The Stability of Chloritoid Below 10 kb PH2o

The stability of chloritoid, FeAl₂SiO₆H₂O, was investigatedat fluid pressures less than 10 kb. At oxygen fugacities definedby the Ni-NiO buffer, chloritoid reacts to Fe-cordierite andhercynite spinel at 550 and 575 °C at 1 and 2 kb fluid pressure.At pressures between 2.5 and 3.5 kb the assemblage aluminousferro-anthophyllite, staurolite and hercynite spinel appears.The breakdown of chloritoid to this assemblage takes place at625, 650, and 675 °C at 5.5, 7.0, and 8.7 kb, respectively.The aluminous ferro-anthophyllite assemblage is stable onlyover 50 °C, reacting with increasing temperature to almandine,staurolite, and hercynite spinel. Under the QFM buffer, thesame equilibria are displaced to higher temperatures and thealuminous ferro-anthophyllite bearing field is further restrictedwith respect to temperature. The 7 Å chamosite assemblage,previously considered to be the metastable equivalent of chloritoidat low pressures, is shown to be unstable and chloritoid canbe synthesized at pressures as low as 1 kb. An analysis of the equilibria and related experimental datapermits the construction of a schematic P-T grid which outlinesthe stability limits of several important mineral assemblagesin this system. Although the experimental and natural systemsare not strictly analogous, there is an excellent degree ofcorrespondence between the defined upper limit of chloritoidstability and previous estimates of the facies boundaries itserves to define.     

An experimental study of the effect of iron on magnesiochloritoid-talc-clinochlore-kyanite stability

The influence of Fe on the reaction clinochlore+kyanite&ding3F;magnesiochloritoid+talc (1) was determined experimentally as a function of pressure (1.6-2.6 GPa) and composition at 600 and 540 °C. Analytical electron microscopy (AEM) was used to determine the compositions of the coexisting phases in these complex multiphase run products. Within the compositional range studied (x_Fe^bulk=0.12-0.34), chlorite solid solutions were always richer in magnesium than the coexisting chloritoid solid solutions. Fractionation of Fe into the chloritoid extends the stability field of the assemblage chloritoid plus talc towards lower pressures. However, the extent of the stability field for increasing x_Fe is less than one would expect for ideal mixing behaviour in chloritoid and chlorite, and indicates a moderate positive deviation from ideality for chloritoid solid solutions. The thermodynamic data for magnesiochloritoid given in two earlier studies are in good agreement with the present experiments, those of a third study yield pressures which are much too high, at least compared with the experiments at 600 °C.     

Fe--Al Oxides: Phase Relationships below 1,000?C

The subsolidus phase relationships of magnetite, hercynite,hematite, corundum, wostite, and iron are described. The phaseswere synthesized from chemical mixtures. Reactions and solidsolution between them were induced under controlled conditionsof composition, temperature, total vapor pressure, and partialpressure of oxygen. Reaction rates are slow, so that the experimentslasted from 1 to 40 days, and quenching is completely successful. A solvus was determined which limits solid solution along themagnetitc-hercynite join at temperatures below 860^o?15^oC. Compositionsof the spinel solid solutions were determined by measuring theshift of the (440) reflection, using a powder X-ray diffractometer.The calibration curve, 20 vs. composition, was made from measurementsof spinel solid solutions synthesized in the one-phase region.The cell edge a_o changes from 8–391?0.002 A (magnetic,Fe⁺²Fe₂⁺²O₄OJ to 8.150?0.004 (hercynite, Fe⁺²Al₂O₄)by a_o?8.391–0.00190x- 0.5X²10^-5 (X is mol per cent FeAl₂O₄ in solid solution). In the system Fe-Al₂-O₃-O there are five univariant assemblages: 1. Hematite-corundum+magnetite +V (vapor) 2. Corundum+magnetite+hercynite+V 3. Magnetite+hercynite+w?stite+V 4. Hercynite+wilstite+iron+V 5. Hercynite+iron+corundum+V The lines were located by determining the composition of themagnetite, hercynite, hematite, and corundum solid solutionsfor each assemblage. The diagrams provide a basis for the discussion of the paragenesisof the oxide minerals. The progressive metamorphism of lateritedeposits can be represented by (1) laterites and bauxites: hematiteH+hydratedaluminum oxides; (2) diasporites: hematite+diaspore+corundum,with magnetite as a rare accessory; (3) emery: corundum+magnetite,with hematite as an accessory. The path of these mineral changeson the diagrams shows the decrease in oxygen content of thesolids with decrease in the partial pressure of oxygen and relatesthe aluminum content of the magnetite to temperature. The occurrences of hercynite are discussed. It is a rare mineralbecause it requires unusual conditions to grow, i.e. relativelylow oxygen pressure and an extremely Fe-Al-rich environment.     

