Prograde Path Research Articles

Using P–T paths to interpret the tectonothermal setting of prograde metamorphism: An example from the northeastern Gawler Craton, South Australia

We present metamorphic analysis from a porphyroblastic garnet–cordierite–spinel pelite from the Palaeo-Mesoproterozoic Mount Woods Inlier in South Australia. Petrographic analysis combined with a series of P–T pseudosections representative of a progressively changing effective bulk composition due to early porphyroblast growth and creation of an Al-rich, Si-poor chemical environment in the porphyroblast cores provides evidence of a prograde path that reached granulite facies conditions. Andalusite porphyroblasts are interpreted to have grown during the early phases of prograde metamorphism at conditions of ∼2–3kbar and ∼550–610°C. Geothermal gradients associated with early stages of prograde metamorphism are estimated to be ∼50–90°Ckm−1. The aluminosilicate porphyroblasts subsequently reacted to form cordierite–spinel symplectites that are chemically restricted from the remainder of the bulk rock by earlier formed cordierite moats. Peak metamorphism is petrographically represented by a phase of garnet growth, and attained conditions of ∼4.7kbar and 750°C, equivalent to a geothermal gradient of 45–50°Ckm−1. Monazite CHIME analysis has yielded an age of ∼1615Ma for the Moonlight Hills samples, and is suggested to represent monazite growth during prograde metamorphism through temperatures of ∼500–600°C. Peak metamorphism was contemporaneous with high-temperature/low-pressure metamorphism elsewhere in the Gawler Craton (Fleurieu Peninsula) and eastern Proterozoic Australia (e.g. Broken Hill Block, Mount Isa Inlier) at ca. 1.61–1.59Ga. The Mount Woods Inlier was heated at low pressure conditions due to intrusion of voluminous magmas to mid- to upper-crustal levels, some of which have been removed during exhumation and erosion. The hot rocks were then buried, at which point peak metamorphic conditions were attained.

Phase equilibria in quartzitic garnet-kyanite-chloritoid micaschist from the Eclogite Zone, Tauern Window, Eastern Alps

In a quartzitic garnet-kyanite-chloritoid micaschist garnet is present as compact, partly euhedral grains and as web-like types with numerous inclusions of quartz. Compact types overgrown by web garnets or alternating layers of both types are also common. Continuous zoning is observed with increase of pyrope and decrease of grossular and spessartine component towards garnet rim. A first stage of garnet growth together with chloritoid is recognized, followed by a garnet rim stage including kyanite. Calculated P-T pseudosections in the KNCFMMnASTH system suggest a prograde path with garnet growth starting in the assemblage chloritoid-chlorite-lawsonite-glaucophane (muscovite-quartz-rutile-H2O excess phases). Later breakdown of chloritoid +chlorite is followed by the reaction chloritoid+quartz = garnet+kyanite+H2O. The calculations suggest that glaucophane was present along a major portion of the prograde P-T path. Conditions of ~20.5 kbar, 540 °C are suggested for growth of garnet core and ~19–21 kbar, 580–590 °C for garnet rim coexisting with the assemblage garnet-kyanite-chloritoid-paragonite. During exhumation, cooling and influx of H2O took place. As a result high-XMg chloritoid was formed in the matrix and lower-XMg chloritoid in garnet inclusions. In a later event at lower P-T conditions, minor amounts of low-Si phengite, biotite, chlorite and clinozoisite replaced high-Si phengite, kyanite, garnet and chloritoid.

Garnet re-equilibration by coupled dissolution-reprecipitation: evidence from textural, major element and oxygen isotope zoning of ‘cloudy’ garnet

The analysis of texture, major element and oxygen isotope compositions of cloudy garnet crystals from a metapelite sampled on Ikaria Island (Greece) is used to assess the model of growth and re-equilibration of these garnet crystals and to reconstruct the pressure–temperature–fluid history of the sample. Garnet crystals show complex textural and chemical zoning. Garnet cores (100–200 μm) are devoid of fluid inclusions. They are characterized by growth zoning demonstrated by a bell-shaped profile of spessartine component (7–3 mol.%), an increase in grossular from 14 to 22 mol.% and δ18O values between 9.5 ± 0.3‰ and 10.4 ± 0.2‰. Garnet inner rims (90–130 μm) are fluid inclusion-rich and show a decreasing grossular component from 22 to 5 mol.%. The trend of the spessartine component observed in the inner rim allows two domains to be distinguished. In contrast to domain I, where the spessartine content shows the same trend as in the core, the spessartine content of domain II increases outwards from 2 to 14 mol.%. The δ18O values decrease towards the margins of the crystals to a lowest value of 7.4 ± 0.2‰. The outer rims (<10 μm) are devoid of fluid inclusions and have the same chemical composition as the outermost part of domain II of the inner rim. Garnet crystals underwent a four-stage history. Stage 1: garnet growth during the prograde path in a closed system for oxygen. Garnet cores are remnants of this growth stage. Stage 2: garnet re-equilibration by coupled dissolution–reprecipitation at the temperature peak (630 < T < 650 °C). This causes the creation of porosity as the coupled dissolution–reprecipitation process allows chemical (Ca) and isotopic (O) exchange between garnet inner rims and the matrix. The formation of the outer rim is related to the closure of porosity. Stage 3: garnet mode decreases during the early retrograde path, but garnet is still a stable phase. The resulting garnet composition is characterized by an increasing Mn content in the inner rim’s domain II caused by intracrystalline diffusion. Stage 4: dissolution of garnet during the late retrograde path as garnet is not a stable phase anymore. This last stage forms corroded garnet. This study shows that coupled dissolution–reprecipitation is a possible re-equilibration process for garnet in metamorphic rocks and that intra-mineral porosity is an efficient pathway for chemical and isotopic exchange between garnet and the matrix, even for otherwise slow diffusing elements.

