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Based on the nature of ferric sillimanite, an activity model for sillimanite containing Fe3+ is constructed, tested and adopted to calculate phase equilibria of pelitic compositions under ultrahigh‐temperature (UHT) conditions. The calculated P–T projections and pseudosections suggest that the incorporation of Fe3+ into sillimanite can fairly solve the current imperfectly topological match between thermodynamic calculations and synthetic experiments, especially at high oxygen fugacity. Fe3+ in sillimanite remarkably elevates the temperature to switch the parageneses of orthopyroxene + sillimanite (Opx + Sil) and sapphirine + quartz (Spr + Qz) in oxidized metapelites, with an increment around 50–70°C. The calculated compatibility diagrams show that the widely approbatory UHT‐diagnostic mineral assemblages of Opx + Sil and Spr + Qz usually occur in metapelites with high Mg/Fe2+ ratios, which depend on both bulk‐rock MgO and oxygen fugacity, whereas the metapelites with low Mg/Fe2+ ratios are characterized by the assemblage of Garnet + sillimanite (Grt + Sil) with or without spinel (Spl) in UHT conditions. Moreover, comprehensive comparisons suggest that the essential petrogenetic framework of natural UHT metapelites is mostly governed by the two metamorphic reactions of Opx + Sil = Spr + Grt and Grt + Sil = Spl + Spr. [ABSTRACT FROM AUTHOR]

The metamorphic evolution of serpentinite has a fundamental bearing on geochemical cycles, mechanical behaviour and seismic properties of the Earth. Phase equilibria in the system MgO–Al2O3–SiO2–H2O (MASH) provide a 'backbone' for phase equilibria in larger systems that are applicable to serpentinites and also many whiteschists. Phase equilibria for MASH, and departures from MASH, are calculated using the Holland and Powell data set, tc‐ds62, and new activity–composition relations for antigorite and talc. Phase relations are presented using PT projections and compatibility diagrams to highlight the role of P, T and composition on the equilibrium mineral assemblages stable in these chemical systems at high pressure. These phase relations provide the context for pseudosection and other modelling focused on individual rock compositions. [ABSTRACT FROM AUTHOR]

Collisional orogens commonly include mono‐metamorphic and poly‐metamorphic units, and their different evolution can be difficult to recognize and reconcile. The Theodul Glacier Unit (TGU) in the Western Alps consists of an association of metasedimentary and metamafic rocks embedded within the Zermatt‐Saas tectonic unit. In spite of recent petrological studies, it remains unclear whether these rocks underwent one or multiple metamorphic cycles. In this study, different lithologies from the TGU unit (mafic schist, mafic granofels, and chloritoid schist) were investigated for petrography, quantitative compositional mapping of garnet, thermodynamic modelling, and Lu–Hf garnet dating. The data reveal a coherent mono‐metamorphic history with a β‐shaped pressure–temperature (P–T) path characteristic of oceanic subduction. Garnet Lu–Hf ages yield a restricted garnet crystallization time window between 50.3 and 48.8 Ma (±0.5%, 2SD). A prograde metamorphic stage recorded in garnet cores yields conditions of 490 ± 15℃ and 1.75 ± 0.05 GPa. Maximum pressure conditions of 2.65 ± 0.10 GPa and 580 ± 15℃ were reached at 50.3 ± 0.3 Ma. Initial exhumation was rapid and led to isothermal decompression to 1.50 ± 0.10 GPa within 1 Ma. This decompression was associated with lawsonite breakdown in mafic schist and in mafic granofels, causing intense fluid–rock interaction within and between different lithologies. This process is recorded in garnet textures and trace element patterns, and in the major element composition of K‐white mica. Initial exhumation was followed by re‐heating of ~30℃ at a pressure of 1.50 ± 0.10 GPa. Perturbation of the subduction‐zone thermal structure may be related to upwelling of hot asthenospheric mantle material and transient storage of the unit at the crust–mantle boundary. [ABSTRACT FROM AUTHOR]

Cryptic geochemical indicators of prograde lawsonite breakdown are established in basic and metasedimentary Eocene blueschist and eclogite in New Caledonia utilizing phase equilibria modelling and synchrotron X‐ray fluorescence mapping. Lawsonite–epidote‐bearing eclogite has garnet with concentric zones depleted in the LREE and MREE, and clinozoisite with high‐Sr rims, which geochemically record a prograde stage of lawsonite stability. These records persist in eclogite that now lacks lawsonite, and may be used to validate texturally based interpretations for prograde lawsonite stability. The compositional trends depend on extrinsic and intrinsic variables, including bulk rock composition, which determine the rock‐forming and accessory minerals that host Sr and the REEs. Distinct prograde reaction histories are recorded by two eclogite terranes in NE New Caledonia due to subtle distinctions in their protolith whole‐rock compositions. [ABSTRACT FROM AUTHOR]

Progressive vapour‐absent partial melting of a closed rock system increases melt pressure due to an expansion in the volume of the mineral plus melt assemblage. For a locally closed system, we quantify the melt pressure increase per increment of partial melting of a metapelite using phase equilibria modelling and combine it with Mohr–Coulomb theory to examine the interplay between melt pressure and fracture behaviour. It is shown that very small increments of vapour‐absent partial melting (<1%) increase melt pore pressure by tens of MPa leading to inevitable brittle failure of locally closed systems. Fracturing will affect these systems, even if initially limited to the scale of a few grains, and a connected microfracture network will enhance permeability as partial melting progresses. This will lead to a conditionally open system, potentially limiting accumulation of melt in the source. Repeated and cyclic fracture as temperature progressively increases will drive migration of the melt into sites of low fluid pressure at all scales. Crystal‐plastic creep processes create deformation‐induced dilatancy gradients that dominate over buoyancy forces at all scales in the melt source. Brittle and ductile deformation therefore cooperate in the extraction of melt. Enhanced porosity and permeability in ductile shear zones result in lower fluid pressure, providing a potentially important driving force for melt migration and drainage 'up' shear zones and along larger scale fluid pressure gradients in the crust. [ABSTRACT FROM AUTHOR]

