberation of aqueous fluids during crystallization of anatectic melts. These fluids partially retrograded previously dehydrated gneisses. In contrast, South Indian high-grade gneisses have mainly prograde amphibolite–granulite-facies transitions which are distinct and well preserved, later than penetrative deformation, and are likely to have been controlled by CO₂ streaming. These amphibolite–granulite-facies transitions are reported to be near-isochemical. It is suggested that there are (at least) two different kinds of granulite-facies metamorphism: a near-isochemical prograde type in stabilized tectonic environments, perhaps controlled by influx of CO₂ (e.g. in South India) and significantly post-dating original crust formation; and a fluid-deficient type with widespread anatexis, hydrous retrogression and metasomatism, which takes place during accretion of continental crust, and in which heat is the governing factor (e.g. in southern West Greenland).     

Metamorphic imprints upon sulphide mineralisation at Boksputs,Northern Cape,South Africa

A small stratiform Fe-Cu-Zn mineralisation has been examined from Boksputs within the metavolcanic Areachap Group (1.5 Ga), along the eastern boundary of the Namaqua Province. Distinctly deformational and metamorphic features within the mineralisation can be separated into an early and a late fabric; the former is related to an early prograde metamorphism (M1) and also to a subsequent mild retrogression (M2) which can be regionally distinguished in the silicate mineralogy, whilst the latter fabric is derived during a severe retrogressive event (M3) that is concentrated especially in shearzones that cut the mineralisation. A release of secondary sulphur-bearing fluids during M3 is evidenced by new growth and redistribution of sulphides and by local replacement of pyrrhotite by pyrite.     

Fluid-rock history of granulite facies humite-marbles from Ambasamudram, southern India.

An extensive humite‐bearing marble horizon within a supracrustal sequence at Ambasamudram, southern India, was studied using petrological and stable isotopic techniques to define its metamorphic history and fluid characteristics. At peak metamorphic temperatures of 775±73°C, based on calcite‐graphite carbon isotope thermometry, the mineral assemblages suggest layer‐by‐layer control of fluid compositions. Clinohumite + calcite‐bearing assemblages suggest X_CO2 < 0.4 (at 700°C and 5 kbar), calcite + forsterite + K‐feldspar‐bearing assemblages suggest X_CO2>0.9 (at 790°C); and local wollastonite + scapolite + grossular‐bearing zones formed at X_CO2 of c. 0.3. Retrograde reaction textures such as scapolite + quartz symplectites after feldspar and calcite and replacement of dolomite + diopside or tremolite+dolomite after calcite+forsterite or calcite+clinohumite are indicative of retrogression under high X_CO2 conditions. Calcite preserves late Proterozoic carbon and oxygen isotopic signatures and the marble lacks evidence for extensive retrograde fluid infiltration, while during prograde metamorphism the possible infiltration of aqueous fluids did not produce significant isotopic resetting. Isotopic zonation of calcite and graphite grains was likely produced by localized CO₂ fluid infiltration during retrogression. Contrary to the widespread occurrence of humite‐marbles related to retrograde aqueous fluid infiltration, the Ambasamudram humite‐marbles record a prograde‐to‐peak metamorphic humite formation and retrogression under conditions of low X_H2O.     

Chemical processes and migration of elements during retrogression of a staurolite-zone assemblage in western North Carolina

