Reaction relationships during retrograde metamorphism at Olary, South Australia

Abstract In metapelitic schists of the north-eastern Weekeroo Inliers, Olary Block, Willyama Supergroup, South Australia, syn-S1 and syn-S2 assemblages involving staurolite, garnet, biotite and another mineral, most probably cordierite, were overgrown by large syn-S3 andalusite porphyroblasts, owing to isobaric heating from metamorphic conditions that existed during the development of S2. Conditions during the development of S3 probably just reached the andalusite—sillimanite transition. During the development of S4, at somewhat lower temperatures than those that accompanied the development of S3, the following reaction occurred:
staurolite + chlorite + muscovite ± biotite + andalusite + quartz + H₂O.
The amount of retrogression is controlled primarily by the amount of H₂O added by infiltration. As the syn-S3 matrix assemblage was stable during the development of S4, but the andalusite porphyroblasts were no longer stable with the matrix when H₂O was added, the retrogression is focused in and around the porphyroblasts. With enough H₂O available, and if quartz was consumed before biotite in a porphyroblast, then the following reaction occurred:
staurolite + chlorite + muscovite + corundum ± biotite + andalusite + H₂O.
This reaction allowed corundum inclusions in the andalusite to grow, regardless of the presence of quartz in the matrix assemblage.     

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

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

Prograde reactions and garnet zoning reversals in staurolite schist, British Columbia: significance for thermobarometric interpretations

Abstract An outcrop of staurolite-bearing pelitic schist from the Solitude Range in the south-western Rocky Mountains, British Columbia, was examined in order to determine the nature of prograde garnet- and staurolite-producing reactions using information from garnet zoning and inclusion mineralogy. Although not present as a matrix phase, chloritoid is present as inclusions in garnet and is interpreted to have participated in the simultaneous growth of garnet and staurolite by a reaction such as chloritoid + quartz = garnet + staurolite + H₂O.
A garnet zoning trend reversal, which is most pronounced with respect to almandine and grossular components, is present in the outer core of garnets. The location of the zoning reversal corresponds to the outer limit of chloritoid inclusions in garnet. As there is no evidence for polymetamorphism, the zoning reversal is interpreted to indicate continued garnet growth by prograde reaction(s) during a single metamorphic event after the exhaustion of chloritoid as a matrix phase.
Metamorphic conditions recorded by mineral rim compositions are 550–600° C at 6–7 kbar. Because there is no evidence for partial resorption of garnet during production of staurolite, we interpret these results to represent peak conditions.     

Epitaxial quartz inclusions in corundum from a sapphirine–garnet boudin,Bamble Sector,SE Norway: SiO2–Al2O3 miscibility at high P–T dry granulite facies conditions

An Al‐rich, SiO₂‐deficient sapphirine–garnet‐bearing rock occurs as a metapelitic boudin within granulite facies Proterozoic charnockitic gneisses and migmatites on the island of Hisøy, Bamble Sector, SE Norway. The boudin is made up of peraluminous sapphirine, garnet, corundum, spinel, orthopyroxene, sillimanite, cordierite, staurolite and biotite in a variety of assemblages. Thermobarometric calculations based on coexisting sapphirine–spinel, garnet–corundum–spinel–sillimanite, sapphirine–orthopyroxene, and garnet–orthopyroxene indicate peak‐metamorphic conditions near to 930 °C at 10 kbar. Corundum occurs as single 200 to 3000 micron sized skeletal crystal intergrowths in cores of optically continuous pristine garnet porphyroblasts. Quartz occurs as 5–60 micron‐sized euhedral to lobate inclusions in the corundum where it is in direct contact with the corundum with no evidence of a reaction texture. Some crystal inclusions exhibit growth zoning, which indicates that textural equilibrium was achieved. Electron Back‐Scatter Diffraction (EBSD) studies reveal that the quartz inclusions share a common c‐axis with the host corundum crystal. The origin of the quartz inclusions in corundum is enigmatic as recent experimental studies have confirmed the instability of quartz–corundum over geologically realistic P–T ranges. The combined EBSD and textural observations suggest the presence of a former silica‐bearing proto‐corundum, which underwent exsolution during post‐peak‐metamorphic uplift and cooling. Exsolution of quartz in corundum is probably confined to fluid‐absent conditions where phase transitions by coupled dissolution–precipitation mechanisms are prevented.     

