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.     

P–T and structural evolution during exhumation of high-T, medium-P basement rocks in the Barberton Mountain Land, South Africa

The P–T evolution of amphibolite facies gneisses and associated supracrustal rocks exposed along the northern margin of the Paleo to MesoArchean Barberton greenstone belt, South Africa, has been reconstructed via detailed structural analysis combined with calculated K(Mn)FMASH pseudosections of aluminous felsic schists. The granitoid‐greenstone contact is characterized by a contact‐parallel high‐strain zone that separates the generally low‐grade, greenschist facies greenstone belt from mid‐crustal basement gneisses. The supracrustal rocks in the hangingwall of this contact are metamorphosed to upper greenschist facies conditions. Supracrustal rocks and granitoid gneisses in the footwall of this contact are metamorphosed to sillimanite grade conditions (600–700 °C and 5 ± 1 kbar), corresponding to elevated geothermal gradients of ～30–40 °C km^?1. The most likely setting for these conditions was a mid‐ or lower crust that was invaded and advectively heated by syntectonic granitoids at c. 3230 Ma. Combined structural and petrological data indicate the burial of the rocks to mid‐crustal levels, followed by crustal exhumation related to the late‐ to post‐collisional extension of the granitoid‐greenstone terrane during one progressive deformation event. Exhumation and decompression commenced under amphibolite facies conditions, as indicated by the synkinematic growth of peak metamorphic minerals during extensional shearing. Derived P–T paths indicate near‐isothermal decompression to conditions of ～500–650 °C and 1–3 kbar, followed by near‐isobaric cooling to temperatures below ～500 °C. In metabasic rock types, this retrograde P–T evolution resulted in the formation of coronitic Ep‐Qtz and Act‐Qtz symplectites that are interpreted to have replaced peak metamorphic plagioclase and clinopyroxene. The last stages of exhumation are characterized by solid‐state doming of the footwall gneisses and strain localization in contact‐parallel greenschist‐facies mylonites that overprint the decompressed basement rocks.     

Formation of gold deposits: a metamorphic devolatilization model

A metamorphic devolatilization model can explain the enrichment, segregation, timing, distribution and character of many goldfields such as those found in Archean greenstone belts, slate‐belts and other gold‐only provinces. In this genetic model, hydrated and carbonated greenschist facies rocks, particularly metabasic rocks, are devolatilized primarily across the greenschist–amphibolite facies boundary in an orogenic setting. Devolatilization operates on the scale of individual mineral grains, extracting not just H₂O and CO₂ but also S and, in turn, Au. Elevated gold in solution is achieved by complexing with reduced S, and by H₂CO₃ weak acid buffering near the optimal fluid pH for gold solubility (the buffering is more important than being at the point of maximum gold solubility). Low salinity ensures low base metal concentrations in the auriferous metamorphic fluid. Migration of this fluid upwards is via shear zones and/or into hydraulic fracture zones in rocks of low tensile strength. The geometry of the shear zones dictates the kilometre‐scale fluid migration paths and the degree of fluid focusing into small enough volumes to form economic accumulations of gold. Deposition of gold from solution necessitates breakdown of the gold–thiosulphide complex and is especially facilitated by fluid reduction in contact with reduced carbon‐bearing host rocks and/or by sulphidation of wallrocks to generate iron‐bearing sulphide and precipitated gold. As such, black slate, carbon seams, banded iron formation, tholeiitic basalt, magnetite‐bearing diorite and differentiated tholeiitic dolerite sills are some of the important hosts to major goldfields. Gold deposition is accompanied by carbonation, sulphidation and muscovite/biotite alteration where the host rock is of suitable bulk composition. The correlation of major gold deposits with rock type, even when the gold is primarily in veins, argues for rock‐dominated depositional systems, not fluid‐dominated ones. As a consequence, a general role in gold deposition for fluid mixing, temperature decrease and/or fluid pressure decrease and boiling is unlikely, although such effects may be involved locally. Several geological features that are recorded at gold‐only deposits today reflect subsequent modifications superimposed upon the products of this generic metamorphic devolatilization process. Overprinting by higher‐grade metamorphism and deformation, and/or (palaeo)‐weathering may provide many of the most‐obvious features of goldfields including their mineralogy, geochemistry, geometry, small‐scale timing features, geophysical response and even mesoscopic gold distribution.     