This paper introduces the software solution Bingo‐Antidote for thermodynamic calculations at equilibrium based on iterative thermodynamic models. It describes a hybrid strategy combining the strength of Gibbs energy minimization (GEM) and inverse thermobarometry models based on the comparison between the modelled and observed mineral assemblage, modes and compositions. The overall technique relies on quantitative compositional maps acquired by electron probe micro‐analyser for obtaining a mutually consistent set of observed data such as bulk rock and mineral compositions. Thus it offers the opportunity to investigate metamorphic rocks on a microscale. The scoring part Bingo integrates three statistical model quality factors Qasm for the assemblage, Qvol for the mineral modes, Qcmp for the mineral compositions combined in a global evaluation criterion Qtotal that quantifies how the model reproduces the observations for the investigated volume. The input parameters of GEM affecting the model quality such as pressure, temperature and eventually some components of the bulk composition (e.g. the molar amount of hydrogen, carbon or oxygen) or activity variables of fluids and gases (e.g. aH2O, aCO2, f(O2)) can be optimized by inversion in Antidote using several mapping stages followed by a direct search optimization. Examples of iterative models based on compositional maps processed with Bingo‐Antidote demonstrate the utility of the program. In contrast to the qualitative interpretation of phase diagrams, the inversion maximizes the benefits of GEM and permits the derivation of statistically 'optimal' pressure–temperature conditions for well‐equilibrated samples. In addition, Bingo‐Antidote opens new avenues for petrological investigations such as the generation of chemical potential landscape maps. [ABSTRACT FROM AUTHOR]

Low‐pressure and high‐temperature (LP–HT) metamorphism of basaltic rocks, which occurs globally and throughout geological time, is rarely constrained by forward phase equilibrium modelling, yet such calculations provide valuable supplementary thermometric information and constraints on anatexis that are not possible to obtain from conventional thermometry. Metabasalts along the southern margin of the Sudbury Igneous Complex (SIC) record evidence of high‐grade contact metamorphism involving partial melting and melt segregation. Peak metamorphic temperatures reached at least ~925°C at ~1–3 kbar near the SIC contact. Preservation of the peak mineral assemblage indicates that most of the generated melt escaped from these rocks leaving a residuum characterized by a plagioclase–orthopyroxene–clinopyroxene–ilmenite‐magnetite±melt assemblage. Peak temperatures reached ~875°C up to 500 m from the SIC lower contact, which marks the transition to metabasalts that only experienced incipient partial melting without melt loss. Metabasalts ~500 to 750 m from the SIC contact are characterized by a similar two‐pyroxene mineral assemblage, but typically contain abundant hornblende that overgrew clino‐ and orthopyroxene along an isobaric cooling path. Metabasalts ~750 to 1,000 m from the SIC contact are characterized by a hornblende–plagioclase–quartz–ilmenite assemblage indicating temperatures up to ~680°C. Mass balance and phase equilibria calculations indicate that anatexis resulted in 10–20% melt generation in the inner ~500 m of the aureole, with even higher degrees of melting towards the contact. Comparison of multiple models, experiments, and natural samples indicates that modelling in the Na2O–CaO–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O2 (NCFMASHTO) system results in the most reliable predictions for the temperature of the solidus. Incorporation of K2O in the most recent amphibole solution model now successfully predicts dehydration melting by the coexistence of high‐Ca amphibole and silicate melt at relatively low pressures (~1.5 kbar). However, inclusion of K2O as a system component results in prediction of the solidus at too low a temperature. Although there are discrepancies between modelling predictions and experimental results, this study demonstrates that the pseudosection approach to mafic rocks is an invaluable tool to constrain metamorphic processes at LP–HT conditions. [ABSTRACT FROM AUTHOR]

The discovery of eclogites is reported within the Great Himalayan Crystalline Complex in the Thongmön area, central Himalaya, and their metamorphic evolution is deciphered by petrographic studies, pseudosection modelling, and zircon dating. For the first time, omphacite has been found in the matrix of eclogites taken from a metamorphic mafic lens. Two groups of garnet have been identified in the Thongmön eclogites on the basis of major and rare earth elements and mineral inclusions. Core and intermediate sections of garnet represent Grt I, in which the major elements (Ca, Mg, and Fe) show a nearly homogenous distribution with little or weak zonation. This Grt I displays an almost flat chondrite‐normalized HREE pattern, and the main inclusions are amphibole, apatite, quartz, and abundant omphacite. Grt II, forms thin rims on large garnet grains, and is characterized by rim‐ward Ca decrease and Mg increase and MREE enrichment relative to HREE and LREE. No amphibole inclusions are found in Grt II, indicating the decomposition of amphibole contributed to its MREE enrichment. Two metamorphic stages, recorded by matrix minerals and inclusions in garnet and zircon, outline the burial of the Thongmön eclogites and progressive metamorphic processes to the pressure peak: (a) the assemblage of amphibole–garnet–omphacite–phengite–rutile–quartz, with the phengite interpreted as having been replaced by Bt+Pl symplectites, represents the prograde amphibole eclogite facies stage M1(1), (b) in the peak eclogite facies [stage M1(2)], amphibole was lost and melting started. Based on the compositions of garnet and omphacite inclusions, M1(1) is constrained to 19–20 kbar and 640–660°C and M1(2) occurred at >21 kbar, >750°C, with appearance of melt and its entrapment in metamorphic zircon. SHRIMP U–Pb dating of zircon from two eclogite samples yielded consistent metamorphic ages of 16.7 ± 0.6 Ma and 17.1 ± 0.4 Ma respectively. The metamorphic zircon grew concurrently with Grt II in the peak eclogite facies. Thongmön eclogites characterized by the prograde metamorphism from amphibolite facies to eclogite facies were formed by the continuing continental subduction of Indian plate beneath the Euro‐Asian continent in the Miocene. [ABSTRACT FROM AUTHOR]