Staurolite Stability and Related Parageneses: Theory, Experiments, and Applications

The results of recent investigations on the stability limitsof staurolite have been combined together with those of thepresent study to develop a semi-quantitative model of the P–T–f_o2–Xrelations of staurolite±quartz±magnetite. Theproblem with respect to the hydroxyl content of staurolite hasbeen analysed; it is concluded that no evidence has yet beenmustered to discount the idealised stoichiometry proposed byNaray-Szabó & Sasvari (1958), at least as a limitingcomposition. The stability limits of staurolite±magnetitehave been calculated from the experimental data for the equilibriainvolving quartz. Also the conditions over which the assemblagecordierite+magnetite+quartz could be stable, as well as a quantitativemodel for the fo₂-P stability of almandine ± quartz havebeen deduced theoretically. An analysis is presented of the paragenetic relations of staurolitein common pelitic schists. It is suggested that the formationof staurolite at the expense of either chloritoid or chlorite,rather than the unqualified first appearance of staurolite asproposed by Winkler (1970), should define a ‘staurolite-in’isograd in the range of 500–575 °C. In regional metamorphism,chloritoid, staurolite, and aluminum silicates should, underequilibrium conditions, be unstable relative to almandine ingraphitic pelitic schists involving magnetite (chloritoid/staurolite/Al₂SiO₆+magnetite+quartzalmandine+O₂+H₂O).The limits of P-T conditions over which staurolite and cordieritemay coexist in natural assemblages have been deduced; it isrestricted, almost entirely within the field of andalusite,between 500–700 °C, and 2–6 kbars, thus definingthe range of P-T conditions for the ‘low-pressure intermediate’—or ‘Buchan’–type amphibolite facies discussedby Miyashiro (1961). In assemblages involving staurolite andandalusite, cordierite rather than almandine should usuallybe stable; the reverse holds for assemblages involving stauroliteand sillimanite.     

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.     

Genese und Paragenese der Minerale Chloritoid und Staurolith in den Ostalpen

Zusammenfassung Die Bildung der Paragenesen von Chloritoid und Staurolith ist nicht nur von den Druck-und Temperaturbedingungen bei der Metamorphose abhängig, sondern auch weitgehend vom Chemismus der Ausgangsgesteine. Es werden die Vorkommen dieser Paragenesen im österreichischen Anteil der Ostalpen beschrieben und ihr Mineral bestand und Chemismus angegeben. Das chemisch bedingte Bildungsfeld des Stauroliths ist weiter ausgedehnt als das des Chloritoids; das erklärt die relativ größere Häufigkeit des Stauroliths. Wenn Biotit nicht vorhanden ist, kann angenommen werden, daß die betrachtete Paragenese im Gleichgewicht gebildet wurde, bei Vorhandensein von Biotit handelt es sich um rückschreitende Metamorphose. Die Ausbildung der Umwandlung von Spinell in Chloritoid konnte bewiesen werden.

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Genesis and paragenesis of the minerals chloritoid and staurolite in the Eastern Alps

Summary The formation of the paragenesis of chloritoid and staurolite depends not only on pressure and temperature during metamorphism, but also on the chemical composition of the parent rocks. The occurrences in the Austrian parts of the Eastern Alps are described. Their mineralogical and chemical composition is given. The chemical influenced field of formation of staurolite is bigger than that one of chloritoid. This proves that staurolite bearing rocks are more frequent than chloritoid bearing rocks. If biotite is not present in the rock, one may assume, that the paragenesis was built in equilibrum. If biotite is present, retrograd metamorphism may be assumed. Formation of the paragenesis of spinel and chloritoid is described.

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Mit 4 Abbildungen     

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.     