The metamorphic history of rocks buried, accreted and exhumed in an accretionary prism: an example from the Otago Schist, New Zealand

AbstractAlthough subgreenschist facies metamorphic rocks are widespread in the upper crust, mineralogical processes affecting these rocks are poorly understood. Subgreenschist mineralogical transitions have been invoked as critical controls on the mechanical behaviour of rocks within the crustal seismogenic zone, calling for further study of very low‐grade metamorphic assemblages. In this study a multi‐technique thermobarometric study of the Chrystalls Beach Complex mélange, which is located within the Otago Schist accretion‐collision assemblage of the South Island of New Zealand, is presented. The Chrystalls Beach Complex comprises highly sheared trench‐fill sedimentary rocks and scattered pillow basalts, and is inferred to have formed during Jurassic subduction under the paleo‐Pacific Gondwana margin. Equilibrium mineral assemblages indicate peak P–T conditions in the range 400–550 MPa and 250–300 °C, which is supported by chlorite thermometry. Relatively high pressures of burial and accretion during foliation development are inferred from phengite content and b0 spacing analyses of white mica. Rare lawsonite occurs in a post‐foliation vein, and illite ‘crystallinity’ measurements indicate a thermal overprint during exhumation. These P–T estimates and their relative chronology indicate that the mineral assemblages developed along a clockwise P–T path. Based on variability in P–T estimates from different techniques, mineral assemblages developed during burial are largely overprinted during exhumation at similar or higher‐T than experienced along the prograde path. Observed subduction‐related subgreenschist assemblages are therefore likely to indicate lower‐P than experienced during subduction, as higher‐P mineral compositions re‐equilibrate during exhumation. The P–T path inferred in this study is similar in shape to P–T paths for higher grade parts of the Otago Schist, and other exhumed accretionary prisms around the world, and is therefore probably common for rocks buried, accreted and exhumed in accretionary prisms.

MENZERITE-(Y), A NEW SPECIES, [(Mg,Fe2+)(Fe3+,Al)](Si3)O12, FROM A FELSIC GRANULITE, PARRY SOUND, ONTARIO, AND A NEW GARNET END-MEMBER, [Mg2](Si3)O12

Menzerite-(Y), a new mineral species, forms reddish brown cores, n = 1.844 (20), up to 70 μm across, rimmed successively by euhedral almandine containing up to 2.7 wt% Y 2 O 3 and by K-feldspar in a felsic granulite on Bonnet Island in the interior Parry Sound domain, Grenville Orogenic Province, Canada. It is named after Georg Menzer (1897–1989), the German crystallographer who solved the crystal structure of garnet. Single-crystal X-ray-diffraction results yielded space group Ia 3 d , a = 11.9947(6) A. An electron-microprobe analysis of the grain richest in Y (16.93 wt% Y 2 O 3 ) gave the following formula, normalized to eight cations and 12 oxygen atoms: {Y 0.83 Gd 0.01 Dy 0.05 Ho 0.02 Er 0.07 Tm 0.01 Yb 0.06 Lu 0.02 Ca 1.37 Fe 2+ 0.49 Mn 0.07 } [Mg 0.55 Fe 2+ 0.42 Fe 3+ 0.58 Al 0.35 V 0.01 Sc 0.01 Ti 0.08 ](Si 2.82 Al 0.18 )O 12 , or {(Y,REE)(Ca,Fe 2+ ) 2 }[(Mg,Fe 2+ )(Fe 3+ ,Al)](Si 3 )O 12 . Synchrotron micro-XANES data gave Fe 3+ /∑Fe = 0.56(10) versus 0.39(2) calculated from stoichiometry. The scattering power refined at the octahedral Y site, 17.68 epfu , indicates that a relatively light element contributes to its occupancy. Magnesium, as determined by electron-microprobe analyses, would be a proper candidate. In addition, considering the complex occupancy of this site, the average Y –O bond length of 2.0244(16) A is in accord with a partial occupancy by Mg. The dominance of divalent cations with Mg > Fe 2+ and the absence of Si at the octahedral Y site (in square brackets) are the primary criteria for distinguishing menzerite-(Y) from other silicate garnet species; the menzerite-(Y) end-member is {Y 2 Ca}[Mg 2 ](Si 3 )O 12 . The contacts of menzerite-(Y) with almandine are generally sharp and, in places, cuspate. It is interpreted to have equilibrated with ferrosilite, augite, quartz, oligoclase, allanite-(Ce), magnetite, ilmenite and fluorapatite, in the absence of almandine, on the prograde path at 7–8.5 kbar and T ≈ 700–800°C, and subsequently dissolved incongruently in an anatectic melt to form almandine, most likely, at P ≈ 8.5–9.5 kbar and T ≈ 800–850°C.