In the North‐East Greenland Caledonides, P–T conditions and textures are consistent with partial melting of ultrahigh‐pressure (UHP) eclogite during exhumation. The eclogite contains a peak assemblage of garnet, omphacite, kyanite, coesite, rutile, and clinozoisite; in addition, phengite is inferred to have been present at peak conditions. An isochemical phase equilibrium diagram, along with garnet isopleths, constrains peak P–T conditions to be subsolidus at 3.4 GPa and 940°C. Zr‐in‐rutile thermometry on inclusions in garnet yields values of ~820°C at 3.4 GPa. In the eclogite, plagioclase may exhibit cuspate textures against surrounding omphacite and has low dihedral angles in plagioclase–clinopyroxene–garnet aggregates, features that are consistent with former melt–solid–solid boundaries and crystallized melt pockets. Graphic intergrowths of plagioclase and amphibole are present in the matrix. Small euhedral neoblasts of garnet against plagioclase are interpreted as formed from a peritectic reaction during partial melting. Polymineralic inclusions of albite+K‐feldspar and clinopyroxene+quartz±kyanite±plagioclase in large anhedral garnet display plagioclase cusps pointing into the host, which are interpreted as crystallized melt pockets. These textures, along with the mineral composition, suggest partial melting of the eclogite by reactions involving phengite and, to a large extent, an epidote‐group mineral. Calculated and experimentally determined phase relations from the literature reveal that partial melting occurred on the exhumation path, at pressures below the coesite to quartz transition. A calculated P–T phase diagram for a former melt‐bearing domain shows that the formation of the peritectic garnet rim occurred at 1.4 GPa and 900°C, with an assemblage of clinopyroxene, amphibole, and plagioclase equilibrated at 1.3 GPa and 720°C. Isochemical phase equilibrium modelling of a symplectite of clinopyroxene, plagioclase, and amphibole after omphacite, combined with the mineral composition, yields a P–T range at 1.0–1. 6 GPa, 680–1,000°C. The assemblage of amphibole and plagioclase is estimated to reach equilibrium at 717–732°C, calculated by amphibole–plagioclase thermometry for the former melt‐bearing domain and symplectite respectively. The results of this study demonstrate that partial melt formed in the UHP eclogite through breakdown of an epidote‐group mineral with minor involvement of phengite during exhumation from peak pressure; melt was subsequently crystallized on the cooling path. [ABSTRACT FROM AUTHOR]

The Th/U ratios of zircon crystals are routinely used to help understand their growth mechanism. Despite the wide application of Th/U ratios in understanding the geological significance of zircon U--Pb ages, the main controls on the Th/U ratio in metamorphic zircon are poorly understood. Here, phase equilibria modelling coupled with solubility expressions for accessory minerals are used to investigate the controls on the Th/U ratios of suprasolidus metamorphic zircon in an average amphibolite facies metapelite composition. We also present a new database of metamorphic Th/U ratios in zircon from Western Australia. Several factors affecting the Th/U ratio are investigated, including the bulk rock concentrations of Th and U, the amount of monazite in the system, and open v. closed system behaviour. Our modelling predicts that the main controls on the Th/U ratio of suprasolidus metamorphic zircon are the concentrations of Th and U in the system, and the breakdown and growth of monazite in equilibrium with zircon. Furthermore, the relative timing of zircon and monazite growth during cooling and melt crystallization has an important role in the Th/U ratio of zircon. Early grown zircon near the peak of metamorphism is expected to have elevated Th/U ratios whereas zircon that grew near the solidus is predicted to have relatively low Th/U ratios, which reflects the coeval growth of monazite during cooling and melt crystallization. Our modelling approach aims to provide an improved understanding of the main controls of Th/U in metamorphic zircon in migmatites and hence better apply this geochemical ratio as a tool to assist in interpretation of the genesis of metamorphic zircon. [ABSTRACT FROM AUTHOR]

High-pressure ( HP) metagreywacke from the Namche Barwa Complex, Eastern Himalayan Syntaxis ( EHS), consists of garnet, biotite, plagioclase, quartz, rutile and ilmenite with or without K-feldspar, sillimanite, cordierite, spinel and orthopyroxene. Two types of metagreywacke are recognized: medium-temperature ( MT) and high-temperature ( HT) types. Garnet in the MT metagreywacke shows significant growth zoning and contains lower MgO than the weakly zoned garnet in the HT metagreywacke. Petrographic observations and phase equilibria modelling for four representative samples indicate that both types of metagreywacke experienced clockwise P-T paths subdivided into three stages: stage I is the pre-peak prograde to pressure peak ( Pmax) stage characterized by progressive increase in P-T conditions. The Pmax conditions are estimated using the garnet composition with maximum CaO, being 12.5-13.5 kbar and 685-725 °C for the MT metagreywacke, and 15-16 kbar and 825-835 °C for the HT one. Stage II is the post- Pmax decompression with heating or near-isothermal to Tmax stage and the Tmax conditions, constrained using the garnet compositions with maximum MgO, are 11 kbar and 760 °C for the MT metagreywacke, and ~12 kbar and 830-845 °C for the HT one. The modelled mineral assemblages at Tmax are garnet + biotite + K-feldspar + rutile + plagioclase ± ilmenite in the presence of melt for both types of metagreywacke, consistent with the petrographic observations. Stage III is the post- Tmax retrograde metamorphism, characterized by decompression and cooling. The modelling suggests that the melts with high Na/K ratios (1.7-5.2) have been produced during stages I and II, which could be responsible for the formation of sodium-rich leucogranites. This study and previous results indicate that the Higher Himalayan Crystallines in the EHS consist of MT- HP and HT- HP metamorphic units separated by a speculated tectonic contact. Petrological and structural discontinuities within the EHS cannot be easily interpreted with 'tectonic aneurysm' model. [ABSTRACT FROM AUTHOR]

