Petrology of some Glaucophane Schists and Related Rocks from Taiwan

Detailed laboratory study has been made on pre-Tertiary coarse-grainedglaucophane schist, garnet-epidote amphibolite, and epidoteamphibolite in the eastern slope of the Central Mountain Range,Taiwan. These petrotectonic assemblages are considered to beexotic tectonic blocks emplaced within the feebly metamorphosedin situ graphite and quartzose schists of the Yuli belt. Thinlenses of Mn-rich metamorphosed tuff are intercalated withinthe metabasaltic rocks. Such high MnO (2 wt. per cent) and lowMgO (3–4 wt. per cent) tuffaceous rocks are similar inbulk composition to some volcanic clays collected in deep oceanbasins. They consist of the characteristic assemblage Mn-bearinggarnet (5–7 wt. per cent MnO and 30 volume per cent inthe rock)+muscovite+epidote+hornblende+quartz+ albite+rutile?pyrite. Successive stages of conversion of garnet-epidote amphiboliteto blueschist assemblages were noticed. The most recrystallizedschists display abundant Mn-bearing garnet, zoned amphibole,phengite, zoned epidote, stilpnomelane, chlorite, quartz, minoralbite, magnetite, and sphene. The recrystallization processis nearly isochemical except the glaucophane schists appearto be more oxidized and contain more Na₂O than the relict amphibolites.Intimately associated amphibolites of basaltic composition,in contrast, contain the assemblage hornblende+paragonite+epidote+chlorite+quartz+albite+rutile. Microprobe analyses of the coexisting minerals in glaucophaneschists, garnet-epidote amphibolites and epidote amphibolitesyield the following results: (1) garnets, consisting of almandine,spessartine, and grossular components, are less Mn and Mg-richcompared to those in in situ metabasalts of the Franciscan;(2) rim epidotes of the glaucophane schists are more pistastic(X_Fe=0?27–0?30) than that of the garnet-epidote amphibolite(0?2–0?22) implying higher fO₂ values for the glaucophanization;(3) phengitic micas of the glaucophane schist have less Al₂O₃content (29 wt. per cent) than those of the garnet-epidote amphibolite(32 wt. per cent) whereas micas of epidote amphibolites areparagonites with K/(K+Na) ratio of 0?04; (4) the zoned amphibolesshow glaucophane occurring marginal to cores of calcic amphibole.Sodic amphiboles with Al₂O₃ of 6-? to 10?4 wt. per cent arecrossite-glaucophane whereas all calcic amphiboles analyzedare barroisite-pargasite (Al₂O₃ greater than 10 wt. per cent). The garnet-epidote-rutile bearing glaucophane schist of Taiwanprobably recrystallized at temperatures above 350 ?C (the epidotezone) whereas the lawsonite-sphene glaucophane schists of theFranciscan equilibrated below 350 ?C (the lawsonite zone). TheMn-rich basaltic tuffs and their associated flows appear tohave been metamorphosed at profound depths and at the relativelyhigh temperatures of the epidote amphibolite facies, succeededlater by glaucophane schist facies metamorphism at lower temperatures.     

Cordierite-Cummingtonite Facies Rocks from the Gold Brick District, Colorado

The unusual association of cordierite and cummingtonite (? gedrite+ chlorite + biotite + ilmenite + plagioclase + quartz) definesa metamorphic facies within aluminous, low-Ca amphibolites fromthe Proterozoic rocks of the Gold Brick District, east of Gunnison,Colorado. More Fe-rich bulk chemistries in the same facies arecharacterized by assemblages consisting of cordierite+-gedrite+ garnet + chlorite + biotite + ilmenite + plagioclase + quartz,whereas more Mg-rich compositions are characterized by cordierite+ anthophyllite + chlorite + biotite + ilmenite ? plagioclase+ quartz. The assemblage gedrite 4- cummingtonite + chlorite+ biotite + ilmenite + plagioclase + quartz was also observed.Coexisting cordierite+ anthophyllite + cummingtonite was notobserved in any rocks, apparently because this assemblage isstable over only a very narrow range of bulk compositions. Metamorphosedpelitic rocks are more iron rich than the assemblage cordierite+ gedrite + garnet + chlorite + biotite + ilmenite + plagioclase+ quartz and consist of garnet ?cordierite ?staurolite ? chlorite? andalusite + biotite + ilmenite + plagioclase + quartz? microclineor muscovite. Mineral rim compositions from cordierite-bearing amphibolitesand metapelites determined by electron microprobe analysis showsystematic Fe/Mg partitioning and define assemblages that occupynon-overlapping regions of the compositional system SiO₂-TiO₂-Al₂O₃-MnO-FeO-MgO-CaO-Na₂O-K₂O-H₂Oas determined by algebraic and statistical methods developedby Braun & Stout (1975) and Fisher (1989). Graphical methods(projections) produced spurious overlaps not confirmed by themore rigorous algebraic tests. The spurious overlaps were generatedbecause standard projective analysis was not able simultaneouslyto account for the important effects of the components Na₂O,CaO, and MnO on the AFM topologies. The results of algebraicand statistical analysis are consistent with an equilibriumorigin at constant values of temperature and pressure. The cordierite-cummingtonite facies encompasses the relativelylow-pressure and moderate-temperature conditions associatedwith the stability field of andalusite. Garnet-biotite geothermo-metry,and garnet, aluminosilicate, silica, plagioclase (GASP) geobarometrysuggest that temperatures and pressures were nearly constantacross the study area at 550( ? 70) ?C and 3 kb, respectively,near the peak of metamorphism. Other geothermometers and geobarometers,and independent pressure and temperature estimates, are compatiblewith garnet-biotite thermometry and GASP geo-barometry. Gradientsin fO₂ or H₂O are not required to explain the compatibilityof these assemblages at constant T and P. Cordierite + cummingtonite-bearingrocks can apparently be derived from anthophyllite +garnet-bearingrocks by increasing temperature or decreasing pressure.     

