The Garnet+Hornblende Isograd in Calcic Schists from an Andalusite-Type Regional Metamorphic Terrain, Panamint Mountains, California

Calcic schists in the andalusite-type regional metamorphic terrainin the Panamint Mountains, California, contain the low-varianceassemblage quartz+epidote+muscovite+biotite+calcic amphibole+chlorite+plagioclase+spheneat low grade. Near the sillimanite isograd, chlorite in thisassemblage is replaced by garnet. The low variance in many calcicschists allows the determination of the nature of the reactionthat resulted in the coexistence of garnet+hornblende. A graphicalanalysis of the mineral assemblages shows that the reactioncan not be of the form biotite+epidote+chlorite+plagioclase+quartz=garnet+hornblende+muscovite+sphene+H₂Obecause garnet+chlorite never coexisted during metamorphismand the chlorite-bearing and garnet-bearing phase volumes donot overlap. The compositions of the minerals show that withincreasing grade amphibole changed from actinolite to pargasitichornblende with no apparent miscibility gap, the partitioningof Fe and Mg between chlorite and hornblende changed from K_D(Mg/Fe, chl&amp) < 1 to K_D > 1, the partitioning betweenbiotite and hornblende changed from K_D (Mg/Fe, bio/amp) <1 in chlorite-zone samples to K_D > 1 in garnet + hornblende-zonesamples, and the transition to the garnet-bearing assemblageoccurred when the composition of plagioclase was between An₅₅and An₈₀. Both the graphical analysis and an analytical analysisof the compositions of the minerals using simplified componentsderived from the natural mineral compositions indicate thatat the garnet+hornblende isograd the composition of hornblendewas colinear with that of plagioclase and biotite, as projectedfrom quartz, epidote, muscovite, and H₂O. During progressivemetamorphism, chlorite+biotite+epidote+quartz continuously brokedown to form hornblende+muscovite+sphene until the degeneracywas reached. At that point, tie lines from hornblende couldextend to garnet without allowing garnet to coexist with chlorite.Thus, the garnet+hornblende isograd was established throughcontinuous reactions within the chlorite-grade assemblage ratherthan through a discontinuous reaction. In this type of isograd,the low-grade diagnostic assemblage occurs only in Mg-rich rocks;whereas the high-grade assemblage occurs only in Fe-rich rocks.This relation accounts for the restricted occurrence of garnet+hornblendeassemblage in low-pressure terrains. In Barrovian terrains,garnet+chlorite commonly occurs, and the first appearana ofgarnet+hornblende can simply result from the continuous shiftof the garnet+chlorite tie line to Mg-rich compositions.     

Metamorphic Conditions and Fluid Compositions of Scapolite-Bearing Rocks from the Lapis Lazuli Deposit at Sare Sang, Afghanistan

Scapolite and other halogen-rich minerals (phlogopite, amphibole,apatite, titanite and clinohumite) occur in some high-pressureamphibolite facies calc-silicates and orthopyroxene-bearingrocks at Sare Sang (Sar e Sang or Sar-e-Sang), NE Afghanistan.The calc-silicates are subdivided into two groups: garnet-bearingand garnet-free, phlogopite-bearing. Besides garnet and/or phlogopite,the amphibolite facies mineral assemblages in the calc-silicatesinclude clinopyroxene, calcite, quartz and one or more of theminerals scapolite, plagioclase, K-feldspar, titanite, apatiteand rarely olivine. Orthopyroxene-bearing rocks consist of clinopyroxene,garnet, plagioclase, scapolite, amphibole, quartz, calcite andaccessory dolomite and alumosilicate (kyanite?). Retrogradephases in the rocks are plagioclase, scapolite, calcite, amphibole,sodalite, haüyne, lazurite, biotite, apatite and dolomite.The clinopyroxene is mostly diopside and rarely also hedenbergite.Aegirine and omphacite with a maximum jadeite content of 29mol % were also found. Garnet from the calc-silicates is Grs_45–95Py_0–2and from the orthopyroxene-bearing rocks is Grs_10–15Py_36–43.Peak P–T metamorphic conditions, calculated using availableexchange thermobarometers and the TWQ program, are 750°Cand 1·3–1·4 GPa. Depending on the rock type,the scapolite exhibits a wide range of composition (from Eq_An= 0·07, X_Cl =0·99 to Eq_An = 0·61, X_Cl =0·07).Equilibria calculated for scapolite and coexisting phases atpeak metamorphic conditions yield X_CO2 = 0·03–0·15.X_NaCl (fluid), obtained for scapolite, ranges between 0·04and 0·99. Partitioning of F and Cl between coexistingphases was calculated for apatite–biotite and amphibole–biotite.Fluorapatite is present in calc-silicates, but orthopyroxene-bearingrocks contain chlorapatite. Cl preferentially partitions intoamphibole with respect to biotite. All these rocks have sufferedvarious degrees of retrogression, which resulted in removalof halogens, CO₂ and S. Halogen- and S-bearing minerals formedduring retrogression and metasomatism are fluorapatite, sodalite,amphibole, scapolite, clinohumite, haüyne, pyrite, andlazurite, which either form veins or replace earlier formedphases. KEY WORDS: scapolite; fluid composition; high-pressure; amphibolite facies; Western Hindukush; Afghanistan     