Hercynite as the product of staurolite decomposition in the contact aureole of Vedrette di Ries,eastern Alps,Italy

Metapelitic hornfelses in the contact aureole of the Vedrette di Ries pluton exhibit the terminal decomposition of Zn-poor Fe-staurolite in a muscovite-quartz-free domain. The reaction takes place only within coarsegrained sillimanite that has replaced andalusite porphyroblasts during prograde metamorphism. The product is a gahnite-poor hercynitic spinel, which occurs as very small grains closely associated in space with resorbed staurolite. Microstructural observations indicate that bereynite growth postdates the pseudomorphs of sillimanite after andalusite. The textural evidence for a genetic relationship between hercynite and staurolite is confirmed by the identical Fe/Mg/Zn ratios of the two minerals, which causes the collinearity of hercynite, staurolite and Al₂SiO₅ in FeO–MgO–ZnO–Al₂O₃–SiO₂–H₂O composition space (FMZASH), and indicates hercynite formed by the reaction: Fe-staurolite = 3.85hercynite + 5.1sillimanite + 2.55quartz + 2H₂O Staurolite inclusions within andalusite did not break down to form hercynite, indicating a kinetic control, as well as little overstepping of the equilibrium conditions, of the reaction forming hercynite. Assuming overstepping did not occur, modelling of the reaction with existing thermodynamic data in the simplified FASH system suggests that the terminal breakdown of staurolite to form hercynite occurred at 2.5–3.75kbar and 585–655°C.     

Progressive metamorphism of pelitic rocks from protolith to granulite facies, Dutchess County, New York, USA: constraints on the timing of fluid infiltration during regional metamorphism

A complete Barrovian sequence ranging from unmetamorphosed shales to sillimanite–K-feldspar zone metapelitic gneisses crops out in a region extending from the Hudson River in south-eastern New York state, USA, to the high-grade core of the Taconic range in western Connecticut. NNE-trending subparallel biotite, garnet, staurolite, kyanite, sillimanite and sillimanite–K-feldspar isograds have been identified, although the assignment of Barrovian zones in the high-grade rocks is complicated by the appearance of fibrolitic sillimanite at the kyanite isograd. Thermobarometric results and reaction textures are used to characterize the metamorphic history of the sequence. Pressure–temperature estimates indicate maximum metamorphic conditions of 475 °C, c. 3–4 kbar in the garnet zone to >720 °C, c. 5–6 kbar in the highest grade rocks exposed. Some samples in the kyanite zone record anomalous (low) peak conditions because garnet composition has been modified by fluid-assisted reactions. There is abundant petrographic and mineral chemical information indicating that the sequence (with the possible exception of the granulite facies zone) was infiltrated by a water-rich fluid after garnet growth was nearly completed. The truncation of fluid inclusion trails in garnet by rim growth or recrystallization, however, indicates that metamorphic reactions involving garnet continued subsequent to initial infiltration. The presence of these textures in some zones of a well-constrained Barrovian sequence allows determination of the timing of fluid infiltration relative to the P–T paths. Thermobarometric results obtained using garnet compositions at the boundary between fluid–inclusion-rich and inclusion-free regions of the garnet are interpreted to represent peak metamorphic conditions, whereas rim compositions record slightly lower pressures and temperatures. Assuming that garnet grew during a single metamorphic event, infiltration must have occurred at or slightly after the peak of metamorphism, i.e. 4–5 kbar and a temperature of c. 525–550 °C for staurolite and kyanite zone rocks.     