Pumpellyite–actinolite facies of the Sanbagawa metamorphism

The pumpellyite–actinolite facies proposed by Hashimoto is defined by the common occurrence of the pumpellyite–actinolite assemblage in basic schists. It can help characterize the paragenesis of basic and intermediate bulk compositions, which are common constituents of various low-grade metamorphic areas. The dataset of mutually consistent thermodynamic properties of minerals gives a positive slope for the boundary between the pumpellyite–actinolite and prehnite–pumpellyite facies in P–T space. In the Sanbagawa belt in Japan, the mineral parageneses of hematite-bearing and -free basic schists, as well as pelitic schists have been well documented. The higher temperature limit of this facies is defined by the disappearance of the pumpellyite+epidote+actinolite+chlorite assemblage in hematite-free basic schists with X_Fe3+ of epidote around 0.20–0.25 and the appearance of epidote+actinolite+chlorite assemblage with X^Ep_Fe3+≤0.20. In hematite-bearing basic schists, there is a continuous change of paragenesis to higher grade, epidote–glaucophane or epidote–blueschist facies. In pelitic schists, the albite+lawsonite+chlorite assemblage does occur but only rarely, and its assemblage cannot be used to determine the regional thermal structure. The lower temperature equivalence of the pumpellyite–actinolite assemblage is not observed in the field. The Mikabu Greenstone complex and the northern margin of the Chichibu complex, which are located to the south of the Sanbagawa belt, are characterized by clinopyroxene+chlorite or lawsonite+actinolite assemblages, which are lower temperature assemblages than the pumpellyite+actinolite assemblage. These three metamorphic complexes belong to the same subduction-metamorphic complex. The pumpellyite–actinolite facies or subfacies can be useful to help reveal the field thermal structure of metamorphic complexes     

Gradients in fluid composition across metacarbonate layers of the Wepawaug Schist, Connecticut, USA

Metamorphic index mineral zones, pressure-temperature (P-T) conditions, and CO₂-H₂O fluid compositions were determined for metacarbonate layers within the Wepawaug Schist, Connecticut, USA. Peak metamorphic conditions were attained in the Acadian orogeny and increase from ~420 °C and ~6.5 kb in the low-grade greenschist facies to ~610 °C and ~9.5 kb in the amphibolite facies. The index minerals oligoclase, biotite, calcic amphibole, and diopside formed with progressive increases in metamorphic intensity. In the upper greenschist facies and in the amphibolite facies, prograde reaction progress is greatest along the margins of metacarbonate layers in contact with surrounding schists, or in reaction selvages bordering syn-metamorphic quartz veins. New index minerals typically appear first in these more highly reacted contact and selvage zones. It has been postulated that this spatial zonation of mineral assemblages resulted from infiltration, largely by diffusion, of water-rich fluids across lithologic contacts or away from fluid conduits like fractures. In this model, the infiltrating fluids drove prograde CO₂ loss and were derived from surrounding dehydrating schists or sources external to the metasedimentary sequence. The model predicts that significant gradients in the mole fraction of CO₂ (X_CO2 X_{CO_2 } ) should have been present during metamorphism, but new estimates of fluid composition indicate that differences in X_CO2 X_{CO_2 } preserved across layers or vein selvages were very small, ~0.02 or less. However, analytical solutions to the two-dimensional advection-dispersion-reaction equation show that only small fluid composition gradients across layers or selvages are needed to drive prograde CO₂ loss by diffusion and mechanical dispersion. These gradients, although typically too small to be measured by field-based techniques, would still be large enough to dominate the effects of fluid flow and reaction along regional T and P gradients. Larger gradients in fluid composition may have existed across some layers during metamorphism, but large gradients favor rapid reaction and would, therefore, seldom be preserved in the rock record. Most of the H₂O needed to drive prograde CO₂ loss probably came from regional dehydration of surrounding metapelitic schists, although H₂O-rich diopside zone conditions may have also required an external fluid component derived from syn-metamorphic intrusions or the metavolcanic rocks that structurally underlie the Wepawaug Schist.     