Granulite-Facies Metamorphism, Granite Falls-Montevideo Area, Minnesota

In the Granite Falls-Montevideo area, Minnesota, granulite faciesmineral assemblages were collected from three major lithologicunits—hornblende-pyroxene gneiss, garnet-biotite gneiss,and granitic gneiss. Mineral assemblages most commonly observedare: plagioclase-hornblende-orthopyroxene-clinopyroxene-magnetite-ilmenite-(biotite-quartz);quartz-plagioclase-biotite-garnet-orthopyroxene-(orthoclase);quartz-plagioclase-biotite-garnet-(orthoclase); quartz-plagioclase-microcline-(garnet-hematite);and quartz-plagio-clase-microcline-biotite-(garnet-hematite-rutile).Partial analyses of isomorphous phases from the hornblende-pyroxenegneiss and the garnet-biotite gneiss were determined with anelectron microprobe. Negligible compositional variation within single grains andof a particular mineral within a given specimen, regular distributionof Fe and Mg between coexisting hornblende, ortho-pyroxene,and clinopyroxene, obedience of the mineral assemblages to theGibbs Phase Rule, and lack of empirically determined incompatiblephases indicate a close approach to chemical equilibrium duringmetamorphism. Plots of coexisting biotite, garnet, and orthopyroxeneon an appropriate phase diagram result in some crossing tielines which cannot be adequately explained by temperature orpressure differences, but suggest that H₂O and/or O₂ were notperfectly mobile components during metamorphism. This is alsoindicated by interlayering of hornblende assemblages with pyroxeneassemblages and by different iron oxide phases in essentiallythe same mineral assemblage. Textural and chemical relationships of retrograde metamorphicassemblages suggest that some retrograde reactions are a resultof cooling following the granulite-facies metamorphism, butthat others may have resulted from recrystallization duringa stage of thermal metamorphism that is reflected in the potassium-argonand rubidium-strontium biotite ages of the metamorphic rocks.     

Petrology of pelitic schists in the oligoclase-biotite zone of the Sanbagawa metamorphic terrain, Japan: phase equilibria in the highest grade zone of a high-pressure intermediate type of metamorphic belt

The oligoclase-biotite zone of the Bessi area, central Shikoku is characterized by sodic plagioclase (X_Ca= 0.10–0.28)-bearing assemblages in pelitic schists, and represents the highest-grade zone of the Sanbagawa metamorphic terrain. Mineral assemblages in pelitic schists of this zone, all with quartz, sodic plagioclase, muscovite and clinozoisite (or zoisite), are garnet + biotite + chlorite + paragonite, garnet + biotite + hornblende + chlorite, and partial assemblages of these two types. Correlations between mineral compositions, mineral assemblages and mineral stability data assuming PH₂O = P_solid suggests that metamorphic conditions of this zone are about 610 ± 25°C and 10 ± 1 kbar.
Based upon a comparative study of mineralogy and chemistry of pelitic schists in the oligoclase-biotite zone of the Sanbagawa terrain with those in the New Caledonia omphacite zone as an example of a typical high-pressure type of metamorphic belt and with those in a generalized'upper staurolite zone'as an example of a medium-pressure type of metamorphic belt, progressive assemblages within these three zones can be related by reactions such as:     

Progressive Regional Metamorphism of Thin Calcareous Bands from the Moinian Rocks of N.W. Scotland