Seafloor hydrothermal alteration at an Archaean mid-ocean ridge

A hydrothermally metamorphosed/altered greenstone complex capped by bedded cherts exposed in the North Pole, Pilbara Carton, Western Australia, is interpreted as an accretionary complex. It is distinctive in being characterised by both duplex structure and an oceanic crust stratigraphy. This complex is shown to represent an Archaean upper oceanic crust with a mid‐ocean ridge hydrothermal metamorphism that increases in grade stratigraphically downward. Three mineral zones have been defined; Zone A of the zeolite facies, the prehnite‐pumpellyite facies or the lower‐greenschist facies at high‐X_CO2 condition, Zone B of the greenschist facies, and Zone C of the greenschist/amphibolite transition facies. In Zone A metabasites, Ca‐Al silicates including Ca‐zeolites, prehnite and pumpellyite are absent and epidote/clinozoisite is extremely rare. Instead, abundant carbonates are present with chlorite suggesting high‐X_CO2 composition in the fluid. On the other hand, in Zones B and C metabasites, where Ca‐amphibole + epidote/clinozoisite + chlorite + Ca‐Na plagioclase are the dominant assemblages, carbonate is not identified. The metamorphic conditions boundary of Zones B/C were estimated to be about 350 °C at a pressure of <0.5 kbar. Fluid compositions coexisting with Archaean greenstones at the transition between Zones B and C were estimated by thermodynamic calculation in the CaFMASCH system (T = 350–370 °C, P = 150–1000 bar) at X_CO2 of 0.012–0.140, such values are higher than present‐day vent fluids collected near mid‐ocean ridges with low‐X_CO2 values, up to 0.005. The Archaean seawater depth at the mid‐ocean ridge was estimated to be 1600 m at X_CO2 = 0.06 using a depth‐to‐boiling point curve for a fluid. The carbonation due to high‐X_CO2 hydrothermal fluids occurred near the ridge‐axis before or was coincident with ridge metamorphism.     

A mineral equilibria study of the hydrothermal alteration in mafic greenschist facies rocks at Kalgoorlie, Western Australia

The influx of a H₂O–CO₂‐dominated fluid into actinolite‐bearing metabasic rocks during greenschist facies metamorphism in the Kalgoorlie area of Western Australia resulted in a zoned alteration halo around inferred fluid conduits that contain gold mineralisation. The alteration halo is divided into two outer zones, the chlorite zone and the carbonate zone, and an inner pyrite zone adjacent to the inferred fluid conduits. Reaction between the fluid and the protolith resulted in the breakdown of actinolite and the development of chlorite, dolomite, calcite and siderite. In addition, rocks in the pyrite zone developed muscovite‐bearing assemblages as a consequence of the introduction of potassium by the fluid. Mineral equilibria calculations undertaken using the computer software thermocalc in the model system Na₂O–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O–CO₂ show that mineral assemblages in the outer zones of the alteration halo are consistent with equilibrium of the protoliths with a fluid of composition X_CO2 = CO₂/(CO₂ + H₂O) = 0.1–0.25 for temperatures of 315–320 °C. The inner zone of the alteration halo reflect equilibrium with a fluid of composition X_CO2≈ 0.25. Fluid‐rock buffering calculations show that the alteration halo is consistent with interaction with a single fluid composition and that the zoned structure of the halo reflects the volume of this fluid with which the rocks reacted. This fluid is likely to have also been the one responsible for the gold mineralisation at Kalgoorlie.     

