Phase Equilibria in Three Assemblages of Kyanite-Zone Pelitic Schists, Lincoln Mountain Quadrangle, Central Vermont

Petrologic and geologic arguments suggested that a close approachto chemical equilibrium at a single temperature and pressuremight be found in the rocks near Mt. Grant in central Vermont.An area 2,800 feet by 4,000 feet was selected and studied indetail. The major assemblages are: kyanite-chloritoid-chlorite-quartz-muscovite-paragonite-rutile;garnet-chloritoid-chlorite-quartz-muscovite-paragonite-ilmenite;and garnet-chlorite-biotite-quartz-muscovite-albite-ilmenite.All minerals were separated from a primary sample for each ofthese assemblages and purified, complete gravimetric and spectrographicanalyses performed, and optical properties, X-ray properties,and densities measured. Chlorite, garnet, and chloritoid orbiotite were separated from an additional nine samples of theseassemblages and spectrographic and partial gravimetric analysesperformed. Distribution coefficients of each mineral pair for Mg/Fe andMn/Fe+ Mg and for the minor elements are similar in nearly everysample for a given assemblage. Distribution coefficients forgarnet-chlorite differ in the two assemblages which containthis pair; this difference is attributed to the difference inAl-content of the chlorite from the two assemblages. The smallrange of distribution coefficients, despite a wide range inthe relative proportions of the ferromagnesian phases, is convincingevidence that an equilibrium partition of the major and minorcations between the phases in each sample had been attainedand that the equilibration temperature was the same for eachsample. The coexistence of kyanite+muscovite+quartz and comparison ofthe composition of coexistent Ca-free muscovite and paragoniteand of O¹⁸/O¹⁰ ratios for coexistent quartz and magnetite withexperimental values indicate that these rocks formed at P_s 11 kb, T 550? C, and aH₂o low enough (Pe(H₂o) sufficiently lessthan P_s) to depress the pyrophyllite breakdown temperature byabout 40? C. The cations and the ao₂ are equilibrated locally(within each sample), but all the samples have equilibratedto a common O¹⁸/O¹⁶ ratio. A possible explanation is that afluid medium had sufficient oxygen in H₂O to control the O¹⁸/O¹⁶ratio of the rock and its phases, but that the rock system hadsufficient buffer capacity to control the ao₂ of the fluid.     

Genesis of Peraluminous Granites I. Experimental Investigation of Melt Compositions at 3 and 5 kb and Various H2O Activities

Melting experiments have been performed on a peraluminous quartzo-feldspathicgneiss from Northern Portugal. This gneiss occurs as xenolithsin the Tourem anatectic complex and is the most probable sourcerock for the surrounding anatectic granites. On the basis offield, petrological, and geochemical data, it can be shown thatanatexis took place in the stability field of cordierite + biotiteand that the evolution of magmas is the result of processesinvolving segregation of partial melt and separation of restiteminerals. Experiments were performed at 700, 750, and 800 ?C, 3 and 5kb, and various H₂O activities (aH₂O) to clarify the influenceof aH₂O and melt fraction on the composition of the generatedmelts. Biotite and cordierite are the two main ferromagnesianphases observed in the run products. Cordierite is formed byincongment melting of biotite. For relatively low melt fractions (< 30–35 wt. %),the partial melts coexisting with quartz, alkali feldspar, andplagioclase have a minimum or near-minimum melt composition.The melts become richer in potassium with decreasing aH₂O. Thisresult using a natural rock as starting material is in goodagreement with results achieved in the synthetic Qz-Ab-Or system.In the stability field of biotite and cordierite, the influenceof aH₂O on melt composition is at least as important as theeffect of changing P and/or T. For higher melt fractions the composition of the melt is stronglycontrolled by the disappearance of alkali feldspar and the meltsbecome richer in An and poorer in Or with increasing degreeof melting. The wide range of compositions (especially for K₂O, which variesfrom 3.5 to 5.4%) observed in the experimental peraluminousmelts demonstrates that a wide variety of granitoid magmas maybe produced from the same source rocks. The application of theexperimental results to the Tourem anatectic complex shows thatmelting occurred at H2O-undersaturated conditions (4–5wt. % H₂O in the melts, corresponding to aH₂O of {small tilde}0.5at 5 kb). Experimental melts similar in composition to the mostleucocratic granite of the Tourem anatectic complex (consideredto approximate the composition of the generated melt) were obtainedaround 800 ?C, suggesting that this temperature was attainedduring the peak of anatexis.     