Zoned Ca-amphibole as a new marker of the Alpine metamorphic evolution of phyllites from the Jubrique unit, Alpujárride Complex, Betic Cordillera, Spain

AbstractZoned Ca-amphibole from metaclastic rocks and synfolial quartz-rich veins of the Jubrique area (Alpujárride complex, Betic Cordillera, Spain), and their significance in the Alpine metamorphic evolution are described here for the first time. Typical Al-rich metapelites from this area show assemblages with white mica and chlorite, with sporadic kyanite and chloritoid. Nevertheless, some Ca-richer phyllites, fine-grained quartzites and quartz veins, show assemblages consisting of Ca-amphibole, plagioclase, epidote, titanite, chlorite and quartz. Ca-amphibole, appearing in large radial grains as well as in smaller subhedral crystals, displays a zonation with actinolite cores, overgrown by magnesiohomblende (and edenite) and minor tschermakite (and pargasite or hastingsite). The zoning suggests growth during prograde metamorphism. Retrogressive rims are variably developed in the several lithotypes. General zonation is well described in terms of the tschermakite, the edenite and the Fe3+Al-1, the MgFe2+-1, the NaK-1 and the OHCl-1 exchanges. Chemical parameters (IVAl, A(Na+K), BNa) indicate an evolution from low- to medium-grade temperature and pressure. The Al-in-amphibole thermobarometer provides a range of prograde temperatures between ∼300°C (for cores) and ∼600°C (for rims), for pressures from ∼1 to ∼6 kbar, respectively. Despite the uncertainty resulting from the fact that the thermobarometer used was calibrated for metabasites, amphibole defines a prograde pressure-temperature path typical of Barrovian-type metamorphism.

Fluid inclusions in quartz related to subsequent stages of foliation development during a single metamorphic cycle (Schneeberg Fault Zone, Eastern Alps, Austria)

Macro- and micro-scale structures, fluid inclusions, quartz rheology and host mineral chemistry were used to establish the P– T– d evolutionary path of monometamorphic metapelites of the Schneeberg Fault Zone. Four selected quartz generations indicate formation conditions during stages of prograde and retrograde metamorphism. Earliest foliation development S1 is outlined by quartz foam microstructures that are armored within foliation-parallel garnet layers which reflect individual growth characteristics during the prograde and early retrograde stage. The prograde path is linked with garnet growth at low differential stress up to peak pressure conditions of ca. 10 kbar. Final garnet growth during increasing temperatures and WNW-directed shearing up to ≤ 600 °C predates exhumation along an isothermal decompressional path and increasing differential stress. Exhumation was accompanied by the formation of younger quartz generations and the formation of a second foliation S2 that was incorporated into intense folding. Minimum pressure condition for S2 is about 4.5 kbar resulting from the gradient of fluid inclusion density isochores estimated from S2-related pressure shadows in combination with the temperature range for dislocation creep in quartz aggregates between 500 and 600 °C. The dominance of carbonic fluid inclusions indicates decarbonation associated with H 2O leakage as a main process during Alpine monometamorphism in the metasedimentary host rocks. Aqueous fluid inclusions are arranged along late fluid planes and reflect a wide range in densities with variable salinities. This study corroborates the successful approach of combining mineral growth, fluid inclusions and macro- to micro-scale deformation stages to constrain a prograde to retrograde growth history of metamorphic minerals like garnet and quartz even though these underwent polyphase deformation and upper amphibolite facies metamorphism. It supports previous arguments about low differential stress at high-pressure conditions and allows speculations about the location of the metasediments of the Schneeberg Fault Zone nearby the low strength plate interface during late stage of Cretaceous subduction and subsequent early exhumation.