A detailed study of retrograde alteration of a staurolite porphyroblast and its surrounding matrix of mica schist has made use of petrographic, modal, and microprobe analysis. Retrogression was to the garnet zone of metamorphism and apparently occurred largely after a temperature decline of 70–100° C. The event caused metasomatic removal of Zn but may have been isochemical relative to other analyzed elements. The best estimate of the overall reaction is: 1 staurolite+3.018 biotite+3.550 quartz+0.629 albite +0.014 anorthite+0.678 NaCl+14.004 H₂O =3.274 Na-rich muscovite+3.561 chlorite +0.273 ilmenite+0.110 chloritoid+0.039 garnet +0.339 ZnCl₂.Non-systematic variation in composition of analyzed minerals is revealed by statistical treatment of replicate analyses. Such variation involves monovalent and divalent cations within many minerals, but is most pronounced within retrograde muscovite. Muscovite variation involves Si and Al as well as FM and alkalis and does not follow a phengite law of charge-coupled substitution.Relative to the core of the retrograded staurolite crystal, zoning is seen in averaged muscovite compositions and in development of incompatible mineral assemblages, which include chloritoid well within retrograded staurolite but biotite within the matrix. A local gradient in the chemical potential of an Al-bearing component was likely present during retrogression.Alteration of staurolite was probably accomplished by reaction and diffusion through the medium of an intergranular fluid phase. Relative to staurolite, migration of elements involved immigration of considerable amounts of Mg, Na, K, and H and expulsion of Al, Fe, Zn, and O. It is inferred that concentration of Al within the fluid phase was considerably lower than those of monovalent and divalent cations.Preservation of considerable staurolite and evidence for a local concentration gradient of Al in the fluid phase suggest that limited amounts of H₂O were available. Expulsion of Zn suggests that much water was not consumed locally but exited the terrane. An attempt at resolution of this dilemna involves fracture-channelized infiltration of H₂O into the rock. A more regional petrographic study of retrogression suggests that H₂O which entered the rock may have been liberated initially by prograde dehydration at a moderately greater depth of 2–3 km.Results of this study, especially the non-phengitic nature of crystal-chemical substitution within muscovite, indicate chemical reaction under conditions of disequilibrium. Apparently, extent of retrogression was controlled by availability of H₂O rather than by thermochemical equilibria.     

Tectonic and polymetamorphic history of the Lesser Himalaya in central Nepal

The Lesser Himalaya in central Nepal consists of Precambrian to early Paleozoic, low- to medium-grade metamorphic rocks of the Nawakot Complex, unconformably overlain by the Upper Carboniferous to Lower Miocene Tansen Group. It is divided tectonically into a Parautochthon, two thrust sheets (Thrust sheets I and II), and a wide shear zone (Main Central Thrust zone) from south to north by the Bari Gad–Kali Gandaki Fault, the Phalebas Thrust and the Lower Main Central Thrust, respectively. The Lesser Himalaya is overthrust by the Higher Himalaya along the Upper Main Central Thrust (UMCT). The Lesser Himalaya forms a foreland-propagating duplex structure, each tectonic unit being a horse bounded by imbricate faults. The UMCT and the Main Boundary Thrust are the roof and floor thrusts, respectively. The duplex is cut-off by an out-of-sequence fault. At least five phases of deformation (D₁–D₅) are recognized in the Lesser Himalaya, two of which (D₁ and D₂) belong to the pre-Himalayan (pre-Tertiary) orogeny. Petrographic, microprobe and illite crystallinity data show polymetamorphic evolution of the Lesser and Higher Himalayas in central Nepal. The Lesser Himalaya suffered a pre-Himalayan (probably early Paleozoic) anchizonal prograde metamorphism (M₀) and a Neohimalayan (syn- to post-UMCT) diagenetic to garnet grade prograde inverted metamorphism (M₂). The Higher Himalaya suffered an Eohimalayan (pre or early-UMCT) kyanite-grade prograde metamorphism (M₁) which was, in turn, overprinted by Neohimalayan (syn-UMCT) retrograde metamorphism (M₂). The isograd inversion from garnet zone in the Lesser Himalaya to kyanite zone in the Higher Himalaya is only apparent due to post-metamorphic thrusting along the UMCT. Both the Lesser and Higher Himalayas have undergone late-stage retrogression (M₃) during exhumation.     

Magmatic and metamorphic controls on chemical variations within the Eiksunddal eclogite complex,Sunnmøre,western Norway