Solid-Fluid Equilibria in the System KAlSi3O8-NaAlSi3O8-Al2SiO5-SiO2-H2O-HCl

Activity diagrams in the system KAlSi₃O₈-NaAlSi₃O₈-Al₂SiO₅-SiO₂-H₂O-HClhave been calculated in terms of a_K+/a_H+ and a_N+/a_H+ from existingexperimental data. They show the effect of temperature, pressure,and a_H2O on the stability fields of the alkali feldspars, micas,and aluminium silicate. These activity diagrams are useful in revealing the bufferingcapacity of mineral assemblages and the chemical potential gradientsestablished by changes in T, P, a_H2O, and mineral assemblage.An analysis of mineral paragenesis in terms of these diagramssuggests that mosaic equilibrium, allowing limited metasomatismand internal buffering of chemical potentials, best describemetamorphic systems. Thus the dehydration reaction: muscovite+quartz=K-feldspar+Al₂SiO₅+H₂O which is most important in closed systems, probably fails todescribe in detail the mechanism of natural muscovite decomposition.Rather the decomposition of muscovite is more likely representedby ionic reactions. The replacement of muscovite by feldspar: muscovite+6 SiO₂+2 K⁺=3 K-feldspar+2 H⁺ muscovite+6 SiO₂+3 Na⁺=3 Albite+K⁺+2 H⁺ is favored at high temperature and low pressure, and may accountfor the crystallization of some feldspars in metamorphic rocks.The reaction involving aluminium silicate replacement of muscovite: 2 muscovite+2 H⁺=3 Al₂SiO₅+3 SiO₂+3 H₂O+2 K⁺ is favored at high temperature and pressure and low a_H2O, andcould contribute to the development of the aluminium silicates.It is concluded that both activity diagrams and AKNa projectionsshould be used together to more completely evaluate mineralparagenesis in terms of mosaic equilibria.     

Prehnite-Pumpellyite to Greenschist Fades Transition in the Karmutsen Metabasites, Vancouver Island, B.C.

Mineral paragenescs in the prehnite-pumpellyite to greenschistfades transition of the Karmutsen metabasites are markedly differentbetween amygdule and matrix, indicating that the size of equilibriumdomain is very small. Characteristic amygdule assemblages (+chlorite + quartz) vary from: (1) prehnite + pumpeUyite + epidote,prehnite + pumpellyite + calcite, and pumpellyite + epidote+ calcite for the prehnite-pumpellyite facies; through (2) calcite+ epidote + prehnite or pumpellyite for the transition zone;to (3) actinolite + epidote + calrite for the greenschist facies.Actinolite first appears in the matrix of the transition zone.Na-rich wairakites containing rare analcime inclusions coexistwith epidote or Al-rich pumpellyite in one prehnite-pumpellyitefacies sample. Phase relations and compositions of these wairakite-bearingassemblages further suggest that pumpellyite may have a compositionalgap between 0.10 and 0.15 X_Fe?. Although the facies boundaries are gradational due to the multi-varianceof the assemblages, several transition equilibria are establishedin the amygdule assemblages. At low X_co2, pumpellyite disappearsprior to prehnite by a discontinuous-type reaction, pumpellyite+ quartz + CO₂ = prehnite + epidote + calcite + chlorite + H₂O,whereas prehnite disappears by a continuous-type reaction, prehnite+ CO₂ = calcite + epidote + quartz-l-H₂O. On the other hand,at higher X_CO2 a prehnite-out reaction, prehnite + chlorite+ H₂O + CO₂ = calcite + pumpellyite + quartz, precedes a pumpellyiteoutreaction, pumpellyite + CO₂ = calcite + epidote + chlorite +quartz + H₂O. The first appearance of the greenschist faciesassemblages is defined at both low and high XCOj by a reaction,calcite + chlorite + quartz = epidote + actinolite+ H₂O + CO₂.Thus, these transition equilibria are highly dependent on bothX_Fe³⁺ + of Ca-Al silicates and X_H20 of the fluid phase. Phaseequilibria together with the compositional data of Ca-Al silicatesindicate that the prehnite-pumpellyite to greenschist faciestransition for the Karmutsen metabasites occurred at approximately1.7 kb and 300?C, and at very low X_co2, probably far less than0.1.     

Phase Relations in Peridotitic and Pyroxenitic Rocks in the Model Systems CMASH and NCMASH

Alpine-type peridotites and associated pyroxenites are foundas lenses in the continental crust in many different orogens.The reconstruction of the pressure–temperature (P–T)evolution of these rocks is, however, difficult or even impossible.With geothermobarometry, usually one point on the overall P–Tpath can be obtained. To use the different mineral assemblagesobserved in ultramafic rocks as P–T indicators, quantitativeP–T phase diagrams are required. This study presents newcalculated phase diagrams for peridotitic and pyroxenitic rocksin the model systems CaO–MgO–Al₂O₃–SiO₂–H₂O(CMASH) and Na₂O–CaO–MgO–Al₂O₃–SiO₂–H₂O(NCMASH), which include the respective solid solutions as continuousexchange vectors. These phase diagrams represent applicablepetrogenetic grids for peridotite and pyroxenite. On the basisof these general petrogenetic grids, phase diagrams for particularperidotite and pyroxenite bulk compositions are constructed.In an example of pyroxenite from the Shackleton Range, Antarctica,the different observed mineral assemblages are reflected bythe phase diagrams. For these rocks, a high-pressure metamorphicstage around 18 kbar and an anticlockwise P–T evolution,not recognized previously, can be inferred. KEY WORDS: Antarctic; high-pressure metamorphism; peridotite; phase diagrams; pyroxenite     