High‐grade metamorphism and partial melting in Archean composite grey gneiss complexes

Much of the exposed Archean crust is composed of composite gneiss which includes a large proportion of intermediate to tonalitic material. These gneiss terranes were typically metamorphosed to amphibolite to granulite facies conditions, with evidence for substantial partial melting at higher grade. Recently published activity–composition (a?x) models for partial melting of metabasic to intermediate compositions allows calculation of the stable metamorphic minerals, melt production and melt composition in such rocks for the first time. Calculated P?T pseudosections are presented for six bulk rock compositions taken from the literature, comprising two metabasic compositions, two intermediate/dioritic compositions and two tonalitic compositions. This range of bulk compositions captures much of the diversity of rock types found in Archean banded gneiss terranes, enabling us to present an overview of metamorphism and partial melting in such terranes. If such rocks are fluid saturated at the solidus, they first begin to melt in the upper amphibolite facies. However, at such conditions, very little (< 5%) melt is produced and this melt is granitic in composition for all rocks. The production of greater proportions of melt requires temperatures ～800–850 °C and is associated with the first appearance of orthopyroxene at pressures below 8–9 kbar or with the appearance and growth of garnet at higher pressures. The temperature at which orthopyroxene appears varies little with composition providing a robust estimate of the amphibolite–granulite facies boundary. Across this boundary, melt production is coincident with the breakdown of hornblende and/or biotite. Melts produced at granulite facies range from tonalite–trondhjemite–granodiorite for the metabasic protoliths, granodiorite to granite for the intermediate protoliths and granite for the tonalitic protoliths. Under fluid‐absent conditions the melt fertility of the different protoliths is largely controlled by the relative proportions of hornblende and quartz at high grade, with the intermediate compositions being the most fertile. The least fertile rocks are the most leucocratic tonalites due to their relatively small proportions of hydrous mafic phases such as hornblende or biotite. In the metabasic rocks, melt production becomes limited by the complete consumption of quartz to higher temperatures. The use of phase equilibrium forward‐modelling provides a thermodynamic framework for understanding melt production, melt loss and intracrustal differentiation during the Archean.     

Mineral assemblages and phase equilibria of metabasites from the prehnite–pumpellyite to amphibolite facies,with the Flin Flon Greenstone Belt (Manitoba) as a type example

An exceptionally well-exposed part of the Flin Flon Greenstone Belt (Manitoba/Saskatchewan) is used to characterize the mineral assemblage evolution associated with prehnite–pumpellyite through amphibolite facies metamorphism of basalts. Data from these rocks are combined with a large literature data set to assess the ability of current thermodynamic models to reproduce natural patterns, evaluate the use of metabasic rocks at these grades to estimate pressure–temperature (P–T) conditions of metamorphism, and to comment on the metamorphic devolatilization that occurs. At Flin Flon, five major isograds (actinolite-in, prehnite- and pumpellyite-out, hornblende-in, oligoclase-in, and actinolite-out) collectively represent passage from prehnite–pumpellyite to lower amphibolite facies conditions. The evolution in mineral assemblages occurs in two narrow (~1,000 m) zones: the prehnite–pumpellyite to greenschist facies (PP-GS) transition and greenschist to amphibolite facies (GS-AM) transition. Across the GS-AM transition, significant increases in the hornblende and oligoclase proportions occur at the expense of actinolite, albite, chlorite, and titanite, whereas there is little change in the proportions of epidote. The majority of this mineral transformation occurs above the oligoclase-in isograd within the hornblende–actinolite–oligoclase zone. Comparison with thermodynamic modelling results suggests data set 5 (DS5) of Holland and Powell (1998, Journal of Metamorphic Geology, 16 (3):309–343) and associated activity–composition (a–x) models is generally successful in reproducing natural observations, whereas data set 6 (DS6) (Holland & Powell, 2011, Journal of Metamorphic Geology, 29 (3):333–383) and associated a–x models fail to reproduce the observed mineral isograds and compositions. When the data from Flin Flon are combined with data from the literature, two main pressure-sensitive facies series for metabasites are revealed, based on prograde passage below or above a hornblende–albite bathograd at ~3.3 kbar: a low-pressure ‘actinolite–oligoclase type’ facies series, characterized by the appearance of oligoclase before hornblende, and a moderate- to high-pressure ‘hornblende–albite type’ facies series, characterized by the appearance of hornblende before oligoclase. Concerning the PP-GS transition, the mineral assemblage evolution in Flin Flon suggests it occurs over a small zone (<1,000 m), in which assemblages containing true transitional assemblages (prehnite and/or pumpellyite coexisting with actinolite) are rare. This contrasts with thermodynamic modelling, using either DS5 or DS6, which predicts a wide PP-GS transition involving the progressive appearance of epidote and actinolite and disappearance of pumpellyite and prehnite. Patterns of mineral assemblages and thermodynamic modelling suggest a useful bathograd (‘CHEPPAQ bathograd’), separating prehnite–pumpellyite-bearing assemblages at low pressures and pumpellyite–actinolite-bearing assemblages at higher pressures, occurs at ~2.3 to 2.6 kbar. Observations from the Flin Flon sequence suggests devolatilization across the GS-AM transition (average: ~1.8 wt% H₂O) occurs over a very narrow interval within the actinolite–hornblende–oligoclase zone, associated with the loss of >75% of the total chlorite. By contrast, modelling of the GS-AM transition zone predicts more progressive dehydration of ~2 wt% H₂O over a >50°C interval. Observations from the field suggest devolatilization across the PP-GS transition occurs over a very narrow interval given the rarity of transitional assemblages. Modelling suggests fluid release of 1.0–1.4 wt% resulting from prehnite breakdown over a ~10°C interval. This fluid may not be entirely lost from the rock package due to involvement in the hydration of igneous mineralogy across the PP-GS transition as observed in the Flin Flon sequence.     

