Separate or shared metamorphic histories of eclogites and surrounding rocks? An example from the Bohemian Massif

Eclogite boudins occur within an orthogneiss sheet enclosed in a Barrovian metapelite‐dominated volcano‐sedimentary sequence within the Velké Vrbno unit, NE Bohemian Massif. A metamorphic and lithological break defines the base of the eclogite‐bearing orthogneiss nappe, with a structurally lower sequence without eclogite exposed in a tectonic window. The typical assemblage of the structurally upper metapelites is garnet–staurolite–kyanite–biotite–plagioclase–muscovite–quartz–ilmenite ± rutile ± silli‐manite and prograde‐zoned garnet includes chloritoid–chlorite–paragonite–margarite, staurolite–chlorite–paragonite–margarite and kyanite–chlorite–rutile. In pseudosection modelling in the system Na₂O–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O (NCKFMASH) using THERMOCALC, the prograde path crosses the discontinuous reaction chloritoid + margarite = chlorite + garnet + staurolite + paragonite (with muscovite + quartz + H₂O) at 9.5 kbar and 570 °C and the metamorphic peak is reached at 11 kbar and 640 °C. Decompression through about 7 kbar is indicated by sillimanite and biotite growing at the expense of garnet. In the tectonic window, the structurally lower metapelites (garnet–staurolite–biotite–muscovite–quartz ± plagioclase ± sillimanite ± kyanite) and amphibolites (garnet–amphibole–plagioclase ± epidote) indicate a metamorphic peak of 10 kbar at 620 °C and 11 kbar and 610–660 °C, respectively, that is consistent with the other metapelites. The eclogites are composed of garnet, omphacite relicts (jadeite = 33%) within plagioclase–clinopyroxene symplectites, epidote and late amphibole–plagioclase domains. Garnet commonly includes rutile–quartz–epidote ± clinopyroxene (jadeite = 43%) ± magnetite ± amphibole and its growth zoning is compatible in the pseudosection with burial under H₂O‐undersaturated conditions to 18 kbar and 680 °C. Plagioclase + amphibole replaces garnet within foliated boudin margins and results in the assemblage epidote–amphibole–plagioclase indicating that decompression occurred under decreasing temperature into garnet‐free epidote–amphibolite facies conditions. The prograde path of eclogites and metapelites up to the metamorphic peak cannot be shared, being along different geothermal gradients, of about 11 and 17 °C km^?1, respectively, to metamorphic pressure peaks that are 6–7 kbar apart. The eclogite–orthogneiss sheet docked with metapelites at about 11 kbar and 650 °C, and from this depth the exhumation of the pile is shared.     

Regression Modelling of Metamorphic Reactions in Metapelites, Snow Peak, Northern Idaho

The second of two periods of regional metamorphism that affectedpelitic rocks near Snow Peak caused complete re-equilibrationof mineral assemblages and resulted in a consistent set of metamorphicisograds. Metamorphic chlorite and biotite occur in the lowestgrade rocks. With increasing grade, garnet, staurolite, andkyanite join the assemblage, resulting in a transition zonecontaining all the above phases. At higher grade, chlorite,and finally staurolite disappear. Mass balance relations at isograds and among minerals of low-varianceassemblages have been modelled by a non-linear least-squaresregression technique. The progressive sequence can be describedin terms of schematic T-X_H2O relations among chlorite, biotite,garnet, staurolite, and kyanite at P_total above the KFMASH invariantpoint involving those phases. The first appearance of garnetwas the result of an Fe-Mg-Mn continuous reaction. As temperaturerose, the garnet zone assemblage encountered the stauroliteisograd reaction, approximated by the model reaction: 3?0 chlorite + 1?5 garnet + 3?3 muscovite + 05 ilmenite = 1?0staurolite + 3?1 biotite + 1?5 plagioclase + 3?3 quartz + 10?3H₂O. The staurolite zone corresponds to buffering along this reactionto the intersection where chlorite, biotite, garnet, staurolite,and kyanite coexist. The transition zone assemblage formed byreaction at this T–X _H2O intersection which migrates towardmore H2O-rich fluid composition with progressive reaction. Thenet reaction at the intersection is approximated by the transitionzone reaction: 1?0 chlorite +1?1 muscovite + 0?2 ilmenite = 2?7 kyanite + 1?0biotite + 0?4 albite + 4?2 H₂O. Chlorite was commonly the first phase to have been exhaustedand the remaining assemblage was buffered along a staurolite-outreaction, represented by the model reaction: 1?0 staurolite + 3?4 quartz + 0?4 anorthite + 1?4 garnet + 0?1ilmenite + 7?9 kyanite + 2?0 H₂O. Consumption of staurolite by this reaction resulted in the highestgrade assemblage, which contains kyanite, garnet, biotite, muscovite,quartz, plagioclase, ilmenite, and graphite.     