The Eiksunddal eclogite complex is one of the larger group B (“country-rock”) eclogites within the Western Gneiss Region (WGR) of Norway. Whole-rock chemical data suggest a magmatic origin for the rocks within the eclogite complex. The chemical variations among the eclogitic rocks were controlled by accumulation of olivine (Fo₈₀) and plagioclase (An₇₀) and probably magnetite. The composition of the interstitial (“trapped”) liquid is derived by linear regression methods and is a high-Al orthopyroxene normative basalt. The most likely conditions of crystallization are: T ≈ 1150°C, P < 10 kbar. The Al-rich nature of the interstitial liquid may have implications for the formation of anorthosites and leucogabbros associated with similar mafic rocks elsewhere in the WGR. Trace-element characteristics are compatible with a continental origin of these rocks.During the subsequent metamorphic history, the marginal parts of the complex have suffered variable degrees of contamination. Abundant amphibole inclusions in large garnets in garnet websterites suggest local hydration at pre-eclogitic stages. This may be associated with introduction of radiogenic Sr as previously reported by W.L. Griffin and H.K. Brueckner. During prograde metamorphism, alkali exchange occurred during growth of garnet, clinopyroxene and biotite at the expense of pargasitic amphibole and orthopyroxene. The most extensive metasomatism however, was probably associated with the introduction of fluid during amphibolitization (retrogression) of the eclogitic rocks. Three model reactions — eqs. 5–7 — qualitatively explain the relation between degree of retrogression and the observed metasomatic effects. Whether Si and Ca are gained or lost during retrogression seems to be controlled by the extent of amphibole growth in diopside + plagioclase symplectites initially formed by breakdown of omphacite.Alumina has behaved in a more or less immobile way during metasomatism. Low mobility of Al is also reflected by the formation of subsilic (Al-rich) amphiboles and Al-rich plagioclase during garnet breakdown. This demonstrates the importance of kinetic factors in controlling mineral chemistry.     

Evidence for multiple burial–partial exhumation cycles from the Onodani eclogites in the Sambagawa metamorphic belt,central Shikoku,Japan

Eclogites from the Onodani area in the Sambagawa metamorphic belt of central Shikoku occur as layers or lenticular bodies within basic schists. These eclogites experienced three different metamorphic episodes during multiple burial and exhumation cycles. The early prograde stage of the first metamorphic event is recorded by relict eclogite facies inclusions within garnet cores (X_Sps 0.80–0.24, X_Alm 0–0.47). These inclusions consist of relatively almandine‐rich garnet (X_Sps 0.13–0.24, X_Alm 0.36–0.45), aegirine‐augite/omphacite (X_Jd 0.08–0.28), epidote, amphiboles (e.g. actinolite, winchite, barroisite and taramite), albite, phengite, chlorite, calcite, titanite, hematite and quartz. The garnet cores also contain polyphase inclusions consisting of almandine‐rich garnet, omphacite (X_Jd 0.27–0.28), amphiboles (e.g. actinolite, winchite, barroisite, taramite and katophorite) and phengite. The peak P–T conditions of the first eclogite facies metamorphism are estimated to be 530–590 °C and 19–21 kbar succeeded by retrogression into greenschist facies. The second prograde metamorphism began at greenschist facies conditions. The peak metamorphic conditions are defined by schistosity‐forming omphacites (X_Jd ≤ 49) and garnet rims containing inclusions of barroisitic amphibole, phengite, rutile and quartz. The estimated peak metamorphic conditions are 630–680 °C and 20–22 kbar followed by a clockwise retrograde P–T path with nearly isothermal decompression to 8–12 kbar. In veins cross‐cutting the eclogite schistosity, resorbed barroisite/Mg‐katophorite occurs as inclusions in glaucophane which is zoned to barroisite, suggesting a prograde metamorphism of the third metamorphic event. The peak P–T conditions of this metamorphic event are estimated to be 540–600 °C and 6.5–8 kbar. These metamorphic conditions are correlated with those of the surrounding non‐eclogitic Sambagawa schists. The Onodani eclogites were formed by subduction of an oceanic plate, and metamorphism occurred beneath an accretionary prism. These high‐P/T type metamorphic events took place in a very short time span between 100 and 90 Ma. Plate reconstructions indicate highly oblique subduction of the Izanagi plate beneath the Eurasian continent at a high spreading rate. This probably resulted in multiple burial and exhumation movements of eclogite bodies, causing plural metamorphic events. The eclogite body was juxtaposed with non‐eclogitic Sambagawa schists at glaucophane stability field conditions. The amalgamated metamorphic sequence including the Onodani eclogites were exhumed to shallow crustal/surface levels in early Eocene times (c. 50 Ma).     

Contrasting metamorphic histories of lenses of high‐pressure rocks and host migmatites with a flat orogenic fabric (Bohemian Massif,Czech Republic): a result of tectonic mixing within horizontal crustal flow?

