Stability Relations of the Ferruginous Biotite, Annite

Annite, KFe₃AISi₃O₁₀(OH)₂ a member of the iron biotites andthe ferrous analogue of phlogopite, has been synthesized andits phase relations have been determined as functions of temperature,fugacity of oxygen (fo₂), and total pressure (P_totalPH₂O+PH₂).A method for controlling fo₂at high total pressures is described,and data for the ‘oxygen buffers’ used are given.Buffers range from quartz+iron+fayalite assemblages (low fo₂)to magnetite-hematite assemblages (high fo₂). Optical propertiesand unit-cell dimensions of synthetic annites depend on theconditions of synthesis. By recalculating published analyses of natural iron-rich biotitesit can be shown that one cannot assume a constant hydrogen contentfor such biotites. Oxidation may have occurred by drying at115?C. Octahedral occupancy therefore cannot be calculated fromsuch data. Phase relations of annite are presented in 2,070 and 1,035 barsections. Depending on fo₂-T values annite was found to decomposeto one of the following assemblages: hematite+ sanidine, magnetite+sanidine,fayalite+leucite+kalsilite, iron+sanidine. All decompositionsare dehydration and redox reactions and are sensitive to changesin fH₂0 and fo₂ (or fH₂0 and fH₂). At 2, 070 bars total pressureannite+magnetite+sanidine can coexist between 425?C and 825?C, depending upon the magnitude of fo₂. In the presence of quartz the stability field of annite is morerestricted. Phase equilibria in the system KAlSiO₄–SiO₂–Fe–O₂–H₂have been summarized schematically. Wherever possible, thermodynamic extrapolations are made totest the internal consistency of the data. Enthalpies of formationare calculated for both annite and phlogopite. Ranges of fo₂values in nature as well as mechanisms for changes in fo₂ areinvestigated. It is useful to distinguish between assemblageswhich are internally buffered with respect to fo₂changes andthose which are not buffered. The applications of individualreactions involving annite to specific geologic problems arediscussed with respect to igneous, metamorphic, and sedimentaryrocks.     

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     

Thermobarometry and Geotectonic Significance of High-Pressure Granulites: Examples from the Moldanubian Zone of the Bohemian Massif in Lower Austria

Petrographic details together with mineral and whole-rock compositiondata are provided for acid-intermediate garnet granulites fromexposed granulite complexes in Lower Austria. Thermobarometricevaluation integrated with available isotopic age data indicatesthe initial equilibration of early Variscan ({small tilde}370Ma) high-pressure granulite assemblages at {small tilde}16 kbarand 1000C and their partial overprinting by retrograde assemblageswhich reflect the blocking of mineral exchange reaction equilibriaat {small tilde}6•5 kbar and 725C during subsequent Variscanuplift and cooling. These calculated P-T estimates, togetherwith general phase equilibria constraints and evidence frompreserved prograde coronitic reaction textures and garnet compositionalzoning profiles, indicate a clockwise P-T-t evolutionary pathof the type expected during crustal thickening in a major platecollision orogen and characterized by near-isothermal decompressionduring initial uplift. Geochemical characterization of the rockprotoliths as calc-alkaline igneous rocks and the high metamorphictemperatures suggest that garnet granulite formation involvedthe subduction of a magmatic arc at a continental plate margin. Reviewed evidence from granulites in the Central European Variscidesruns counter to suggestions by Bohlen (1987, 1991) that high-pressuregranulites are of little regional geotectonic significance incomparison with low- to medium-pressure granulites. The differentevolutionary P-T paths for these two important groups of granulitespoint to formation in contrasting plate settings. However, questionsare raised regarding petrogenetic models for low- to medium-pressuregranulites which have emphasized the importance of magmatic,rather than tectonic, crustal thickening and the recognitionof stabilization along deduced anti-clockwise P-T-t paths characterizedby post-peak near-isobaric cooling. It is suggested here that the reality of stabilization of atleast some low- to medium-pressure granulites in a collisionaltectonic regime may have been concealed either because lower-pressureassemblages have overprinted mineralogical evidence for an earlierhigh-pressure history at deeper crustal levels or through invaliddeduction of near-isobaric cooling trajectories as a resultof the different closure temperatures for the mineral reactionsused to monitor the equilibration temperatures and pressuresin granulites. However, the sequential underthrusting modelfavoured for the tectonometamorphic evolution of the Variscannappe pile in the Bohemian Massif renders it unlikely that alllate Variscan low- to medium-pressure granulites have experiencedthe early Variscan high-pressure metamorphism.     