Petrogenesis of Glaucophane Schists

Some glaucophane schists are chemically indistinguishable fromgreenschists and epidote amphibolites. Provided all three rocktypes represent equilibrium assemblages, they must have formedunder differing physical conditions. The mineralogy of suchglaucophane schists taken in conjunction with experimental evidencesuggests that these rocks formed at low temperatures and atrelatively elevated pressures. The relatively high-pressure,low-temperature phases lawsonite, jadeitic pyroxene, and metamorphicaragonite are diagnostic of physical conditions attending thismetamorphism. Differential stress may aid in the attainmentof the appropriate mean pressure necessary for the productionof these phases. Graphic analysis and approximated thermodynamic calculationsindicate that relatively elevated pressures, or relatively lowtemperatures, or both, promote the formation of glaucophanein rocks of a wide range of bulk compositions while restrictingthe compositional range of albite-bearing rocks. It is concludedthat the coexistence of glaucophane with carbonate, calcium-aluminumsilicate or paragonite results from such physical conditions,and it is on the basis of these associations or, equally well,the presence of lawsonite, jadeitic pyroxene, or metamorphicaragonite that the blueschist facies should be defined. High pressures are not required for the production of glaucophaneitself. It is stable under physical conditions present in thegreenschist and epidote amphibolite facies in rocks deficientin CaO and rich in Na₂O and MgO relative to A1₂O₃. Such bulkcompositions might result from exchange of material betweenserpentinite and albite-bearing country rocks, and could accountfor glaucophane aureoles around, and inclusions of glaucophanerock within, some serpentinites.     

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

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

Mineralogy and Petrology of the Metamorphosed Wabush Iron Formation, Southwestern Labrador

The Wabush Iron Formation, of late Precambrian (Proterozoic)age is part of the Labrador Trough in southwestern Labrador,Canada. It is the regionally metamorphosed equivalent of lowgrade metamorphic (chlorite zone) iron-rich sediments of thecentral part of the Labrador Trough. The metamorphic grade iskyanite-staurolite zone, as concluded from conformably underlyingpelitic schist assemblages. Sedimentary textural features suchas very pronounced banding and a very rare occurrence of relicgranules are still preserved. The iron formation consists mainly of quartz, specularite, magnetite,cummingtonite-grunerite, and ferrodolomite-ankerite. Less commonare actinolite, anthophyllite, riebeckitetremolite, magnesioriebeckite,ferrosalite, orthopyroxene, aegirine-augite, aegirine, rhodonite,garnet (almandine, spessartine, calderite), siderite, rhodochrosite,calcite, and kutnahorite. Conventional wet chemical analyses or electron microprobe analyseshave been made of thirty-four phases belonging to the abovelist. Six additional electron probe analyses have been madeof phases from the underlying pelitic schists. All conventionallyanalyzed phases are characterized by complete optical, unitcell parameter, and density measurments. The analyzed assemblages from the silicate and silicate-carbonateiron formation include grunerite-ferrosalite, grunerite-eulite-siderite,grunerite-actinolite, grunerite-almandine, cummingtonite-spessartine,rhodonite-kutnahorite-calderite, aegirine-augite-riebeckite-tremolite,magnesioriebeckite-cummingtonite-rhodonite, aegirine-augite-rhodonite-rhodo-chrosite,and aegirine-rhodonite-calderite-rhodochrosite. The assemblages are concluded to be equilibrium assemblages.Of the volatile components, O₂, CO₂, and H₂O, O₂, is concludedto have behaved as an inert (buffered) component. Variationsin the activity of CO₂ are concluded to have existed betweensilicate-oxide and carbonate-oxide members of the iron formation.It is not clear, however, whether CO₂ has acted as a perfectlymobile component with strong aco₂ gradients throughout the area,or as an inert component in some parts of the area. H₂O is consideredto have been perfectly mobile. An increase in Mg/(Mg+Fe) ratioin ferromagnesian silicates is correlated with an increase inthe oxidation state of the assemblage. A similar increase in(Mg+Mn)/(Mg+Mn+Fe) is found in manganoan ferromagnesian silicateswith increasing activity of O₂. A number of ferromagnesian silicatescontain large amounts of Na⁺ and Fe³⁺ as a result of the verylow Al₂O₃ content of the iron formation. The P and T conditionsof metamorphism are deduced from experimental studies applicableto the underlying pelitic schists.     