A petrogenetic grid for pelitic schists in the system SiO2-Al2O3-FeO-MgO-K2O-H2O

A quantitative petrogenetic grid for pelitic schists in the system KFMASH that includes the phases garnet, chlorite, biotite, chloritoid, cordierite, staurolite, talc, kyanite, andalusite, sillimanite, and pyrophyllite (with quartz, H₂O and muscovite or K-feldspar in excess) is presented. The grid is based on thermodynamic data of Berman et al. (1985) and Berman (1988) for endmember KFASH and KMASH equilibria and natural Fe-Mg partitioning for the KFMASH system. Calculation of P-T slopes and the change in Fe/(Fe+Mg) along reactions in the KFMASH system were made using the Gibbs method. In addition, the effect on the grid of MnO and CaO is evaluated quantitatively. The resulting grid is consistent with typical Buchan and Barrovian parageneses at medium to high grades. At low grades, the grid predicts an extensive stability field for the paragenesis chloritoid+biotite which arises because of the unusual facing of the reaction chloritoid+biotite + quartz+H₂O = garnet+chlorite+muscovite, which proceeds to the right with increasing T in the KFMASH system. However, the reaction proceeds to the left with increasing T in the MnKFASH system so the assemblage chloritoid + biotite is restricted to bulk compositions with high Fe/(Fe+Mg+Mn). Typical metapelites will therefore contain garnet+chlorite at low grades rather than chloritoid + biotite.     

Regional metamorphism of pelitic rocks around Kandra,Singhbhum, Bihar

A suite of pelitic rocks around Kandra, Singhbhum District, Bihar, displays a metamorphic gradient registered by the index minerals chlorite, biotite, garnet, staurolite and sillimanite in a Barrovian sequence. Metamorphism was by and large coeval with folding movements, and correlating the internal fabric of minerals and deformational characters, a regular sequence of the index minerals is derived. It is argued that the chronological order by itself is not sufficient to prove that metamorphism was progressive in time.Among the index minerals, garnet appears to have formed by the reaction chlorite+biotite_a+quartz garnet+biotite_b+H₂O. For the origin of sillimanite, a new reaction, 3 staurolite+muscovite+quartz=7 sillimanite+biotite+3H₂O, is suggested on the basis of significant textural features. Textural and petrological indications regarding the formation of staurolite are in discordance. Staurolite was either derived from the biotite zone phases, or should be taken to have formed, against textural evidences, from chloritoids of the garnet zone.Graphical analysis of the assemblages by Thompson's AFM projection reveals that chlorite and staurolite are excess phases owing to retrogression and incomplete reaction. Shifting of apices of triangular fields and intersection of garnet-biotite tie lines within a zone can be satisfactorily explained in terms of extra components CaO and MnO or their ratios. It is pointed out that if MgO/(MgO + FeO) between two phases show a linear relation, their tie lines will be concurrent on the AF side of the projection, the point of concurrence reflecting equilibrium and temperature of recrystallisation.     