Untersuchungen zum Stabilitätsbereich von Chloritoid und Staurolith

Zusammenfassung Zur experimentellen Erfassung der natürlichen Bildungsbedingungen von Chloritoid und Staurolith wurde zunächst das Auftreten dieser beiden Minerale in der Natur untersucht. An Hand von chemischen Analysen aus Literaturangaben wurde der Zusammensetzungsbereich chloritoidführender und staurolithf ührender Gesteine ermittelt. Diese weisen im Vergleich zu tonigen und sandigen Sedimenten bzw. deren metamorphen Äquivalenten häufig folgende Unterschiede auf: geringere Alkaligehalte, geringere Ca-Gehalte, geringere Werte für das Verhältnis MgFe, höhere Al-Gehalte. Die Unterschiede sind bei chloritoidführenden Gesteinen größer als bei staurolithführenden Gesteinen. Eine Folge davon ist, daß Staurolith bei der progressiven Metamorphose nicht nur aus chloritoidführenden Paragenesen hervorgeht, sondern auch aus der Paragenese Quarz + Muskovit + Biotit + Chlorit. Die Bildung von Staurolith aus dieser Paragenese, welche in natürlichen Gesteinen der Grünschieferfazies verbreitet vorkommt, bedingt offenbar auch das häufigere Auftreten von Staurolith, verglichen mit Chloritoid.Aus den Naturbeobachtungen ergibt sich, daß chloritoidführende Gesteine überwiegend folgende Paragenese aufweisen: Chloritoid + Quarz + Muskovit + Chlorit±Akzessorien. Almandin und Disthen bzw. Andalusit treten manchmal zusätzlich auf. Einige Minerale, welche häufig bei der Metamorphose toniger und sandiger Sedimente gebildet werden, treten in chloritoidführenden Gesteinen nicht auf. Es sind dies: Stilpnomelan, Kalifeldspat und Albit. Biotit tritt im größten Teil des Stabilitätsbereiches von Chloritoid ebenfalls nicht mit diesem zusammen auf. Das Auftreten dieser Minerale in Gesteinen der Grünschieferfazies kann als Hinweis gewertet werden, daß ein für die Bildung von Chloritoid ungeeigneter Chemismus vorliegt.Staurolithführende Gesteine weisen meist folgende Paragenese auf: Staurolith + Quarz + Muskovit + Biotit + Almandin + Plagioklas±Akzessorien. Disthen, Sillimanit oder Andalusit können zusätzlich auftreten. Dagegen kann das Auftreten von Kalifeldspat und von Cordierit in muskovitführenden Gesteinen der unteren Amphibolitfazies als Hinweis gewertet werden, daß Staurolith infolge eines ungeeigneten Chemismus nicht gebildet wurde.Der Druckbereich, innerhalb dessen Chloritoid nach bisherigen Naturbeobachtungen gebildet wird, reicht von niedrigen Drucken, entsprechend der Kontaktmetamorphose, bis zu hohen Drucken, entsprechend der glaukophanitischen Grünschieferfazies der Regionalmetamorphose. Für Staurolith ist auf Grund von Naturbeobachtungen ein ähnlich großer Druckbereich anzunehmen, welcher von den entsprechenden Drucken der Kontaktmetamorphose bis zu den hohen Drucken der Regionalmetamorphose vom Barrow-Typ reicht. Der Temperaturbereich, innerhalb dessen Chloritoid in den häufigen natürlichen Paragenesen stabil ist, erstreckt sich zumindest über den Bereich der gesamten Grünschieferfazies; Staurolith ist in den häufigen natürlichen Paragenesen zumindest über den unteren Teil der Amphibolitfazies stabil. In natürlichen Gesteinen können viele Mineralreaktionen unter Beteiligung von Chloritoid oder Staurolith ablaufen, häufig dagegen dürften nur wenige von ihnen sein, und zwar: Chlorit + Kaolinit = Chloritoid + Quarz + Wasser Chloritoid + Chlorit + Quarz = Staurolith + Almandin + Wasser Chloritoid + Muskovit = Staurolith + Biotit + Almandin + Wasser Chlorit + Muskovit = Staurolith + Biotit + Quarz + Wasser Staurolith + Muskovit + Quarz = Al- Silikat + Biotit + Wasser Experimentell konnten diese oben angeführten Reaktionen noch nicht vollständig beobachtet werden; weitere Versuche dazu sind im Gange. Dagegen konnte der Ablauf einer Reaktion Chloritoid + Al-Silikat = Staurolith + Quarz + Wasser im Bereich von 4000–8000 Bar bei 545±20° C reversibel nachgewiesen werden. Diese Reaktion wird zwar infolge des Mineralbestands chloritoidführender Gesteine in der Natur relativ selten stattfinden; jedoch ist mit ihrer experimentellen Durchführung erstmalig eine Reaktion unter Beteiligung von Chloritoid und Staurolith nachgewiesen worden, welche in dem von Winkler (1965) angegebenen p, T-Bereich für die Grenze Grünschieferfazies/Amphibolitfazies abläuft. Die Phasengrenze der in der Natur häufiger ablaufenden Reaktion, wobei Staurolith + Biotit gebildet und Chlorit + Muskovit abgebaut werden, dürte nach bisherigen Ergebnissen von zur Zeit laufenden Versuchen ebenfalls in diesem p, T-Bereich liegen. Die Lage der Phasengrenzen dieser Reaktionen stimmt daher gut mit petrographischen Beobachtungen an Gesteinen des Grenzbereiches Grünschieferfazies/Amphibolitfazies überein. Eine weitere Bestätigung der experimentellen Ergebnisse lieferten Untersuchungen von Althaus (1966a, b, c) über die Stabilitätsbereiche von Andalusit, Sillimanit, Disthen und Pyrophyllit. Danach kann Chloritoid stabil zusammen mit Disthen, Andalusit oder Pyrophyllit auftreten, dagegen nicht mit Sillimanit. Diese Schlußfolgerung wird durch die natürlichen Paragenesen bestätigt.Die für die obere Stabilitätsgrenze von Staurolith angegebene Reaktion Staurolith + Quarz = Almandin + Al-Silikat + Wasser (Turner u. Verhoogen, 1960; Winkler, 1965), konnte in der eigenen Untersuchung nicht nachgewiesen werden. Nach Versuchen von Newton (schrift. Mitt., 1966) liegt diese Phasengrenze im Bereich 10000–20000 Bar um 700° C, d.h. in einem Temperaturbereich, welcher bei den eigenen Experimenten nur wenig untersucht wurde. Auf Grund von petrographischen Beobachtungen dürfte jedoch der Abbau von Staurolith in natürlichen Gesteinen meist nach einer anderen Reaktion, nämlich nach der Gleichung Staurolith + Muskovit + Quarz = Al-Silikat + Biotit + Wasser vor sich gehen. Über die Lage der Phasengrenze dieser Reaktion ist noch nichts bekannt.Aus der Untersuchung ergab sich ferner, daß entgegen der Annahme von Winkler (1965) Chloritoid kein geeigneter Indikator für die Druckbedingungen einer Metamorphose ist, da dieses Mineral nur in Gesteinen mit einem speziellen Chemismus auftritt und nach bisherigen Naturbeobachtungen über einen weiten Druckbereich hinweg gebildet werden kann. Aus den gleichen Gründen kann auch Staurolith nicht als geeigneter Druckindikator angesehen werden. Es muß vermutet werden, daß die Bereiche chemischer Gesteinszusammensetzungen innerhalb derer Chloritoid bzw. Staurolith gebildet werden können eine Abhängigkeit von Druck und Temperatur zeigen, und zwar in ähnlicher Weise wie dies nach Chinner (1962) für die Bildung von Almandin zutreffen soll. Diese Bereiche geeigneter Gesteinszusammensetzungen dürften bei relativ niedrigen Drucken beschränkter sein als bei hohen Drucken, und zwar als Folge einer stetigen Änderung des Chemismus koexistierender Minerale mit wechselnden p, T-Bedingungen.

