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Rocks of basic and intermediate bulk composition occur in orogenic terranes from all geological time periods and are thought to represent significant petrological components of the middle and lower continental crust. However, the former lack of appropriate thermodynamic models for silicate melt, amphibole and clinopyroxene that can be applied to such lithologies at high temperature has inhibited effective phase equilibrium modelling of their petrological evolution during amphibolite- and granulite facies metamorphism. In this work, we present phase diagrams calculated in the Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2-O2 (NCKFMASHTO) compositional system for a range of natural basic and intermediate bulk compositions for conditions of 2-12 kbar and 600-1050 ∘C using newly parameterized activity-composition relationships detailed in a companion paper by Green et al. in this issue. Particular attention is given to mid-ocean ridge basalt (MORB) and diorite protolith bulk compositions. Calculated subsolidus mineral assemblages in all basic and intermediate rock types are modally dominated by hornblende and plagioclase, with variable proportions of epidote, clinopyroxene, garnet, biotite, muscovite, quartz, titanite or ilmenite present at different pressures. The H2O-saturated (wet) solidus has a negative P− T slope and occurs between ∼620-690 ∘C at mid- to lower-crustal pressures of 5-10 kbar. The lowest- T melts generated close to the wet solidus are calculated to have granitic major-element oxide compositions. Melting at higher temperature is attributed primarily to multivariate hydrate-breakdown reactions involving biotite and/or hornblende. Partial melt compositions calculated at 800-1050 ∘C for MORB show good correlation with analysed compositions of experimental glasses produced via hydrate-breakdown melting of natural and synthetic basic protoliths, with Niggli norms indicating that they would crystallize to trondhjemite or tonalite. Diorite is shown to be significantly more fertile than MORB and is calculated to produce high- T melts (>800 ∘C) of granodioritic composition. Subsolidus and suprasolidus mineral assemblages show no significant variation between different members of the basalt family, although the P− T conditions at which orthopyroxene stabilizes, thus defining the prograde amphibolite-granulite transition, is strongly dependent on bulk-rock oxidation state and water content. The petrological effects of open- and closed-system processes on the mineral assemblages produced during prograde metamorphism and preserved during retrograde metamorphism are also examined via a case-study analysis of a natural Archean amphibolite from the Lewisian Complex, northwest Scotland. [ABSTRACT FROM AUTHOR]

Subduction of hydrated lithospheric mantle introduces H [ABSTRACT FROM AUTHOR]

During regional prograde metamorphism, H2O generated by ongoing dehydration reactions is likely to be continuously lost from a rock by compaction. Classical melting experiments cannot easily simulate this phenomenon, because ideally, all run products are conserved within the experimental charge, although significant equilibration and H2O generation may occur during heating. Phase equilibria modelling is used to consider the effect of subsolidus water loss ( SWL) on subsequent melting relationships of felsic lithologies (including metapelite, metagreywacke and metatonalite) in the suprasolidus. SWL drives the bulk composition towards the minimum saturation point on the boundary of the wet-melting field and results in significantly reduced subsequent melt generation when compared to melting experiments involving conservation of subsolidus H2O. This effect is most significant at P- T conditions just above the solidus. For initially hydrated rocks, the reduction in melt production causes rheologically critical thresholds (e.g. melt connectivity threshold, melt escape threshold and the solid-to-liquid transition) to be intersected at temperatures generally more than 100 °C, higher than predicted by idealized experimental melting curves. [ABSTRACT FROM AUTHOR]

The a- x relations recently presented in White et al. (, Journal of Metamorphic Geology, 32, 261-286) are extended to include MnO. This provides a set of internally consistent a- x relations for metapelitic rocks in the MnO-Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2-O2 (MnNCKFMASHTO) system. The mixing parameters for the Mn-bearing minerals were estimated using the micro- ϕ approach of Powell et al. (, Journal of Metamorphic Geology, 32, 245-260). Then the Mn-end-member thermodynamic properties were calibrated using a database of co-existing minerals involving literature data from rocks and from experiments on natural materials. Mn-end-members were calibrated for orthopyroxene, cordierite, staurolite, chloritoid, chlorite, biotite, ilmenite and hematite, assuming known properties for the garnet end-member spessartine. The addition of MnO to phase diagram calculations results in a marked expansion of the stability of garnet-bearing assemblages. At greenschist facies conditions garnet stability is extended down temperature. At amphibolite facies conditions, the garnet-in boundary shifts to lower pressure. While the addition of MnO greatly influences the stability of garnet, it has relatively little effect on the stability of other common metapelitic minerals, with the resultant diagrams being topologically very similar to those calculated without MnO. Furthermore, the addition of MnO in the amounts measured in most metapelites has only a small effect on the mode of garnet, with calculated garnet modes remaining smaller than 1% in the P- T range outside its predicted Mn-free P- T range. [ABSTRACT FROM AUTHOR]