Low-grade Metamorphism of the Karmutsen Volcanics, Vancouver Island, British Columbia

Low-variance assemblages occurring within amygdules of Karmutsenlavas from the Elk Creek and Upper Campbell Lake areas, VancouverIsland, British Columbia, provide important constraints on thepressure and temperature of metamorphism as well as on the compositionof the attendant fluid. The P-T stability of the assemblagesepidote-muscovite-K-feldspar-prehnite and epidote-prehnite-quartz-wairakitecoupled with mean isochores derived from homogenization temperaturesof H₂O inclusions within amygdaloidal quartz indicate that theUpper Campbell Lake area was subjected to metamorphism at 1?5kb (?0?5 kb), 260 ?C (? 15?C) and the Elk Creek area at somewhatlower P or higher T. Isobaric T-a(CO₂) diagrams show that the occurrence of epidote-oligoclase,prehnite-orthoclase-albite, and prehnite-andesine assemblagescollected from the Elk Creek area is consistent with the P-Tconstraints and that these assemblages formed in water-richfluids containing very low concentrations of CO₂. The presenceof Ca-zeolite-epidote assemblages in the Upper Campbell Lakearea is also compatible with P-T estimates. The consistencyof epidote and prehnite rim compositions in low-variance assemblagesand the lack of incompatible phases in these assemblages demonstratethat equilibrium was obtained on limited domains within amygdules. Because epidote and prehnite compositions in low-variance assemblagesare very sensitive to changes in concentration of CO2, low-varianceassemblages involving these phases can serve as monitors offluid composition. It is postulated that low-variance assemblagesin Karmutsen flows originated by reaction of previously formed,high-variance assemblages with infiltrating CO₂-bearing aqueousfluids during a subsequent hydrothermal event. These fluidspreferentially exploited more permeable amygdaloidal portionsof the Karmutsen flows. The low-variance assemblages not onlyrecord the extremely H₂O-rich composition of the permeatingfluid, but also outline the paths the fluid took. ^* Offprint requests to B. R. Frost     

The Graphical Analysis of Greenschist to Amphibolite Facies Mineral Assemblages in Metabasites

The mineral assemblages of greenschist to amphibolite faciesmetabasites may usually be represented in a system of principalcomponents: CaO–Al₂O₃–(Fe₂O₃)–FeO–MgO–Na₂O–SiO₂–CO₂–H₂O Assemblages co-existing with quartz, ‘albite’, ‘epidote’and a fluid of restricted composition, may be shown by projectionin a CAFM subsystem from ‘epidote’ onto an extendedAFM plane. This projection is analogous to the Thompson projectionfor pelites and is particularly useful in displaying the effectsof Fe/Mg and Al substitution in the silicates as well as incorporatingCaO; it is illustrated by plotting assemblages from the SouthernAlps of New Zealand and the Scottish Highlands and demonstrateschanges occurring with grade in the assemblages. Some commonisograds and facies boundaries are seen to be strongly dependenton bulk rock composition. In some cases MnO must be consideredas an additional component. A model of P_solids=P_fluid, where the fluid is composed of CO₂+H₂Ois consistent with many greenschist to amphibolite facies metabasicassemblages. Natural assemblages indicate this fluid phase tohave restricted mobility. Theoretical consideration of mineralreactions resulting from increasing X_co2, in conjunction withdata from natural mineral assemblages, leads to the distinctionof five principal types of assemblage which may be expectedas a function of varying X_Co2. Recognition of these assemblagetypes provides a useful guide to relative X_Co2 during metamorphism. ^* Present Address: Department of Geology, University of California, 405 Hilgard Avenue, Los Angeles, California 90024.     

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.     