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Compared with the bulk chemical composition of the shales, sandstones and their metamorphic equivalents, chloritoid- and staurolite-bearing rocks have a restricted chemical composition; they are poorer in alkalies and CaO, have comparatively lower MgFe ratio and higher Al₂O₃-content than most of the metamorphic rocks devoid of these two minerals. Further, the bulk composition of the chloritoid bearing-rocks is more restricted than that of the staurolithe-bearing ones. Consequently, in course of a progressive metamorphism, staurolite is produced not only at the cost of the assemblage chloritoid+quartz+muskovite + chlorite but also at the cost of the assemblage quartz+muskovite+biotite+chlorite. This explains why staurolite is more frequent than chloritoid. From empirical petrographical observation it is known that chloritoid and staurolite are found both in contact as well as in regional metamorphic areas. This fact as well as the special bulk composition necessary for their formation make them unsuitable as indicators of pressure acting during the metamorphism.The lower stability limit of chloritoid could not be worked out by hydrothermal experimentation. However the phase transition chloritoid+Al-silicate=staurolite+quartz+water was observed around 545±20° C at pressures between 4000–8000 bars. The reversal of the reaction was also successful. The p, T conditions of this reaction, therefore, compare favourably with the greenschist/amphibolite facies boundary given by Winkler (1965). Tentative results show that another reaction, namely the formation of staurolite in the assemblage chlorite+muskovite+quartz also takes place at the same p, T conditions of that facies boundary. The upper stability limit of staurolite could not yet been established experimentally in our laboratory. Petrographic observations show that in natural assemblages, staurolite breaks down more probably through reactions with muskovite + quartz rather than through the more simple reaction staurolite + quartz to Al-silicate + almandine.