Eclogite Facies Regional Metamorphism of Hydrous Mafic Rocks in the Central Alpine Adula Nappe

The Adula Nappe is a slice of Pre-Mesozoic continental basementaffected by Early Alpine (Mesozoic or Lower Tertiary) high-pressuremetamorphism. Mineral compositions in mafic rocks containingomphacite + garnet + quartz record a continuous regional trendof increasing recrystalliza tion temperatures and pressuresthat can be ascribed to this regional high-pressure metamorphicevent. P-T estimates derived from mineral compositions gradefrom about 12 kb and 500 ?C or less in the north of the nappeto more than 20 kb/800 ?C in the south. The regional P-T trend is associated with a mineralogical transitionfrom assemblages containing additional albite and abundant amphiboles,epidote minerals, and white micas in the north (omphacite-garnetamphibolites) to kyanite eclogites containing smaller amountsof hornblende and zoi.site in the south. Textures and mineralcompositional data show that these hydrous and anhydrous silicatesassociated with omphacite + garnet + quartz arc primary partsof the high-pressure assem blages. Observed phase relationsbetween these primary silicates, theoretical Schreinemakersanalysis, and the thermobarometric results, together indicatethat the regional transition from omphacite amphibolites tokyanite eclogites can be described by two simplified reactions: alb+epi+hbl=omp+kya+qtz+par (H₂O-conserving) (15) par+epi+hbl+qtz=omp+kya+H₂O (dehydration) (12) which have the character of isograd reactions. Local variations of water activity (a_H2O) as indicated by isofacialmineral assemblages, and the H₂O character of the reaction (15),are interpreted to reflect largely H and predominantly fluid-absenthigh-pressure metamorphism within the northern part of the nappe.The omphacite amphibolites and paragonite eclogites in thisarea are thought to have formed by H₂O reactions from Pre-Mesozoichigh-grade amphibolites, i.e. from protoliths of similar bulkH₂O-countent. The second ‘isograd’ (12) is interpreted to markthe regional transition from largely fluid-absent metamorphismin the north to fluid-present metamorphism in the south, wheremetamorphic pressures and temperatures in excess of 12-15kband 500-600?C were sufficient for prograde in-situ dehydrationof similar hydrous protoliths to kyanite eclogites. The observationof abundant veins, filled with quartz+kyanite+omphacite, suggeststhat a free fluid coexisted locally with the kyanite eclogitesof the southern Adula Nappe at some time during progressivedehydration.     

An Experimentally Constrained Petrogenetic Grid in the Silica-Saturated Portion of the System KFMASH at High Temperatures and Pressures

Experiments in the quartz-saturated part of the system KFMASHunder fO₂ conditions of the haematite–magnetite bufferand using bulk compositions with X_Mg of 0·81, 0·72,0·53 define the stability limits of several mineral assemblageswithin the P–T field 9–12 kbar, 850–1100°C.The stability limits of the mineral assemblages orthopyroxene+ spinel + cordierite ± sapphirine, orthopyroxene + garnet+ sapphirine, sapphirine + cordierite + orthopyroxene and garnet+ orthopyroxene + spinel have been delineated on the basis ofP–T and T–X pseudosections. Sapphirine did not appearin the bulk composition of X_Mg = 0·53. A partial petrogeneticgrid applicable to high Mg–Al granulites metamorphosedat high fO₂, developed in our earlier work, was extended tohigher pressures. The experimental results were successfullyapplied to several high-grade terranes to estimate P–Tconditions and retrograde P–T trajectories. KEY WORDS: KFMASH equilibria; experimental petrogenetic grid at high fO₂     

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.     