Transition from the Zeolite to Prehnite-Pumpellyite Facies in the Karmutsen Metabasites, Vancouver Island, British Columbia

The upper Triassic Karmutsen metabasites from northeast VancouverIsland, B.C., are thermally metamorphosed by the intrusion ofthe Coast Range Batholith. The amygdaloidal metabasites developedin the outer portion of the contact aureole show a progressivemetamorphism from zeolite to prehnite-pumpellyite facies. Thesize of an equilibrium domain is extremely small for these metabasites,and the individual amygdule assemblages are assumed to be inequilibrium. Two major calcite-free assemblages (+chlorite+quartz)are characteristic: (i) laumontite+pumpellyite+epidote in thezeolite facies and (ii) prehnite+pumpellyite+epidote in theprehnite-pumpellyite facies. The assemblages and compositionsof Ca-Al silicates are chemographically and theoretically interpretedon the basis of the predicted P-T grid for the model basalticsystem, CaO-MgO-A1₂O₃-Fe₂O₃-SiO₂-H₂O. The results indicate:(1) local equilibrium has been approached in mineral assemblagesand compositions; (2) the X_Fe³⁺ values in the coexisting Ca-Alsilicates decrease from epidote, through pumpellyite to prehnite;(3) with increasing metamorphic grade, the Fe³⁺ contents ofepidotes in reaction assemblages decrease in the zeolite facies,then increase in the prehnite-pumpellyite facies rocks. Suchvariations in the assemblages and mineral compositions are controlledby a sequence of continuous and discontinuous reactions, andallow delineation of T-X_Fe³⁺ relations at constant pressure.The transition from the zeolite to prehnite-pumpellyite faciesof the Karmutsen metabasites is defined by a discontinuous reaction:0·18 laumontite+pumpellyite+0·15 quartz = 1·31prehnite+ 0·78 epidote+0·2 chlorite+ 1·72H₂O, where the X_Fe³⁺ values of prehnite, pumpellyite and epidoteare 0·03, 0·10 and 0·18, respectively.These values together with available thermodynamic data andour preliminary experimental data are used to calculate theP-T condition for the discontinuous reaction as P = 1·1±0·5 kb and T = 190±30°C. The effectsof pressure on the upper stability of the zeolite facies assemblagesare discussed utilizing T-X_Fe³⁺ diagrams. The stability of thelaumontite-bearing assemblages for the zeolite facies metamorphismof basaltic rocks may be defined by either continuous or discontinuousreactions depending on the imposed metamorphic field gradient.Hence, the zeolite and prehnite-pumpellyite facies transitionboundary is multivariant.     

Beginning of melting and composition of first melts in the system Qz-Ab-Or-H2O-CO2

Solidus temperatures have been determined for minimum melt compositions in the system Qz(SiO₂)-Ab(NaAlSi₃O₈)-Or(KAlSi₃O₈) at P(fluid)=2,5 and 10 kbar and at various water activities. The dry solidus was investigated in a dry argon environment. Water activities (aH₂O) between 0.0 and 1.0 were obtained by using H₂O-CO₂ mixtures. The Or/Ab+Or ratio of first melts increases considerably with decreasing water activity. At 10 kbar it is 0.28 in the water-saturated system and 0.56 at water activity 0.1. The Qz-content does not change with changing water activities. The Ab-content of minimum melts formed at high pressures and low aH₂O may remain almost constant in ascending magmas that are cooling and crystallizing. Qz-content increases at the expense of the Or-component. Solidus temperatures decrease considerably when aH₂O increases slightly from zero. At 10 kbar, the temperature difference between dry melting and the solidus for aH₂O=0.1 is 120°C. The influence of pure CO₂ on the solidus is very limited in the investigated P-T range. The solidus is approximatively 760°C at aH₂O=0.5 between 2 and 10 kbar and approximatively 830°C at aH₂O=0.3. This means that melting of quartz-feldspar assemblages may induce dehydration reactions at P-T conditions of the granulite facies.     

‘Derived’ and observed sulphosalt-sulphide phase assemblages compared — A case study from Rajpura-Dariba,India

Log a_{S 2}-T diagrams for several sulphosalts (and sulphides) are constructed from published thermochemical data. Additionally, extension of published data on the Cu-S system (Barton, 1973) to the univariant phase equilibria involving fahlore and high-chalcocite in the Cu-Sb-As-S system (Luce et al., 1977) permits construction of approximate As-isopleths for fahlore in log a_{S 2}-T space. Phase relationships thus derived are compared with the sulphosalt-sulphide assemblages present in the biphase, polymetallic mineralization at Rajpura-Dariba where geology, genesis and mineral-chemistry are well documented. A gratifying correspondence between the two is observed. Biphase origin of the orebody, inferred from geological data, is confirmed, and rigorous narrow limits of variation of a_{S 2} and T could be set for some assemblages belonging to the second phase of mineralization. The phase diagrams also indicate that the observed pyrargyrite-proustite textural relation and the decomposition fabric of some fahlores, independently, point to an intermittent increase in the activities of S₂ and As during the second phase mineralization.     