Chemical and textural equilibration of garnet during amphibolite facies metamorphism: The influence of coupled dissolution–reprecipitation

Metamorphic equilibration requires chemical communication between minerals and may be inhibited through sluggish volume diffusion and or slow rates of dissolution in a fluid phase. Relatively slow diffusion and the perceived robust nature of chemical growth zoning may preclude garnet porphyroblasts from readily participating in low‐temperature amphibolite facies metamorphic reactions. Garnet is widely assumed to be a reactant in staurolite‐isograd reactions, and the evidence for this has been assessed in the Late Proterozoic Dalradian pelitic schists of the Scottish Highlands. The 3D imaging of garnet porphyroblasts in staurolite‐bearing schists reveals a good crystal shape and little evidence of marginal dissolution; however, there is also lack of evidence for the involvement of either chlorite or chloritoid in the reaction. Staurolite forms directly adjacent to the garnet, and its nucleation is strongly associated with deformation of the muscovite‐rich fabrics around the porphyroblasts. “Cloudy” fluid inclusion‐rich garnet forms in both marginal and internal parts of the garnet porphyroblast and is linked both to the production of staurolite and to the introduction of abundant quartz inclusions within the garnet. Such cloudy garnet typically has a Mg‐rich, Mn‐poor composition and is interpreted to have formed during a coupled dissolution–reprecipitation process, triggered by a local influx of fluid. All garnet in the muscovite‐bearing schists present in this area is potentially reactive, irrespective of the garnet composition, but very few of the schists contain staurolite. The staurolite‐producing reaction appears to be substantially overstepped during the relatively high‐pressure Barrovian regional metamorphism reflecting the limited permeability of the schists in peak metamorphic conditions. Fluid influx and hence reaction progress appear to be strongly controlled by subtle differences in deformation history. The remaining garnet fails to achieve chemical equilibrium during the reaction creating distinctive patchy compositional zoning. Such zoning in metamorphic garnet created during coupled dissolution–reprecipitation reactions may be difficult to recognize in higher grade pelites due to subsequent diffusive re‐equilibration. Fundamental assumptions about metamorphic processes are questioned by the lack of chemical equilibrium during this reaction and the restricted permeability of the regional metamorphic pelitic schists. In addition, the partial loss of prograde chemical and textural information from the garnet porphyroblasts cautions against their routine use as a reliable monitor of metamorphic history. However, the partial re‐equilibration of the porphyroblasts during coupled dissolution–reprecipitation opens possibilities of mapping reaction progress in garnet as a means of assessing fluid access during peak metamorphic conditions.     

Prograde evolution of Sulu UHP metamorphic rock in Yangzhuang,Junan region,deduced by combined Raman and petrological studies

The prograde metamorphic history of the Sulu ultrahigh‐pressure metamorphic terrane has been revealed using Raman‐based barometry of the SiO₂ phases and other mineral inclusions in garnet porphyroblasts of a coesite eclogite from Yangzhuang, Junan region, eastern China. Garnet porphyroblasts have inner and outer segments with the boundary being marked by discontinuous changes in the grossular content. In the inner segment, the SiO₂ phase inclusions are α‐quartz with no coesite or relict features such as radial cracks. The residual pressures retained by the quartz inclusions systematically increase from the crystal centre to the margin of the inner segment. The metamorphic conditions estimated by calculation from the residual pressure and conventional thermodynamic calculation range from 500 to 630 °C and 1.3 to 2.3 GPa for the stage of the inner segment. Coesite and its pseudomorph occur as inclusions in the outer segment of the garnet and matrix omphacite. This occurrence of coesite is consistent with the pressure and temperature conditions of 660–725 °C and 3.1 GPa estimated by conventional geothermobarometry. Our results suggest that the quartz inclusions in the inner segment were trapped by garnet under α‐quartz‐stable conditions and survived phase transition to coesite at the peak metamorphic stage. The SiO₂ phases and other inclusions in the garnet have retained evidence of the pre‐eclogite prograde stage even during exhumation stage. The combined Raman spectroscopic and petrological approaches used here offers a powerful means for obtaining more robust constraints prograde stages involving garnet growth where different SiO₂ phases are present as inclusions.     