Migmatites with sub‐horizontal fabrics at the eastern margin of the Variscan orogenic root in the Bohemian Massif host lenses of eclogite, kyanite‐K‐feldspar granulite and marble within a matrix of migmatitic paragneiss and amphibolite. Petrological study and pseudosection modelling have been used to establish whether the whole area experienced terrane‐wide exhumation of lower orogenic crust, or whether smaller portions of higher‐pressure lower crust were combined with a lower‐pressure matrix. Kyanite‐K‐feldspar granulite shows peak conditions of 16.5 kbar and 850 °C with no clear indications of prograde path, whereas in the eclogite the prograde path indicates burial from 10 kbar and 700 °C to a peak of 18 kbar and 800 °C. Two contrasting prograde paths are identified within the host migmatitic paragneiss. The first path is inferred from the presence of staurolite and kyanite inclusions in garnet that contains preserved prograde zoning that indicates burial with simultaneous heating to 11 kbar and 800 °C. The second path is inferred from garnet overgrowths of a flat foliation defined by sillimanite and biotite. Garnet growth in such an assemblage is possible only if the sample is heated at 7–8 kbar to around 700–840 °C. Decompression is associated with strong structural reworking in the flat fabric that involves growth of sillimanite in paragneiss and kyanite‐K‐feldspar granulite at 7–10 kbar and 750–850 °C. The contrasting prograde metamorphic histories indicate that kilometre‐scale portions of high‐pressure lower orogenic crust were exhumed to middle crustal levels, dismembered and mixed with a middle crustal migmatite matrix, with the simultaneous development of a flat foliation. The contrasting P–T paths with different pressure peaks show that tectonic models explaining high‐pressure boudins in such a fabric cannot be the result of heterogeneous retrogression during ductile rebound of the whole orogenic root. The P–T paths are compatible with a model of heterogeneous vertical extrusion of lower crust into middle crust, followed by sub‐horizontal flow.     

Dehydration melting and devolatilization during exhumation of high-grade metapelites: the Tatra Mountains, Western Carpathians

Partial melting and retrogression related to Variscan tectonic exhumation have been recognized in the high-grade metapelites of the Tatra Mountains, Western Carpathians. Staurolite and kyanite relics document an early stage of the prograde metamorphism at c. 600 °C and 9–10 kbar. An increase in temperature to >730 °C at 11–12 kbar resulted in partial melting and incipient migmatization in the stability field of kyanite. Further heating at decreasing pressure during the earliest stage of exhumation led to the dehydration-melting of muscovite and biotite at >750–800 °C and 6–10 kbar, producing garnet-bearing granite as leucosomes in migmatite. Subsequent cooling is documented by garnet resorption by biotite and sillimanite (a reversal of the prograde biotite dehydration-melting reaction). This was followed by nearly isothermal decompression to c. 4–5 kbar producing cordierite and some melt due to biotite decomposition. Later nearly isobaric cooling led to cordierite pinitization and formation of orthoamphibole, chlorite and carbonates. Densities of primary, monophase CO₂–N₂ inclusions (0.69–1.06 g cm^?3) from the migmatite leucosome are consistent with the near-peak and retrograde conditions. Highly varying N₂ contents (5–30 mol%) are thought to result from the nitrogen uptake in retrograde K-bearing minerals, or dilution by CO₂ liberated during interaction of melt-derived water with metapelite graphite. The relatively high nitrogen content, not observed until now in migmatites, could have been inherited from the high-pressure metamorphism stage. It is assumed that the water-absent composition of fluid inclusions is not representative of the bulk water content (X_H2O≤0.7), which was masked by mechanical separation of the CO₂- and H₂O-dominated immiscible phases, and/or by post-entrapment modifications of the fluid inclusions. Decompression and the final stage of exhumation were accomplished by top-to-the-south thrusting as well as west–east (orogen-parallel) extension. They were most probably related to regional uplift and gravitational collapse of thermally weakened Variscan crust.     