Bands and pods of calc-silicate rock a few centimetres thickare widely distributed throughout the Late Precambrian Moiniansequence in N.W. Scotland. They probably originated as late-diageneticcalcareous concretions and were subsequently affected by greenschistto upper middle amphibolite facies (Barrovian) regional metamorphismduring the Caledonian (s.l.) orogeny. The calc-silicate rocks described here are from Inverness-shirein the Western Highlands of Scotland. Distinctive prograde mineralassemblages define four narrow zones which run broadly north-southand increase in grade eastwards. Plagioclase composition changesprogressively from albite to near anorthite with increasinggrade and, together with the presence or absence of zoisitebiotite, and hornblende, is the basis of the zonal divisions.Zoisite Ca₂(Al_0.96, Fe_0.04)₃Si₃O₁₂(OH) with biotite is commonin the lower zones and plagioclase ‘jumps’ in compositionfrom calcic andesine to bytownite with the exit of all, or mostof, the zoisite. Similarly biotite-bearing assemblages giveway to those containing ferro-horn-blende and/or pyroxene. Almandinegarnet with approximately 30–40 per cent grossular ispresent throughout and clinozoisite becomes more common in thehigher grade assemblages. Three main episodes of folding (F1–F3) are recognizedin the area, with the calc-silicate assemblages having developedduring the second deformation (MS2) and immediately followingit (MP2). The metamorphic zones (largely of MP2 age) are foldedby major third folds and there is evidence of widespread superimposed‘retrogression’ of probable MP3 age to the east. New analyses of 21 whole rocks by X.R.F., and of 20 mineralsby microprobe, are presented. AI₂O₃ content varies over a smallrange in the whole rock analyses and aluminium is used as astandard for comparing variations in the other elements. CaO/Al₂O₃ratios show little variation but a progressive decrease in Na₂O/Al₂O₃and K₂O/Al₂O₃ with increasingly calcic plagioclase composition(as assessed by measurements on separated fractions), and hence with metamorphic grade, isnoted. Coupled with the fact that the metamorphic zones cross-cutpreviously folded stratigraphic boundaries, this suggests thatboth Na₂O and K₂O have been in part lost from the higher gradecalc-silicate assemblages during progressive metamorphism. Reactions are proposed for the observed mineral changes, andit is concluded that these are most likely achieved by cationexchange through the medium of the pore fluid and result inpartial elimination from the local system of certain elements.No evidence is found that the reaction often quoted as thatresponsible for the exit of zoisite in semi-calcareous rocks,namely 4zo+qtz5an+gross+vap, has taken place.     

Geothermobarometry in metasediments of the southern Grossvenediger area (Tauern Window, Austria)

Metasediments in the southern Grossvenediger area (Tauern Window, Austria) were studied along a cross-section through rocks of increasing metamorphic grade from the margin of the Tauern Window in the south to the base of the Upper Schieferhülle, including the Eclogite Zone, in the north. In the southern part of the cross-section there is no evidence for a pre-late Alpine metamorphic history in the form of high-pressure relics or pseudomorphs. Mineral assemblages are characterized by the stability of tremolite + calcite, biotite + calcite and biotite + chlorite + calcite. In the northern part a more complete Alpine metamorphic evolution is preserved. Primary high-pressure assemblages are dolomite + quartz, tremolite + zoisite, zoisite + dolomite + quartz + phengite I and probably tremolite + dolomite + phengite I. Secondary, post-kinematic assemblages [tremolite + calcite, talc + calcite, phengite II + chlorite + calcite (+ quartz), biotite + chlorite + calcite, biotite + zoisite + calcite] formed as a result of the dominant late Alpine metamorphic overprint. The occurrence of biotite + zoisite + calcite is confined to the northernmost area and defines a biotite–zoisite–calcite isograd. P–T estimates based on standard thermobarometric techniques and on stability relationships of tremolite + calcite + dolomite + quartz and zoisite give consistent results. P–T conditions of the main Tertiary metamorphic overprint were 525° C, P= 7.5 ± 1 kbar in the northern part of the cross-section. The southern part was metamorphosed at lower temperatures of 430–470° C. The Si-content of phengites from this area is almost as high as that of phengites from the Eclogite Zone (Si^max= 3.4 pfu). Pressures > 10 kbar at 420° C are suggested by phengite barometry according to Massone & Schreyer (1987). In the absence of high-pressure relics or pseudomorphs, these phengites, which lack late Alpine re-equilibration, are the only record that rocks of the southern part probably also experienced an early non-eclogitic high-pressure metamorphism.     

Fluid-Absent Melting Behavior of an F-Rich Tonalitic Gneiss at Mid-Crustal Pressures: Implications for the Generation of Anorogenic Granites

Fluid-absent melting experiments on a biotite (20 wt.%) andhornblende (2 wt.%) bearing tonalitic gneiss were conductedat 6 kbar (900–975C), 10 kbar (875–1075C), and14 kbar (950–975C) to study melt productivity from weaklyperaluminous quartzofeldspathic metamorphic rocks. At 6 kbar,biotite dehydration–melting is completed at 975C viaincongruent melting reactions that produce orthopyroxene, twooxides, and {small tilde}25 wt.% granitic melt. At 6 kbar, hornblendedisappears at 900C, probably in reaction with biotite. At 10kbar, biotite dehydration–melting produces <10 wt.%melt up to 950C via incongruent melting reactions that produceorthopyroxene, garnet, and granitic melt. Hornblende disappearsin the satne temperature interval either by resorption or byreaction with biotite. Widespread biotite dehydration–meltingoccurs between 950 and 975C and produces orthopyroxene, twooxides, and {small tilde}20 wt.% fluorine-rich (up to 0•31wt.%) granitic melt. At 14 kbar only a trace of melt is presentat 950C, and the amounts of hornblende and biotite are virtuallythe same as in the starting material. At 975C, hornblende isgone and {small tilde}10 wt.% granitic melt is produced by meltingof both biotite and hornblende. Our results show that hornblende-bearing assemblages cannotgo through dehydration–melting on their own (althoughthey can in combination with biotite) if the Ca content in thesource rock is too low to stabilize clinopyroxene. In such rocks,hornblende will either resorb or melt by reaction with biotite.Under fluid-absent conditions, intrusion of hot, mantle-derivedmagmas into the lower crust is necessary to initiate widespreaddehydration–melting in rocks with compositions similarto those discussed here. We argue that the high thermal stabilityof biotite in our starting material is caused mainly by theincorporation of fluorine. The relatively high F content inbiotite in the starting material (0•47 wt.%) suggests thatthe rock has experienced dehydroxylation in its past. F enrichmentby a previous fluid-absent partial melting event is excludedbecause of the lack of phases such as orthopyroxene and garnetwhich would have been produced. Our experiments show that thedehydration–melting of such F-enriched biotite producesF-rich granitic liquids, with compositions within the rangeof A-types granites, and leaves behind a granulitic residuedominated by orthopyroxene, quartz, and plagioclase. This studytherefore supports the notion that A-type granites can be generatedby H₂O-undersaturated melting of rocks of tonalitic composition(Creaser et al., 1991), but does not require that these sourcerocks should be residual after a previous melting event.     