Chloritoid, and the Isochemical Character of Barrow's Zones

It is argued that despite poverty of outcrop the apparent restrictionof chloritoid to a wedge-shaped area at the north-eastern extremityof Barrow's zones is real. Two possible interpretations of thisrestriction are considered: (a) That the chloritoid producingreaction (as yet unidentified) was characterized by a lowerP/T than that of the reaction muscovite+ chlorite+chloritoid+quartz staurolite+biotite+H²O, whereby, with increasing grade, chloritoidgives way to staurolite. A pressure gradient increasing fromnorth-east to south-west (postulated on separate grounds, Chinner,1966) would then result in the convergence of the chloritoidand staurolite isograds towards the south-west, and the eventualsuppression of the chloritoid isograd to give the wedge-shapedoutcrop actually found, (b) The lack of low-grade hydrous assemblagesaluminous enough to give chloritoid or staurolite with increasinggrade suggests that the low-grade limit of chloritoid (and,to the south-west, of staurolite) may not be an isograd, buta chemical boundary. Such a boundary could either be metasedimentary,or metasomatic, representing an alkali gradient of the typestudied by Orville, in which, essentially, potassium and waterreleased within the high-grade metamorphic zones have migratedto low-grade zones to form more micaceous assemblages. The widespreadexistence of ‘shimmer aggregate‘ muscovite alterationof aluminous minerals in thesillimanite, kyanite, and staurolitezones provides evidence of potassium transfer during the waneof metamorphic temperatures on a scale comparable to that which,during the main metamorphic imprint, would have been requiredto mask the development of peraluminous assemblages in the chlorite,biotite, and garnet zones.     

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

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

Reversal of Fe-Mg Partitioning Between Garnet and Staurolite in Eclogite-facies Metapelites from the Champtoceaux Nappe (Brittany, France)

This paper concentrates on the petrology of eclogite-faciesmetapelites and, particularly, the significance of staurolitein these rocks. A natural example of staurolite-bearing eclogitic micaschistsfrom the Champtoceaux nappe (Brittany, France) is first described.The Champtoceaux metapelites present, in addition to quartz,phengite, and rutile, two successive parageneses: (1) chloritoid+staurolite+garnetcores, and (2) garnet rims+kyanite?chloritoid. Detailed microprobe analyses show that garnet and chloritoidevolve towards more magnesian compositions and that stauroliteis more Fe-rich than coexisting garnet. A comparison of thestudied rocks with other known occurrences of eclogitic metapelitesshows that whereas staurolite is always more Fe-rich than garnetin high-pressure eclogites, the reverse is true in low- to medium-pressuremicaschists. Phase relations between garnet, staurolite, chloritoid, biotite,and chlorite are analysed in the KFMASH system (with excessquartz, phengite, rutile, and H₂O). The topology of univariantreactions is depicted for a normal and a reverse Fe-Mg partitioningbetween garnet and staurolite. Mineral compositional changesare also predicted for varying bulk-rock chemistries. In the studied micaschists, the zonal arrangement of garnetinclusions and the progressive compositional changes of ferromagnesianphases record part of the prograde P–T path, before theattainment of ‘peak’ metamorphic conditions (atabout 65O–7OO?C, 18–20 kb). The retrograde path,which records the uplift of the Champtoceaux nappe, occurs underdecreasing temperatures.     

Pseudomorphs and associated microstructures of western Maine,USA

In metapelitic rocks of western Maine, a pluton-related M3 metamorphic gradient ranging in grade from garnet to upper sillimanite zone was superposed on a fairly uniform M2 regional metamorphic terrain characterized by the assemblage andalusite+staurolite+biotite+/−garnet. As a result, M2 assemblages re-equilibrated to the P, T, and aH₂O conditions of M3, and both prograde and retrograde pseudomorphism of M2 porphyroblasts occurred. The type of pseudomorph and degree of development is directly related to the rock's position within the M3 metamorphic gradient, a function of its proximity to the Mooselookmeguntic pluton. Several ‘hinge’ zones occur in which the M3 minerals that pseudomorphed a particular M2 phase change. For example, M2 garnet was replaced by M3 chlorite or biotite, depending on its position within the M3 gradient. Similarly, in a transition zone between M3 upper staurolite and lower sillimanite zones, M2 staurolite was stable and shows M3 growth rims. Downgrade from this transition zone, staurolite was pseudomorphed by chlorite and muscovite, whereas upgrade, the pseudomorphs contain muscovite and some biotite. M3 pseudomorphs commonly retain crystal shapes of the original M2 porphyroblasts, reflecting relatively low regional deviatoric stress during and after M3. Although evidence for textural disequilibrium is common, chemical equilibrium was closely approached during M3. This study demonstrates for M3 that: (1) the pseudomorphic replacement was a constant volume process, and (2) fabrics produced by tectonic events can be erased by subsequent deformation and/or sufficiently intense subsequent recrystallization.     