     

Thermobarometry, Geochronology and the Interpretation of P-T-t Data in the Britt Domain, Ontario Grenville Orogen, Canada

Textural evidence, thermobarometry, and geochronology were usedto constrain the pressure-temperature-time (P—T—t)history of the southern portion of the Britt domain in the CentralGneiss Belt, Ontario Grenville Province. Typical metapeliticassemblages are quartz+plagioclase+ biotite + garnet + kyanite alkali feldspar sillimanite rutile ilmenite staurolite gahnite muscovite. Metatonalitic assemblages have quartz+ plagioclase + garnet biotite + hornblende + rutile + ilmenite.Metagabbroic rocks contain plagioclase + garnet + clinopyroxene+ biotite + ilmenite hornblende rutile quartz. Notabletextural features include overgrowths of sillimanite on kyaniteand of spinel on staurolite. The spinel overgrowths can be modeledby the breakdown of staurolite via the reaction Fe-staurolite= hercynite +kyanite + quartz + H₂O. The decomposition of stauroliteto her-cynite has a steep dP/dT slope and constrains the lateprograde path of a staurolite metapelite. Garnet—Al₂SiO₅—plagioclase—quartz(GASP) barometry applied to metapelitic garnets that preservecalcium zoning reveals a pressure decrease from 11 to 6 kbat an assumed temperature of 700 C. Garnet—plagioclase—ilmenite—rutile—quartzand garnet—clinopyroxene—plagioclase—quartzbarometry is in good agreement with pressures obtained withthe GASP barometer. Geochronologic data from garnet, allanite,and monazite in metapelitic rocks give ages that fall into twogroups, 1–4 Ga and 1.1 Ga, suggesting the presence ofat least two metamorphic events in the area. It is most reasonableto assign the 1.4 Ga age to the high-pressure data and the 1.1Ga age to the lower-pressure data. Collectively the P—T—tdata indicate a complex and protracted history rather than asingle cycle of burial and uplift for this part of the GrenvilleProvince.     

Chemistry and origin of chloritoid rocks from eastern Taiwan

Chloritoid-rich rocks occur as thin pods or lenses in the well-foliated marble and greenschist of the pre-Tertiary metamorphic complex of eastern central Taiwan. They contain unusually abundant chloritoid: 60–90% in the massive rocks and 30–50% in the schistose rocks. Compared to the adjacent basaltic greenschist, the chloritoid rocks contain extremely low SiO₂, CaO and Na₂O and very high Al₂O₃, total Fe as Fe₂O₃, and TiO₂. Both monoclinic and triclinic chloritoids were identified. These rocks may have been derived originally from fossil lateritic soils formed from a basaltic layer in a limestone terrain. These soils and the bedded rocks were then metamorphosed in the chlorite-biotite zone of the greenschist facies.     

Petrology of Biotite-Cordierite-Garnet Gneiss of the McCullough Range, Nevada I. Evidence for Proterozoic Low-Pressure Fluid-Absent Granulite Grade Metamorphism in the Southern Cordillera

Proterozoic migmatitic paragneisses exposed in the McCulloughRange, southern Nevada, consist of cordierite+almanditic garnet+biotite+sillimanite+plagioclase+K-feldspar+quartz+ilmenite+hercynite.This assemblage is indicative of a low-pressure fades seriesat hornblende-granulite grade. Textures record a single metamorphicevent involving crystallization of cordierite at the expenseof biotite and sillimanite. Thermobarometry utilizing cation exchange between garnet, biotite,cordierite, hercynite, and plagioclase yields a preferred temperaturerange of 590–750?C and a pressure range of 3–4 kb.Equilibrium among biotite, sillimanite, quartz, garnet, andK-feldspar records a_H2O between 0?03 and 0?26. The low a_H2Otogetherwith low f_O2 (QFM) and optical properties of cordierite indicatemetamorphism under fluid-absent conditions. Preserved mineralcompositions are not consistent with equilibrium with a meltphase. Earlier limited partial melting was apparently extensiveenough to cause desiccation of the pelitic assemblage. The relatively low pressures attending high-grade metamorphismof the McCullough Range paragneisses allies this terrane withbiotite-cordierite-garnet granulites in other orogenic belts.aosure pressures and temperatures require a transient apparentthermal gradient ofat least 50?C/km during part of this Proterozoicevent in the southern Cordillera. ^*Present address: Institute of Geophysics and Planetary Physics, University of California, Los Angeles, CA 90024-1567     

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.     