Paragonite–hornblende assemblages and their petrological significance: an example from the Austroalpine Schneeberg Complex, Southern Tyrol, Italy

ABSTRACT Paragonite-bearing amphibolites occur interbedded with a garbenschist-micaschist sequence in the Austroalpine Schneeberg Complex, southern Tyrol. The mineral assemblage mainly comprises paragonite + Mg-hornblende/tschermakite + quartz + plagioclase + biotite + ankerite + Ti-phase + garnet ± muscovite. Equilibrium P–T conditions for this assemblage are 550–600°C and 8–10 kbar estimated from garnet–amphibole–plagioclase–ilmenite–rutile and Si contents of phengitic muscovites. In the vicinity of amphibole, paragonite is replaced by symplectitic chlorite + plagioclase + margarite +± biotite assemblages. Muscovite in the vicinity of amphibole reacts to form plagioclase + biotite + margarite symplectites. The reaction of white mica + hornblende is the result of decompression during uplift of the Schneeberg Complex. The breakdown of paragonite + hornblende is a water-consuming reaction and therefore it is controlled by the availability of fluid on the retrogressive P–T path. Paragonite + hornblende is a high-temperature equivalent of the common blueschist-assemblage paragonite + glaucophane in Ca-bearing systems and represents restricted P–T conditions just below omphacite stability in a mafic bulk system. While paragonite + glaucophane breakdown to chlorite + albite marks the blueschist/greenschist transition, the paragonite + hornblende breakdown observed in Schneeberg Complex rocks is indicative of a transition from epidote-amphibolite facies to greenschist facies conditions at a flatter P–T gradient of the metamorphic path compared to subduction-zone environments. Ar/Ar dating of paragonite yields an age of 84.5 ± 1 Ma, corroborating an Eoalpine high-pressure metamorphic event within the Austroalpine unit west of the Tauern Window. Eclogites that occur in the Ötztal Crystalline Basement south of the Schneeberg Complex are thought to be associated with this Eoalpine metamorphic event.     

Petrology of ultrahigh-pressure rocks from the southern Su-Lu region, eastern China

Abstract In the Su-Lu ultrahigh- P terrane, eastern China, many coesite-bearing eclogite pods and layers within biotite gneiss occur together with interlayered metasediments now represented by garnet-quartz-jadeite rock and kyanite quartzite. In addition to garnet + omphacite + rutile + coesite, other peak-stage minerals in some eclogites include kyanite, phengite, epidote, zoisite, talc, nyböite and high-Al titanite. The garnet-quartz-jadeite rock and kyanite quartzite contain jadeite + quartz + garnet + rutile ± zoisite ± apatite and quartz + kyanite + garnet + epidote + phengite + rutile ± omphacite assemblages, respectively. Coesite and quartz pseudomorphs after coesite occur as inclusions in garnet, omphacite, jadeite, kyanite and epidote from both eclogites and metasediments. Study of major elements indicates that the protolith of the garnet-quartz jadeite rock and the kyanite quartzite was supracrustal sediments. Most eclogites have basaltic composition; some have experienced variable 'crustal'contamination or metasomatism, and others may have had a basaltic tuff or pyroclastic rock protolith.
The Su-Lu ultrahigh- P rocks have been subjected to multi-stage recrystallization and exhibit a clockwise P-T path. Inclusion assemblages within garnet record a pre-eclogite epidote amphibolite facies metamorphic event. Ultrahigh- P peak metamorphism took place at 700–890° C and P >28 kbar at c . 210–230 Ma. The symplectitic assemblage plagioclase + hornblende ± epidote ± biotite + titanite implies amphibolite facies retrogressive metamorphism during exhumation at c . 180–200 Ma. Metasedimentary and metamafic lithologies have similar P-T paths. Several lines of evidence indicate that the supracrustal rocks were subducted to mantle depths and experienced in-situ ultrahigh- P metamorphism during the Triassic collision between the Sino-Korean and Yangtze cratons.     