Very low-grade metamorphism of sedimentary rocks of the Meliata unit,Western Carpathians,Slovakia: implications of phyllosilicate characteristics

The Meliata unit represents a mélange-like accretionary wedge, containing blueschist facies tectonic blocks and slices in a Triassic and Jurassic sedimentary matrix. The blueschist facies rocks are tectonic remnants of the subducted parts of the Meliata-Hallstatt branch of the Tethys. The phyllosilicate assemblages in very low-grade metapelites represent metastable disequilibrium stages which the assemblages have reached during reaction progress. Therefore, temperature and pressure values of low-T metamorphism of the sedimentary series and the late stages of decompressional cooling of blueschist facies rocks, obtained by phyllosilicate "crystallinity", chlorite thermometric and white K-mica geobarometric methods, can be regarded as semiquantitative estimates. However, results of chlorite–white mica thermobarometry suggest that local equilibrium was approached at a microscopic scale. For deciphering the age relations of prograde and retrograde events, K–Ar isotope geochronological methods were applied. The sedimentary series and related basalts of the Meliata unit experienced high-T anchizonal prograde regional metamorphism, the temperature and pressure of which can vary between ca. 280 and 350 °C and ca. 2.5 and 5 kbar. White K-mica b geobarometry suggests possible minimal pressures of ca. 1.5 to 3 kbar. The mylonitic retrogression of blueschist facies phyllites is characterised by 340 °C and 4 kbar (minimal P). The low-T prograde metamorphism was synchronous with the retrograde metamorphism of the blueschists. The ages of these two events may be between ca. 150 and 120 Ma, culminating most probably at around 140–145 Ma. Thus, the Upper Jurassic (lowermost Cretaceous) very low-grade metamorphism of the Meliata unit is younger than the subduction-related, 160–155 Ma blueschist facies event, and definitely older than the Cretaceous (100–90 Ma) metamorphism of the footwall Gemer Palaeozoic.     

Biotite breakdown and the formation of gahnite in metapelitic rocks from Kemiö, southwest Finland

The formation of ferroan gahnite by breakdown of zinc-bearing biotite has been studied in metapelitic gneisses from the Kemiö region in southwestern Finland. Spinel compositions range from herc₅₀ gahn₄₀spin₁₀ to herc₁₅gahn₈₂spin₃; biotite contains up to 0.24 wt.% ZnO. Individual spinel grains show a gradual compositional zoning owing to increasing zinc contents towards the grain margins.Chemical and textural evidence indicate that spinel has been formed by breakdown of zinc-bearing biotite. During prograde metamorphism ferroan gahnite formed according to the reaction: biotite+sillimanite +quartz=cordierite+spinel+fluid, and during retrogressive metamorphism ferroan gahnite was formed by chloritization of biotite. It is proposed that the zinc-saturation limit of biotite depends on the metamorphic conditions. Decrease of this limit during retrogression to values below the zinc content of biotite initiated release of zinc and formation of ferroan gahnite. The activity of the Fe and Mg end members of retrogressively formed spinel are fixed by the coexisting biotite.     

Alteration,mineralization, and genesis of the zoned Tongshan skarn-type copper deposit,Anhui, China

The Tongshan skarn-type copper deposit is located in the Anqing–Guichi ore cluster of the iron–copper metallogenic belt which occurs along the Middle–Lower Yangtze River Valley, China. In the study area, skarnization and mineralization took place along the contact zone between carbonates and granodiorite porphyries. The contact zone shows significant horizontal and vertical variations in alteration and mineralization. In the horizontal direction, the garnet content is high in the skarns near the intrusive body (proximal skarns), the diopside content is high farther from the intrusive body (distal skarns), and hedenbergite is concentrated in the skarns adjacent to the marble zone. Limestones located far from the marble zone experienced a strong silicification. In the vertical direction (from higher to lower levels), the rocks change from hornfels to calcareous skarn to magnesian skarn. Mineralogical studies show that the skarns near the intrusion are relatively oxidized, and the garnet in the skarns is relatively andradite rich. High concentrations of Cu are found in the porphyries with quartz veins, as well as in the calcic skarns, magnesian skarns, hornfelses, and marbles, which are located at distances of 13, 10, 43 and 25 m from the porphyries, respectively. High concentrations of Zn are found in silicified limestones and skarns located even farther from the porphyries. The present findings suggest that the Tongshan deposit was subjected to prograde alteration and mineralization, followed by retrogression. The alteration can be divided into a sequence of stages: contact metamorphism, prograde metasomatism, early retrogression, and late retrogression. The copper mineralization occurred mainly during the early retrogression, and the copper was further enriched in quartz veins within the porphyries during the late stages of magma evolution.     