Phase equilibrium modelling of the amphibolite to granulite facies transition in metabasic rocks (Ivrea Zone,NW Italy)

The development of thermodynamic models for tonalitic melt and the updated clinopyroxene and amphibole models now allow the use of phase equilibrium modelling to estimate P–T conditions and melt production for anatectic mafic and intermediate rock types at high‐T conditions. The Permian mid‐lower crustal section of the Ivrea Zone preserves a metamorphic field gradient from mid amphibolite facies to granulite facies, and thus records the onset of partial melting in metabasic rocks. Interlayered metabasic and metapelitic rocks allows the direct comparison of P–T estimates and partial melting between both rock types with the same metamorphic evolution. Pseudosections for metabasic compositions calculated in the Na₂O–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O–TiO₂–O (NCKFMASHTO) system are presented and compared with those of metapelitic rocks calculated with consistent end‐member data and a–x models. The results presented in this study show that P–T conditions obtained by phase equilibria modelling of both metabasic and metapelitic rocks give consistent results within uncertainties, allowing integration of results obtained for both rock types. In combination, the calculations for both metabasic and metapelitic rocks allows an updated and more precisely constrained metamorphic field gradient for Val Strona di Omegna to be defined. The new field gradient has a slightly lower dP/dT which is in better agreement with the onset of crustal thinning of the Adriatic margin during the Permian inferred in recent studies.     

Nature and source of the ore-forming fluids associated with orogenic gold deposits in the Dharwar Craton

Neoarchean orogenic gold deposits, associated with the greenstone-granite milieus in the Dharwar Craton include(1) the famous Kolar mine and the world class Hutti deposit;(2) small mines at HiraBuddini, Uti, Ajjanahalli, and Guddadarangavanahalli;(3) prospects at Jonnagiri; and(4) old mining camps in the Gadag and Ramagiri-Penakacherla belts. The existing diametric views on the source of ore fluid for formation of these deposits include fluids exsolved from granitic melts and extracted by metamorphic devolatilization of the greenstone sequences. Lode gold mineralization occurs in structurally controlled higher order splays in variety of host rocks such as mafic/felsic greenstones, banded iron formations, volcaniclastic rocks and granitoids. Estimated metamorphic conditions of the greenstones vary from lower greenschist facies to mid-amphibolite facies and mineralizations in all the camps are associated with distinct hydrothermal alterations. Fluid inclusion microthermometric and Raman spectroscopic studies document low salinity aqueous-gaseous(H_2O + CO_2 ± CH_4 + NaCl) ore fluids,which precipitated gold and altered the host rocks in a narrow P-T window of 0.7-2.5 kbar and 215-320℃. While the calculated fluid O-and C-isotopic values are ambiguous, S-isotopic compositions of pyrite-precipitating fluid show distinct craton-scale uniformity in terms of its reduced nature and a suggested crustal sulfur source.Available ages on greenstone metamorphism, granitoid plutonism and mineralization in the Hutti Belt are tantamount, making a geochronology-based resolution of the existing debate on the metamorphic vs.magmatic fluid source impossible. In contrast, tourmaline geochemistry suggests involvement of single fluid in formation of gold mineralization, primarily derived by metamorphic devolatilization of mafic greenstones and interlayered sedimentary rocks, with minor magmatic contributions. Similarly, compositions of scheelite, pyrite and arsenopyrite point toward operation of fault-valves that caused pressure fluctuation-induced fluid phase separation, which acted as the dominant process of gold precipitation,apart from fluid-rock sulfidation reactions. Therefore, results from geochemistry of hydrothermal minerals and those from fluid inclusion microthermometry corroborate in constraining source of ore fluid,nature of gold transport(by Au-bisulfide complex) and mechanism of gold ore formation in the Dharwar Craton.     