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.     

Petrology of an Andalusite-Type Regional Metamorphic Terrane, Panamint Mountains, California

The terrane in the Panamint Mountains, California, was regionallymetamorphosed under low-pressure conditions and subsequentlyunderwent retrograde metamorphism. Prograde metamorphic isogradsthat mark the stability of tremolite + calcite, diopside, andsillimanite indicate a westward increase in grade. The studywas undertaken to determine the effects of the addition of Caon the types of assemblages that may occur in pelitic schists,to contribute to the understanding of the stability limits inP – T – aH₂O – X_Fe of the pelitic assemblagechlorite + muscovite + quartz, and to estimate the change inenvironment from prograde to retrograde metamorphism. Peliticassemblages are characterized by andalusite + biotite + stauroliteand andalusite + biotite + cordierite. Within a small changein grade, chlorite breaks down over nearly the entire rangein Mg/(Mg + Fe) to biotite + aluminous mineral. Chlorite withMg/(Mg + Fe) = 0.55 is stable to the highest grade, and thegeneralized terminal reaction is chlorite + muscovite + quartz= andalusite + biotite + cordierite + H₂O. Calcic schists arecharacterized by the assemblage epidote + muscovite + quartz+ chlorite + actinolite + biotite + calcite + plagioclase atlow grades and by epidote + muscovite + quartz + garnet + hornblende+ biotite + calcite + plagioclase at high grades. Epidote doesnot coexist with any AFM phase that is more aluminous than garnetor chlorite. Lithostatic pressure ranged from 2.3 kb to 3.0kb. During prograde-metamorphism temperatures ranged from lessthan 400° to nearly 700°C, and X_H2O (assuming P_H2O +P_CO3 = P_total) is estimated to be 0.25 in siliceous dolomite,0.8 in pelitic schist, and 1.0 in calcic schist. Temperatureduring retrograde metamorphism was 450° ± 50°C,and all fluid were H₂O-rich. A flux of H₂O-rich fluid duringfolding is believed to have caused retrograde metamorphism.The petrogenetic grid of Albee (1965b) is modified to positionthe (A, Cd) invariant point relative to the aluminosilicatetriple point, which allows the comparison of facies series thatinvolve different chloritoid-reactions.     

Genesis of Peraluminous Granites II. Mineralogy and Chemistry of the Tourem Complex (North Portugal). Sequential Melting vs. Restite Unmixing

The Tourem granitic complex (North Portugal) consists of quartz-and alkali-feldspar-rich felsic granites, biotite- and plagioclase-richheterogeneous granites, and cordierite-biotite granites, containingnumerous enclaves of orthogneisses and metapelitic schists.Mineralogical, chemical, and experimental data suggest thatall the granites and the orthogneiss enclaves are geneticallyrelated. The felsic granites are characterized by normally zoned plagioclase,absence of cordierite, high SiO₂ and K₂O (72–74 wt.% and5?4–6?4 wt.%, respectively), moderate P₂O₅ and REE (0?22–0?24%and 85?0–95?7 ppm), and low Fe₂O₃^* and Zr contents (1?3–1?5%and 80–90 ppm). These features are consistent with thoseof restite-free melts formed by low extents of melting. Meltingexperiments show that these felsic granites are likely to bederived by melting of a source material similar to the orthogneissenclaves under low water activities (0?5), at relatively hightemperature ( 800?C) and <30% melting. The heterogeneous and cordierite-biotite granites display highcordierite contents (up to 30%) in addition to biotite (5–25%),complexly zoned plagioclase, and high Fe₂O₃ (2?72–6?99%),CaO (0?56–1?95%), Zr (101–213 ppm), and Ce (39?8–98?1ppm) contents, suggesting that the melts contained significantproportions of residual biotite, cordierite, plagioclase, andaccessories. Experimental data indicate that the melts weregenerated under water-undersaturated conditions but by higherextents of melting (30–60% melting) with probably a largeramount of available water compared with the felsic granites. The major and trace element chemical trends of the granites,which do not define single arrays on two-element variation diagrams,and experimental data show that the generation of the Touremanatectic complex cannot be explained by the restite unmixingmodel but could have resulted from sequential low extents ofmelting with efficient melt segregation followed by higher extentsof melting with restite retention.     