Garnet amphibolites can provide valuable insights into geological processes of orogenic belts, but their metamorphic evolution is still poorly constrained. Garnet amphibolites from the Wutai-Hengshan area of the North China Craton mainly consist of garnet, hornblende, plagioclase, quartz, rutile and ilmenite, with or without titanite and epidote. Four samples selected in a south-north profile were studied by the pseudosection approach in order to elucidate the characteristics of their metamorphic evolution, and to better reveal the northwards prograde change in P-T conditions as established previously. For the sample from the lower Wutai Subgroup, garnet exhibits obvious two-substage growth zoning characteristic of pyrope ( Xpy) increasing but grossular ( Xgr) decreasing outwards in the core, and both Xpy and Xgr increasing outwards in the rim. Phase modelling using thermocalc suggests that the garnet cores were formed by chlorite breakdown over 7-9 kbar at 530-600 °C, and rims grew from hornblende and epidote breakdown over 9.5-11.5 kbar at 600-670 °C. The isopleths of the minimum An in plagioclase and maximum Xpy in garnet were used to constrain the peak P-T conditions of ~11.5 kbar/670 °C. The modelled peak assemblage garnet + hornblende + epidote+ plagioclase + rutile + quartz matches well the observed one. Plagioclase-hornblende coronae around garnet indicate post-peak decompression and fluid ingress. For the samples from the south Hengshan Complex, the garnet zoning weaken gradually, reflecting modifications during decompression of the rocks. Using the same approach, the rocks are inferred to have suprasolidus peak conditions, increasing northwards from 11.5 kbar/745 °C, 12.5 kbar/780 °C to 13 kbar/800 °C. Their modelled peak assemblages involve diopside, garnet, hornblende, plagioclase, rutile and quartz, yet diopside is not observed petrographically. The post-peak decompression is characterized by diopside + garnet + quartz + melt = hornblende + plagioclase, causing the diopside consumption and garnet compositions to be largely modified. Thus, the pesudosection approach is expected to provide better pressure results than conventional thermobarometry, because the later approach cannot be applied with confidence to rocks with multi-generation assemblages. U-Pb dating of zircon in the Wutai sample records a protolith age of c. 2.50 Ga, and a metamorphic age of c. 1.95 Ga, while zircon in the Hengshan samples records metamorphic ages of c. 1.92 Ga. The c. 1.95 Ga is interpreted to represent the pre-peak or peak metamorphic stages, and the ages of c. 1.92 Ga are assigned to represent the cooling stages. All rocks in the Wutai-Hengshan area share similar clockwise P-T morphologies. They may represent metamorphic products at different crustal depths in one orogenic event, which included a main thickening stage at c. 1.95 Ga followed by a prolonged uplift and cooling after 1.92 Ga. [ABSTRACT FROM AUTHOR]

Contact metamorphism of greenschist facies Neoproterozoic turbidites by the Cretaceous Bugaboo Batholith in southeastern British Columbia has resulted in a well-developed contact aureole. The aureole is about 1 km wide and can be divided into three main zones: (i) spotted phyllite zone, extending from the first appearance of spots of cordierite or andalusite to the last occurrence of primary chlorite; (ii) cordierite + andalusite + biotite zone, comprising hornfelses or schists with abundant porphyroblasts of cordierite and andalusite and, at higher grades, fibrolitic sillimanite; and (iii) K-feldspar zone, characterized by hornfelses and schists that, in the inner part of this zone, are variably migmatitic. Four parts of the aureole were examined, three of which are characterized by schists, and one of which (Cobalt Lake area) is characterized by hornfelses and has exceptional exposure and comparatively unaltered rocks. Petrographic, modal, mineral-compositional and whole rock-compositional data were collected from the Cobalt Lake transect, allowing the prograde reaction sequence to be inferred. Notable features of the aureole at Cobalt Lake include: initial development of andalusite and plagioclase at the expense of paragonite-rich white mica; a narrow interval across which cordierite, andalusite and biotite increase markedly at the expense of chlorite; gradual development of andalusite and biotite at the expense of cordierite and muscovite upgrade of chlorite consumption; and near-simultaneous development of andalusite + K-feldspar and sillimanite, the latter indicating a pressure of contact metamorphism of ~3 kbar. In other parts of the aureole, the development of sillimanite downgrade of the initial development of K-feldspar suggests slightly higher pressures of contact metamorphism. Lack of correspondence between the observed sequence of reactions in the aureole and those predicted thermodynamically suggests that modifications to some of the thermodynamic data or activity-composition models may be required. Textural features in the aureole suggest the influence of kinetic factors on metamorphic recrystallization, including: (i) deformation-catalysed reaction in the schists compared to the hornfelses, as indicated by different mineral-growth sequences inferred from microstructures, and (ii) heating rate-controlled recrystallization, as indicated by the decrease in grain size of hornfelses with increasing metamorphic grade. [ABSTRACT FROM AUTHOR]