Staurolite-quartzite bands in kyanite quartzite at big rock,Rio Arriba County,New Mexico

Concordant igneous-looking bands of ferruginous bulk composition occur in a highly aluminous Precambrian metasedimentary series composed predominantly of kyanite quartzite. The bands consist of quartz, staurolite, and magnetite (partially martitized) with accessory amounts of muscovite, chlorite (pseudomorphous after biotite), chloritoid, apatite, and monazite. Quartz is found in three types (I–III) differing in appearance as well as in origin. Staurolite, in combination with quartz-II, shows peculiar radial sieve textures caused by mimetic crystallization after preexisting chloritoid rosettes. The chloritoid has been largely consumed, either by a reaction with hypothetical former kyanite to produce staurolite+ quartz with rock composition unchanged, or, possibly, by metasomatic introduction of oxygen (oxidation) to yield staurolite+quartz+magnetite; the remaining chloritoid, however, persisted in stable equilibrium with the other minerals of the rock. The staurolite quartzites are thus considered to represent original sedimentary bands which have undergone several stages of recrystallisation and (possibly) metasomatic modification during their metamorphic history. Their igneous aspect results from annealing crystallisation during a late static, i.e. postdeformational, thermal event of regional metamorphism.Chemical analysis of the staurolite shows no unusual features. For all staurolites plotted there is a positive relationship of the excess H⁺ over 2.0 and the Si⁺⁴-deficiency in the unit cell. This suggests partial substitution of 4 H⁺ for Si⁺⁴.The formation of staurolite in regional metamorphic rocks with excess silica, low alkali contents, and (FeO+MgO)/Al₂O₃ ratios < 1 showing chloritoid at lower grades appears to be governed, in many cases, by the reaction chloritoid+Al-silicate=staurolite+quartz+H₂O.The assemblage chloritoid-staurolite may be stable in regional metamorphism over a limited pressure-temperature range.     

The origin of chloritoid – 3‐mica pseudomorph growth in staurolite–muscovite schist,Bangriposi (Eastern India)

At Bangriposi, variable stages in replacement of staurolite by chloritoid – Na–K–Ca mica shimmer aggregates in muscovite schists provides insight into the complex interplay between fluid flow, mass transfer, and dissolution–precipitation during pseudomorph growth. Idioblastic chloritoid growing into mica caps without causing visible deformation, and monomineralic chloritoid veins (up to 300 μm wide) within shimmer aggregates replacing staurolite attest to chloritoid nucleation in fluid‐filled conduits along staurolite grain boundaries and crystallographic planes. The growth of shimmer aggregates initiated along staurolite margins, and advanced inwards into decomposing staurolite along networks of crystallographically controlled fluid‐filled conduits. Coalescence among alteration zones adjacent to channel fills led to dismemberment and the eventual demise of staurolite. Mass balance calculation within a volume‐fixed, silica‐conserved reference frame indicate the shimmer aggregates grew via precipitation from fluids in response to mass transport that led to the addition of H₂O, K₂O, Na₂O and CaO in the reaction zone, and Al₂O₃ was transported outward from the inward‐retreating margin of decomposing staurolite. This aided precipitation of chloritoid in veins and in the outer collars, and as disseminated grains in the shimmer aggregates at mid‐crustal condition (~520 ± 20 °C, 5.5 ± 2.0 kbar). Computation using one‐dimensional transport equation suggests that staurolite decomposition involved advection dominating over diffusive transport; the permeation of externally derived H₂O caused flattening of chemical potential gradients in H₂O and aqueous species, for example, and , computed using the Gibbs method. This suggests that staurolite decomposition was promoted by the infiltration of a large volume of H₂O that flattened existing chemical potential gradients. In the initial stages of replacement, chloritoid super‐saturation in fluid caused preferential nucleation and growth of chloritoid at staurolite grain boundaries and in crystallographic planes. As reaction progressed, further chloritoid nucleation was halted, but chloritoid continued to grow as the 3‐mica aggregates continued to replace the remaining staurolite in situ, while the chloritoid‐compatible elements were transported in the water‐rich phase facilitating continued growth of the existing chloritoid grains.     