High-Grade Fluid Metasomatism on both a Local and a Regional Scale: the Seward Peninsula, Alaska, and the Val Strona di Omegna, Ivrea-Verbano Zone, Northern Italy. Part I: Petrography and Silicate Mineral Chemistry

K-feldspar–plagioclase–quartz mineral textures aswell as biotite and hornblende compositions are compared forsuites of metamorphosed mafic rocks from two widely separatedtraverses. A portion of either traverse has experienced a high-gradedehydration event transforming it from an H₂O-rich, hornblende-bearingzone to an H₂O-poor, hornblende-free, orthopyroxene-bearing,‘granulite facies’ zone at 700–800°C and7–8 kbar. In the Kigluaik Mountains, Seward Peninsula,Alaska, dehydration took place over an 85 cm thick layer ofmetatonalite in contact with a marble during regional metamorphismand involved a CO₂-rich fluid, whereas for the Val Strona diOmegna traverse, Ivrea–Verbano Zone, northern Italy, dehydrationtook place over a 3–4 km thick sequence of metabasitesinterlayered with metapelites in a contact metamorphic eventinvolving basaltic magmas intruded at the base of the sequence.Orthopyroxene-bearing samples from both dehydration zones showmicro-veins of K-feldspar along quartz and plagioclase grainboundaries as well as replacement antiperthite in plagioclase.K came primarily from the breakdown of hornblende + quartz toorthopyroxene ± clinopyroxene, feldspar and fluid. Biotiteeither was stabilized or formed in the dehydration zones andis enriched in Ti, Mg, F and Cl relative to biotite in the amphibolitefacies zone. KEY WORDS: KCl–NaCl brines; metasomatism; granulite facies metamorphism; charnockite–enderbite; orthopyroxene; K-feldspar; biotite; hornblende     

A Biotite Isograd in South-Central Maine, U.S.A.: Mineral Reactions, Fluid Transfer, and Heat Transfer

The biotite isograd in pelitic schists of the Waterville Formationinvolved reaction of muscovite + ankerite + rutile + pyrite+graphite + siderite or calcite to form biotite + plagioclase+ ilmenite. There was no single reaction in all pelites; eachrock experienced a unique reaction depending on the mineralogyand proportions of minerals in the chlorite-zone equivalentfrom which it evolved. Quartz, chlorite, and pyrrhotite werereactants in some rocks and products in others. All inferredbiotite-forming reactions involved decarbonation and desulfidation;some were dehydration reactions and others were hydration reactions.P-T conditions at the biotite isograd were near 3500 bars and400 °C. C-O-H-S fluids in equilibrium with the pelitic rockswere close to binary CO₂-H₂O mixtures with X_CO2 = 0.02–0.04.During the biotite-forming reaction, pelitic rocks (a) decreasedby 2–5 percent in volume, (b) performed – (4–11)kcal/liter P-V work on their surroundings, (c) absorbed 38–85kcal/liter heat from their surroundings, and (d) were infiltratedby at least 0.9–2.2 rock volumes H₂O fluid. The biotite isograd sharply marks the limit of a decarbonationfront that passed through the terrane during regional metamorphism.Decarbonation converted meta-shales with 6–10 per centcarbonate to carbonate-free pelitic schists. One essential causeof the decarbonation event was pervasive infiltration of theterrane by at least 1–2 rock volumes H₂O fluid early inthe metamorphic event under P-T conditions of the biotite isograd.Average shale contains 4–13 per cent siderite, ankerite,and/or calcite, but average pelitic schist is devoid of carbonateminerals. If the Waterville Formation serves as a general modelfor the metamorphism of pelitic rocks, it is likely that worldwidemany pelitic schists developed by decarbonation of shale caused,in part, by pervasive infiltration of metamorphic terranes byseveral rock volumes of aqueous fluid during an early stageof the metamorphic event.     

Phase relations of a simplified Marly rock system with application to the Western Hohe Tauern (Austria)

The regional distribution of metamorphic mineral assemblages in Mesozoic carbonate rocks of the Western Hohe Tauern allows the mapping of isograds based on the appearance of biotite+calcite and biotite+zoisite+calcite. The latter isograd corresponds approximately to the thermal maximum of the alpidic metamorphism in the central part of this area. An estimate of P, T, X _fluid conditions can be obtained from phase relations among muscovite, biotite, chlorite, margarite, tremolite, zoisite, anorthite, quartz, calcite, and dolomite in the system K₂O-CaO-MgO-Al₂O₃-SiO₂-H₂O-CO₂ which approximates the composition of marls. Calculations based on various experimental and thermodynamic data have been made with emphasis on phase relations pertinent to a group of carbonate rocks with very low Fe and Na contents in non-opaque minerals. Significant and opposite deviations from the phase relations for stochiometric end member mineral compositions are due to the substitutions F-OH and Mg+Si-2Al. Consistency of observed and calculated phase relations is favoured by high F-contents. For the majority of carbonate rocks in the high metamorphic zone, maximum temperatures around 550° C, minimum pressures of 4–6 kb, and relatively low XCO₂ values within the stability field of zoisite and of biotite+calcite+quartz are indicated.     