Metamorphic reprocessing of a serpentinized carbonate-bearing peridotite after detachment from the mantle wedge: A P–T path constrained from textures and phase diagrams in the system CaO–MgO–Al 2O 3–SiO 2–CO 2–H 2O

In the central Rhodope mountains of Greece a carbonate-bearing metaperidotite lens ∼ 200 × 500 m in size crops out as part of the high- to ultrahigh-pressure metamorphic Upper Sidironero Complex ∼ 500 m SE of the Gorgona Village, north of Xanthi town. It is composed primarily of coarse grained (3–20 mm in size) olivine and orthopyroxene, medium grained clinohumite and medium to fine grained tremolite, chlorite, dolomite, magnesite, talc, antigorite and various spinel phases. Whole-rock chemistry, mineral textures and compositions, and phase diagram calculations show that the metaperidotite was subjected to a prograde HP metamorphism, isofacial with the surrounding migmatitic gneisses, metapelites and amphibolites. The prograde character of metamorphism is demonstrated by inclusions of talc, antigorite, chlorite, dolomite, magnesite and Ti-clinohumite in ferrit-chromite, olivine, and orthopyroxene, as well as of olivine in orthopyroxene, and by the typical change in composition of zoned spinel minerals from ferrit-chromite in the core to chromian spinel at the rim. The prograde path is characterized by successive growth of amphibole, Ti-clinohumite, olivine and orthopyroxene, followed by the breakdown of Ti-clinohumite to olivine + Mg-ilmenite and of chlorite to olivine + spinel, probably during exhumation. The construction of a partial petrogenetic P– T grid in the system CaO–MgO–Al 2O 3–SiO 2–CO 2–H 2O (CMASCH) for Ca-poor ultramafic bulk compositions has proven highly useful for the reconstruction of the metamorphic evolution and a P– T path, indicating that the use of univariant reactions in mixed volatile systems is highly warranted. The P– T path is clearly constrained to pressures below 1.5–1.7 GPa by the absence of clinopyroxene. These pressures are slightly lower than those recorded in the closely associated Jurassic eclogites and much lower than those recorded in the diamond-bearing gneisses 5 km to the south in the same tectonic unit. The carbonate-bearing metaperidotite from Gorgona probably represents a fragment of the hydrated mantle wedge. This is indicated by the REE compositions which differ from those of ophiolitic peridotites and resemble those of spinel or garnet peridotites of sub-continental origin. The ultramafic slice was incorporated tectonically into the subduction channel, most likely by tectonic erosion in the Early Jurassic, but did not experience ultrahigh-pressure metamorphism like the nearby metapelites that exhumed along the same subduction channel.

Metamorphism of High/ultrahigh-pressure Pelitic-Felsic Schist in the South Tianshan Orogen, NW China: Phase Equilibria and P-T Path

The South Tianshan high/ultrahigh-pressure (HP^UHP) metamorphic belt, NW China is characterized by extensive pelitic^felsic schists with numerous blocks of eclogite, ultramafic rock and marble.The pelitic^felsic schists preserve two generations of garnetbearing mineral assemblages: (1) an early generation that involves garnet, phengite, glaucophane, and quartz/coesite with or without paragonite; (2) a late generation that involves albite, hornblendic amphibole, quartz and grossular-rich, pyrope-poor garnet. Early generation garnet has growth zoning involving core to rim increases in pyrope content, coupled with increased or constant grossular content. Pseudosection modeling of these growth zoning textures in three samples (T311,T314, andT316) reflects prograde paths and peak conditions of c. 32 kbar at 550^5708C, c. 22.5 kbar at 5508C, and c. 22^23 kbar at 540^5508C, respectively.These P^T estimates are consistent with those recovered from eclogite blocks hosted by the schists. Phase equilibrium modeling predicts that the early garnet would have mainly grown in mineral assemblages involving lawsonite, jadeite and chloritoid, with or without coesite or carpholite, distinct from the assemblages now in the matrix. The post-peak decompression of the pelitic^felsic schists is inferred to have involved two stages.The early stage decompression is characterized by dehydration reactions involving lawsonite and carpholite at P420 kbar, coupled with mode and compositional changes of garnet, glaucophane and phengite. The late-stage decompression after lawsonite disappearance led to the rocks being fluid-absent. Under such fluid-absent conditions, the solid transition of jadeite to albite occurs at P 14 kbar, and hornblendic amphibole forms at 12^13 kbar. Most garnet grains are somewhat changed in composition, to produce a pyrope-poor and grossular-rich outer rim. In contrast to above-solidus conditions, decompression of the pelitic^felsic schists at subsolidus conditions does not destabilize phengitic muscovite and tends to preserve their peak mineral assemblages if there is no intensive fluid infiltration. In the SouthTianshan belt, buoyancy of subducted metasediments with respect to mantle rocks could be one of the major reasons for fast exhumation of the HP^UHP rocks.