Constant Ps-T Amphibolite to Granulite Facies Transition in Agto (West Greenland) Metadolerites: Implications and Applications

Within the Isortoq Complex of central West Greenland, doleritedykes have been metamorphosed in upper amphibolite to garnetgranulite facies. Detailed sampling and petrographic study ofthese dykes demonstrate that the entire sequence of assemblagesmay occur in individual dykes, with the amphibolite facies assemblagesnormally restricted to dyke margins, garnet granulite faciesrocks occurring in the dyke cores, and pyroxene granulite faciesrocks intervening between these two. Electron microprobe analysisof all coexisting phases demonstrates that the progression fromamphibolite facies to granulite facies assemblages results inthe extraction of edenite and tschermakite components from theamphibole in approximately a 2: 1 ratio. As amphibole breakdownproceeds, systematic changes in element abundance within amphiboleoccur when reactions are crossed, due to the development ofnew phases. The algebraic removal of small but systematic compositionalvariations within the dykes allows derivation of six equilibria,which provide a general model for metamorphism of basaltic rocksunder high grade metamorphic conditions. This model takes theform of the idealized assemblage sequences which will developalong (regional) gradients in X_H2O, temperature and/or solidpressure. Analysis of these equilibria demonstrates that variablesilica activity and Fe²⁺–Mg²⁺ exchange will modify reactionstoichiometry, will affect the ratio of edenite to tschermakitein amphibole breakdown reactions, and will modify the compositionof the product plagioclase. These variations together with variationsin water fugacity will locally modify the ideal assemblage sequences,and will explain the complex assemblage relationships observedin metabasic rocks of natural high grade terranes. ^*Present address: Instituttet for Teknisk Geologi, Danish Technical University, Building 204, DK–2800 Lyngby, Denmark     

Degree of Oxidation of Adirondack Iron Oxide and Iron-Titanium Oxide Minerals in Relation to Petrogeny

Partial chemical analyses of the accessory iron oxide and iron-titaniumoxide minerals from more than one hundred samples of Adirondackigneous, metamorphic, and metasomatic rocks portray the degreeof oxidation of the minerals and afford a basis for a discussionof their relation to the accompanying silicate assemblage andpetrogeny. Among the granitic igneous rocks, the green pyroxenicfacies have the least oxidized iron-titanium oxide mineralsand a pink potassium-rich microperthitic microcline facies themost oxidized. Granites and syenites may crystallize with eitheran iron-rich variety of hornblende (or pyroxene) plus ilmenite,or with an iron-magnesium variety of hornblende (or of pyroxene)plus magnetite plus ilmenite, depending on the degree of oxidation.The granite facies of the orthogneisses of the Diana Complex(Precambrian) have a more oxidized mineral assemblage than thesyenitic and quartz syenitic facies. The anorthosites and gabbroanorthosites have relatively oxygen-rich oxide minerals. Orthogneissesin the granulite facies have a lower ratio of Fe₂O₃/FeO thansimilar gneisses in the amphibolite facies. At least part ofthis lower ratio is due to metamorphism at higher temperaturesand pressures. A regional belt of granitic orthogneiss metamorphosedin the amphibolite facies has a magnetite-sphene instead ofa magnetite-ilmenite assemblage. Metasomatism of biotite-quartz-plagioclasegneiss to sillimanitic quartz-microcline gneisses is accompaniedby decrease in mafic silicates and a series of changes of mineralsin intermediate stages such that, in general, biotite yieldswith increasing degrees of oxidation successively such assemblagesas (1) biotite, garnet, magnetite, hemo-ilmenite, and ilmeno-hematite;(2) pale brown mica, sillimanite, magnetite, and ilmeno-hematite;and (3) rutilo-hematite, rutile, and meagre silicates includingsillimanite, muscovite, and chlorite. Diabase altered to monzodioriteby potassium-bearing solutions has ilmeno-magnetite partly alteredto ilmeno-maghemite.     