Interaction of aqueous fluids with calcareous metasediments during high-T, low-P regional metamorphism in the Qadda area, southern Arabian Shield

The S.W. Nabitah Mobile Belt, Saudi Arabia, contains a Proterozoic island-arc complex. In the Qadda area, the metavolcanic-dominated supracrustal sequence records amphibolite facies regional metamorphism of high-T , low-P type. Calcsilicate rocks and aluminous dolomitic marbles within the supracrustal sequence have been studied in detail to refine estimates of peak metamorphic P–T conditions and assess the role of fluids during prograde and retrograde metamorphism. Fluid-independent thermobarometers (including the calcite–dolomite thermometer and P-sensitive equilibria involving grossular, wollastonite, anorthite, meionite, quartz and calcite) yield peak P–T conditions of c. 650–660 °C, 4 kbar, both higher than previous estimates, giving a revised average thermal gradient of c. 45 °C km^–1. The close match between the peak temperatures implied by calcite–dolomite thermometry and those recorded by univariant devolatilization equilibria suggests that the calcareous rocks were fluid-bearing during late-prograde and peak metamorphic stages. These fluids were essentially binary H₂O–CO₂ mixtures with low NaCl and HF concentrations. Most were H₂O-rich, with X_CO2 between 0.02 and 0.2, but values of c. 0.6 are recorded by two samples. High modal abundances of the solid products of decarbonation reactions (e.g. c. 10–50% wollastonite) in many of the rocks that record low-X_CO2 equilibrium fluids implies infiltration of significant quantities of externally derived aqueous fluid during late-prograde metamorphism, but not enough to exhaust the buffering capacity of the rocks. Calculated minimum time-integrated fluid-to-rock ratios of five wollastonite-bearing calcsilicate rocks range from 0.7±0.22 to 1.39±0.46 (1σ); those of six marbles range from c. 0 to 4±1.4. The latter variation occurs on a metre-scale, implying focusing of fluid flow. Diopside-rich rocks record fluid-to-rock ratios of up to 88±48. Penetrative wollastonite lineations indicate a temporal link between infiltration and distributed ductile deformation. Infiltrating fluids were probably derived both from the prograde dehydration of adjacent metabasalts and metatuffs and from crystallization of voluminous pretectonic granitoid intrusions. In general, fluid-to-rock ratios deduced for the metavolcanic-dominated Qadda area are similar to those recorded by rocks in the metasediment-dominated terrane of N. New England. The occurrence of post-tectonic retrograde hydration textures in both carbonate-bearing and carbonate-free rocks otherwise lacking hydrous minerals testifies to infiltration of aqueous fluids during retrograde metamorphism in the absence of penetrative deformation. Minimum fluid-to-rock ratios calculated for secondary grossular reaction rims in some calcsilicates are c. 0.04. Later patchy hydration of scapolite probably utilized static, pore-filling fluids remaining after the early retrograde infiltration.     