The Graphical Analysis of Greenschist to Amphibolite Facies Mineral Assemblages in Metabasites

The mineral assemblages of greenschist to amphibolite faciesmetabasites may usually be represented in a system of principalcomponents: CaO-Al₂O₃-(Fe₂O₃)-FeO-MgO-Na₂O-SiO₂-C0₂-H₂O Assemblages co-existing with quartz, ‘albite’, ‘epidote’and a fluid of restricted composition, may be shown by projectionin a CAFM subsystem from ‘epidote’ onto an extendedAFM plane. This projection is analogous to the Thompson projectionfor pelites and is particularly useful in displaying the effectsof Fe/Mg and Al substitution in the silicates as well as incorporatingCaO; it is illustrated by plotting assemblages from the SouthernAlps of New Zealand and the Scottish Highlands and demonstrateschanges occurring with grade in the assemblages. Some commonisograds and facies boundaries are seen to be strongly dependenton bulk rock composition. In some cases MnO must be consideredas an additional component. A model of P_solid=P_fluid, where the fluid is composed of CO₂+H₂Ois consistent with many greenschist to amphibolite facies metabasicassemblages. Natural assemblages indicate this fluid phase tohave restricted mobility. Theoretical consideration of mineralreactions resulting from increasing X_CO2 in conjunction withdata from natural mineral assemblages, leads to the distinctionof five principal types of assemblage which may be expectedas a function of varying X_CO2. Recognition of these assemblagetypes provides a useful guide to relative X_CO2 during metamorphism.     

Metamorphism of Calcareous Rocks in Three Roof Pendants in the Sierra Nevada, California

Two roof pendants in the Hope Valley area, Alpine County, containabundant calc-silicate assemblages which can be related to univariantor invariant equilibria in the CaO-Al₃O₃-SiO₂-H₂O-CO₂ system.Such assemblages are considered to represent components of reactionsthat buffered the chemistry of the pore fluid. Through dataobtained from microprobe analysis it is concluded that solidsolution in plagioclase, garnet, and clinozoisite are importantvariables such that on a TXco₂ projection each sample had aunique path during metamorphism. Differences in the plagioclasecomposition of nearby samples with assemblages related by thereaction: grossularite_(s.s)+quartz = anorthite_(s.s.)+wollastonite, suggest unique equilibration temperatures for assemblages inlocal domains. In the Twin Lakes pendant in Fresno County, thereaction: clinohumite+calcite+CO₂= 4forsterite+dolomite+H₂O, is importantin magnesian marbles. Contrasting parageneses, which are relatedby this equilibrium, are considered to reflect variations influid composition. Constrasting assemblages in calc-silicaterocks, which are linked by the reactions: calcite+quartz= wollastonite+CO₂, tremolite+calcite= dolomite+diopside+CO₂+H₂O, exist down to the scale of a thin section. Variation in Ti contentof idocrase may be an important factor in assemblages linkedby reactions involving this phase. This study suggests that during contact metamorphism of calcareousrocks in the Sierra Nevada, H₂O and CO₂ behaved as ‘initialvalue components’ (Zen, 1963) whose activities were controlledby reactions withion local systems.     

Water-Saturated and -Undersaturated Melting of Metaluminous and Peraluminous Crustal Compositions at 10 kb: Evidence for the Origin of Silicic Magmas in the Taupo Volcanic Zone, New Zealand, and Other Occurrences