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.     

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.     

Talc-Phengite: a Widespread Assemblage in High-Grade Pelitic Blueschists of the Western Alps

Talc-phengite, an assemblage hitherto believed to be rare, isfound in regional distribution in the Gran Paradiso area, whereit occurs in the characteristic mineral association chloritoid-talc-phengite(Si_3·4–_3·5). Talc contains up to 15 moleper cent minnesotaite, and chloritoid up to 45 mole per centof the magnesium end member. The talc-phengite stability resultsbasically from the disappearance of chlorite + quartz in rockswith low and moderate MgO/FeO ratios through the divariant reactionsfirst recognized here: Fe-Mg-Chlorite+quartz talc + garnet + H₂O and Fe-Mg-chlorite + quartz talc + Chloritoid + H₂O These reactions imply the disappearance of the join biotite-chloritein the presence of quartz and thus open a talc-phengite stabilityfield (±garnet or chloritoid or Mg-chlorite) which extends,with increasing P and T, toward Mg-richer compositions. Whetheror not it reaches the magnesian subsystem in the Gran Paradisoarea cannot be ascertained. However, the sporadic occurrenceof the high-pressure assemblage talc-kyanite-chloritoid 50 to70 km further northeast in the vicinity of the Monte Rosa massifwithin the same lithological unit (Zermatt-Saas Fee zone s.l.)indicates the instability of any chlorite in quartz-bearingrocks, and implies that talc-phengite must also be stable forpurely magnesian compositions in that area. This progressivestabilization of talc-phengite with increasing metamorphic gradesupports Abraham & Schreyer's (1976) hypothesis of a high-pressurefield for this assemblage, and rules out Chernosky's construction(1978) implying a low-pressure field. The following paragenetic sequence is proposed for pelitic compositionswith intermediate Mg/Fe ratios and excess quartz subjected tohigh-pressure metamorphism with maximum temperatures near 400–500°C: chlorite-illite chlorite-phengite chloritoid-talc-phengite.The absence of biotite is a compositional effect due to thehigh degree of phengite substitution in the white mica. ^*Present address: Institut fr Mineralogic, Ruhr-Universitt, Postfach 10 21 48, D-4630 Bochum 1, Federal Republic of Germany.     

High-pressure metamorphism in gneisses and pelitic schists in the East Rhodope zone (N. Greece)

Summary The high-alumina metapelites and the orthogneisses of the lower tectonic unit of East Rhodope underwent high P/T metamorphism followed by partial reequilibration during decompression under epidote-amphibolite/amphibolite facies to greenschist facies conditions. The high P/T mineral paragenesis in the orthogneisses is: quartz + albite + microcline + phengite (Si_max = 7 atoms p.f.u.) + biotite and in the high alumina metapelites: garnet + chloritoid + chlorite + phengite (Si_max. = 6.85 atoms p.f u.) + paragonite + quartz. Pressures between 14 and 15.5 kbar, for T_min = 550°C, are estimated for the high P/T metamorphism. During continuing uplift, staurolite + chlorite, staurolite + biotite and finally andalusite + Fe-ripidolite are grown at the expense of chloritoid in metapelites, while in the orthogneisses oligoclase, still coexisting with albite, is formed; in both rock types the Si content of white K-mica decreases considerably from almost pure phengite to pure muscovite.