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.     

Coexisting chloritoid-staurolite from the Sillimanite (fibrolite) zone,Sini, district Singhbhum,India

Microprobe analyses of the minerals from an unusual chloritoid-staurolite-garnet (+ muscovite + quartz + ilmenite) assemblage from the sillimanite (fibrolite) zone of Sini, India are presented and the petrological significance of the paragenesis is discussed. The X Mg in the different minerals from the chloritoid-staurolite-bearing rock varies in the order, muscovite > chlorite > chloritoid > staurolite > garnet > ilmenite, and from the associated sillimanite-bearing schists: muscovite > biotite > staurolite > garnet rim > garnet core > ilmenite. A graphical representation of the mineral compositions in an AFM projection displays a consistent topology if the effects of non-AFM components such as Zn in the staurolite and Mn in the garnet are taken into account. Petrographic and mineralogical data are consistent with a prograde formation of the chloritoid-staurolite-garnet assemblage. It is suggested that the paragenesis has been formed at similar PT conditions to the associated sillimanite (fibrolite)-staurolite-garnet-mica schists. These conditions are estimated to be 600–625°C/6±0.5 Kb.     

Spiral and staircase inclusion trail axes within garnet and staurolite porphyroblasts from schists of the Bolton Syncline, Connecticut: timing of porphyroblast growth and the effects of fold development

In the Littleton Formation, garnet porphyroblasts preserve three generations of growth that occurred before formation of the Bolton Syncline. Inclusion trails of foliations overgrown by these porphyroblasts are always truncated by the matrix foliation suggesting that garnet growth predated the matrix foliation. In contrast, many staurolite porphyroblasts grew synchronously with formation of the Bolton Syncline. However, local rim overgrowths of the matrix foliation suggest that some staurolite porphyroblasts continued to grow after development of the fold during younger crenulation producing deformations. The axes of curvature or intersection of foliations defined by inclusion trails inside the garnet porphyroblasts lie oblique to the axial plane of the Bolton Syncline but do not change orientation across it. This suggests the garnets were not rotated during the subsequent deformation associated with fold development or during even younger crenulation events. Three samples also contain a different set of axes defined by curvature of inclusion trails in the cores of garnet porphyroblasts suggesting a protracted history of garnet growth. Foliation intersection axes in staurolite porphyroblasts are consistently orientated close to the trend of the axial plane of the Bolton Syncline on both limbs of the fold. In contrast, axes defined by curvature or intersection of foliations in the rims of staurolite porphyroblasts in two samples exhibit a different trend. This phase of staurolite growth is associated with a crenulation producing deformation that postdated formation of the Bolton Syncline. Measurement of foliation intersection axes defined by inclusion trails in both garnet and staurolite porphyroblasts has enabled the timing of growth relative to one another and to the development of the Bolton Syncline to be distinguished in rocks where other approaches have not been successful. Consistent orientation of foliation intersection axes across a range of younger structures suggests that the porphyroblasts did not rotate relative to geographical coordinates during subsequent ductile deformation. Foliation intersection axes in porphyroblasts are thus useful for correlating phases of porphyroblastic growth in this region.     