The melting relations of two proposed crustal source compositionsfor rhyolitic magmas of the Taupo Volcanic Zone (TVZ), New Zealand,have been studied in a piston-cylinder apparatus at 10 kb totalpressure and a range of water activities generated by H₂O-CO₂vapour. Starting materials were glasses of intermediate composition(65 wt.% Si0₂ representing a metaluminous ‘I-type’dacite and a peraluminous ‘S-type’ greywacke. Crystallizationexperiments were carried out over the temperature range 675to 975?C, with aH₂O values of approximately 1?0, 0?75, 0?5,and 0?25. Talc-pyrex furnace assemblies imposed oxygen fugacitiesclose to quartz-fayalite-magnetite buffer conditions. Assemblages in both compositions remain saturated with quartzand plagioclase through 675–700?C at high aH₂O, 725–750?Cat aH₂O0?5, and 800–875?C at aH₂O0?25, corresponding to<60–70% melting. Concentrations of refractory mineralcomponents (Fe, Mg, Mn, P, Ti) in liquids increase throughoutthis melting interval with increasing temperature and decreasingaH₂O. Biotite and hornblende are the only mafic phases presentnear the solidus in the dacite, compared with biotite, garnet,gedritic orthoamphibole, and tschermakitic clinoamphibole inthe greywacke. Near-solidus melting reactions are of the type:biotite + quartz + plagioclase = amphibole ? garnet, potentiallyreleasing H₂O for dehydration melting in the greywacke, butproducing larger amounts of hornblende and releasing littleH₂O in the dacite. At aH₂O0?25 and temperatures 825–850?C,amphibole dehydration produces anhydrous mineral phases typicalof granulite fades assemblages (clinopyroxene, orthopyroxene,plagioclase?quartz in the dacite; garnet, orthopyroxene, plagioclase?quartzin the greywacke) coexisting with melt proportions as low as40%. Hornblendce-saturated liquids in the dacite are weaklyperaluminous (0?3–1?6 wt.% normative C—within therange of peraluminous TVZ rhyolites), whereas, at aH₂O0?25 andtemperatures 925?C, metaluminous partial melt compositions (upto 1?8 wt.% normative Di) coexist with plagioclase, orthopyroxene,and clinopyroxene. At all water activities, partial melts ofthe greywacke are uniformly more peraluminous (1?5–2?6wt.% normative C), reflecting their saturation in the componentsof more aluminous mafic minerals, particularly garnet and Al-richorthopyroxene. A metaluminous source for the predominantly Di-normativeTVZ rhyolites is therefore indicated. With decreasing aH₂O the stability fields of plagioclase andquartz expand, whereas that of biotite contracts. These changesare reflected in the proportions of normative salic componentsin partial melts of both the dacite and greywacke. At high aH₂O,partial melts are rich in An and Ab and poor in Or (trondhjemitic-tonalitic);with decreasing aH₂O they become notably poorer in An and richerin Or (granodioritic-granitic). These systematic variationsin salic components observed in experimental metaluminous tostrongly peraluminous melts demonstrate that a wide varietyof granitoid magmas may be produced from similar source rocksdepending upon P-T-aH₂O conditions attending partial melting.Some peraluminous granitoids, notably trondhjemitic leucosomesin migmatites, and sodic granodiorites and granites emplacedat deep crustal levels, have bulk compositions similar to nearsolidus melt compositions in both the dacite and greywacke,indicating possible derivation by anatexis without the involvementof a significant restite component.     

Low-Temperature, Low-Pressure Metamorphism of Mn-rich Rocks in the Lienne Syncline, Venn--Stavelot Massif (Belgian Ardennes), and the Role of Carpholite

Low-grade Mn-rich metamorphic rocks of the Lienne syncline (westernpart of the Venn–Stavelot Massif, Belgian Ardennes) havebeen re-examined to evaluate the petrological significance ofcarpholite proper, Mn^2$ Al₂[Si₂O₆](OH)₄. Metamorphic P–Tconditions of these rocks are estimated to be {small tilde}300C1–2 kbar, which is in accordance with the exclusive occurrenceof carpholite in low-P rocks such as hydrothermal environmentselsewhere. Carpholite of the Lienne syncline exclusively occursin quartz-rich segregations. Its composition is close to end-member.Thermodynamic calculations confirm that carpholite is a stablephase at low-pressure–low-temperature conditions, in contrastto ferro- and magnesiocarpholite, which are high-pressure minerals.No information is available on the high-P behaviour of carpholite.The occurrence of carpholite is partly closely associated withspessartine-bearing country rocks, or carpholite is alteredto assemblages with spessartine, sudoite, chlorite, muscoviteand paragonite. Spessartine in these rocks contains minor amountsof hydrogarnet component {(H/4)/[Si$(H/4)] = 0.03–0.06}.The presence of carpholite-spessartine assemblages in theselow-P rocks is in contrast to high-pressure metamorphic rocksfrom other areas, where parageneses such as fem/magnesiocarpholite–chloritoidor magnesiocarpholite–chlorite–kyanite occur. Theappearance of carpholite–garnet assemblages in low-P Mn-richrocks can be explained by contrasting phase relations becauseof a high Mn–Mg partition coefficient between the mineralsunder consideration. In rhodo-chrosite-bearing veins in theLienne syncline, nearly complete replacement of carpholite byspessartine and chlorite is due to the continuous reaction carpholite$ rhodochrosite $ quartz = spessartine $ chlorite $ H₂O $ CO₂,which defines a very low X_co, in the temperature range underconsideration. It is suggested that spessartine (possibly containingsome hydrogarnet component), during prograde metamorphism atlow pressure, becomes stable at a temperature of {small tilde}300C KEY WORDS: carpholite; spessartine; sudoite; Venn–Stavelot Massif; Ardemes ^*Corresponding author. Fax: x49/531/3918131. e-mail: t.theye{at}tu.bs.de     

Stability Relations of the Pyroxenes and Olivine in Certain High Grade Metamorphic Rocks