Thermal-baric structure and P–T history of the Brooks Range metamorphic core, Alaska

Documentation of pressure–temperature (P–T) histories across an epidote‐amphibolite facies culmination provides new insight into the tectono‐thermal evolution of the Brooks Range collisional orogen. Thermobarometry reveals that the highest grade rocks formed at peak temperatures of 560–600 °C and at pressures of 8–9.5 kbar. The thermal culmination coincides with the apex of a structural dome defined by oppositely dipping S2 crenulation cleavages suggesting post‐metamorphic doming. South of the thermal culmination, greenschist facies and lowermost epidote‐amphibolite facies rocks preserve widespread evidence for an early blueschist facies metamorphism. In contrast, no evidence for an early blueschist facies metamorphism was found in similar grade rocks of the northern flank, indicating that the southern flank underwent initial deeper burial during southward underthrusting of the continental margin. Thus, while the dome shows a symmetric distribution of peak temperatures, the P–T paths followed by the two flanks must have varied. This variation suggests that final thermal re‐equilibration to greenschist and epidote–amphibolite facies conditions did not result from a simple process of southward underthrusting followed by thermal re‐equilibration from the bottom upward. The new data are inconsistent with a previous model that invokes such re‐equilibration, along with northward thrusting of epidote–amphibolite facies rocks over lower grade rocks presently on the southern flank of the culmination, to produce an inverted metamorphic field gradient. Instead, it is suggested that following blueschist facies metamorphism, rocks of the southern and northern flanks were juxtaposed, during which time the more deeply buried south flank was partially emplaced above rocks to the north, where they escaped Albian epidote–amphibolite facies overprinting. Porphyroblast growth, which post‐dates the main fabric on the north flank of the culmination may be the result of Albian thermal re‐equilibration following this deformation. Post‐metamorphic doming resulted from a combination of Albian‐Cenomanian extension and Tertiary deformation.     

Compositional evolution in Ca-amphibole in the Karmutsen metabasites, Vancouver Island, British Columbia, Canada

Abstract The 6-km-thick Karmutsen metabasites, exposed over much of Vancouver Island, were thermally metamorphosed by intrusions of Jurassic granodiorite and granite. Observations of about 800 thin sections from the Campbell River and Buttle Lake area show that the metabasites provide a complete succession of mineral assemblages ranging from the zeolite to pyroxene hornfels facies around the intrusion. The most important observations are as follows. (1) The compositional change of Ca-amphiboles with increasing metamorphic grade is not straightforward. The tremolite component decreases from the prehnite–actinolite facies to the greenschist facies with a compensating tschermak component increase, but the tendency is not clear thereafter. Instead, the edenite component increases from the amphibolite facies to the pyroxene hornfels facies. (2) The most pargasitic Ca-amphibole occurs in high-Fe²⁺/Mg metabasite from the greenschist/amphibolite transition zone. (3) The reasons for such irregular compositional trends, even in the rather uniform MORB-like composition of the Karmutsen metabasites, are non-ideal solid solutions of Ca-amphibole at low temperature and the effective control by bulk rock composition in the amphibolite facies. (4) The data from this study support, but do not prove, a transition loop for the actinolite–hornblende compositional gap rather than a solvus. If the gap is a solvus, its shape is asymmetric, and is highly dependent on the other compositional parameters such as Fe³⁺/Al and Fe²⁺/Mg. (5) The X_NaA/X_A±X_Ab) ratios between Ca-amphibole and plagioclase are most useful as an indicator of metamorphic grade even within the amphibolite facies, and these change systematically from 0.2 to 0.5 from the greenschist to pyroxene hornfels facies. (6) The compositional trend of Ca-amphibole from the Karmutsen metabasites indicates a typical low-P/T metamorphic facies series on a Rbk–Gln–Tr–Ts diagram.     

P–T path of high-pressure/low-temperature rocks and tectonic implications in the western Tianshan Mountains, NW China