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.     

Microstructure, metamorphism and tectonics of the western Cape Breton Highlands, Nova Scotia

ABSTRACT Microstructural and petrological data from the Jumping Brook metamorphic suite, western Cape Breton Highlands, suggest that a single episode of syntectonic prograde metamorphism, followed by uplift, cooling and associated retrogression, affected these rocks during mid-Palaeozoic times. Microstructures indicative of progressive crenulation foliation development can be traced from low-grade (chlorite zone) through high-grade (kyanite zone) rocks, allowing a clear sequence of porphyroblast growth to be established. Metamorphic reactions and P-T calculations suggest metamorphic conditions of 700-750°C at 8-10 kbar were achieved in kyanite zone rocks. Although a complete P-T-t path was not defined, combined petrological and geochronological data can be used to constrain computed P-T-t models. These models suggest that a component of post-metamorphic tectonic exhumation is required to explain the observed times of cooling and uplift. The microstructural and petrological data to not support the interpretation that the high-grade rocks represent pre-existing crystalline basement. Indeed, the metamorphic history, geochronology and computed tectonic models all point to a single, short-lived episode of Silurian-Devonian volcanism, intrusion, convergence, regional metamorphism and uplift, probably resulting from collision tectonics at an irregular continental margin.     

Garnet and staurolite producing reactions in a chlorite-chloritoid schist

During prograde metamorphism garnet and, in some higher grade samples, staurolite were produced in a chlorite-chloritoid schist, part of the Precambrian Z to Cambrian Hoosac Formation near Jamaica, VT. Garnet grew during two prograde events separated by a retrogression. This sequence resulted in distinctive inclusion textures and zoning anomalies in garnet produced by diffusive alteration. Textures, reaction space analysis, and mineral compositional variations constrain the possible sequence of reactions in these rocks. Below the staurolite isograd, and to some unknown extent above it, garnet grew by the reaction chloritoid+chlorite+quartz→garnet+H₂O. With increasing grade the mineral compositions are displaced towards lower Mn/Fe and higher Mg/Fe ratios. The data are compatible with equilibrium with respect to exchange reactions for the matrix assemblages on a thin section scale and with minerals having closely followed equilibrium paths during reaction. The staurolite isograd coincides with the reaction chloritoid+quartz→garnet+staurolite+chlorite+H₂O. This reaction is continuous and trivariant with ZnO becoming an additional component concentrated in staurolite. During this reaction both the Mn/Fe and Mg/Fe ratios of the phases appear to have decreased. This new chemical trend is recorded by garnet zoning profiles and is compatible with trends predicted from phase diagrams. Thus there are two distinct types of garnet zoning reversals in these samples. One is near the textural unconformity and is best explained by diffusive alteration during partial resorption of first stage garnet. The other occurs near the outer rim of garnet in staurolite zone samples and marks the onset of a new prograde garnet producing reaction.     

Preservation of high‐P rocks coupled to rock composition and the absence of metamorphic fluids

Eclogites, blueschists and greenschists are found in close proximity to one another along a 1‐km coastal section where the Cyclades Blueschist Unit (CBU) is exposed on SE Syros, Greece. Here, we show that the eclogites and blueschists experienced the same metamorphic history: prograde lawsonite blueschist facies metamorphism at 1.2–1.9 GPa and 410–530°C followed, at 43–38 Ma, by peak blueschist/eclogite facies metamorphism at 1.5–2.1 GPa and 520–580°C. We explain co‐existence of eclogites and blueschists by compositional variation probably reflecting original compositional layering. It is also shown that the greenschists record retrogression at 0.34 ± 0.21 GPa and T = 456 ± 68°C. This was spatially associated with a shear zone on a scales of 10–100‐m and veins on a scale of 1–10‐cm. Greenschist facies metamorphism ended at (or shortly after) 27 Ma. We thus infer a period of metamorphic quiescence after eclogite/blueschist facies metamorphism and before greenschist facies retrogression which lasted up to 11–16 million years. We suggest that this reflects an absence of metamorphic fluid flow at that time and conclude that greenschist facies retrogression only occurred when and where metamorphic fluids were present. From a tectonic perspective, our findings are consistent with studies showing that the CBU is (a) a high‐P nappe stack consisting of belts in which high‐P metamorphism and exhumation occurred at different times and (b) affected by greenschist facies metamorphism during the Oligocene, prior to the onset of regional tectonic extension.     