Origin of Grandite Garnet in Calc-Silicate Granulites: Mineral-Fluid Equilibria and Petrogenetic Grids

The role of clinopyroxene in producing grandite garnet is evaluatedusing data from an ultrahigh-temperature metamorphosed calc-silicategranulite occurrence in the Eastern Ghats Belt, India. ‘Peak’pressure–temperature conditions of metamorphism were previouslyconstrained from associated high Mg–Al granulites as c.0·9 GPa, >950°C, and the rocks were near-isobaricallycooled to c. 750°C. Grandite garnet of variable compositionwas produced by a number of reactions involving phases suchas clinopyroxene, scapolite, plagioclase, wollastonite and calcite,in closely spaced domains. Compositional heterogeneity is preservedeven on a microscale. This precludes pervasive fluid fluxingduring either the peak or the retrograde stage of metamorphism,and is further corroborated by computation of fluid–rockratios. With the help of detailed textural and mineral compositionalstudies leading to formulation of balanced reactions, and usingan internally consistent thermodynamic dataset and relevantactivity–composition relationships, new petrogenetic gridsare developed involving clinopyroxene in the system CaO–Al₂O₃–FeO–SiO₂–CO₂–O₂in T–aCO₂–fO₂ space to demonstrate the importanceof these factors in the formation of grandite garnet. Two singularcompositions in garnet-producing reactions in this system arededuced, which explain apparently anomalous textural relations.The possible role of an esseneite component in clinopyroxenein the production of grandite garnet is evaluated. It is concludedthat temperature and fO₂ are the most crucial variables controllinggarnet composition in calc-silicate granulites. fO₂, however,behaves as a dependent variable of CO₂ in the fluid phase. Externalfluid fluxing of any composition is not necessary to producechemical heterogeneity of garnet solid solution. KEY WORDS: grandite garnet; role of clinopyroxene; internal buffering; oxidation–decarbonation equilibria     

Synthesis of Staurolite in Melting Experiments of a Natural Metapelite: Consequences for the Phase Relations in Low-Temperature Pelitic Migmatites

We document experiments on a natural metapelite in the range650–775°C, 6–14 kbar, 10 wt % of added water,and 700–850°C, 4–10 kbar, no added water. Staurolitesystematically formed in the fluid-present melting experimentsabove 675°C, but formed only sporadically in the fluid-absentmelting experiments. The analysis of textures, phase assemblages,and variation of phase composition and Fe–Mg partitioningwith P and T suggests that supersolidus staurolite formed at(near-) equilibrium during fluid-present melting reactions.The experimental results are used to work out the phase relationsin the system K₂O–Na₂O–FeO–MgO–Al₂O₃–SiO₂–H₂Oappropriate for initial melting of metapelites at the upperamphibolite facies. The P–T grid developed predicts theexistence of a stable P–T field for supersolidus staurolitethat should be encountered by aluminous Fe-rich metapelitesduring fluid-present melting at relatively low temperature andintermediate pressures (675–700°C, 6–10 kbarfor X_H2O = 1, in the KNFMASH system), but not during fluid-absentmelting. The implications of these findings for the scarcityof staurolite in migmatites are discussed. KEY WORDS: metapelites; migmatites; partial melting; P–T grid; staurolite     