TheP–Tpath of the ultra‐high pressure Lago Di Cignana and adjoining high‐pressure meta‐ophiolitic units: insights into the evolution of the subducting Tethyan slab

AbstractThe Lago di Cignana ultra‐high‐pressure unit (LCU), which consists of coesite–eclogite facies metabasics and metasediments, preserves the most deeply subducted oceanic rocks worldwide. New constraints on the prograde and early retrograde evolution of this ultra‐high pressure unit and adjoining units provide important insights into the evolution of the Piemontese–Ligurian palaeo‐subduction zone, active in Paleocene–Eocene times. In the LCU, a first prograde metamorphic assemblage, consisting of omphacite + Ca‐amphibole + epidote + rare biotite + ilmenite, formed during burial at estimatedP &lt; 1.7 GPa and 350 &lt; T &lt; 480 °C. Similar metamorphic conditions of 400 &lt; T &lt; 650 °C and 1.0 &lt; P &lt; 1.7 GPa have been estimated for the meta‐ophiolitic rocks juxtaposed to the LCU. The prograde assemblage is partially re‐equilibrated into the peak assemblage garnet + omphacite + Na‐amphibole + lawsonite + coesite + rutile, whose conditions were estimated at 590 &lt; T &lt; 605 °C andP &gt; 3.2 GPa. The prograde path was characterized by a gradual decrease in the thermal gradient from ∼9–10 to ∼5–6 °C km−1. This variation is interpreted as the evidence of an increase in the rate of subduction of the Piemonte–Ligurian oceanic slab in the Eocene. Accretion of the Piemontese oceanic rocks to the Alpine orogen and thermal relaxation were probably related to the arrival of more buoyant continental crust at the subduction zone. Subsequent deformation of the orogenic wedge is responsible for the present position of the LCU, sandwiched between two tectonic slices of meta‐ophiolites, named the Lower and Upper Units, which experienced peak pressures of 2.7–2.8 and &lt;2.4 GPa respectively.

Forward modeling of the effects of mixed volatile reaction, volume diffusion, and formation of submicroscopic exsolution lamellae on calcite-dolomite thermometry

This paper reports the results of several thousand analyses of the Mg content of calcite from 31 samples from the Ubehebe Peak contact aureole, Death Valley, California. All data reported are from metamorphic calcite formed during mixed volatile-mineral reactions in which dolomite remained in the rock. The Mg content generally increases toward the intrusive contact and bend with increasing temperature, but it varies strongly. Indeed, probability distributions for each sample are near Gaussian, possess a relatively small skewness (−1.72 to 3.32), and a variance that is a multiple of the estimated measuring uncertainty. These findings complicate direct application of the Mg content in calcite for use as an accurate thermometer. The purpose of the study presented in the second part of the paper is to explore the significance of these systematic variations of Mg composition of calcite to aid the interpretation of contact metamorphic temperatures recorded in carbonates. We developed forward models to evaluate the effect of growth zoning, volume diffusion, and the formation of submicroscopic exsolution lamellae (<1 μm) on the measured Mg distribution in individual calcite crystals and compared the modeling results to the field data. Modeled Mg distributions were transformed into histograms by taking into account intersection probabilities and random microprobe analyses. Modeling results reveal that the original prograde Mg zoning in calcite crystal will be reset if the calcite crystal is assumed to grow slowly along a prograde path. Original low-Mg compositions can only be preserved if the entire grain forms over a small temperature interval, as can be expected for infiltration-driven mineral reactions. It is shown that all three mechanisms combined give an adequate model for the Mg-content data. We demonstrate that Mg distributions in calcite grains of the Ubehebe Peak contact aureole are the consequence of rapid crystal growth in combination with diffusion and exsolution.

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?

AbstractMigmatites 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.

Granulite facies metamorphism and subsolidus fluid‐absent reworking, Strangways Range, Arunta Block, central Australia

AbstractOrthopyroxene‐rich quartz‐saturated granulites of the Strangways Range, Arunta Block, central Australia, record evidence of two high‐grade metamorphic events. Initial granulite facies metamorphism (M1, at c. 1.7 Ga) involved partial melting and migmatization culminating in conditions of 8.5 kbar and 850 °C. Preservation of the peak M1 mineral assemblages from these conditions indicates that most of the generated melt was lost from these rocks at or near peak metamorphic conditions. Subsequent reworking (M2, at c. 1.65 Ga) is characterized by intense deformation, the absence of partial melting and the development of orthopyroxene–sillimanite ± gedrite‐bearing mineral assemblages. Gedrite is only present in cordierite‐rich lithologies where it preferentially replaces M1 cordierite porphyroblasts. Pseudosection calculations indicate that M2 occurred at subsolidus fluid‐absent conditions (aH2o ∼ 0.2) at 6–7.5 kbar and 670–720 °C. The mineral assemblages in the reworked rocks are consistent with closed system behaviour with respect to H2O subsequent to M1 melt loss. M2 reworking was primarily driven by increased temperature from the stable geotherm reached after cooling from M1 and deformation‐induced recrystallization and re‐equilibration, rather than rehydration from an externally derived fluid. The development of the M2 assemblages is strongly dependent on the intensity of deformation, not only for promoting equilibration, but also for equalizing the volume changes that result from metamorphic reactions. Calculations suggest that the protoliths of the orthopyroxene‐rich granulites were cordierite–orthoamphibole gneisses, rather than pelites, and that the unusual bulk compositions of these rocks were inherited from the protoliths. Melt loss is insufficient to account for the genesis of these rocks from more typical pelitic compositions. In quartz‐rich gneisses, however, melt loss along the M1 prograde path was able to modify the bulk rock composition sufficiently to stabilize peak metamorphic assemblages different from those that would have otherwise developed.