Calculated Phase Relations in the System Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O with Applications to UHP Eclogites and Whiteschists

Pressure–temperature grids in the system Na₂O–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O and its subsystems have been calculatedin the range 15–45 kbar and 550–900°C, usingan internally consistent thermodynamic dataset and new thermodynamicmodels for amphibole, white mica, and clinopyroxene, with thesoftware THERMOCALC. Minerals considered for the grids includegarnet, omphacite, diopside, jadeite, hornblende, actinolite,glaucophane, zoisite, lawsonite, kyanite, coesite, quartz, talc,muscovite, paragonite, biotite, chlorite, and plagioclase. Compatibilitydiagrams are used to illustrate the phase relationships in thegrids. Coesite-bearing eclogites and a whiteschist from Chinaare used to demonstrate the ability of pseudosections to modelphase relationships in natural ultrahigh-pressure metamorphicrocks. Under water-saturated conditions, chlorite-bearing assemblagesin Mg- and Al-rich eclogites are stable at lower temperaturesthan in Fe-rich eclogites. The relative temperature stabilityof the three amphiboles is hornblende > actinolite > glaucophane(amphibole names used sensu lato). Talc-bearing assemblagesare stable only at low temperature and high pressure in Mg-and Al-rich eclogites. For most eclogite compositions, talccoexists with lawsonite, but not zoisite, in the stability fieldof coesite. Water content contouring of pressure–temperaturepseudosections, along with appropriate geotherms, provides newconstraints concerning dehydration of such rocks in subductingslabs. Chlorite and lawsonite are two important H₂O-carriersin subducting slabs. Depending on bulk composition and pressure–temperaturepath, amphibole may or may not be a major H₂O-carrier to depth.In most cases, dehydration to make ultrahigh-pressure eclogitestakes place gradually, with H₂O content controlled by divariantor higher variance assemblages. Therefore, fluid fluxes in subductionzones are likely to be continuous, with the rate of dehydrationchanging with changing pressure and temperature. Further, eclogitesof different bulk compositions dehydrate differently. Dehydrationof Fe-rich eclogite is nearly complete at relatively shallowdepth, whereas Mg- and Al-rich eclogites dehydrate continuouslydown to greater depth. KEY WORDS: dehydration; eclogites; phase relations; THERMOCALC; UHP metamorphism; whiteschists     

Metamorphism and Structure of Penfold Creek Area, near Quesnel Lake, British Columbia

The Quesnel Lake area lies within the Omineca Crystalline Belt,and is underlain by the northern extremity of the Shuswap MetamorphicComplex. Closely spaced and steeply dipping isograds mark themargins of the metamorphic belt. In the Penfold Creek area onlyone and one-half miles separate the biotite and sillimaniteisograds. Related to this sharp increase in metamorphic gradethere is a marked change in the fold style. In the chloritezone are similar folds, showing a strong axial-plane cleavage,and tight refolded isoclines dominate in the sillimanite zone.Three periods of deformation and two periods of prograde metamorphismhave been recognized, with the first metamorphic period beingassociated with Phase 2 deformation and the second being post-Phase2. Regression analysis of possible mineral reactions in these rocksshows that they formed under conditions of a gradient in temperatureand possibly H₂O activity, and that most of the assemblages,although containing many phases, have at least two degrees offreedom. Thermodynamic analysis of mineral equilibria indicatesthat equilibrium was closely approached between solid solutionsinvolving garnet, plagioclase, biotite, white mica, and staurolitein the presence of Al₂SiO₅, quartz, and H₂O. The conditionsof metamorphism of rocks containing Al₂SiO₅ are estimated tobe 7000 ± 1500 bars, 680 ± 30 °C, activityof H₂O = 0.80 (approx.). Partial melting did not occur becauseof the reduced activity of H₂O. ^*Present address: Overseas Division, Institute of Geological Sciences, 5 Princes Gate, London SW7, England.     