The Formation of Chrysoberyl in Metamorphosed Pegmatites

Mineral assemblages in pegmatite samples from Kolsva, Swedenand Marikov, Czechoslovakia show that chrysoberyl is alwaysaccompanied by quartz, and is a breakdown product of primarypegmatitic beryl. Textures and the mineral-forming process forthe Kolsva pegmatite are explained by the reactions beryl +K-feldspar + H+ = chrysoberyl + quartz + SiO_{2, aq} + K⁺ + H₂Oor alternatively beryl —K—feldspar + H₂O = chrysoberyl+ quartz + melt. Mineral assemblages from mica-rich parts ofthe pegmatite include sillimanite—K—feldspar, muscovite—K—feldspar—sillimanite,and annite—magnetite—spinel—sillimanite—garnet.Details about the composition and the textural relationshipsof these minerals are given; they indicate a post-pegmatiticmetamorphic event at P—T conditions near to the anatecticregime. The samples from Marikov show textures, which are explainedby the reactions beryl + albite + H⁺ = chrysoberyl + quartz+ Na⁺ + H₂O or alternatively beryl + albite + H₂O = chrysoberyl+ quartz + melt. Breakdown of muscovite produces sillimaniteaccording to the reactions beryl + albite + muscovite + H⁺ =chrysoberyl + quartz + sillimanite + Na⁺ + K⁺ + H₂O or alternativelyberyl + albite + muscovite + H₂O = chrysoberyl + quartz + sillimanite+ melt. Similar reaction textures and mineral assemblages were foundin other chrysoberyl-bearing pegmatites (Maroankora, Madagascar;Helsinki, Finland; Haddam, Greenfield, Greenwood, U.S.A.). Hydrothermal experiments located the reaction beryl + alkalifeldspar + H₂O = chrysoberyl + phenakite + melt at P—Tconditions between the K—feldspar—quartz—H₂Osolidus and the K—feldspar—albite—quartz-H₂Osolidus. It is concluded that the formation of Al-rich minerals likechrysoberyl and sillimanite in pegmatites is due to a post-pegmatiticevent at high P—T conditions. The question as to whichof the alternative set of reactions is more likely, the ionicequilibria or the anatectic chrysoberyl formation, must be leftopen. The previous hypothesis of a desilification of a pegmatitewhich intruded into SiO₂-poor country rocks, or of the assimilationof Al₂O₃-rich country rocks, cannot explain the mineral assemblagesof the two pegmatites.     

Petrology of Franciscan Metabasites Along the Jadeite-Glaucophane Type Facies Series, Cazadero, California

The Cazadero blueschist allochthon lies within the Central MelangeBelt of the Franciscan assemblage in the northern Coast Rangeof California. Mineral compositions and assemblages of morethan 200 blueschists from Ward Creek were investigated. Theresults delineate lawsonite-, pumpellyite-, and epidote-zones.The lawsonite and pumpellyite zones are equivalent to the TypeII metabasites of Coleman & Lee (1963) and are characterizedby well-preserved igneous textures, relict augite, and pillowstructures, whereas epidote zone rocks are equivalent to theType III strongly deformed and schistose metabasites. Chlorite,phengite, aragonite, sphene, and minor quartz and albite areubiquitous. The lawsonite zone metabasites contain lawsonite ( < 3 wt.per cent Fe₂O₃), riebeckite-crossite, chlorite, and Ca-Na-pyroxene;some rocks have two distinct clinopyroxenes separated by a compositionalgap. The clinopyroxene of the lowest grade metabasites containsvery low X_jd. In pumpellyite zone metabasites, the most commonassemblages contain Pm + Cpx + Gl + Chl and some samples withhigher Al₂O₃ and/or Fe₂O₃ have Pm + Lw + Cpx + Chl, Actinolitejoins the above assemblage in the upper pumpellyite zone wherethe actinolite-glaucophane compositional gap is well defined.The epidote zone metabasites are characterized by the assemblagesEp + Cpx + two amphiboles + Chl, Lw + Pm + Act + Chl, and Ep+ Pm + two amphiboles + Chl depending on the Fe₂O₃ content ofthe rock. In the upper epidote zone, winchite appears, Fe-freelawsonite is stable, pumpellyite disappears and omphacite containsvery low Ac component. Therefore, the common assemblages areEp + winchite + Lw, and Lw + Omp + winchite. With further increasein metamorphic grade, epidote becomes Al-rich and lawsoniteis no longer stable. Hence Ep + winchite + omphacite ? garnetis characteristic. Mineral assemblages and paragenetic sequences delineate threediscontinuous reactions: (1) pumpellyite-in; (2) actinolite-in;and (3) epidote-in reactions. Using the temperatures estimatedby Taylor & Coleman (1968) and phase equilibria for Ca-Na-pyroxenes,the P–T positions of these reactions and the metamorphicgradient are located. All three metabasite zones occur withinthe aragonite stability field and are bounded by the maximumpressure curve of Ab = Jd + Qz and the maximum stabilities ofpumpellyite and lawsonite. The lawsonite zone appears to bestable at T below 200?C with a pressure range of 4–6?5kb; the pumpellyite zone between 200 and 290?C and the epidotezone above 290?C with pressure variation between 6?5 and 9 kb.The metamorphic field gradient appears to have a convex naturetowards higher pressure. A speculative model of underplatingseamounts is used to explain such feature.     