The Chinese western Tianshan high-pressure/low-temperature (HP–LT) metamorphic belt, which extends for about 200 km along the South Central Tianshan suture zone, is composed of mainly metabasic blueschists, eclogites and greenschist facies rocks. The metabasic blueschists occur as small discrete blocks, lenses, bands, laminae or thick beds in meta-sedimentary greenschist facies country rocks. Eclogites are intercalated within blueschist layers as lenses, laminae, thick beds or large massive blocks (up to 2 km² in plan view). Metabasic blueschists consist of mainly garnet, sodic amphibole, phengite, paragonite, clinozoisite, epidote, chlorite, albite, accessory titanite and ilmenite. Eclogites are predominantly composed of garnet, omphacite, sodic–calcic amphibole, clinozoisite, phengite, paragonite, quartz with accessory minerals such as rutile, titanite, ilmenite, calcite and apatite. Garnet in eclogite has a composition of 53–79 mol% almandine, 8.5–30 mol% grossular, 5–24 mol% pyrope and 0.6–13 mol% spessartine. Garnet in blueschists shows similar composition. Sodic amphiboles include glaucophane, ferro-glaucophane and crossite, whereas the sodic–calcic amphiboles mainly comprise barroisite and winchite. The jadeite content of omphacite varies from 35–54 mol%. Peak eclogite facies temperatures are estimated as 480–580 °C for a pressure range of 14–21 kbar. The conditions of pre-peak, epidote–blueschist facies metamorphism are estimated to be 350–450 °C and 8–12 kbar. All rock types have experienced a clockwise P–T path through pre-peak lawsonite/epidote-blueschist to eclogite facies conditions. The retrograde part of the P–T path is represented by the transition of epidote-blueschist to greenschist facies conditions. The P–T path indicates that the high-pressure rocks formed in a B-type subduction zone along the northern margin of the Palaeozoic South Tianshan ocean between the Tarim and Yili-central Tianshan plates.     

Scheme of mineral facies of metamorphic rocks

The proposed PT grid of mineral facies of metamorphic rocks, which retains the commonly adopted nomenclature (greenschist, epidote-amphibolite, amphibolite, granulite, glaucophane-schist, and eclogite facies), is based on original calculations and the published calculated and experimental data on mineral equilibria. To validate the facies and subfacies PT boundaries, the mineral equilibria in metapelitic and metabasic rocks have been used.     

Iranshahr blueschist: subduction of the inner Makran oceanic crust

The Makran accretionary prism in SE Iran and SW Pakistan is one of the most extensive subduction accretions on Earth. It is characterized by intense folding, thrust faulting and dislocation of the Cenozoic units that consist of sedimentary, igneous and metamorphic rocks. Rock units forming the northern Makran ophiolites are amalgamated as a mélange. Metamorphic rocks, including greenschist, amphibolite and blueschist, resulted from metamorphism of mafic rocks and serpentinites. In spite of the geodynamic significance of blueschist in this area, it has been rarely studied. Peak metamorphic phases of the northern Makran mafic blueschist in the Iranshahr area are glaucophane, phengite, quartz±omphacite+epidote. Post peak minerals are chlorite, albite and calcic amphibole. Blueschist facies metasedimentary rocks contain garnet, phengite, albite and epidote in the matrix and as inclusions in glaucophane. The calculated P–T pseudosection for a representative metabasic glaucophane schist yields peak pressure and temperature of 11.5–15 kbar at 400–510 °C. These rocks experienced retrograde metamorphism from blueschist to greenschist facies (350–450 °C and 7–8 kbar) during exhumation. A back arc basin was formed due to northward subduction of Neotethys under Eurasia (Lut block). Exhumation of the high‐pressure metamorphic rocks in northern Makran occurred contemporarily with subduction. Several reverse faults played an important role in exhumation of the ophiolitic and HP‐LT rocks. The presence of serpentinite shows the possible role of a serpentinite diapir for exhumation of the blueschist. A tectonic model is proposed here for metamorphism and exhumation of oceanic crust and accretionary sedimentary rocks of the Makran area. Vast accretion of subducted materials caused southward migration of the shore.     

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.     