New activity-composition ( a- x) relations for minerals commonly occurring in metapelites are presented for use with the internally consistent thermodynamic dataset of Holland & Powell (, Journal of Metamorphic Geology, 29, 333-383). The a- x relations include a broader consideration of Fe2O3 in minerals, changes to the formalism of several phases and order-disorder in all ferromagnesian minerals where Fe-Mg mixing occurs on multiple sites. The a- x relations for chlorite, biotite, garnet, chloritoid, staurolite, cordierite, orthopyroxene, muscovite, paragonite and margarite have been substantially reparameterized using the approach outlined in the companion paper in this issue. For the first time, the entire set of a- x relations for the common ferromagnesian minerals in metapelitic rocks is parameterized simultaneously, with attention paid to ensuring that they can be used together to calculate phase diagrams of geologically appropriate topology. The a- x relations developed are for use in the Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2-O2 (NCKFMASHTO) system for both subsolidus and suprasolidus conditions. Petrogenetic grids in KFMASH and KFMASHTO are similar in topology to those produced with earlier end-member datasets and a- x relations, but with some notable differences. In particular, in subsolidus equilibria, the FeO/(FeO + MgO) of garnet is now greater than in coexisting staurolite, bringing a number of key staurolite-bearing equilibria into better agreement with inferences from field and petrographic observations. Furthermore, the addition of Fe3+ and Ti to a number of silicate phases allows more plausible equilibria to be calculated in relevant systems. Pseudosections calculated with the new a- x relations are also topologically similar to equivalent diagrams using earlier a- x relations, although with many low variance fields shifting in P- T space to somewhat lower pressure conditions. [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]

Activity-composition ( a-x) models have been generated for zirconium-bearing haplogranitic silicate melt and garnet from experimental data on zircon dissolution and natural rock data, respectively. Additionally including the recently proposed a-x model for Zr-bearing rutile [Tomkins et al., Journal of Metamorphic Geology (2007) 401], calculated phase diagrams that explicitly include ZrO in the bulk composition predict the growth and dissolution of zircon at sub- and supra-solidus conditions. This occurs within the context of the evolution of major metamorphic minerals and mineral assemblages in pressure-temperature-composition space for a metapelitic rock composition. The stability of zircon is a function of the bulk ZrO content. Garnet contains insufficient Zr to affect the stability of zircon whereas rutile does contain sufficient Zr that zircon stability can be curtailed in rocks with significant rutile. Silicate melt contains appreciable Zr and zircon abundance varies inversely with melt abundance. Thermometers based on the Zr-content of rutile (and potentially garnet) can be graphically portrayed as compositional contours in mineral assemblage fields on the phase diagrams, thereby potentially adding to the utilization of such thermometers. The ability to calculate phase diagrams explicitly including Zr is a major step towards more systematically linking zircon growth - and zircon geochronology - and accessory phase thermometry in a readily adaptable way to the metamorphic evolution of major silicate minerals in a wide range of rocks. [ABSTRACT FROM AUTHOR]