Counterclockwise P– T evolution of the Aghil Range: Metamorphic record of an accretionary melange between Kunlun and Karakorum (SW Sinkiang, China)

This paper describes the metamorphic evolution and the tectonic significance of the Aghil Range, a poorly known terrane located between Kunlun and Karakorum north of K2 in the framework of western Tibet. The Aghil Range consists of different units separated by syn- to late-metamorphic thrusts and post-metamorphic faults of similar attitude; among other units, the Surukwat Complex is a composite sequence of thrust sheets trending WNW–ESE and steeply dipping SSW showing a general increase of metamorphic grade from lower to higher structural levels. P– T pseudosections and conventional thermobarometry of metapelites at the top of the Surukwat Complex tightly constrain the P– T path of this highest-grade metamorphic portion of the Aghil Range. The prograde path is characterized by an early increase in both P and T and by a later, nearly isothermal, increase in P, from 500 < T < 530 °C and 0.25 < P < 0.40 GPa to 580 < T < 600 °C and 0.80 < P < 0.90 GPa. The peak metamorphic event is constrained at 550 < T < 590 °C and 0.77 < P < 0.91 GPa. The retrograde path is characterized by decompression associated to a slight cooling to T ~ 500 °C and P < 0.5 GPa. Altogether, the petrology of the studied rocks suggests a P– T path with a narrow counterclockwise shape. The studied sequence could represent the result of the early subduction of an accretionary complex, interpreted further eastward according to a melange underthrusting model. As concerning the tectonic setting of this terrane, a number of geologic and petrologic similarities link the Aghil Range and the central Qiangtang metamorphic belt, suggesting that the Aghil Range is the possible NW extension of the Qiangtang microplate separating Kunlun from Karakorum.

Prograde muscovite-rich pseudomorphs as indicators of conditions during metamorphism: An example from NW Maine

During metamorphism, evidence of the prograde path is commonly obliterated by continued recrystallization as temperatures increase. However, prograde pseudomorphs that are common in many terrains may provide insight into this portion of the metamorphic path if their mineralogy can be accurately interpreted. Seventeen sillimanite-zone metapelitic samples, each containing 2-5 muscovite-rich pseudomorphs, from the Farmington Quadrangle, Maine, U.S.A., were investigated to evaluate their use as indicators of conditions during prograde metamorphism. SEM-CL images, X-ray maps, image analysis, and electron microprobe analyses characterize the mineral distribution, modes, and compositions within the pseudomorph and the surrounding matrix. Based on modal mineralogy determined from image analyses, the muscovite-rich pseudomorphs are divided into four major types: muscovite-rich (&gt;70% muscovite), quartz-muscovite (60-70% muscovite with 10-25% quartz), plagioclase-muscovite (58-72% muscovite with 10-20% plagioclase), and sillimaniteplagioclase- muscovite (50-60% muscovite with 10-20% each plagioclase and sillimanite). A biotiterich, muscovite-poor mantle surrounds many pseudomorphs. All pseudomorphs are interpreted to be prograde, based on their texture, and are after staurolite because of the partial replacement of staurolite by coarse muscovite at lower grades. Textural modeling of reaction mechanisms required to reproduce the observed mineralogy in the pseudomorphs indicates that each major pseudomorph type holds clues to the prograde path and represents a different mechanism of formation. Muscovite-rich and quartz-muscovite pseudomorphs formed by the breakdown of staurolite containing different modal amounts of poikiloblastic quartz. Quartz-muscovite pseudomorphs likely reflect a quartz-rich initial rock composition. Plagioclasemuscovite pseudomorphs require the infiltration of Na-bearing fluids. Sillimanite-plagioclase-muscovite pseudomorphs require a two-stage process; the infiltration of Na-rich fluids during staurolite breakdown followed by sillimanite growth. The subtle mineralogical differences recorded in the pseudomorphs studied here provide evidence of previously unrecognized controls along the prograde path during metamorphism.