Sequential, syndeformational porphyroblast growth during Hercynian low-pressure/high-temperature metamorphism in the Canigou massif, Pyrenees

The sequence of growth of garnet, staurolite and aluminosilicate in Fe-rich metapelitic rocks from the Canigou massif, Pyrenees, is established using evidence of inclusion, reaction and pseudomorphing textures between the different minerals, compositional zoning patterns in garnet and staurolite (that can be related to the KFMASH reaction grid), and the geometric relations between inclusion trails in the porphyroblasts and the matrix microstructures. The evidence indicates that garnet and staurolite commenced growth before aluminosilicate in all cases, even where all three are in textural equilibrium. Interpretation of the reaction textures between the porphyroblasts and of the compositional zoning in garnet and staurolite in terms of the KFMASH reaction grid indicates the importance of continuous reactions in the development of these phases. Some garnet and staurolite porphyroblasts underwent renewed growth during breakdown, producing rims enriched in Mn and Zn respectively. The presence of aluminosilicate in these assemblages (i.e. the absence of a clear andalusite-absent zone in the field) is attributed to a strong pressure-dependence for the aluminosilicate-producing reactions. Porphyroblast-matrix microstructural relations indicate that Hercynian metamorphism in the massif was synchronous with the development of the regional subhorizontal foliation (S3).     

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.     

Submicron polyphase inclusions in garnet from the Tananao Metamorphic Complex, Taiwan: a key to unravelling otherwise unrecognized metamorphic events

Kyanite and staurolite occur in the Tananao Metamorphic Complex as submicron inclusions in almandine‐rich garnet from a metamorphosed palaeosol weathering horizon, near Hoping, eastern Taiwan. Quartz, rutile/brookite and zircon are also found as associated submicron inclusions in garnet. Employing the reaction ilmenite+kyanite+quartz=almandine+rutile, and the breakdown of staurolite and quartz as thermobarometers, these submicron‐scale minerals formed at >8.3–8.8 kbar and < 660–690 °C. This P–T estimate is different from that (i.e. 5–7 kbar and 530–550 °C) derived from matrix minerals, which include almandine‐rich garnet, muscovite, chlorite, chloritoid, plagioclase, quartz and ilmenite. These results suggest that submicron inclusions in garnet‐like materials may record portions of the otherwise undocumented prograde path or provide information about previous metamorphic events and thus yield new insights into orogenic belts.     

http://www.sciencedirect.com/science/article/pii/S1674987112000060

The Khondalite Belt within the Inner Mongolia Suture Zone（IMSZ） in the North China Craton is a classic example for Paleoproterozoic ultrahigh-temperature（UHT） metamorphism.Here we report new spinel-bearing metapelitic granulites from a new locality at Xumayao within the southern domain of the IMSZ.Petrological studies and thermodynamic modeling of the spinel+quartz-bearing assemblage shows that these rocks experienced extreme metamorphism at UHT conditions.Spinel occurs in two textural settings:（1） high X_Zn（Zn/（Mg+Fe^Ⅱ+Zn）=0.071-0.232） spinel with perthitic K-feld-spar. sillimanite and quartz in the rock matrix;and（2） low X_Zn（0.045—0.070） spinel as inclusions within garnet porphyroblasts in association with quartz and sillimanite. Our phase equilibria modeling indicates two main stages during the metamorphic evolution of these rocks:（1） near-isobaric cooling from 975℃to 875℃around 8 kbar.represented by the formation of garnet porphyroblasts from spinel and quartz;and（2）cooling and decompression from 850℃.8 kbar to below 750℃.6.5 kbar,represented by the break-down of garnet.The spinel+quartz assemblage is considered to have been stable at peak metamorphisni.formed through the break-down of cordierite.indicating a near isothermal compression process.Our study confirms the regional extent of UHT metamorphisni within the IMSZ associated with the Paleoproterozoic subduction-collision process.     