Considering physical segregation of melt from its residue, the chemical potentials of the components (oxides) are the same in both when segregation occurs. Then, as P– T conditions change, gradients in chemical potential are established between the melt-rich domains and residue permitting diffusional interaction to occur. In particular, on cooling, the chemical potential of H2O becomes higher in the melt segregation than in the residue, particularly when biotite becomes stable in the residue assemblage. Diffusion of water from the melt to the residue promotes crystallization of anhydrous products from the melt and hydrous products in the residue. This diffusive process, when coupled with melt loss from the rocks subsequent to some degree of crystallization, can result in a significant degree of anhydrous leucosome being preserved in a migmatite with only minor retrogression of the residue. If H2O can diffuse between the melt segregation and all of the residue, then no apparent selvedge between the two will be observed. Alternatively, if H2O can diffuse between the melt segregation and only part of the residue, then a distinct selvedge may be produced. Diffusion of H2O into the residue may be in part responsible for the commonly anhydrous nature of leucosomes, especially in granulite facies migmatites. Diffusion of other relatively mobile species such as Na2O and K2O has a lesser effect on overall melt crystallization but can change the proportion of quartz, plagioclase and K-feldspar in the resultant leucosome. The diffusion of H2O out of the melt results in the enhanced crystallization of the melt in the segregation and increases the amount of resulting anhydrous leucosome relative to the amount produced if melt crystallized in chemical isolation from the residue. For high residue:melt ratios, the proportion of resulting near-anhydrous leucosome can approach that of the proportion of melt present at the onset of cooling with only minor loss of melt from a given segregation required. Crystallization of melt segregations via the diffusion of H2O out of them into the host may also play a major role in driving melt-rich segregations across key rheological transitions that would allow the expulsion of remaining melt from the system. [ABSTRACT FROM AUTHOR]

Kyanite-bearing paragneisses from the Manicouagan Imbricate Zone and its footwall (high- P belt of the central Grenville Province) preserve evidence of partial melting with development of metamorphic textures involving biotite–garnet ± kyanite ± plagioclase ± K-feldspar–quartz. Garnet in these rocks displays a variety of zoning patterns with respect to Ca. Pseudosection modelling in the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O (NCKFMASHTO) system using measured bulk rock compositions accounts for the textural evolution of two aluminous and two sub-aluminous samples from the presumed thermal peak to conditions at which retained melt solidified. The prograde features are best explained by pseudosections calculated with compositions to account for melt loss. The intersection of isopleths of grossular content and Fe/(Fe + Mg) relating to large porphyroblasts of garnet provide constraints on the P– T conditions of the metamorphic peak. These P– T estimates are considered to be minima because of the potential for diffusional modification of the composition of garnet at high- T and during the early stages of cooling. However, they are consistent with textural observations and pseudosection topology, with peak assemblages best preserved in rocks for which the calculated pseudosections predict only small changes in mineral proportions in the P– T interval, in which retrograde reactions are inferred to have occurred between the thermal peak and the solidus. Maximum P– T conditions (14.5–15.5 kbar and 840–890 °C) and steep retrograde P– T paths inferred for rocks from the Manicouagan Imbricate Zone are comparable with those determined for mafic rocks from the same area. In contrast, maximum P– T conditions of 12.5–13 kbar and 815–830 °C and flatter P– T paths are inferred for the rocks of the footwall to the Manicouagan Imbricate Zone. The general consistency between textures, mineral compositions and the topologies of the calculated pseudosections suggests that the pseudosection approach is an appropriate tool for inferring the P– T evolution of high- P anatectic quartzo-feldspathic rocks. [ABSTRACT FROM AUTHOR]

Greywacke occurs in most regionally metamorphosed orogenic terranes, with depositional ages from Archean to recent. It is commonly the dominant siliciclastic rock type, many times more abundant than pelite. Using calculated pseudosections in the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O system, the partial melting of metagreywacke is investigated using several natural protolith compositions that reflect the main observed compositional variations. At conditions appropriate for regional metamorphism at mid-crustal depths (6–8 kbar), high- T subsolidus assemblages are dominated by quartz, plagioclase and biotite with minor garnet, orthoamphibole, sillimanite, muscovite and/or K-feldspar (±Fe–Ti oxides). Modelled solidus temperatures are dependent on bulk composition and vary from 640 to 690 °C. Assuming minimal melting at the H2O-saturated solidus, initial prograde anatexis at temperatures up to ∼800 °C is characterized by very low melt productivity. Significant melt production in commonly occurring (intermediate) metagreywacke compositions is controlled by the breakdown of biotite and production of orthopyroxene (±K-feldspar) across multivariant fields until biotite is exhausted at 850–900 °C. Assuming some melt is retained in the source, then at temperatures beyond that of biotite stability, melt production occurs via the consumption of plagioclase, quartz and any remaining K-feldspar as the melt becomes progressively more Ca-rich and H2O-undersaturated. Melt productivity with increasing temperature across the melting interval in metagreywacke is generally gradational when compared to metapelite, which is characterized by more step-like melt production. Comparison of the calculated phase relations with experimental data shows good consistency once the latter are considered in terms of the variance of the equilibria involved. Calculations on the presumed protolith compositions of residual granulite facies metagreywacke from the Archean Ashuanipi subprovince (Quebec) show good agreement with observed phase relations. The degree of melt production and subsequent melt loss is consistent with the previously inferred petrogenesis based on geochemical mass balance. The results show that, for temperatures above 850 °C, metagreywacke is sufficiently fertile to produce large volumes of melt, the separation from source and ascent of which may result in large-scale crustal differentiation if metagreywacke is abundant. [ABSTRACT FROM AUTHOR]