Phase equilibria modelling, laser-ablation split-stream ( LASS)- ICP- MS petrochronology and garnet trace-element geochemistry are integrated to constrain the P-T-t history of the footwall of the Priest River metamorphic core complex, northern Idaho. Metapelitic, migmatitic gneisses of the Hauser Lake Gneiss contain the peak assemblage garnet + sillimanite + biotite ± muscovite + plagioclase + K-feldspar ± rutile ± ilmenite + quartz. Interpreted P-T paths predict maximum pressures and peak metamorphic temperatures of ~9.6-10.3 kbar and ~785-790 °C. Monazite and xenotime 208Pb/232Th dates from porphyroblast inclusions indicate that metamorphism occurred at c. 74-54 Ma. Dates from HREE-depleted monazite formed during prograde growth constrain peak metamorphism at c. 64 Ma near the centre of the complex, while dates from HREE-enriched monazite constrain the timing of garnet breakdown during near-isothermal decompression at c. 60-57 Ma. Near-isothermal decompression to ~5.0-4.4 kbar was followed by cooling and further decompression. The youngest, HREE-enriched monazite records leucosome crystallization at mid-crustal levels c. 54-44 Ma. The northernmost sample records regional metamorphism during the emplacement of the Selkirk igneous complex ( c. 94-81 Ma), Cretaceous-Tertiary metamorphism and limited Eocene exhumation. Similarities between the Priest River complex and other complexes of the northern North American Cordillera suggest shared regional metamorphic and exhumation histories; however, in contrast to complexes to the north, the Priest River contains less partial melt and no evidence for diapiric exhumation. Improved constraints on metamorphism, deformation, anatexis and exhumation provide greater insight into the initiation and evolution of metamorphic core complexes in the northern Cordillera, and in similar tectonic settings elsewhere. [ABSTRACT FROM AUTHOR]

In the Chinese southwestern Tianshan (U)HP belt, former lawsonite presence has been predicted for many (U)HP metamorphic eclogites, but only a very few lawsonite grains have been found so far. We discovered armoured lawsonite relicts included in quartz, which, on its part, is enclosed in porphyroblastic garnet in an epidote eclogite H711-14 and a paragonite eclogite H711-29. H711-14 is mainly composed of garnet, omphacite, epidote and titanite, with minor quartz, paragonite and secondary barroisite and glaucophane. Coarse-grained titanite occasionally occurs in millimetre-wide veins in equilibrium with epidote and omphacite, and relict rutile is only preserved as inclusions in matrix titanite and garnet. H711-29 shows the mineral assemblage of garnet, omphacite, glaucophane, paragonite, quartz, dolomite, rutile and minor epidote. Dolomite and rutile are commonly rimed by secondary calcite and titanite respectively. Porphyroblastic garnet in both eclogites is compositionally zoned and exhibits an inclusion-rich core overgrown by an inclusion-poor rim. Phase equilibria modelling predicts that garnet cores formed at the P-peak (490-505 °C and 23-25.5 kbar) and coexisted with the lawsonite eclogite facies assemblage of omphacite + glaucophane + lawsonite + quartz. Garnet rims (550-570 °C and ~20 kbar) grew subsequently during a post-peak epidote eclogite facies metamorphism and coexisted with omphacite + quartz ± glaucophane ± epidote ± paragonite. The results confirm the former presence of a cold subduction zone environment in the Chinese southwestern Tianshan. The P-T evolution of the eclogites is characterized by a clockwise P-T path with a heating stage during early exhumation (thermal relaxation). The preservation of lawsonite in these eclogites is attributed to isolation from the matrix by quartz and rigid garnet, which should be considered as a new type of lawsonite preservation in eclogites. The complete rutile-titanite transition in H711-14 took place in the epidote eclogite facies stage in the presence of an extremely CO2-poor fluid with X( CO2) [ CO2/( CO2 + H2O) in the fluid] <<0.008. In contrast, the incomplete rutile-titanite transition in H711-29 may have occurred after the epidote eclogite facies stage and the presence of dolomite reflects a higher X( CO2) (>0.01) in the coexisting fluid at the epidote eclogite facies stage. [ABSTRACT FROM AUTHOR]

We have investigated the effects of different Fe2O3 bulk contents on the calculated phase equilibria of low- T/intermediate- P metasedimentary rocks. Thermodynamic modelling within the MnO-Na2O-K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2-O (Mn NKFMASHTO) chemical system of chloritoid-bearing hematite-rich metasedimentary rocks from the Variscan basement of the Pisani Mountains (Northern Apennines, Italy) fails to reproduce the observed mineral compositions when the bulk Fe2O3 is determined through titration. The mismatch between observed and computed mineral compositions and assemblage is resolved by tuning the effective ferric iron content by P-XFe2O3 diagrams, obtaining equilibration conditions of 475 °C and 9-10 kbar related to a post-compressional phase of the Alpine collision. The introduction of ferric iron affects the stability of the main rock-forming silicates that often yield important thermobaric information. In Fe2O3-rich compositions, garnet- and carpholite-in curves shift towards higher temperatures with respect to the Fe2O3-free systems. The presence of a ferric-iron oxide (hematite) prevents the formation of biotite in the mineral assemblage even at temperatures approaching 550 °C. The use of P-T-XFe2O3 phase diagrams may also provide P-T information in common greenschist facies metasedimentary rocks. [ABSTRACT FROM AUTHOR]

The activity-composition ( a- x) relations of sapphirine are re-evaluated in the light of a recent new internally-consistent data set of phase end-members for use in phase equilibria modelling, particularly of ultra-high-temperature (UHT) rocks. This is achieved with the aid of relatively oxidized sapphirine+quartz-bearing granulites from Wilson Lake, Canada. Calculated P- T projections and compatibility diagrams in the K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2-Fe2O3 (KFMASHTO) system are used to illustrate sapphirine+quartz-bearing phase equilibria in the context of UHT metamorphism. These new a- x relations for sapphirine should allow pseudosection thermobarometry in NCKFMASHTO for estimating peak P- T conditions of sapphirine-bearing rocks. [ABSTRACT FROM AUTHOR]

The activity-composition ( a- x) relations of sapphirine are re-evaluated in the light of a recent new internally-consistent data set of phase end-members for use in phase equilibria modelling, particularly of ultra-high-temperature (UHT) rocks. This is achieved with the aid of relatively oxidized sapphirine+quartz-bearing granulites from Wilson Lake, Canada. Calculated P- T projections and compatibility diagrams in the K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2-Fe2O3 (KFMASHTO) system are used to illustrate sapphirine+quartz-bearing phase equilibria in the context of UHT metamorphism. These new a- x relations for sapphirine should allow pseudosection thermobarometry in NCKFMASHTO for estimating peak P- T conditions of sapphirine-bearing rocks. [ABSTRACT FROM AUTHOR]