The effect of whole-rock MnO content on the stability of garnet in pelitic schists during metamorphism

Garnet-bearing mineral assemblages are commonly observed in pelitic schists regionally metamorphosed to upper greenschist and amphibolite facies conditions. Modelling of thermodynamic data for minerals in the system Na₂O–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O, however, predicts that garnet should be observed only in rocks of a narrow range of very high Fe/Mg bulk compositions. Traditionally, the nearly ubiquitous presence of garnet in medium- to high-grade pelitic schists is attributed qualitatively to the stabilizing effect of MnO, based on the observed strong partitioning of MnO into garnet relative to other minerals. In order to quantify the dependence of garnet stability on whole-rock MnO content, we have calculated mineral stabilities for pelitic rocks in the system MnO–Na₂O–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O for a moderate range of MnO contents from a set of non-linear equations that specify mass balance and chemical equilibrium among minerals and fluid. The model pelitic system includes quartz, muscovite. albite, pyrophyllite, chlorite, chloritoid, biotite, garnet, staurolite, cordierite, andalusite, kyanite. sillimanite, K-feldspar and H₂O fluid. In the MnO-free system, garnet is restricted to high Fe/Mg bulk compositions, and commonly observed mineral assemblages such as garnet–chlorite and garnet–kyanite are not predicted at any pressure and temperature. In bulk compositions with X_Mn= Mn/(Fe + Mg + Mn) > 0.01, however, the predicted garnet-bearing mineral assemblages are the same as the sequence of prograde mineral assemblages typically observed in regional metamorphic terranes. Temperatures predicted for the first appearance of garnet in model pelitic schist are also strongly dependent on whole-rock MnO content. The small MnO contents of normal pelitic schists (X_Mn= 0.01–0.04) are both sufficient and necessary to account for the observed stability of garnet.     

Modeling of prograde mineral δ18O changes in metamorphic systems

A general model has been developed to calculate changes of ¹⁸O of minerals in addition to their composition and modal abundance in metamorphic systems. A complete set of differential equations can be written to describe any chemical system in terms of the variables dP, dT, dX, dM, and d¹⁸O (X, M, and ¹⁸O refer to the chemical composition, number of moles, and oxygen isotope composition of each phase respectively). This set is composed of the differentials of five subsets of equations: (1) conditions of heterogeneous equilibrium; (2) compositional stoichiometry for each mineral; (3) mass balance for each oxide component; (4) oxygen isotope partitioning between phases; (5) conservation of the oxygen isotope ratio of the system. The variance of the complete set of equations is 2, and changes of ¹⁸O, composition, and modal abundance for each mineral can be calculated for arbitrary changes of P and T. Applications to a typical pelitic bulk composition at amphibolite and lower granulite facies conditions suggest that for systems dominated by continuous reactions such as: (a) chlorite + quartz = garnet+H₂O; (b) staurolite + biotite = garnet + muscovite + H₂O; or (c) garnet + muscovite = sillimanite + biotite, isopleths of mineral ¹⁸O are nearly independent of pressure, and have a spacing of about 0.1 per 10–20°C. For nearly discontinuous reactions such as: (d) garnet + chlorite + muscovite = biotite + staurolite+H₂O; (e) staurolite + muscovite = biotite + aluminosilicate + garnet+H₂O; or (f) muscovite + quartz = sillimanite + K-feldspar+H₂O, isopleths of mineral ¹⁸O have slopes more nearly parallel to endmember reaction boundaries and ¹⁸O of phases can have a greater temperature dependence (e.g., 0.1 per 2°C for reaction d). This behavior results from relatively large amounts of reaction progress for small changes of P or T. However, the calculated exhaustion of a reactant within 0.1–5°C ensures that the predicted effects of such reactions on mineral ¹⁸O will not exceed 0.25 for typical bulk compositions. Models that allow for fractional crystallization of garnet suggest that prograde garnet zoning in pelitic assemblages will be relatively smooth until staurolite becomes unstable. At higher temperatures, garnet may develop a step of as much as 0.6 in its core-rim zoning as a result of combined garnet resorption during the continuous reaction garnet + muscovite = sillimanite + biotite and repartitioning of the garnet rim composition to relatively heavy ¹⁸O. The models are insensitive to the degree to which garnet fractionally crystallizes and to the isotope fractionation factors used; only extreme changes in modal abundance or bulk composition for a given mineral assemblage can produce significant changes in the predicted trends. In the absence of infiltration, isotopic shifts resulting from net transfer reactions for minerals in typical amphibolite, eclogite, and lower granulite facies metapelites and metabasites are inferred from the models to be 1 or less for 150°C of heating.     