Thermobarometry, the estimation of the pressure–temperature ( P– T) conditions of metamorphism, can be undertaken by using pseudosection calculations as well as by conventional methods. Conventional thermobarometry uses only the equilibrium thermodynamics of balanced reactions between end-members of minerals, combined with the observed compositions of minerals. In contrast, pseudosections involve a forward calculation of mineral equilibria for a given rock composition. When related to observed rock data such as mineral assemblages, mineral proportions and mineral compositions, pseudosections have the power to provide valuable additional thermobarometric information. This is because the rock composition provides added constraints on P– T, unavailable in conventional thermobarometry, such as when minerals in the mineral assemblage are no longer stable, or when additional minerals join the mineral assemblage. Considering both conventional and pseudosection thermobarometry, a minimum requirement is that they use the same thermodynamic data and activity–composition models for the minerals involved. A newthermocalc facility is introduced that allows pseudosection datafile coding to be used for conventional thermobarometry. Guidelines are given and pitfalls discussed relating to pseudosection modelling and conventional thermobarometry. We argue that, commonly, pseudosection modelling provides the most powerful thermobarometric tools. [ABSTRACT FROM AUTHOR]

Evolving pressure–temperature conditions during metamorphism drive changes in the stable mineral assemblage, mineral proportions and mineral compositions in rocks. These changes are achieved via the diffusion of components between minerals, fluid and melt, the driving force for diffusion being the gradients in chemical potential of the components developed spatially within the rock. This study utilises recent developments in the softwarethermocalc to investigate quantitatively chemical potential relationships in rocks, with the phases involved being (solid) solutions. Phase diagrams with chemical potentials as axes are used to understand better the spatial rearrangement of components during the metamorphic evolution of rocks and the metamorphic textures that result. In contrast to qualitative chemical potential diagrams, quantitative diagrams can be contoured for mineral composition, allowing consideration of chemical zoning in minerals. Furthermore, the amount of material required to diffuse to equalise chemical potentials can be calculated. We start by demonstrating the approach via an example of retrograde corona development in an ultra-high-temperature granulite. Whereas the use of chemical potentials to consider the retrograde development of corona textures is well known, they are also significant in considering the prograde history. The role of chemical potentials in prograde metamorphic textural evolution is highlighted in consideration of the consumption and growth of aluminosilicates during the kyanite-to-sillimanite reaction, and the growth of garnet porphyroblasts. [ABSTRACT FROM AUTHOR]

Improved activity–composition relationships for biotite, garnet and silicate liquid are used to construct updated P– T grids and pseudosections for high-grade metapelites. The biotite model involves Ti charge-balanced by hydrogen deprotonation on the hydroxyl site, following the substitution , where HD represents the hydroxyl site. Relative to equivalent biotite-breakdown melting reactions in P– T grids in K2O–FeO–MgO–Al2O3–SiO2–H2O (KFMASH), those in K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O2 (KFMASHTO) occur at temperatures close to 50 °C higher. A further consequence of the updated activity models is that spinel-bearing equilibria occur to higher temperature and higher pressure. In contrast, the addition of Na2O and CaO to KFMASH to make the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O (NCKFMASH) system lowers key biotite-breakdown melting reactions in P– T space relative to KFMASH. Combination of the KFMASHTO and NCKFMASH systems to make Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O2 (NCKFMASHTO) results in key biotite-breakdown melting reactions occurring at temperatures intermediate between those in KFMASHTO and those in NCKFMASH. Given such differences, the choice of model system will be critical to inferred P– T conditions in the application of mineral equilibria modelling to rocks. Further, pseudosections constructed in KFMASH, NCKFMASH and NCKFMASHTO for several representative rock compositions show substantial differences not only in the P– T conditions of key metamorphic assemblages but also overall topology, with the calculations in NCKFMASHTO more reliably reflecting equilibria in rocks. Application of mineral equilibria modelling to rocks should be undertaken in the most comprehensive system possible, if reliable quantitative P– T information is to be derived. [ABSTRACT FROM AUTHOR]