Medium-temperature ultrahigh pressure ( MT- UHP) eclogites from the south Dabie orogen, as represented by samples from the Jinheqiao, Shuanghe and Bixiling areas, consist of garnet, omphacite, phengite, epidote, hornblendic amphibole, quartz/coesite and rutile with or without kyanite and talc. Garnet is mostly anhedral and unzoned, but a few porphyroblasts are weakly zoned with core-mantle increasing grossular ( Xgr) and decreasing pyrope ( Xpy) contents. Garnet compositions are closely correlated with the bulk compositions. For instance, the Xpy and Xgr contents are positively correlated with the bulk MgO and CaO contents. Phengite is occasionally zoned with core-rim deceasing Si content, and phengite grains as inclusions in garnet show higher Si than in the matrix, suggesting differently resetting during post-peak stages. The maximum Si contents are mostly 3.60-3.63 p.f.u. for the three areas. Pseudosections calculated using THERMOCALC suggest that the MT- UHP eclogites should have a peak assemblage of garnet + omphacite + lawsonite + phengite + coesite in most rocks of higher MgO content. In this assemblage, the Xpy in garnet mostly depends on bulk compositions, whereas the Xgr in garnet and the Si contents in phengite regularly increase, respectively, as temperature and as pressure rise, and thus, can provide robust thermobarometric constraints. Using the Xgr and Si isopleths in pseudosections, the peak P-T conditions were estimated to be 40 kbar/730 °C for the Jinheqiao, 41 kbar/726 °C for the Shuanghe, and 37-52 kbar and 700-830 °C for the Bixiling eclogites. Some eclogites with higher FeO are predicted to have a peak assemblage of garnet + omphacite + coesite ± phengite without lawsonite, where the garnet and phengite compositions highly depend on bulk compositions and generally cannot give available thermobarometric constraints. Decompression of the eclogites with lawsonite in the peak stage is inferred to be accompanied with cooling and involves two stages: an early-stage decompression is dominated by lawsonite dehydration, resulting in increase in the mode of anhydrous minerals, or further eclogitization, and formation of epidote porphyroblasts and kyanite-bearing quartz veins in eclogite. As lawsonite dehydration can facilitate evolution of assemblages under fluid-present conditions, it is difficult to recover real peak P-T conditions for UHP eclogites with lawsonite. This may be a reason why the P-T conditions estimated for eclogites using thermobarometers are mostly lower than those estimated for the coherent ultramafic rocks, and lower than those suggested from the inclusion assemblages in zircon from marble. A late-stage decompression is dominated by formation of hornblendic amphibole and plagioclase with fluid infiltration. The lawsonite-absent MT- UHP eclogites have only experienced a decompression metamorphism corresponding to the later stage and generally lack the epidote overprinting. [ABSTRACT FROM AUTHOR]

Eclogites from the Kebuerte Valley, Chinese South Tianshan, consist of garnet, omphacite, phengite, paragonite, glaucophane, hornblendic amphibole, epidote, quartz and accessory rutile, titanite, apatite and carbonate minerals with occasional presence of coesite or quartz pseudomorphs after coesite. The eclogites are grouped into two: type I contains porphyroblastic garnet, epidote, paragonite and glaucophane in a matrix dominated by omphacite where the proportion of omphacite and garnet is >50 vol.%; and type II contains porphyroblastic epidote in a matrix consisting mainly of fine-grained garnet, omphacite and glaucophane where the proportion of omphacite and garnet is <50 vol.%. Garnet in both types of eclogites mostly exhibits core-rim zoning with increasing grossular ( Xgr) and pyrope ( Xpy) contents, but a few porphyroblastic garnet grains in type I eclogite shows core-mantle zoning with increasing Xpy and a slight decrease in Xgr, and mantle-rim zoning with increases in both Xgr and Xpy. Garnet rims in type I eclogite have higher Xpy than in type II. Petrographic observations and phase equilibria modelling with pseudosections calculated using thermocalc in the NCKMnFMASHO system for three representative samples suggest that the eclogites have experienced four stages of metamorphism: stage I is the pre-peak temperature prograde heating to the pressure peak ( Pmax) which was recognized by the garnet core-mantle zoning with increasing Xpy and decreasing Xgr. The P- T conditions at Pmax constrained from garnet mantle or core compositions with minimum Xgr content are 29-30 kbar at 526-540 °C for type I and 28.2 kbar at 518 °C for type II, suggesting an apparent thermal gradient of ∼5.5 °C km−1. Stage II is the post- Pmax decompression and heating to the temperature peak ( Tmax), which was modelled from the garnet zoning with increasing Xgr and Xpy contents. The P- T conditions at Tmax, defined using the garnet rim compositions with maximum Xpy content and the Si content in phengite, are 24-27 kbar at 590 °C for type I and 22 kbar at 540 °C for type II. Stage III is the post- Tmax isothermal decompression characterized by the decomposition of lawsonite, which may have resulted in the release of a large amount of fluid bound in the rocks, leading to the formation of epidote, paragonite and glaucophane porphyroblasts. Stage IV is the late retrograde evolution characterized by the overprint of hornblendic amphibole in eclogite and the occurrence of epidote-amphibole facies mineral assemblages in the margins or in the strongly foliated domains of eclogite blocks due to fluid infiltration. The P- T estimates obtained from conventional garnet-clinopyroxene-phengite thermobarometry for the Tianshan eclogites are roughly consistent with the P-T conditions of stage II at Tmax, but with large uncertainties in temperature. On the basis of these metamorphic stages or P-T paths, we reinterpreted that the recently reported zircon U-Pb ages for eclogite may date the Tmax stage or the later decompression stage, and the widely distributed (rutile-bearing) quartz veins in the eclogite terrane may have originated from the lawsonite decomposition during the decompression stage rather than from the transition from blueschist to eclogite as previously proposed. [ABSTRACT FROM AUTHOR]