Subduction-related metamorphism in the Alps: review of isotopic ages based on petrology and their geodynamic consequences

Abstract We summarize ages of the high‐pressure/low‐temperature (HP/LT) metamorphic evolution of the central and the western Alps. The individual isotopic mineral ages are interpreted to represent either: (1) early growth of metamorphic minerals on the prograde path; (2) timing close to peak metamorphism; or (3) retrograde resetting of the chronometers at still‐elevated pressures. Therefore, each individual age cannot easily be transferred to a geodynamic setting at a certain time. These different data indicate a subduction‐related metamorphism between 62 and 35 Ma in different units (e.g. Voltri Massif, Schistes Lustrés of the western Alps, Tauern Window). Oceanic and continental basement units show isotope ages related to eclogitic or blueschist facies metamorphism between 75 and 40 Ma. Most of these ages may record equilibration along the retrograde path, except of some Lu/Hf garnet ages and some zircon SHRIMP ages, which provide information on the prograde path. These different isotope ages are interpreted as different steps along pressure–time paths and so may provide some information on the geodynamic evolution. The data record a continuous subduction, which is ongoing for several tens of millions years. In a large‐scale picture, we have to assume fragmentation of the downgoing plate in order to explain the available P–T and t data. This interpretation questions the ongoing driving force for subduction during the disappearance of the Alpine Tethys.

Cycling of B, Li, and LILE (K, Cs, Rb, Ba, Sr) into subduction zones: SIMS evidence from micas in high- P/ T metasedimentary rocks

The Catalina Schist (California) affords a unique opportunity to evaluate the effects of varying prograde P– T paths on the magnitudes of devolatilization and chemical/isotopic alteration of subducting sediments. Analysis of a suite of trace elements (Li, B, Rb, Sr, Cs, Ba) in micas obtained by secondary ion mass spectrometry (SIMS) complements previously published whole-rock data for metasedimentary rocks in the Catalina Schist and circumvents problems associated with chemical/isotopic overprinting during retrogression, further elucidating trace element redistribution during devolatilization along prograde P– T paths. SIMS mineral analyses confirm the inference from the whole-rock data that white mica (and in a few higher-grade samples, also biotite) is the dominant reservoir for B, Rb, Ba, and Cs. The mineral residency of Li and Sr is more complex, as Li is also highly concentrated in chlorite (where present) and in some amphiboles, and Sr whole-rock budgets are strongly influenced by the presence or absence of Ca-rich phases (e.g., lawsonite, clinozoisite, minor carbonate and titanite). Changes of concentrations of B and Cs, in white micas, with increasing metamorphic grade parallel the whole-rock trends, further demonstrating the loss of these elements from recrystallizing white micas during prograde devolatilization reactions. Trace element data for other minerals in the metasedimentary samples (Ca–Al silicates, plagioclase, chlorite, biotite, garnet, kyanite) confirm expected concentrations as related to ionic radius and charge and demonstrate element redistribution during prograde (and retrograde) reactions. For amphibolite-facies metasedimentary rocks, multiple generations of white mica, including a finer-grained, higher-Si generation related to retrogradation, have distinct trace element compositions. In pegmatites produced by partial melting at amphibolite-facies conditions, B, Li, and the LILE are similarly concentrated in coarse-grained muscovite, indicating inheritance of trace element compositions from their metasedimentary sources. These SIMS data, combined with the whole-rock data for the same samples, demonstrate that the deep transfer of relatively “fluid-mobile” trace elements is strongly dependent upon the prograde P– T path experienced by the subducting sediments. Efficient retention of B, Cs, and N in micas, to depths of > 40 km, is evident from study of these and other forearc metasedimentary suites that experienced low- T prograde paths representative of most modern subduction zones, and is consistent with models invoking additions of these elements from the slab into arc source regions, perhaps to contribute to the across-arc chemical trends (Be/Be, B/La, Cs/Th, Ba/Th) observed in some margins.

Variations in the transient prograde geothermal gradient from chloritoid‐staurolite equilibria: a case study from the Barrovian and Buchan‐type domains in the Bohemian Massif