Lawsonite equilibria are predicted to occur over a broad P–T spectrum developed during subduction, yet lawsonite-bearing assemblages are rare. In the context of mafic mineral equilibria modelled for the range of common crustal metamorphism (4–23 kbar, 400–750 °C) using the system Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O and the softwarethermocalc, unusually high water contents are demanded by lawsonite assemblages. As a consequence, lawsonite assemblages are predicted to have difficulty forming and lawsonite equilibria to be uncommon. Metabasalt undergoing cooler subduction may experience substantial periods involving the metastable persistence of mineral assemblages because of water under-saturation with non-occurrence of recrystallization. If formed, lawsonite-bearing assemblages are observed to be highly unstable; their preservation requires that exhumation be accompanied by substantial cooling. The amount of structurally bound H2O in minerals plays a critical role in the formation and preservation of mineral assemblages, controlling key changes in rocks undergoing subduction. [ABSTRACT FROM AUTHOR]

The production of large volumes of fluid from metabasic rocks, particularly in greenstone terranes heated across the greenschist–amphibolite facies transition, is widely accepted yet poorly characterized. The presence of carbonate minerals in such rocks, commonly as a consequence of sea-floor alteration, has a strong influence, via fluid-rock buffering, on the mineral equilibria evolution and fluid composition. Mineral equilibria modelling of metabasic rocks in the system Na2O-CaO-FeO-MgO-Al2O3-SiO2-CO2-H2O (NCaFMASCH) is used to constrain the stability of common metabasic assemblages. Calculated buffering paths on T– XCO2 pseudosections, illustrate the evolution of greenstone terranes during heating across the greenschist-amphibolite transition. The calculated paths constrain the volume and the composition of fluid produced by devolatilization and buffering. The calculated amount and composition of fluid produced are shown to vary depending on P– T conditions, the proportion of carbonate minerals and the XCO2 of the rocks prior to prograde metamorphism. In rocks with an initially low proportion of carbonate minerals, the greenschist to amphibolite facies transition is the primary period of fluid production, producing fluid with a low XCO2. Rocks with greater initial proportions of carbonate minerals experience a second fluid production event at temperatures above the greenschist to amphibolite facies transition, producing a more CO2-rich fluid ( XCO2 = 0.2–0.3). Rocks may achieve these higher proportions of carbonate minerals either via more extensive seafloor alteration or via infiltration of fluids. Fluid produced via devolatilization of rocks at deeper crustal levels may infiltrate and react with overlying lower temperature rocks, resulting in external buffering of those rocks to higher XCO2 and proportions of carbonate minerals. Subsequent heating and devolatilization of these overlying rocks results in buffering paths that produce large proportions of fluid at XCO2 = 0.2–0.3. The production of fluid of this composition is of importance to models of gold transport in Archean greenstone gold deposits occurring within extensive fluid alteration haloes, as these haloes represent the influx of fluid of XCO2 = 0.2–0.3 into the upper crust. [ABSTRACT FROM AUTHOR]

A new quantitative approach to constraining mineral equilibria in sapphirine-bearing ultrahigh-temperature (UHT) granulites through the use of pseudosections and compatibility diagrams is presented, using a recently published thermodynamic model for sapphirine. The approach is illustrated with an example from an UHT locality in the Anápolis–Itauçu Complex, central Brazil, where modelling of mineral equilibria indicates peak metamorphic conditions of about 9 kbar and 1000 °C. The early formed, coarse-grained assemblage is garnet–orthopyroxene–sillimanite–quartz, which was subsequently modified following peak conditions. The retrograde pressure–temperature ( P– T) path of this locality involves decompression across the FeO–MgO–Al2O3–SiO2 (FMAS) univariant reaction orthopyroxene + sillimanite = garnet + sapphirine + quartz, resulting in the growth of sapphirine–quartz, followed by cooling and recrossing of this reaction. The resulting microstructures are modelled using compatibility diagrams, and pseudosections calculated for specific grain boundaries considered as chemical domains. The sequence of microstructures preserved in the rocks constrains a two-stage isothermal decompression–isobaric cooling path. The stability of cordierite along the retrograde path is examined using a domainal approach and pseudosections for orthopyroxene–quartz and garnet–quartz grain boundaries. This analysis indicates that the presence or absence of cordierite may be explained by local variation in aH2O. This study has important implications for thermobarometric studies of UHT granulites, mainly through showing that traditional FMAS petrogenetic grids based on experiments alone may overestimate P– T conditions. Such grids are effectively constant aH2O sections in FMAS-H2O (FMASH), for which the corresponding aH2O is commonly higher than that experienced by UHT granulites. A corollary of this dependence of mineral equilibria on aH2O is that local variations in aH2O may explain the formation of cordierite without significant changes in P– T conditions, particularly without marked decompression. [ABSTRACT FROM AUTHOR]