Subduction initiation in the Neo‐Tethys: constraints from counterclockwise P–T paths in amphibolite rocks of the Nagaland Ophiolite Complex,India

High‐P metamorphic rocks that are formed at the onset of oceanic subduction usually record a single cycle of subduction and exhumation along counterclockwise (CCW) P–T paths. Conceptual and thermo‐mechanical models, however, predict multiple burial–exhumation cycles, but direct observations of these from natural rocks are rare. In this study, we provide a new insight into this complexity of subduction channel dynamics from a fragment of Middle‐Late Jurassic Neo‐Tethys in the Nagaland Ophiolite Complex, northeastern India. Based on integrated textural, mineral compositional, metamorphic reaction history and geothermobarometric studies of a medium‐grade amphibolite tectonic unit within a serpentinite mélange, we establish two overprinting metamorphic cycles (M₁–M₂). These cycles with CCW P–T trajectories are part of a single tectonothermal event. We relate the M₁ metamorphic sequence to prograde burial and heating through greenschist and epidote blueschist facies to peak metamorphism, transitional between amphibolite and hornblende‐eclogite facies at 13.8 ± 2.6 kbar, 625 ± 45 °C (error 2σ values) and subsequent cooling and partial exhumation to greenschist facies. The M₂ metamorphic cycle reflects epidote blueschist facies prograde re‐burial of the partially exhumed M₁ cycle rocks to peak metamorphism at 14.4 ± 2 kbar, 540 ± 35 °C and their final exhumation to greenschist facies along a relatively cooler exhumation path. We interpret the M₁ metamorphism as the first evidence for initiation of subduction of the Neo‐Tethys from the eastern segment of the Indus‐Tsangpo suture zone. Reburial and final exhumation during M₂ are explained in terms of material transport in a large‐scale convective circulation system in the subduction channel as the latter evolves from a warm nascent to a cold and more mature stage of subduction. This Neo‐Tethys example suggests that multiple burial and exhumation cycles involving the first subducted oceanic crust may be more common than presently known.     

Dynamothermal transition zone between Archaean greenstone and granitoid gneiss at Lake Dundas,Western Australia

A 500 m wide shear zone occurs between the base of an Archaean greenstone sequence and adjacent granitoid gneiss complex on the shores of Lake Dundas, Western Australia. The dynamothermal margin remains distinguishable due to the preservation of upper amphibolite facies assemblages, related granitoid anatectites and mylonitic, schistose and gneissose fabrics developed parallel to the contact, which itself lies subparallel to the greenstone bedding surface. The margin contrasts with less deformed greenschist to low amphibolite facies assemblages which characterize lithologies within the greenstone belt, many of which retain igneous textures and relict primary phases. Structural, petrological and textural evidence indicates that the dynamothermal contact originally evolved as a subhorizontal ductile shear zone during juxtaposition of the greenstone pile with granitoid gneiss and that its formation preceded regional folding, greenschist facies overprinting and granitoid intrusion which occurred at about 2700 Ma. The amount of heat generated within the transition zone during thrusting was limited to maximum temperatures of c. 650°C due to the buffering effect of granitoid anatexis.     

Metamorphic evolution of the central Southern Cross Province,Yilgarn Craton,Western Australia

Two contrasting styles of metamorphism are preserved in the central Southern Cross Province. An early, low‐grade and low‐strain event prevailed in the central parts of the Marda greenstone belt and was broadly synchronous with the first major folding event (D₁) in the region. Mineral assemblages similar to those encountered in sea‐floor alteration are indicative of mostly prehnite‐pumpellyite facies conditions, but locally actinolite‐bearing assemblages suggest conditions up to mid‐greenschist facies. Geothermobarometry indicates that peak metamorphic conditions were of the order of 250–300°C at pressures below 180 MPa in the prehnite‐pumpellyite facies, but may have been as high as 400°C at 220 MPa in the greenschist facies. A later, higher grade, high‐strain metamorphic event was largely confined to the margins of the greenstone belts. Mineral assemblages and geothermobarometry suggest conditions from upper greenschist facies at P–T conditions of about 500°C and 220 MPa to upper amphibolite facies at 670°C and 400 MPa. Critical mineral reactions in metapelitic rocks suggest clockwise P–T paths. Metamorphism was diachronous across the metamorphic domains. Peak metamorphic conditions were reached relatively early in the low‐grade terrains, but outlasted most of the deformation in the higher grade terrains. Early metamorphism is interpreted to be a low‐strain, ocean‐floor‐style alteration event in a basin with high heat flow. In contrast, differential uplift of the granitoids and greenstones, with conductive heat input from the granitoids into the greenstones, is the preferred explanation for the distribution and timing of the high‐strain metamorphism in this region.