AbstractThermodynamic modelling of metamorphic rocks increases the possibilities of deciphering prograde paths that provide important insights into early orogenic evolution. It is shown that the chloritoid–staurolite transition is not only an indicator of temperature on prograde P–T paths, but also a useful indicator of pressure. The approach is applied to the Moravo‐Silesian eastern external belt of the Bohemian Massif, where metamorphic zones range from biotite to staurolite‐sillimanite. In the staurolite zone, inclusions of chloritoid occur in garnet cores, while staurolite is included at garnet rims and is widespread in the matrix. Chloritoid XFe = 0.91 indicates transition to staurolite at 5 kbar and 550 °C and consequently, an early transient prograde geothermal gradient of 29 °C km−1. The overall elevated thermal evolution is then reflected in the prograde transition of staurolite to sillimanite and in the achievement of peak temperature of 660 °C at a relatively low pressure of 6.5 kbar. To the south and to the west of the studied area, high‐grade metamorphic zones record a prograde path evolution from staurolite to kyanite and development of sillimanite on decompression. Transition of chloritoid to staurolite was reported in two places, with chloritoid XFe = 0.75–0.80, occurring at 8–10 kbar and 560–580 °C, and indicating a transient prograde geothermal gradient of 16–18 °C km−1. These data show variable barric evolutions along strike and across the Moravo‐Silesian domain. Elevated prograde geothermal gradient coincides with areas of Devonian sedimentation and volcanism, and syn‐ to late Carboniferous intrusions. Therefore, we interpret it as a result of heat inherited from Devonian rifting, further fuelled by syntectonic Carboniferous intrusions.

Contrasting behaviour of rare earth and major elements during partial melting in granulite facies migmatites, Wuluma Hills, Arunta Block, central Australia

AbstractLow‐P granulite facies metapelitic migmatites in the Wuluma Hills, Strangways Metamorphic Complex, Arunta Block, preserve evidence of polyphase deformation and migmatite formation which is of the same age of the c. 1730 Ma Wuluma granite. Mineral equilibria modelling of garnet‐orthoproxene‐cordierite‐bearing assemblages using thermocalc is consistent with peak S3 conditions of 6.0–6.5 kbar and 850–900 °C. The growth of orthopyroxene and garnet was primarily controlled by biotite breakdown during partial melting reactions. Whereas orthopyroxene in the cordierite‐biotite mesosome shows enrichment of heavy‐REE (HREE) relative to medium‐REE (MREE), orthopyroxene in adjacent garnet‐bearing leucosome shows depletion of HREE relative to MREE. There is no appreciable difference in major element contents of minerals common to both the mesosome and leucosome. The REE variations can be satisfactorily explained by decoupling of major element and REE partitioning, in the context of appropriate phase‐equilibria modelling of a prograde path at ∼6 kbar. Sparse garnet nucleii formed at ∼760 °C, along with concentrated leucosome development and preferentially partitioned HREE. Further heating to ∼800 °C at constant or subtly increasing pressure conditions additionally stabilized orthopyroxene and decreased the garnet mode. Orthopyroxene in the leucosome inherited an REE pattern consequent to the partial consumption of garnet, it being distinct from the REE pattern in mesosome orthoproxene that was mostly controlled by biotite breakdown. Such within‐sample variability in the enrichment of heavy REE indicates that caution needs to be exercised in the application of common elemental partitioning coefficients in spatially complex metamorphic rocks.

Dating of prograde metamorphic events deciphered from episodic zircon growth in rocks of the Dabie–Sulu UHP complex, China

The timing of ultra-high pressure (UHP) metamorphism has been difficult to determine because of a lack of age constraints on crucial events, especially those occurring on the prograde path. New Sensitive High-Resolution Ion Microprobe (SHRIMP) U–Pb age and rare-earth element (REE) data of zircon are presented for UHP metamorphic rocks (eclogite, garnet peridotite, garnet pyroxenite, jadeite quartzite and garnet gneiss) along the Dabie–Sulu UHP complex of China. With multiphase metamorphic textures and index mineral inclusions within zircon, the Dabie data define three episodes of eclogite-facies metamorphism, best estimated at 242.1 ± 0.4 Ma, 227.2 ± 0.8 Ma and 219.8 ± 0.8 Ma. Eclogite-facies zircons of the Sulu UHP complex grew during two major episodes at 242.7 ± 1.2 and 227.5 ± 1.3 Ma, which are indistinguishable from corresponding events in the Dabie UHP complex. A pre-eclogite metamorphic phase at 244.0 ± 2.6 Ma was obtained from two Sulu zircon samples which contain low pressure–temperature (plagioclase, stable below the quartz/Ab transformation) and hydrous (e.g., amphibole, stable below ∼ 2.5 Gpa) mineral inclusions. In terms of Fe–Mg exchange of trapped garnet–clinopyroxene pairs within zircon domains, we are able to determine the Pressure–Temperature ( PT) conditions for a specific episode of metamorphic zircon growth. We suggest that mineral phase transformations and associated dehydration led to episodic eclogite-facies zircon growth during UHP metamorphism (∼ 2.7 Gpa) began at 242.2 ± 0.4 Ma ( n = 74, pooling the Dabie–Sulu data), followed by peak UHP metamorphism (> ∼ 4 Gpa) at 227.3 ± 0.7 Ma ( n = 72), before exhumation (< ∼ 220 Ma) to quartz stability (~ 1.8 Gpa). The Dabie–Sulu UHP metamorphism lasted for about 15 Ma, equivalent to a minimum subduction rate of 6 mm/year for the descending continental crust.