Talc-, Magnesite- and Ti-Clinohumite-Bearing Ultrahigh-Pressure Meta-Mafic and Ultramafic Complex in the Dabie Mountains, China

The Bixiling mafic-ultramafic metamorphic complex is a 1•5km² tectonic block within biotite gneiss in the southern Dabieultrahigh-pressure terrane, central China. The complex consistsof banded eclogites that contain thin layers of garnet-bearingcumulate ultramafic rock. Except for common eclogitic phases(garnet, omphacite, kyanite, phengite, zoisite and rutilc),banded eclogites contain additional talc and abundant coesiteinclusions in omphacite, zoisite, kyanite and garnet. Some metaultramaficrocks contain magnesite and Ti-clinohumite. Both eclogites andmeta-ultramafic rocks have undergone multi-stage metamorphism.Eclogite facies metamorphisrn occurred at 610–700C andP>27 kbar, whereas amphibolite facies retrograde metamorphismis characterized by symplectites of plagioclase and hornblendeafter omphacite and replacement of tremolite after talc at P<6–15kbar and T <600C. The meta-ultramafic assemblages such asolivine + enstatite + diopside + garnet and Ti-clinohumite +diopside + enstatite + garnet + magnesite olivine formed at700–800C and 47–67 kbar. Investigation of the phaserelations for the system CaO-MgO-SiO₂-H₂O-CO₂ and the experimentallydetermined stabilities of talc, magnesite and Ti-clinohumiteindicate that (1) UHP talc assemblages are restricted to Mg-Algabbro composition and cannot be an important water-bearingphase in the ultramafic mantle, and (2) Ti-clinohumite and magnesiteare stable H₂O-bearing and CO₂-bearing phases at depths >100km. The mafic-ultramafic cumulates were initially emplaced atcrustal levels, then subducted to great depths during the Triassiccollision of the Sine-Korean and Yangtze cratons. KEY WORDS: eclogite; magnesite; meta-ultramafics; talc; ultrahigh-P metamorphism ^*Corresponding author     

The Petrology of an Adamellite-Porphyrite from the New England Bathylith (New South Wales)

The paper presents an account of the occurrence, petrography,mineralogy, and geochemistry of several intrusions of adamellite-porphyrite,an early member of the New England Bathylith (N.S.W.). The rockis composed of phenocrysts of quartz, andesine, hornblende,biotite, augite (and minor orthopyroxene) which are set in afine-grained quartzofeldspathic groundmass. Compositions ofsilicate phenocrysts (which reveal widespread evidence of cataclasis)accord with coexisting phases of an assemblage appropriate tointermediate rocks. Xenolith mineralogy is similar to equivalentminerals occurring as phenocrysts. The recalculated bulk compositionof a rock represented by the modal andesine, hornblende, biotite,augite, and magnetite of the adamellite-porphyrite is similarto the composition of a biotite–diorite; the compositionof the quartzo-feldspathic groundmass plots in the low-meltingregion of the NaAISi₃O₈-KAISi₃O₈-SiO₂-H₂O system. Evidence ispresented to indicate a hybrid origin for the adamellite-porphyritewhereby disrupted biotite-diorite was invaded and mixed withlow-melting silicic alkalic liquid.     

Tschermak Substitution in Low- and Middle-grade Pelitic Schists

In low- and middle-grade pelitic metamorphic rocks, the extentsof Tschermak substitution in muscovite, chlorite and biotitechange regularly with bulk-rock composition and external conditions.This paper gives a theoretical analysis of the changes. From equipotential lines for Al₂O₃ plotted on AFM diagrams,we have derived a series of Thompson-type muscovite compositiondiagrams, which show how the celadonite content of muscovitevaries with the associated ferromagnesian minerals or with thebulk-rock composition under constant external conditions. Thedistribution coefficient of the exchange reaction for Tschermaksubstitution between muscovite and chlorite varies greatly notonly with temperature but also with the extent of this substitutionin the two minerals because of their strong deviation from ideality.Muscovites with a high celadonite content (phengites) occurin low-temperature rocks in any of the high-, medium- and low-pressuretypes of metamorphism, probably because the exchange equilibriumfor Tschermak substitution between muscovite and chlorite isnot sensitive to pressure. When both Mg-Fe and Tschermak substitutionoccur in muscovite and some other silicates, a metapelite containingthree AFM phases together with muscovite and quartz has at leasttwo independent net-transfer reactions that take place sideby side with changing external conditions. The mass balancerequirement in the rock is imposed on a linear combination ofthe two reaction equations, leading to a constraint on the stoichiometricequations among phase components and the progressive compositionalchanges of muscovite, chlorite and biotite. From such a viewpoint,we examine reactions and progressive mineralogical changes inmetapelites, beginning with K-feldspar-bearing low-grade metapeliticrocks, in which biotite appears by reaction of K-feldspar withchlorite at a temperature lower than that of the biotite isograddefined for K-feldspar-free pelitic rocks. When both Mg-Fe and Tschermak substitution occur in muscoviteand some other silicates, a metapelite containing three AFMphases together with muscovite and quartz has at least two independentnet-transfer reactions that take place side by side with changingexternal conditions. The mass balance requirement in the rockis imposed on a linear combination of the two reaction equations,leading to a constraint on the stoichiometric equations amongphase components and the progressive compositional changes ofmuscovite, chlorite and biotite. From such a viewpoint, we examinereactions and progressive mineralogical changes in metapelites,beginning with K-feldspar-bearing low-grade metapelitic rocks,in which biotite appears by reaction of K-feldspar with chloriteat a temperature lower than that of the biotite isoerad definedfor K-feldsoar-free oelitic rocks. The equations for the reactions that produce biotite or biotite? garnet in K-feldspar-free metapelites have been derived. Combinedwith the composition relations of coexisting muscovite, chlorite,biotite and garnet, they lead to the inference that progressof these reactions with rising temperature causes a decreaseof the celadonite content of muscovite, the antigorite contentof chlorite and the phlogopite content of biotite in the metapelitesof the chlorite, biotite and lower garnet zones, and that thistrend may not continue into the staurolite zone. This inferencehas been verified by examination of the analytical data of mineralsin four progressive metamorphic terranes covering the high-,medium- and low-pressure types. The composition of chlorite is buffered by the garnet-producingreaction so that chlorites in garnet-bearing metamorphic rocksdevelop (Mg ? Fe)/Al₂ ratios close to that of almandine at elevatedtemperatures. The maximum possible celadonite content in muscoviteunder given external conditions is realized in K-feldspar-bearingrocks, and decreases with rising temperature.     