Pumpellyite-Actinolite Facies Schists of the Taveyanne Formation near Lo?che, Valais, Switzerland

Electron microprobe analyses are presented for new-formed mineralsfrom a small exposure of semi-schistose Taveyanne Formationof the pumpellyite-actinolite facies near Lo?che, Valais. Comparisonsare drawn with minerals of other low-grade metamorphic areas,especially in southern New Zealand. Sphene shows considerablesubstitution of Ca(Al,Fe)SiO₄(OH) for CaTiSiO₅. Epidotes aresharply divided into early pistacitic (Ps = 0.28–0.37)and later clinozoisitic varieties (Ps = 0.11–0.19). Pumpellyitesrange from pumpellyite-(Fe) to pumpellyite-(Al) and are generallyless Fe-rich than those of zeolite and prehnite-pumpellyitefacies. Pumpellyite inclusions in albitized plagioclase areparticularly low in Mg. Actinolites are low in A1₂O₃, TiO₂,and Na₂O, essentially identical compositions being nucleatedon detrital augite, hornblende, and in the matrix. Phengitesare also extremely low in Na₂O and TiO₂. Chlorites are ripidolites.Albitized clastic plagioclase has the composition An0.7–1.6and albite in clinozoisite-calcite-albite-phengite-chloriteveins An2.1–2.3. Calcites carry minor Mn > Fe ? Mg.New-formed iron oxides are absent, whereas pyrrhotite and minorpyrite occur in one rock, buffering fs₂ and indicating low fo₂. Ratios Mg: Fe^* (Fe^* = total Fe) in coexisting chlorites andA1, Na-poor actinolites vary sympathetically both in the Lo?cheand southern New Zealand rocks here considered, giving K_D =(Mg/Fe^*) _actlnolIte/(Mg/Fe^*)_chlorle = 1.72. Mg/Fe^* ratios inpumpellyites tend to vary sympathetically with those of coexistingchlorites and actinolites but are more variable. Substitutionof (Fe, Mg)Si for A1₂ in phengitic micas and chlorites variessympathetically in the same suites between mafic volcanic andmore pelitic extremes. Various minor elements also behave ina consistent fashion, indicating an encouraging tendency towardsequilibrium. Variable (though small) A1₂O₃ contents of actinolite,Fe: Al ratios in epidotes and pumpellyites, and Mg: Fe^* ratiosin phengites, even within a single grain, are evidence of short-rangedisequilibrium; metamorphic equilibration is evidently easierbetween some crystal structures and structural sites than betweenothers. In phase rule analysis of assemblages in such rocks it is commonlynecessary to treat Fe²O₃, FeO, and MgO as separate componentsand it may also be necessary to regard CO² as an inert componentand/or to interpret observed assemblages as of low variance.The presence of the Ca-Al silicates and sphene indicates verylow X_co2 in the metamorphic fluids in all rocks examined exceptan albite-chlorite-calcite-quartz-anatase assemblage. But higherAn in albites than in isofacial and in greenschist facies rocksof southern New Zealand can be ascribed to significantly higherXco₂ at Lo?che, especially in the veins, than in New Zealand. Pumpellyite and epidotes of the pumpellyite-actinolite faciestend to be lower in Fe and richer in Al than those of lowergrade facies. Important reactions include those of the formpumpellyite-(Fe³⁺)+chlorite+quartz+H₂=pumpellyite-(Al)+actinolite,and pumpellyite+chlorite+quartz- ‘epidote’+actinolite+water.Careful selection of pumpellyite and chlorite compositions isrequired for experimental and chemographic analysis of pumpellyitestability. In the absence of critical data, temperatures ofabout 250–350? and pressures of several kilobars are provisionallysuggested for the Lo?che metamorphism.     

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.     

Fluids in granulites of the Southern Marginal Zone of the Limpopo Belt, South Africa

The Southern Marginal Zone of the Limpopo Belt in South Africa is characterised by a granulite and retrograde hydrated granulite terrane. The Southern Marginal Zone is, therefore, perfectly suitable to study fluids during and after granulite facies metamorphism by means of fluid inclusions and equilibrium calculations. Isolated and clustered high-salinity aqueous and CO₂(-CH₄) fluid inclusions within quartz inclusions in garnet in metapelites demonstrate that these immiscible low H₂O activity fluids were present under peak metamorphic conditions (800-850 °C, 7.5-8.5 kbar). The absence of widespread high-temperature metasomatic alteration indicates that the brine fluid was probably only locally present in small quantities. Thermocalc calculations demonstrate that the peak metamorphic mineral assemblage in mafic granulites was in equilibrium with a fluid with a low H₂O activity (0.2-0.3). The absence of water in CO₂-rich fluid inclusions is due to either observation difficulties or selective water leakage. The density of CO₂ inclusions in trails suggests a retrograde P-T path dominated by decompression at T<600 °C. Re-evaluation of previously published data demonstrates that retrograde hydration of the granulites at 600 °C occurred in the presence of H₂O and CO₂-rich fluids under P-T conditions of 5-6 kbar and ~600 °C. The different compositions of the hydrating fluid suggest more than one fluid source.     