Preservation of evidence for prograde metamorphism in ultrahigh-temperature, high-pressure kyanite-bearing granulites, South Harris, Scotland

Mineralogical and mineral chemical evidence for prograde metamorphism is rarely preserved in rocks that have reached ultrahigh‐temperature (UHT) conditions (>900 °C) because high diffusion and reaction rates erase evidence for earlier assemblages. The UHT, high‐pressure (HP) metasedimentary rocks of the Leverburgh belt of South Harris, Scotland, are unusual in that evidence for the prograde history is preserved, despite having reached temperatures of ～955 °C or more. Two lithologies from the belt are investigated here and quantitatively modelled in the system NaO–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O: a garnet‐kyanite‐K‐feldspar‐quartz gneiss (X_Mg = 37, A/AFM = 0.41), and an orthopyroxene‐garnet‐kyanite‐K‐feldspar quartzite (X_Mg = 89 A/AFM = 0.68). The garnet‐kyanite gneiss contains garnet porphyroblasts that grew on the prograde path, and captured inclusion assemblages of biotite, sillimanite, plagioclase and quartz (<790 °C, <9.5 kbar). These porphyroblasts preserve spectacular calcium zonation features with an early growth pattern overgrown by high‐Ca rims formed during high‐P metamorphism in the kyanite stability field. In contrast, Fe‐Mg zonation in the same garnet porphyroblasts reflects retrograde re‐equilibration, as a result of the relatively faster diffusivity of these ions. Peak P–T are constrained by the occurrence of coexisting orthopyroxene and aluminosilicate in the quartzite. Orthopyroxene porphyroblasts [y(opx) = 0.17–0.22] contain sillimanite inclusions, indicative of maximum conditions of 955 ± 45 °C at 10.0 ± 1.5 kbar. Subsequently, orthopyroxene, kyanite, K‐feldspar and quartz developed in equilibrated textures, constraining the maximum pressure conditions to 12.5 ± 0.8 kbar at 905 ± 25 °C. P–T–X modelling reveals that the mineral assemblage orthopyroxene‐kyanite‐quartz is compositionally restricted to rocks of X_Mg > 84, consistent with its very rare occurrence in nature. The preservation of unusual high P–T mineral assemblages and chemical disequilibrium features in these UHT HP rocks is attributed to a rapid tectonometamorphic cycle involving arc subduction and terminating in exhumation.     

The initial garnet‐in reaction involving siderite–rhodochrosite,garnet re‐equilibration and P–T–t paths of graphitic schists in the Black Hills orogen,South Dakota,USA

It is generally thought that garnet in metapelites is produced by continuous reactions involving chlorite or chloritoid. Recent publications have suggested that the equilibrium temperatures of garnet‐in reactions may be significantly overstepped in regionally metamorphosed terranes. The growth of small spessartine–almandine garnet crystals on Mn‐siderite at the garnet isograd in graphitic metapelites in the Proterozoic Black Hills orogen, South Dakota, demonstrates that Mn‐siderite was the principal reactant that produced the initial garnet in the schists. Moreover, the positions of garnet compositions in isobaric, T–(C/H) pseudosections for the schists show that the temperature of the garnet‐in reaction from Mn‐siderite was overstepped minimally at the most. In the Black Hills, garnet was initially produced during regional metamorphism beginning at c. 1755 Ma due to the collision of Wyoming and Superior cratons, and was subsequently partially or fully re‐equilibrated at more elevated temperatures and pressures during intrusion of the Harney Peak Granite (HPG) at c. 1715 Ma. Garnet occurs in graphitic schists in garnet, staurolite and sillimanite zones, the latter being a product of contact metamorphism by HPG. During metamorphism, coexisting fluid contained both CO₂ and CH₄. In the garnet zone, garnet crystals contain petrographically distinct cores with inclusions of quartz, graphite and other minerals. Centres of the cores have distinctly elevated Y concentrations that mark the positions of garnet nucleation. The elevated Y is thought to have come from the Mn‐siderite onto which Y was probably absorbed during precipitation in an ocean. In the upper garnet and staurolite zones, the cores were overgrown by inclusion‐poor mantles. Mantles are highly zoned and have more elevated Fe and Mg and lower Mn and Ca than cores. The growth of mantles is attributed to late‐orogenic heating by leucogranite magmas and attendant influx of H₂O that caused consumption of graphite in rock matrices. A portion of the Proterozoic terrane that includes the HPG is surrounded by four large faults. In this ‘HPG block’, garnet is inclusion‐poor and its composition does not preserve its early growth history. This garnet appears to have re‐equilibrated by internal diffusion of its major components and/or recrystallization of an earlier inclusion‐rich garnet. It has equilibrated within the kyanite stability range, and together with remnant kyanite in the high‐strain aureole of the HPG, indicates that the HPG block had a ≥6 kbar history. The HPG block has undergone decompression during emplacement of the HPG. The decompression is evident in occurrences of retrograde andalusite and cordierite in the thermal aureole of the HPG. The data support a polybaric metamorphic history of the Black Hills orogen with different segments of the orogen having their own clockwise P–T–t paths.     