Petrology and geochemistry of prograde deserpentinized peridotites from Happo-O’ne, Japan: Evidence of element mobility during deserpentinization

The prograde deserpentinized peridotites from the talc zone in the Happo-O’ne complex, central Japan, show differences in their field relation and mineral assemblage with the high-P retrograde peridotites of the other part of the complex. They show a mineral assemblage, olivine + talc + antigorite ± prograde tremolite ± chlorite, formed by thermal metamorphism around the granitic intrusion at T, 500-650 °C and P < 7 kbar. The olivine has numerous opaque inclusions and high Fo (91.5-96.5) relative to the retrograde olivine, reflecting its formation by deserpentinization. The prograde tremolite, which is low in Al₂O₃ (<1.0 wt.%), Cr₂O₃ (<0.35 wt.%), and Na₂O (<0.6 wt.%) but high in Mg# (up to 0.98) and SiO₂ (up to 59.9 wt.%), is different in size, shape and chemistry from the retrograde tremolite. The prograde peridotites display a U-shaped REE pattern (0.02-0.5 times PM), similar to diopside-zone retrograde metaperidotites, possible protoliths. They are enriched in LILE (e.g., Cs, Pb, Sr, Rb) relative to HFSE (e.g., Ta, Hf, Zr, Nb), like their protoliths, because of their local re-equilibration with the fluid released during dehydration of the protoliths. They have high contents of REE and some trace elements (e.g., Cs, Th, U, Ta) relative to their protoliths because of an external-element addition from the granitic magma. In-situ analyses of peridotitic silicates confirmed that the prograde tremolite and talc display a spoon-shaped primitive mantle (PM)-normalized REE pattern (0.1-3 times PM) in which LREE are higher than HREE contents. The prograde tremolite is depleted in Al, Na, Cr, Sc, V, Ti, B, HREE and Li, but is enriched in Si, Cs, U, Th, HFSE (Hf, Zr, Nb, Ta), Rb and Ba relative to the retrograde tremolite; the immobile-element depletion in this tremolite is inherited from its source (antigorite + secondary diopside), whereas the depletion of mobile elements (e.g., Li, B, Na, Al) is ascribed to their mobility during the deserpentinization and/or the depleted character of the source of tremolite. The enrichment of HFSE and LILE in the prograde tremolite is related to an external addition of these elements from fluid/melt of the surrounding granitic magma and/or in situ equilibrium with LILE-bearing fluid released during dehydration of serpentinized retrograde metaperidotites and olivine-bearing serpentinites (protoliths). The prograde olivine is higher in REE and most trace-element contents than the retrograde one due to the external addition of these elements; it is enriched in B, Co and Ni, but depleted in Li that was liberated during deserpentinization by prograde metamorphism.     

The geochemistry of prograde and retrograde charnockite-gneiss reactions in southern India

Exposures in many quarries in southern India exhibit field evidence for incipient charnockitization of tonalitic and granitic gneiss (prograde relationship), or retrogression of charnockite to produce tonalitic gneiss (retrograde relationship). Few systematic geochemical relationships exist between adjacent gneisscharnockite sample pairs during either prograde or retrograde reactions. Most elements and element ratios exhibit inconsistent variations; however, prograde chamockites appear enriched in Ta, Pb, volatiles (chiefly CO₂), and in transition metals relative to Mg, and depleted in REE and Y compared to adjacent gneiss protoliths. Retrograde gneisses have higher Rb, Pb, Th, Hf, Zn relative to Co, Nb relative to Ta, Hf relative to Zr, and volatiles (chiefly H₂O) compared to parental charnockites. Of those elements (U, Th, Rb, Cs, Pb) significantly depleted in high-pressure charnockites exposed south of the prograde transition zone, only Pb is significantly replenished during retrogression. Evidence suggests that prograde fluids are relatively rich in CO₂ and retrograde fluids in H₂O and that the typical non-systematic geochemical variations during prograde and retrograde reactions reflect local effects at the wave front.