The Effects of Fe3? and Mn3? on Aluminum Silicate Phase Relations in North-Central New Mexico, U.S.A.

Aluminum silicate triple-point occurrences are common in metamorphicrocks of northern New Mexico. The three polymorphs show extensivesolid solution, with Fe and Mn substituting for Al. Mineraltextures, the spatial distribution of phases, and the systematicpartitioning of Fe and Mn indicate that the kyanite-andalusite-sillimaniteassemblages crystallized in equilibrium. The compositions ofminerals in the three-phase assemblage vary across the studyarea, recording regional variations in the pressures and temperaturesof metamorphism. The highest-pressure rocks, exposed at RioMora, contain kyanite at higher elevations and sillimanite atlower elevations. A sub-horizontal isograd separates the twominerals. Kyanite and sillimanite have nearly identical Fe contentwhich varies systematically with X_Fe2O3 in hematite or ilmenite.Andalusite occurs only along a single manganiferous layer, incrystals rich in MnAlSiO₅ and saturated in FeAlSiO₅. Triple-pointassemblages can be found wherever the folded manganiferous layercrosses the unfolded kyanite-sillimanite isograd. The TruchasRange, preserving slightly lower pressures of metamorphism,shows kyanite-andalusite-sillimanite in rocks with titaniferoushematite. Andalusite is enriched in Fe relative to kyanite andsillimanite, but no polymorphs contain Mn. Rocks with lowerX_Fe2O3 in hematite have kyanite and sillimanite without andalusite,whereas rocks with pure hematite contain only andalusite. Theshallowest erosional levels are preserved in the western PicurisRange where the three polymorphs occur as pure minerals in ilmenite-bearingrocks. Hematitic samples contain only andalusite which showsextensive solid solution of both Fe and Mn. The assemblage kyanite-andalusite-sillimanite is not invariant.Iron and manganese both add degrees of freedom. These transitionmetals have stabilized the three-phase assemblage, in apparentchemical equilibrium, across a P-T interval of 500-540 ?C, 3?8-4?6kb in rocks from New Mexico. The saturation level of FeAlSiO5in andalusite does not vary with Mn content but does vary withpressure and temperature. Calculations indicate that a 2-3 kbdecrease in pressure or a 25-50 ?C increase in temperature resultsin a 1 mole per cent increase in X_FeAlSiO5 in iron-saturatedandalusite.     

Eclogites and Associated Albite-Epidote-Garnet Paragneisses between Yambe and Cape Colnett, New Caledonia

High-pressure metamorphic rocks form a coastal belt, 175 kmby 35 km, in northeastern New Caledonia. Metamorphic grade rangesfrom lawsonite-albite schists through glaucophane-epidote schiststo omphacite-garnet-quartz gneisses. In the eclogitic terrane,metabasites, locally containing relict pillow structure andigneous textures, with well-preserved eclogitic mineral assemblages,are intercalated with metasedimentary gneisses containing albite-epidote-garnet? glaucophane and barroisite. Omphacite is partly retrogressedto albite and ferromagnesian minerals in almost every paragneiss.The paragneisses show strong evidence of penetrative foldingand microfracturing and were more permeable to metamorphic fluidsthan were the metabasites. The metabasites are inferred to havebeen relatively ‘dry’ and free of penetrative deformationduring the latter stages of metamorphism and thus were preservedmetastably during uplift, erosion, and cooling. Fe-Mg exchange thermometry between omphacite and garnet suggeststemperatures between 520 and 600 ?C. Omphacite + quartz (molper cent jadeite = 37–43) does not coexist stably withalbite suggesting minimum pressures near 12 kb at 550 ?C. Remnantsof more jadeite-rich pyroxenes in paragneisses (jd_50–60)suggest even higher pressure. The stable coexistence of chloritoidalmandine-quartz in paragneisses suggests relatively H₂O-rich fluids werein equilibrium with this assemblage. The widespread stable occurrenceof sphene suggests relatively low f_co2 during metamorphism.Late stage healed fractures in quartz contain H₂O-rich fluidinclusions with relatively low density isochores. Limited geochronologicdata combined with these petrologic data suggest a fairly rapidinitial rate of uplift followed by a much slower rate of uplift     

Tracking fluid flow during deep crustal anatexis: metasomatism of peridotites (Naxos, Greece)