西昆仑库地韧性剪切带的40Ar/39Ar年龄

西昆仑库地以南有一套变质变形较强的岩系,前人依照区域对比关系将其划为前寒武的古老基底。对西昆仑早期构造演化的论述均基于该观点,但没有提供确凿的同位素年代学证据。笔者通过野外观察、室内研究,确认库地以南的变质变形岩系是大型韧性推覆剪切作用的产物。通过对新生变质矿物角闪石和黑云母单矿物的⁴⁰Ar/³⁹Ar年龄分析,确定剪切变质年龄为426-451Ma,说明库地的变质变形岩系是形成于早古生代晚期的一条大型韧性剪切带,这对于解释西昆仑的早期构造演化具有重要意义。     

Melting of Biotite + Plagioclase + Quartz Gneisses: the Role of H2O in the Stability of Amphibole

Biotite + plagioclase + quartz (BPQ) is a common assemblagein gneisses, metasediments and metamorphosed granitic to granodioriticintrusions. Melting experiments on an assemblage consistingof 24 vol. % quartz, 25 vol. % biotite (XMg = 0·38–0·40),42 vol. % plagioclase (An_26–29), 9 vol. % alkali feldsparand minor apatite, titanite and epidote were conducted at 10,15 and 20 kbar between 800 and 900°C under fluid-absentconditions and with small amounts (2 and 4 wt %) of water addedto the system. At 10 kbar when 4 wt % of water was added tothe system the biotite melting reaction occurred below 800°Cand produced garnet + amphibole + melt. At 15 kbar the meltingreaction produced garnet + amphibole + melt with 2 wt % addedwater. At 20 kbar the amphibole occurred only at high temperature(900°C) and with 4 wt % added water. In this last case themelting reaction produced amphibole + clinopyroxene ±garnet + melt. Under fluid-absent conditions the melting reactionproduced garnet + plagioclase II + melt and left behind a plagioclaseI ± quartz residuum, with an increase in the modal amountof garnet with increasing pressure. The results show that itis not possible to generate hornblende in such compositionswithout the addition of at least 2–4 wt % H₂O. This reflectsthe fact that conditions of low aH₂O may prevent hornblendefrom being produced with peraluminous granitic liquids fromthe melting of biotite gneiss. Thus growth of hornblende inanatectic BPQ gneisses is an indication of addition of externalH₂O-rich fluids during the partial melting event. KEY WORDS: biotite; dehydration; gneisses; hornblende; melt     

Origin of biotite-hornblende-garnet coronas between oxides and plagioclase in olivine metagabbros,Adirondack region,New York

Complex multivariant reactions involving Fe-Ti oxide minerals, plagioclase and olivine have produced coronas of biotite, hornblende and garnet between ilmenite and plagioclase in Adirondack olivine metagabbros. Both the biotite (6–10% TiO₂) and the hornblende (3–6% TiO₂) are exceptionally Titanium-rich. The garnet is nearly identical in composition to the garnet in coronas around olivine in the same rocks. The coronas form in two stages:

1. Plagioclase+Fe-Ti Oxides+Olivine+water =Hornblende+Spinel+Orthopyroxene±Biotite +more-sodic Plagioclase
2. Hornblende+Orthopyroxene±Spinel+Plagioclase =Garnet+Clinopyroxene+more-sodic Plagioclase

The Orthopyroxene and part of the clinopyroxene form adjacent to olivine. Both reactions are linked by exchange of Mg²⁺ and Fe²⁺ with the reactions forming pyroxene and garnet coronas around olivine in the same rocks. The reactions occur under granulite fades metamorphic conditions, either during isobaric cooling or with increasing pressure at high temperature.