Deformation-induced cordierite breakdown–An example from western Maine,USA

Preservation of partially completed metamorphic reactions in the form of partial pseudomorphs is very important as it provides direct insight onto the reaction mechanism and the phases involved in the reaction. The staurolite and andalusite grade rocks in western Maine, USA, contain cordierite porphyroblasts partly pseudomorphed by coarse-grained muscovite and biotite. The pseudomorphs consist of a cordierite core surrounded by a reaction rim. Modal mineralogy, calculated using the ImageJ processing software based on backscatter images and X-ray compositional maps, reveals that the core consists of cordierite (53.5%), muscovite (22.8%), biotite (9.1%), quartz (1 0.4%), plagioclase (3.1%) and ilmenite/pyrrhotite and apatite (1.1%) whereas the reaction rim consists of cordierite (1.8%), muscovite (51.6%), biotite (30.4%), quartz (4.3%), plagioclase (10%), garnet (1.2%), ilmenite/pyrrhotite and apatite (0.8%). The net effect of the cordierite breakdown reaction is an increase of 226% in muscovite, 334% in biotite and 323% in plagioclase content and a decrease of 97% in cordierite. The reaction involved exchange of components with the matrix requiring addition of H₂O, K⁺, Na⁺ and Ti⁴⁺ and removal of SiO₂, Mg²⁺ and PO₄^3- from the reaction site. PT estimates using the garnet–biotite, Ti-in-biotite, Na-in-cordierite thermometers and the garnet–biotite–muscovite–plagioclase barometer indicate that cordierite breakdown occurred at ~550°C and 3.5 kbar. thermocalc modelling using the bulk rock composition suggests that cordierite is not stable at these conditions, whereas modelling using a thin section-derived bulk composition indicates that cordierite stability extends to higher pressures, and most likely that the cordierite breakdown was not PT dependent. The incorporation of Na (up to 0.18 a.f.u.) into the cordierite structure has the effect of stabilizing the cordierite under a variety of H₂O activity and limiting the role of fluids into destabilizing it. The cordierite cores contain evidence of plastic and brittle deformation in the form of subgrains and microcracks, which facilitated the infiltration of fluids that destabilized cordierite at constant PT conditions by leaching Na and introducing K. New mica growth along these structural heterogeneities suggests that deformation played an important role promoting breakdown of cordierite to muscovite and biotite.     

New mineral activity–composition relations for thermodynamic calculations in metapelitic systems

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 ( 2011 , 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.     