Horizons of ultramafic lenses were metamorphosed with host felsic gneisses at upper amphibolite facies conditions during the M2 event on the island of Naxos, Greece. The synkinematic peak M2 Ol-Opx-Hbl-Chl-Spl assemblage of the Main, migmatite-associated, Ultramafic Horizon (MUH) retains mantle-like chemical and oxygen isotope compositions and thus shows no evidence of infiltration of fluids from the host rocks. A bimodal distribution of temperatures, grouped at 700 and 1,200 °C, is given by oxygen isotope Opx-Ol thermometry in the MUH meta-peridotites and indicates partial oxygen exchange during M2 superposed over previous mantle fractionation. The Agia Ultramafic Horizon (AUH), a coarser-grained and unfoliated peridotite, occurs within sillimanite gneisses in northwest Naxos and contains talc-enstatite and olivine domains. Recrystallization of the AUH peridotite during post-peak M2 infiltration of silica-rich, high '¹⁸O fluids is indicated by lack of deformation, increased activity of silica required to stabilize the talc-enstatite assemblage, extremely high '¹⁸O values of Ol and Opx and (¹⁸O_(Opx-Ol) temperatures of 520-650 °C. The source for these fluids is inferred to be aplitic and pegmatitic dikes emanating from the migmatitic core of Naxos and intruding the AUH. At peak M2 temperatures and during anatexis of gneisses, volumes of fluid were small and fluid composition was locally buffered in the deeper part of the Naxos section. Crystallization of melts within the migmatitic core released siliceous fluids and initiated an episode of retrograde hydrous metamorphism in the overlying sequences, as observed in the AUH.     

Primary Igneous Graphite in Ultramafic Xenoliths: I Petrology of the Cumulate Suite in Alkali Basalt Near Tissemt (Egg?r?, Alegerian Sahara)

Ultramafic xenoliths (harzburgite, olivine-orthopyroxenite,orthopyroxenite, websterite and clinopyroxenite) in a Plio-Quaternarystrombolian cone near Tissemt (Egg?r?, Hoggar, Algerian Sahara)contain large (up to 1 mm in diameter) euhedral flakes of graphite.These xenoliths are associated with mafic granulites free ofgraphite. Petrological, mineralogical, and geochemical dataindicate that these rocks have been scavenged from a Precambrianlayered intrusion emplaced in the deep crust. Textural evidencesuggests that the graphite could have crystallized relativelyearly from a silica-saturated melt: following cumulus crystallizationof olivine and orthopyroxene, the graphite crystallized, togetherwith olivine, orthopyroxene, and spinel, as a component of theintercumulus assemblage. The crystallization of graphite directlyfrom the melt is related to relatively high pressure (c. 5 kb)of carbon-rich fluid (CO+CO₂+H₂O) at relatively low oxygen fugacity(–logf_o2, 10 at 1200 ?C).     

Stability of Yoderite in the Absence and in the Presence of Quartz: an Experimental Study in the System MgO-Al2O3-Fe2O3-SiO2-H2O

The water-pressure temperature stability field of yoderite,ideally Mg₂Al_5.6Fe^{3 +} _0.4Si₄O₁₈(OH)₂, was determined at highoxygen fugacities by high-pressure bracketing runs on eightpossible breakdown reactions involving the phases chlorite,kyanite, talc, staurolite, pyrope, enstatite, boron-free kornerupine,cordierite, quartz, and invariably an excess of hematite. Yoderitewas found to be stable over the surprisingly large PT rangefrom 6 to 25 kbar water pressure and 590 to 795 C. It is thusa high-pressure mineral covering the upper amphibolite and portionsof the eclogite facies. In the presence of quartz its upperpressure stability is reduced to some 15 kbar, and its uppertemperature stability to 715 C. Two of the yoderite-producingreactions are anomalous as they show dehydration in the directiontowards lower temperatures. Importantly, this is also true forthe reaction kyanite + talc + hematite+H₂O=yoderite+quartz whichis responsible for the only yoderite occurrence in nature atMautia Hill, Tanzania. Preliminary thermodynamic calculationsindicate that—owing to this unusual dehydration behavior—thestability field for the assemblage yoderite+quartz disappearsfor water activities lower than 0.5. The rarity of yoderitein natural rocks, which is in contrast to its large PT stabilityfield, must be explained on chemical rather than on physicalgrounds. Yoderite can only occur in whiteschist-type bulk compositionsrich in MgO, Al₂O₃, SiO₂, and containing some iron, but poorin alkalis and CaO. Oxygen fugacities must be unusually highto keep Fe trivalent, and—at least for rocks with excessquartz—the water activity must be high as well. In anenvironment of this kind, yoderite formation in the Mautia Hillwhiteschist may have occurred even at constant total pressureand temperature simply by an influx of hydrous fluid duringthe late stages of metamorphism under amphibolite facies conditions.