Silica-undersaturated, sapphirine-bearing granulites occur in a large number of localities worldwide. Such rocks have historically been under-utilized for estimatingP– Tevolution histories because of limited experimental work, and a consequent poor understanding of the topology andP– Tlocation of silica-undersaturated mineral equilibria. Here, a calculatedP– Tprojection for sapphirine-bearing, silica-undersaturated metapelitic rock compositions is constructed using THERMOCALC for the FeO-MgO-Al2O3-SiO2 (FMAS) and KFMASH (+K2O + H2O) chemical systems, allowing quantitative analysis of silica-undersaturated mineral assemblages. This study builds on that for KFMASH sapphirine + quartz equilibria [Kelseyet al. (2004)Journal of Metamorphic Geology, vol. 22, pp. 559–578]. FMAS equilibria are significantly displaced inP– Tspace from silicate melt-bearing KFMASH equilibria. The large number of univariant silica-undersaturated KFMASH equilibria result in aP– Tprojection that is topologically more complex than could be established on the basis of experiments and/or natural assemblages. Coexisting sapphirine and silicate melt (with or without corundum) occur down toc.900 °C in KFMASH, some 100 °C lower than in silica-saturated compositions, and from pressures ofc.≤1 to≥12 kbar. Mineral compositions and composition ranges for the calculated phases are consistent with natural examples. Bulk silica has a significant effect on the stability of sapphirine-bearing assemblages at a givenP– T, resulting in a wide variety of possible granulite facies assemblages in silica-undersaturated metapelites. Calculated pseudosections are able to reproduce many naturally occurring silica-undersaturated assemblages, either within a single assemblage field or as the product of aP– Ttrajectory crossing several fields. With an understanding of the importance of bulk composition on sapphirine stability and textural development, silica-undersaturated assemblages may be utilized in a quantitative manner in the detailed metamorphic investigation of high-grade terranes. [ABSTRACT FROM AUTHOR]

Sapphirine, coexisting with quartz, is an indicator mineral for ultrahigh-temperature metamorphism in aluminous rock compositions. Here a new activity-composition model for sapphirine is combined with the internally consistent thermodynamic dataset used by THERMOCALC, for calculations primarily in K2O-FeO-MgO-Al2O3-SiO2-H2O (KFMASH). A discrepancy between published experimentally derived FMAS grids and our calculations is understood with reference to H2O. Published FMAS grids effectively represent constant aH2O sections, thereby limiting their detailed use for the interpretation of mineral reaction textures in compositions with differing H2O. For the calculated KFMASH univariant reaction grid, sapphirine + quartz assemblages occur at P–T in excess of 6–7 kbar and 1005 °C. Sapphirine compositions and composition ranges are consistent with natural examples. However, as many univariant equilibria are typically not ‘seen’ by a specific bulk composition, the univariant reaction grid may reveal little about the detailed topology of multi-variant equilibria, and therefore is of limited use for interpreting the P–T evolution of mineral assemblages and reaction sequences. Calculated pseudosections, which quantify bulk composition and multi-variant equilibria, predict experimentally determined KFMASH mineral assemblages with consistent topology, and also indicate that sapphirine stabilizes at increasingly higher pressure and temperature as XMg increases. Although coexisting sapphirine and quartz can occur in relatively iron-rich rocks if the bulk chemistry is sufficiently aluminous, the P–T window of stability shrinks with decreasing XMg. An array of mineral assemblages and mineral reaction sequences from natural sapphirine + quartz and other rocks from Enderby Land, Antarctica, are reproducible with calculated pseudosections. That consistent phase diagram calculations involving sapphirine can be performed allows for a more thorough assessment of the metamorphic evolution of high-temperature granulite facies terranes than was previously possible. The establishment of a a-x model for sapphirine provides the basis for expansion to larger, more geologically realistic chemical systems (e.g. involving Fe3+). [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]