The stability relations of mineral assemblages consisting ofvarious combinations of Capyroxene, orthopyroxene, olivine,quartz, dolomite, calcite, and graphite are treated by analyticalmeans with emphasis on reactions of the following type: MgSiO₃CaCO₃SiO₂CaMgSi₂O₆CO₂ (1) 3MgSiO₃CaCO₃ Mg₂SiO₄CaMgSi₂O₆CO₂ (2) and the associated reactions involving ferrous iron. In particular,reaction (1) is discussed in detail in terms of uncertaintiesinherent in the thermochemical data and the limits placed onits equilibrium constant by the geological conditions of theoccurrence of the observed assemblages. It is shown from theequation of equilibrium of (1) that the composition of orthopyroxeneis a linear function of the carbon dioxide fugacity and thatthe latter is lowered by the presence of the FeSiO₃ component.The latter conclusion is compatible with the rarity of the associationcalcite-orthopyroxcne and its apparent confinement to iron-richrocks. An analysis of the same assemblage involved in reaction (1)indicates that when graphite coexists with the iron-rich pyroxene,the fugacity of oxygen falls near the center of the magnetitefield for a wide range of CO₂ fugacities and temperatures. It is shown that reaction (2) is also displaced strongly tothe right under most metamorphic conditions, which is compatiblewith fairly common occurrence of the association of Capyroxeneand olivine. Finally, a critique is presented on the possibility of the useof the pyroxene-bearing assemblages as indicators of metamorphicgrade and it is concluded that this is not rigorously possiblesince the systems almost invariably possess too many degreesof freedom.     

The Distribution of H2O between Cordierite and Granitic Melt: H2O Incorporation in Cordierite and its Application to High-grade Metamorphism and Crustal Anatexis

Experiments defining the distribution of H₂O [D_w = wt % H₂O(melt)/wt% H₂O(crd)]) between granitic melt and coexisting cordieriteover a range of melt H₂O contents from saturated (i.e. coexistingcordierite + melt + vapour) to highly undersaturated (cordierite+ melt) have been conducted at 3–7 kbar and 800–1000°C.H₂O contents in cordierites and granitic melts were determinedusing secondary ion mass spectrometry (SIMS). For H₂O vapour-saturatedconditions D_w ranges from 4·3 to 7 and increases withrising temperature. When the system is volatile undersaturatedD_w decreases to minimum values of 2·6–5·0at moderate to low cordierite H₂O contents (0·6–1·1wt %). At very low aH₂O, cordierite contains less than 0·2–0·3wt % H₂O and D_w increases sharply. The D_w results are consistentwith melt H₂O solubility models in which aH₂O is proportionalto X_w² (where X_w is the mole fraction of H₂O in eight-oxygenunit melt) at X_w     

Selected Phase Relationships in the System Al-Mn-Fe-Si-O-H: A Model for Garnet Equilibria

The bulk compositions 3FeO_x.Al₂O₃.3SiO₂ $ excess H₂O and 3MnO.Al₂O₃.3SiO₂$ excess H₂O were investigated employing conventional hydrothermaltechniques. Almandine and spessartine were synthesized and stabilityrelationships determined in terms of temperature, fluid pressure,and oxygen fugacity. Synthetic almandine has unit cell edge, a₀ = 11.528 0.001 index of refraction, N_D = 1.829 0.003. No systematic variationsof these values with respect to temperature, fluid pressure,and oxygen fugacity were observed. Spessartine, synthesizedat high temperatures, has average values of a₀ = 11.614 0.001 and N_D = 1.799 0.003. However, below about 600 C a₀ graduallyincreases to 11.635 0.001 and N_D decreases to 1.772 0.003with decreasing temperature, irrespective of fluid pressureand oxygen fugacity. These changes appear to reflect the productionof hydrospessartine below about 600 C. The stability of almandine strongly depends on the oxygen fugacity.It is stable up to the vicinity of oxygen fugacities definedby the fayalite–magnetite$quartz buffer; the low f_o2,range has not been determined, but lies at oxygen fugacitiesless than those defined by the ironquartz–fayalite buffer.The stability field of almandine$fluid is bounded by the followingP_fluid-T values. At low oxidation states, the low temperature hydrous assemblageof equivalent composition consists of quartz$iron chlorite ($magnetite)$fluidand the high temperature equivalent assemblage consists of fayalite$ironcordierite$hercynite₈₈$fluid. Where f_O2 approximates or is inexcess of that defined by the fayalite–magnetite$quartzbuffer the low temperature hydrous assemblages consist of quartz$ironchlorite$magnetite$fluid, iron chlorite$pyrophyllite$magnetite$fluid,magnetite$mullite$pyrophyllite$fluid, and hematite$mullite$pyrophyllite$fluid;the anhydrous equivalent assemblages consist of quartz$hercynite₈₈,$magnetite₈₈$fluid, quartz$mullite$magnetite$fluid, and quartz$mullite$hematite$fluid,both in order of increasing oxygen fugacity. The stability of spessartine, in contrast to that of almandine,is essentially independent of oxygen fugacity at least up tothat defined by the magnetite-hematite buffer. Spessartine isstable up to the highest temperature, 930 C, employed in thisinvestigation at P_fluid = 500 bars. However, it decomposes toa hydrous assemblage consisting of quartz$manganese chlorite$fluidat the following P_fluid-T values: 414 5 C and 3000 bars;405 5C and 2000 bars; 386 10 C and 1000 bars; 3645C and 500 bars. Garnets are rare constituents of igneous rocks; those whichdo occur are predominantly spessartine-rich, and are virtuallyconfined to felsic magmas. Garnets are absent from mafic igneousrocks because the thermal stability ranges of iron-rich membersare below the solidus. The near absence of almandine in contactmetamorphosed pelitic rocks may reflect a relatively high oxidationstate in the aureoles rather than inappropriate P-T conditions.It is argued that the compositions of pyralspite garnets inpelitic schists are subject to various physical and chemicalfactors, including f_O2. With appropriate provisions, the Mn/Feratios of garnet coexisting with chlorite and quartz might beused as a temperature indicator. The rarity of spessartine in igneous and metamorphic rocks apparentlystems from the departure of rock bulk composition from Mn-richvalues rather than from the absence of appropriate physicalconditions.     