Kyanite- and phengite-bearing eclogites have better potential to constrain the peak metamorphic P-T conditions from phase equilibria between garnet + omphacite + kyanite + phengite + quartz/coesite than common, mostly bimineralic (garnet + omphacite) eclogites, as exemplified by this study. Textural relationships, conventional geothermobarometry and thermodynamic modelling have been used to constrain the metamorphic evolution of the Tromsdalstind eclogite from the Tromsø Nappe, one of the biggest exposures of eclogite in the Scandinavian Caledonides. The phase relationships demonstrate that the rock progressively dehydrated, resulting in breakdown of amphibole and zoisite at increasing pressure. The peak-pressure mineral assemblage was garnet + omphacite + kyanite + phengite + coesite, inferred from polycrystalline quartz included in radially fractured omphacite. This omphacite, with up to 37 mol.% of jadeite and 3% of the Ca-Eskola component, contains oriented rods of silica composition. Garnet shows higher grossular ( XGrs = 0.25-0.29), but lower pyrope-content ( XPrp = 0. 37-0.39) in the core than the rim, while phengite contains up to 3.5 Si pfu. The compositional isopleths for garnet core, phengite and omphacite constrain the P-T conditions to 3.2-3.5 GPa and 720-800 °C, in good agreement with the results obtained from conventional geothermobarometry (3.2-3.5 GPa & 730-780 °C). Peak-pressure assemblage is variably overprinted by symplectites of diopside + plagioclase after omphacite, biotite and plagioclase after phengite, and sapphirine + spinel + corundum + plagioclase after kyanite. Exhumation from ultrahigh-pressure (UHP) conditions to 1.3-1.5 GPa at 740-770 °C is constrained by the garnet rim ( XCaGrt = 0.18-0.21) and symplectite clinopyroxene ( XNaCpx = 0.13-0.21), and to 0.5-0.7 GPa at 700-800 °C by sapphirine ( XMg = 0.86-0.87) and spinel ( XMg = 0.60-0.62) compositional isopleths. UHP metamorphism in the Tromsø Nappe is more widespread than previously known. Available data suggest that UHP eclogites were uplifted to lower crustal levels rapidly, within a short time interval (452-449 Ma) prior to the Scandian collision between Laurentia and Baltica. The Tromsø Nappe as the highest tectonic unit of the North Norwegian Caledonides is considered to be of Laurentian origin and UHP metamorphism could have resulted from subduction along the Laurentian continental margin. An alternative is that the Tromsø Nappe belonged to a continental margin of Baltica, which had already been subducted before the terminal Scandian collision, and was emplaced as an out-of-sequence thrust during the Scandian lateral transport of nappes. [ABSTRACT FROM AUTHOR]

This Special Issue comprises a selection of the papers given at a two-day discussion meeting held at the University of Melbourne, Australia in June 2009 to celebrate Roger Powell’s 60th birthday. At this milestone, it is fitting to review Roger’s career to date. He has published ∼200 scientific papers on topics that range from low- to high-grade metamorphism, from low- to ultrahigh-pressure (UHP) metamorphism, and from thermodynamics to kinetics. Most of Roger’s papers are multi-authored and address important questions in the petrogenesis of metamorphic rocks. Roger is widely known for his work with Tim Holland to develop the most complete internally consistent dataset of thermodynamic properties of end members of phases necessary to undertake calculations on the conditions of formation and modification of metamorphic rocks. Additionally, Roger and Tim have developed activity–composition models for many of these phases, building on their important methodological developments in formulating such models. Roger is also responsible for the ongoing development ofthermocalc, a thermodynamic calculation software package that may be used to undertake a wide range of phase diagram calculations, including P– T projections, P– T, P– X and T– X, compatibility diagrams and μ–μ diagrams. Together, Roger and Tim have changed the way we carry out quantitative phase equilibria studies. However, Roger’s contributions to metamorphic petrology go well beyond the development of phase equilibria methods and mineral thermodynamics. He has contributed significantly to our understanding of a range of metamorphic processes, and with an extensive array of co-authors has shown how phase equilibria can be used to understand the evolution of metamorphic rocks in general terms as well as in specific terranes. The papers in this Special Issue cover the range from the stabilization of the continents to understanding the formation of orogenic gold deposits, from the stability of sapphirine–quartz-bearing assemblages to the crystallization of melt in migmatites, from the effects of ferric iron and sulphur on the stability of metamorphic mineral assemblages in general to the effects of ferric iron and H2O on the stability of eclogite in particular, and to the quantification of UHP metamorphism. It is our hope that in reading these contributions, you will be stimulated to seek a better understanding of metamorphic processes and to improve our quantification of the variables in metamorphism. [ABSTRACT FROM AUTHOR]

Recent crystallographic data indicate that in biotite Ti orders preferentially onto the M2 octahedral site rather than onto the M1 site as assumed in previous solution models for K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O2 (KFMASHTO) biotite. In view of these data, we reformulate and reparameterize former biotite solution models. Our reparameterization takes into account Fe–Mg order–disorder and ferric iron contents of natural biotite as well as both natural and experimental observations on biotite Ti-content over a wide range of physicochemical conditions. In comparison with previous biotite models, the new model reproduces the Ti-content and stability field of biotite as constrained by experiments with significantly better accuracy. The predictive power of the model is tested by comparison with petrologically well-characterized natural samples of SiO2-saturated and SiO2-undersaturated rocks that were not used in the parameterization. In all these tests, the reformulated model performs well. [ABSTRACT FROM AUTHOR]