Garnet and staurolite producing reactions in a chlorite-chloritoid schist

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

The nature and significance of the mechanism of sillimanite growth in the Connemara schists,Ireland

During prograde metamorphism of the Connemara pelites, sillimanite first develops in biotite immediately adjacent to, or replacing, garnet. In some rocks, breakdown of garnet+muscovite and staurolite +muscovite+quartz leads to the development of fibrolite pseudomorphs after garnet. The textures indicate a constant volume replacement of garnet with movement of Al from staurolite and muscovite in the matrix towards the few, widely scattered, garnet sites. The complex ionic reaction patterns are the result of the strong preference of sillimanite to grow on biotite that is replacing garnet, and this pattern of preferred nucleations is taken to indicate that the equilibrium conditions for the reaction were only overstepped by the minimum required for initial sillimanite nucleation. Chemical movements were controlled by the heterogeneous nucleation pattern, not by intrinsic properties of the moving species. In order for extensive reaction to occur under near-equilibrium conditions, the rate at which the total thermal reaction proceeded must have been controlled by the supply of heat to the rocks rather than by diffusion or local reaction steps.     

Calculated phase equilibria for low- and medium-pressure metapelites in the KFMASH and KMnFMASH systems

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–M_Mn [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 M_Mn 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.     

Fluid flow and Al transport during quartz-kyanite vein formation, Unst, Shetland Islands, Scotland

Quartz‐kyanite veins, adjacent alteration selvages and surrounding ‘precursor’ wall rocks in the Dalradian Saxa Vord Pelite of Unst in the Shetland Islands (Scotland) were investigated to constrain the geochemical alteration and mobility of Al associated with channelized metamorphic fluid infiltration during the Caledonian Orogeny. Thirty‐eight samples of veins, selvages and precursors were collected, examined using the petrographic microscope and electron microprobe, and geochemically analysed. With increasing grade, typical precursor mineral assemblages include, but are not limited to, chlorite+chloritoid, chlorite+chloritoid+kyanite, chlorite+chloritoid+staurolite and garnet+staurolite+kyanite+chloritoid. These assemblages coexist with quartz, white mica (muscovite, paragonite, margarite), and Fe‐Ti oxides. The mineral assemblage of the selvages does not change noticeably with metamorphic grade, and consists of chloritoid, kyanite, chlorite, quartz, white mica and Fe‐Ti oxides. Pseudosections for selvage and precursor bulk compositions indicate that the observed mineral assemblages were stable at regional metamorphic conditions of 550–600 °C and 0.8–1.1 GPa. A mass balance analysis was performed to assess the nature and magnitude of geochemical alteration that produced the selvages adjacent to the veins. On average, selvages lost about −26% mass relative to precursors. Mass losses of Na, K, Ca, Rb, Sr, Cs, Ba and volatiles were −30 to −60% and resulted from the destruction of white mica. Si was depleted from most selvages and transported locally to adjacent veins; average selvage Si losses were about −50%. Y and rare earth elements were added due to the growth of monazite in cracks cutting apatite. The mass balance analysis also suggests some addition of Ti occurred, consistent with the presence of rutile and hematite‐ilmenite solid solutions in veins. No major losses of Al from selvages were observed, but Al was added in some cases. Consequently, the Al needed to precipitate vein kyanite was not derived locally from the selvages. Veins more than an order of magnitude thicker than those typically observed in the field would be necessary to accommodate the Na and K lost from the selvages during alteration. Therefore, regional transport of Na and K out of the local rock system is inferred. In addition, to account for the observed abundances of kyanite in the veins, large fluid‐rock ratios (10²–10³ m³_fluid m⁻³_rock) and time‐integrated fluid fluxes in excess of ∼10⁴ m³_fluid m⁻²_rock are required owing to the small concentrations of Al in aqueous fluids. It is concluded that the quartz‐kyanite veins and their selvages were produced by regional‐scale advective mass transfer by means of focused fluid flow along a thrust fault zone. The results of this study provide field evidence for considerable Al mass transport at greenschist to amphibolite facies metamorphic conditions, possibly as a result of elevated concentrations of Al in metamorphic fluids due to alkali‐Al silicate complexing at high pressures.     