Reactions Leading to the Disappearance of Pumpellyite in Low-grade Metamorphic Rocks of the Sanbagawa Metamorphic Belt in Central Shikoku, Japan

The major mineral assemblages of the metabasites of the Omoiji-Nagasawaarea in central Shikoku are hematite+epidote+chlorite+actinolite,riebeckitic actinolite+epidote+chlorite, epidote+chlorite+actinolite,and pumpellyite+epidote+chlorite+actinolite. The constituentminerals are often heterogeneous and assemblages in the fieldof a thin section sometimes do not obey the phase rule, butif grains apparently in non-equilibrium with others are excludedand domains of chemical equilibrium are appropriately chosenthe assemblages approximately obey the phase rule. The stability of hematite, pumpellyite, and epidote associatedwith chlorite and actinolite can be dealt with in terms of aternary system with appropriate excess phases. By fixing theFe²⁺/(Fe²⁺ +Mg) ratio of chlorite, it is dealt with in termsof stability relations in the system Ca₂Al₃Si₃O₁₂(OH)–Ca₂AlFe₂Si₃O₁₂(OH)with excess chlorite, actinolite, quartz, and controlled P_H2O.The maximum and minimum Fe³⁺ contents of epidote in this modelsystem are determined by hematite+epidote+chlorite+actinoliteand pumpellyite+epidote+chlorite+actinolite assemblages. Themaximum Fe³⁺ of the three phase assemblage epidote+chlorite+actinoliteis insensitive to temperature, but the minimum Fe³⁺ contentof epidote is sensitive to temperature and can be used to definethe metamorphic grade by a continuous quantity related to temperature.The phase relations expected for the model system are in goodagreement with the parageneses of the Sanbagawa terrain in centralShikoku and offer an explanation to the rule of Miyashiro &Seki (1958a) that the compositional range of epidote enlargeswith increasing temperature. The model also makes it possibleto estimate semi-quantitatively the temperature range in whichthe assemblage pumpellyite+epidote+chlorite+actinolite is stable.The possible maximum range is about 120 ?C, but the assemblageis stable in metabasite only for about 90 ?C. The higher temperaturelimit of the pumpellyite-actinolite facies defined by the disappearanceof pumpellyite in metabasite corresponds to the temperatureat which epidote with Fe³⁺/(Fe³⁺ +Al) = 0.10 0.15 coexistswith pumpellyite, actinolite, and chlorite. The compositions of epidotes in the metabasites of the Omoiji-Nagasawaarea cluster around Fe³⁺/(Fe³⁺ +Al) = 0.33. The grade of thisarea is close to the lower temperature stability limit of thepumpellyite+epidote+chlorite+actinolite assemblage.     

Chlorite-Biotite Equilibrium Reactions in a Carbonate-Free System

In a system open to both potassium and water, the conditionsfor two different chloritebiotite equilibrium reactions maybe depicted on an isobaric temperature–(K)/(H) diagram.At moderate values of the (K)/(H) ratio in the fluid phasepervading the system, muscovite is involved in the equilibriumreaction: chloritequartzKOH= biotitemuscoviteH₂O. At high values of (K)/(H) K-feldspar is involved in the equilibriumreaction: chlorite quartzKOH = biotiteK-feldsparH₂O. When the same system is closed to potassium, assemblages formingat lower values of (K/(H) resemble those developed on progressivemetamorphism, whereas higher (K)/ (H assemblages in the closedsystem resemble certain retrogressive mineral assemblages. Theassemblage muscovitebiotite is characteristic of higher temperaturesand lowyer values of µ_K/µ_S than the assemblageK-feldsparchlorite in carbonate-free rocks where both potassiumand water were perfectly mobile components and SiO₂ was presentin excess.