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Hochdruck-Metamorphose in Gneisen und pelitischen Schiefern der östlichen RhodopeZone, Nord-Griechenland

Zusammenfassung Die Aluminium-reichen Metapelite und die Orthogneise der unteren tektonischen Einheit der östlichen Rhodope-Zone wurde unter hohen Drucken und Temperaturen metamorphosiert. Darauf folgte eine teilweise Reequilibration unter Druck-Entlastung bei Bedingungen der Epidot-Amphibolit/Amphibolit bis Grünschiefer Fazies. Die Hoch-P/T Mineral-Assoziation in den Orthogneisen besteht aus: Quarz + Albit + Mikroklin + Phengit (Si_max = 7 atoms p.f.u.) + Biotit und in den Aluminium-reichen Metapeliten: Granat + Chloritoid + Chlorit + Phengit (Si_max = 6.85 atoms p.f.u.) + Paragonit + Quarz. Drucke zwischen 14 and 15.5 kbar für T_min = 550°C wurden für die Hoch- P/T Metamorphose berechnet. Während andauernder Anhebung bildeten sich Staurolit + Chlorit, Staurolit + Biotit und schließlich Andalusit + Fe-Ripidolit auf Kosten von Chloritoid in den Metapeliten, während in den Orthogneisen Oligoklas der noch mit Albit koexistiert, gebildet wurde; in beiden Gesteinstypen nimmt der Si-Gehalt heller Kaliglimmer von fast reinem Phengit bis zu reinem Muskowit ab.

     

The Four-Phase AFM Assemblage Staurolite--Aluminum Silicate--Biotite--Garnet: Extra Components and Implications for Staurolite--Out Isograds

The prograde disappearance of staurolite can be described inthe model system K₂O-FeO-MgO-Al₂O₃-SiO₂-H₂O (KFMASH) by thereaction: staurolite + muscovite + quartz = biotite + aluminumsilicate + garnet + water. The common occurrence and world—widedistribution of the assemblage staurolite-biotite-aluminum silicate-garnet(SBAG) in quartz-mica-schist suggest that the model reactionmay be over-simplified. Previous workers have suggested thatthe SBAG assemblage (1) is a strictly divariant assemblage thatbuffered water activity, (2) is stabilized by non-KFMASH components,and (3) did not attain equilibrium. We used least-squares regression to show that balanced reactionsdo not exist among the minerals in samples of SBAG assemblagesfrom Califonia and New England. The absence of reaction relationshipscan be explained by imbalances in two or three of the minorelements Zn, Mn, and either Ca or Na. The assemblage is apparentlystabilized by non-KFMASH components. Criteria for mapping staurolite-out isograds that representthe conditions of the KFMASH staurolite-out reaction dependon which of the four phases is the ‘extra’ phase,and require an understanding of the thermodynamic effects ofall the ‘extra’ components. Our results suggestthat transition zones of SBAG assemblages near staurolite-outisograds are the result of ‘extra’ components. However,it is uncertain whether µH²O of fluids in equilibriumwith SBAG assemblages varied across such zones.     

An EMP and TEM--AEM Study of Margarite, Muscovite and Paragonite in Polymetamorphic Metabauxites of Naxos Cyclades, Greece) and the Implications of Fine-scale Mica Interlayering and Multiple Mica Generations

Coexisting white micas and plagioclase were studied by electronmicroprobe (EMP), and transmission and analytical electron microscopy(TEM—AEM) in greenschist- to amphibolite-grade metabauxitesfrom Naxos. The TEM—AEM studies indicate that sub-micronscale (0.01–1.0 µm thick) semicoherent intergrowthsof margarite, paragonite and muscovite are common up to loweramphibolite conditions. If unrecognized, such small-scale micainterlayering can easily lead to incorrect interpretation ofEMP data. Muscovite and paragonite in M₂ greenschist-grade Naxosrocks are mainly relics of an earlier high-pressure metamorphism(M₁). Owing to the medium-pressure M₂ event, margante occursin middle greenschist-grade metabauxites and gradually is replacedby plagioclase + corundum in amphibolite-grade metabauxites.The margarite displays minor ^IVAl_{3 VI}(Fe³⁺, Al) Si_-3 ^VI-_-1 andconsiderable (Na, K) SiCa_-1Al_-1 substitution, resulting in upto 44 mol% paragonite and 6 mol % muscovite in solution. Thecompositional variation of muscovite is mainly described by^VI(Fe²⁺, Mg) Si ^VI Al_-1^VI Al_-1 and _VI(Fe₃₊Al^-1) exchanges, thelatter becoming dominant at amphibolite grade, Muscovite issignificantly richer in Fe than margarite or paragonite. Ca—Na—Kpartitioning data indicate that margarite commonly has a significantlyhigher Na/(Na+ K+Ca) value than coexisting muscovite or plagioclase.Exceptions are found in several greenschist-grade rocks, inwhich M₁-formed mussovite may have failed to equilibrate withM₂ margarite. The sluggishness of K-rich micas to recrystallizeand adjust composidonally to changing P-T conditions is alsoreflected in the results of mus-covite-paragonite solvus thermometry.Chemical data for Ca—Na micas from this study and literaturedata indicate that naturally coexisting margarite—paragonitepairs display considerably less mutual solubility than suggestedby experimental work. The variable and irregular Na partitioningbetween margarite and muscovite as observed in many metamorphicrocks could largely be related to opposing effects of pressureon Na solubility in margarite and paragonite and/or non-equilibriumbetween micas. KEY WORDS: Ca—Na—K mica; margarite; metabauxite; Naxos; sub-micron-scale mica interlayering     