Corundum + quartz and Mg-staurolite bearing granulite from the Limpopo Belt, southern Africa: Implications for a P–T path

A new occurrence of the rare corundum + quartz assemblage and magnesian staurolite has been found in a gedrite–garnet rock from the Central Zone of the Neoarchean Limpopo Belt in Zimbabwe. Poikiloblastic garnet in the sample contains numerous inclusions of corundum + quartz ± sillimanite, magnesian staurolite + sapphirine ± orthopyroxene, and sapphirine + sillimanite assemblages, as well as monophase inclusions. Corundum, often containing subhedral to rounded quartz, occurs as subhedral to euhedral inclusions in the garnet. Quartz and corundum occur in direct grain contact with no evidence of a reaction texture. The textures and Fe–Mg ratios of staurolite inclusions and the host garnet suggest a prograde dehydration reaction of St → Grt + Crn + Qtz + H₂O to give the corundum + quartz assemblage. Peak conditions of 890–930 °C at 9–10 kbar are obtained from orthopyroxene + sapphirine and garnet + staurolite assemblages. A clockwise P–T path is inferred, with peak conditions being followed by retrograde conditions of 4–6 kbar and 500–570 °C. The presence of unusually magnesian staurolite (Mg / [Fe + Mg] = 0.47–0.53) and corundum + garnet assemblages provides evidence for early high-pressure metamorphism in the Central Zone, possibly close to eclogite facies. The prograde high-pressure event followed by high- to ultrahigh-temperature metamorphism and rapid uplifting of the Limpopo Belt could have occurred as a result of Neoarchean collisional orogeny involving the Zimbabwe and Kaapvaal Cratons.     

A corundum–quartz assemblage from the Eastern Ghats Granulite Belt,India: evidence for high P–T metamorphism?

Corundum+quartz-bearing assemblages occur in small lenses in granulite facies metapelites in Rayagada, north-central part of the Eastern Ghats Granulite Belt, India. Corundum porphyroblasts and quartz coexist with porphyroblastic almandine-rich garnet, hercynite spinel, ilmenite and magnetite. Corundum and quartz are separated by sillimanite or a composite corona consisting of sillimanite and garnet, whereas corundum shows sharp grain boundaries with spinel, ilmenite and magnetite. Porphyroblastic corundum contains prismatic sillimanite inclusions in which irregularly shaped quartz is enclosed. Two distinct reactions are inferred from the textural features: corundum+quartz=sillimanite and spinel+quartz=garnet+sillimanite. From the petrographical features, we infer that corundum–quartz–garnet–spinel was the peak metamorphic assemblage. Although large uncertainties exist regarding the positions of the respective reactions in P–T  space, from several published experimental results and theoretical calculations a peak metamorphic condition of 12  kbar and 1100  °C is estimated as the lower stability limit of the corundum–quartz assemblage. Decompression from the peak P–T  condition to c .  9  kbar, 950  °C is inferred.     

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.