Equilibria within the mineral assemblage quartz + muscovite + biotite + garnet + plagioclase,and implications for the mixing properties of octahedrally-coordinated cations in muscovite and biotite

Interaction parameters derived using empirical calibration methods indicate strong non-ideality in the mixing of octahedrally-coordinated cations in muscovite and biotite. The data set used for calibration comprises mineral compositions from 49 samples containing quartz, muscovite, biotite, garnet, plagioclase and Al₂SiO₅ (kyanite or sillimanite). Pressures and temperatures in the data set were determined through the simultaneous application of geothermometry based on the garnet-biotite FeMg₁ exchange equilibrium and geobarometry based on the anorthite-breakdown equilibrium. Two equilibria yielded simple expressions from which binary interaction parameters for octahedrally-coordinated cations in biotite could be directly determined. A four-component (Fe²⁺, Mg, Al, Ti) regular symmetric mixing model was assumed for biotite. One equilibrium yielded a simple expression from which an interaction parameter for the mixing of the MgAl-celadonite component in muscovite could be directly determined. Two sets of calculations were performed utilizing different calibrations of the garnet-biotite geothermometer and the anothite-breakdown geobarometer and different garnet activity models. Both placed samples within or near the stability field of the Al₂SiO₅ phase present in each sample and both yielded similar values for the interaction parameters within narrow uncertainties, indicating that the values are insensitive to differences in the underlying methods. Using the derived interaction parameters, activity models were formulated for the annite, phlogopite, eastonite, and siderophyllite components of biotite, and for the MgAl-celadonite component of muscovite. These were utilized for the empirical calibration of 45 fluid-independent equilibria involving unique combinations of phase components from the mineral assemblage garnet + plagioclase±biotite±muscovite±quartz. Forty-three of the equilibria may be applied as geobarometers to equilibrium assemblages of quartz + muscovite + biotite + garnet + plagioclase when care is taken to insure that applications are restricted to valid compositional ranges. For these, the calibrations yielded multiple correlation coefficients ranging from 0.953 to 0.998 and standard deviations of the residuals ranging from 597 to 118 bars.     

Application of lithogeochemistry to exploration for deep VMS deposits in high grade metamorphic rocks, Snow Lake, Manitoba

Volcanic-associated massive sulphide deposits in the Snow Lake area of Manitoba are related to mineralogically and chemically distinct alteration zones. It is generally accepted that these zones represent crosscutting, subconformable or conformable synvolcanic alteration zones, which were coeval with and have been metamorphosed with the massive sulphides. Metamorphism ranges from upper greenschist facies to middle amphibolite facies. Surface lithogeochemical anomalies led to the discovery of small massive sulphide lenses at a vertical depth of 250 m in the Raindrop Lake area, southwest of Snow Lake, Manitoba. Variations in mineral assemblages of middle amphibolite facies alteration zones and analysis of variations in major and trace element chemistry were used to guide deep drilling at Raindrop Lake. The massive sulphide lenses are stratigraphically underlain by a low angle crosscutting “pipe” and a conformable footwall “apron” alteration.The alteration zones are composed of assemblages of garnet, staurolite and chlorite, and, less significantly, biotite, muscovite and kyanite. They are characterized by loss of Na and Ca, and addition of Fe, Mg, Cu and Zn. Mapping the alteration is aided by the application of the metamorphic AFM phase diagram for the appropriate metamorphic facies. Increasing intensity of alteration can be identified by the first appearance of new mineral phases, which are represented on the AFM diagram. These mineral trends coincide with loss of Na and Ca relative to Al, and increased Mg and Fe. Chemical alteration indices ACNK (molecular proportion Al₂O₃/(CaO + Na₂O + K₂O) and AI = 100 × [(MgO + K₂O)/(MgO + K₂O + CaO + Na₂O)] combined with Cu and Zn variation helped to quantify the intensity of alteration, despite being insensitive to Fe.The crosscutting pipe is dominantly Fe enriched, with a Cu-enriched core, Zn enriched margins and widespread Na and Ca depletion. Mineralogically it is identified by garnet, staurolite and chlorite and follows an iron and aluminum enrichment trend on the AFM diagram. The conformable alteration zone is characterized by local strong Mg enrichment, extensive Na and Ca depletion and variable Cu and Zn. Mineralogically it is characterized by the presence of chlorite and kyanite and follows a magnesium and aluminum enrichment trend on the AFM diagram.