Mössbauer and infrared spectroscopic studies of Belgian chloritoids

Three chloritoid samples from the Stavelot massif (Belgium) and one sample from the Serpont massif (Belgium) have been characterized by chemical analyses and differential X-ray diffraction. A classification of chloritoid is proposed. Mössbauer spectra at temperatures between 78 and 360 K and in external magnetic fields were obtained for a triclinic and for a monoclinic specimen. The spectra show a superposition of a weak Fe³⁺ doublet (less than 10%) and an intense Fe²⁺ doublet. It is found that a decomposition of the ferrous absorption into two distinct quadrupole doublets leads to smaller deviations between experimental and calculated line shapes, especially at low temperatures. This suggests that Fe²⁺ is present in both cis and trans O₂(OH)₄ octahedral positions in the trioctahedral layer. However, the structural data derived from the temperature dependence of isomer shifts and quadrupole splittings, are found to be inconsistent with known crystallographic data. It is therefore concluded that Fe²⁺ is present in only one type of lattice site and that the numerically imposed decomposition into two ferrous doublets is merely an artifact due to thickness saturation effects and to the distributive character of the hyperfine parameters. The negative sign of the electric field gradient further confirms the assignment of the Fe²⁺ doublet to a cis octahedral configuration. Finally, only minor differences between the Mössbauer results for triclinic and monoclinic chloritoid are observed. The infrared absorption spectra of the four samples are almost identical except in the region around 600 cm^?1 at which the monoclinic phase exhibits two absorption bands instead of one band for the triclinic samples. All absorption bands can be well assigned to the different vibrations. Inter-layer hydrogen bonding is evidenced by the occurrence of two v _O-H absorption bands. Furthermore, the specific nature of the infrared spectra enables a fast identification of chloritoid samples.     

Contact Metamorphism of Serpentinite, Chloritic Blackwall and Rodingite at Paddy-Go-Easy Pass, Central Cascades, Washington

The 2 km wide contact aureole produced from serpentinite bythe intrusion of the Mount Stuart Batholith into the IngallsComplex at Paddy-Go-Easy Pass contains the following ultramaficassemblages, in order of increasing grade: serpentine-forsterite-diopside,serpentine-forsterite-tremolite, forsterite-talc, forsterite-anthophyllite,forsterite-enstatite-anthophyllite, forsterite-enstatite-chlorite,forsterite-enstatite-spinel. Associated metarodingites displayfive metamorphic zones, the diagnostic assemblages of whichare, in increasing grade: grossular-idocrase-chlorite, grossular-diopside-chlorite,epidote-diopside-chlorite, epidote-diopside-spinel, plagioclase-grossular-diopside.Mafic hornfels in the aureole contains no orthopyroxene, indicatingthat the conditions of pyroxene hornfels facies were not reached. The breakdown of chlorite is best displayed in aluminous blackwallreaction zones around mafic inclusions in the peridotite. Attemperatures above those of the anthophyllite-out isograd, butwithin the field of forsterite+tremolite, these chlorite-richrocks react to form the assemblage: forsterite-enstatite-spinel.Calculations show that cordierite did not form as a result ofchlorite breakdown in the natural system because impurities,such as iron and chromium, displaced the equilibrium: forsterite+cordierite=enstatite+spinelto much lower pressures than the three kilobars found in thepure system. The primary chromite of the peridotite has been altered to chrome-magnetitein the serpentinite. This alteration seems to be isochemicalover the whole rock, as true chromite, formed by metamorphism,occurs at grades above that of the forsterite-enstatite-anthophylliteassemblage. Calcic amphibole in high-grade metaperidotite is tremolite,even in the presence of aluminous chromite, whereas that inmetamorphosed blackwall rock grades from tremolite into hornblende.The pattern of substitution appears to be: Mg₂Si₃(Na,K)(Al^VI)₂(Al^IV)₃.