Chloritoid–glaucophane-bearing rocks are widespread in the high-pressure belt of the north Qilian orogen, NW China. They are interbedded and cofacial with felsic schists originated from greywackes, mafic garnet blueschists and low- T eclogites. Two representative chloritoid–glaucophane-bearing assemblages are chloritoid + glaucophane + garnet + talc + quartz (sample Q5-49) and chloritoid + glaucophane + garnet + phengite + epidote + quartz (sample Q5-12). Garnet in sample Q5-49 is coarse-, medium- and fine-grained and shows two types of zonation patterns. In pattern I, Xgrs is constant as Xpy rises, and in pattern II Xgrs decreases as Xpy rises. Phase equilibrium modelling in the NC(K)MnFMASH system with Thermocalc 3.25 indicates that pattern I can be formed during progressive metamorphism in lawsonite-stable assemblages, while pattern II zonation can be formed with further heating after lawsonite has been consumed. Garnet growth in Q5-49 is consistent with a continuous progressive metamorphic process from ∼14.5 kbar at 470 °C to ∼22.5 kbar at 560 °C. Garnet in sample Q5-12 develops with pattern I zonation, which is consistent with a progressive metamorphic process from ∼21 kbar at 540 °C to ∼23.5 kbar at 580 °C with lawsonite present in the whole garnet growth. The latter sample shows the highest P– T conditions of the reported chloritoid–glaucophane-bearing assemblages. Phase equilibrium calculation in the NCKFMASH system with a recent mixing model of amphibole indicates that chloritoid + glaucophane paragenesis does not have a low-pressure limit of 18–19 kbar as previously suggested, but has a much larger pressure range from 7–8 to 27–30 kbar, with the low-pressure part being within the stability field of albite. [ABSTRACT FROM AUTHOR]

Petrogenetic grids in the KFMASH and KMnFMASH model systems calculated with the software thermocalc 3.1 are presented for the P–T range 0.5–12 kbar and 450–900 °C, for assemblages involving garnet, muscovite, chloritoid, biotite, chlorite, staurolite, cordierite, spinel, orthopyroxene, K-feldspar, Al2SiO5 phases, quartz, water and melt. Based on calculated compatibility diagrams and P–T and T–MMn [Mn/(Mg + Fe + Mn)] pseudosections for different metapelitic bulk compositions, the principal conclusions are that the addition of Mn to the KFMASH system: (i) enhances the stability of garnet, and, to a lesser extent, aluminosilicates; (ii) reduces the stability of staurolite, cordierite and, to a lesser extent, chlorite; and (iii) extends the medium pressure stability of muscovite and the low- P stability field of K-feldspar. The influence of Mn on individual mineral stabilities is strongly related to rock composition, in particular, to the relative contents of Al2O3 and K2O. For metapelites of a range of compositions and MMn values, P–T pseudosections in the KFMASH system, in most cases, do not adequately predict the mineral assemblages observed in natural assemblages under medium and low-pressure conditions. In contrast, the P–T pseudosections in the KMnFMASH system generally provide more satisfactory results, suggesting that MnO is one of the non-KFMASH components that should not be neglected in documenting the phase equilibria of medium- and low- P metapelites. [ABSTRACT FROM AUTHOR]

Abstract Eclogites from the south Tianshan, NW China are grouped into two types: glaucophane and hornblende eclogites, composed, respectively, of garnet + omphacite + glaucophane + paragonite + epidote + quartz and garnet + omphacite + hornblende (sensu lato ) + paragonite + epidote + quartz, plus accessory rutile and ilmenite. These eclogites are diverse both in mineral composition and texture not only between the two types but also among the different selected samples within the glaucophane eclogite. Using thermocalc 3.1 and recent models of activity–composition relation for minerals, a P –T projection and a series of P–T pseudosections for specific samples of eclogite have been calculated in the system Na2 O–CaO–FeO–MgO–Al2 O3 –SiO2 –H2 O (NCFMASH) with quartz and water taken to be in excess. On the basis of these phase diagrams, the phase relations and P–T conditions are well delineated. The three selected samples of glaucophane eclogite AK05, AK11 and AK17 are estimated to have peak P–T conditions, respectively, of 540–550 °C at c . 16 kbar, c . 560 °C at 15–17 kbar and c. 580 °C at 15–19 kbar, and two samples of hornblende eclogite AK10 and AK30 of 610–630 °C and 17–18 kbar. Together with H2 O-content contours in the related P–T pseudosections and textural relations, both types of eclogite are inferred to show clockwise P–T paths, with the hornblende eclogite being transformed from the glaucophane eclogite assemblage dominantly through increasing temperature. [ABSTRACT FROM AUTHOR]

Abstract The reaction muscovite+cordierite→biotite+Al2SiO5 +quartz+H2O is of considerable importance in the low pressure metamorphism of pelitic rocks: (1) its operation is implied in the widespread assemblage Ms + Crd +And± Sil + Bt + Qtz, a common mineral assemblage in contact aureoles and low pressure regional terranes; (2) it is potentially an important equilibrium for pressure estimation in low pressure assemblages lacking garnet; and (3) it has been used to distinguish between clockwise and anticlockwise P–T paths in low pressure metamorphic settings. Experiments and thermodynamic databases provide conflicting constraints on the slope and position of the reaction, with most thermodynamic databases predicting a positive slope for the reaction. Evidence from mineral assemblages and microtextures from a large number of natural prograde sequences, in particular contact aureoles, is most consistent with a negative slope (andalusite and/or sillimanite occurs upgrade of, and may show evidence for replacement of, cordierite). Mineral compositional trends as a function of grade are variable but taken as a whole are more consistent with a negative slope than a positive slope. Thermodynamic modelling of reaction 1 and associated equilibria results in a low pressure metapelitic petrogenetic grid in the system K2O–FeO–MgO–Al2O3–SiO2–H2O (KFMASH) which satisfies most of the natural and experimental constraints. Contouring of the Fe–Mg divariant interval represented by reaction 1 allows for pressure estimation in garnet-absent andalusite+cordierite-bearing schists and hornfelses. The revised topology of reaction 1 allows for improved analysis of P–T paths from mineral assemblage sequences and microtextures in the same rocks. [ABSTRACT FROM AUTHOR]