Phase Equilibria for Calcic Scapolite, and Implications of Variable Al-Si Disorder for P-T, T-XCO2, and a-X Relations

Carbonate scapolite is a potentially powerful mineral for calculatingCO₂ activities in non-calcareous rocks, but an analysis of thethermodynamics and phase equilibria of carbonate scapolite isfirst necessary. This includes an evaluation of Al-Si disorderin meionite, as this has the greatest effect on derived phaserelations. Available experimental data on meionite stability,X-ray diffraction refinements and nuclear magnetic resonancespectra for calcic scapolite do not uniquely constrain the Al-Siordering state of synthetic meionite. However, the data aremost consistent with a high degree of Al-Si disorder and inconsistentwith long-range Al-Si order. An internally consistent thermodynamicdata set was derived and used to calculate P-T and T-X_CO2 equilibriainvolving meionite in the CaO-Al₂O₃-SiO₂-CO₂-H₂O (CASCH) system.The effect of Al-Si disorder is illustrated by calculating thephase equilibria using an ordered, an arbitrary intermediatedisordered, and a completely Al-Si disordered standard statefor meionite. The Gibbs free energy of meionite was calculatedfrom reversals (at 790–815?C, 2–15 kb) on the reaction 3 Anorthite +Calcite =Meionite The _fG?_{m, 298} for each of the standard states is –13 146?6,–13128?8, and –130930kJ/mol, respectively. Becauseof the steep slope of reaction (1) and limited temperature rangeover which it breaks down, meionite used in the experimentsto constrain reaction (1) must possess a limited range of Al-Sidisorder. The P-T slope of reaction (1) increases, and the slopeof meionite decarbonation equilibria changes from positive tonegative in T-X_CO2 and P-T space, as a function of increasingAl-Si disorder. Meionite has a wide stability field at highT in T-X space at 5 and 10 kb (P_Total=P_Fluid), being stableto X_CO2=0?06. Meionite alone breaks down to undersaturated gehleniteand/or corundum-bearing assemblages at 5 kb, and to clinozoisiteat 10 kb. The effect of solid solutions on the T-X stabilityof meionite is similar to that of increasing pressure, stabilizingmeionite to lower temperature. Variable Al-Si disorder doesnot significantly affect the upper limit of meionite stabilityin T-X_CO2 space. Activity-composition relations for meionitein carbonate scapolite were calculated relative to reaction(1) from data on natural scapolite-plagioclase-calcite assemblages.The extent of departure from ideality varies as a function ofAl-Si disorder. Negative deviations from ideality are indicatedfor natural scapolite solid solutions at T<750?C, based ona disordered Al-Si standard state for meionite. This is likelyto reflect a more ordered Al-Si distribution in natural scapolitescompared with the synthetic endmember standard state. Present address: Department of Earth and Space Sciences, State University of New York, Stony Brook, New York 11794-2100     

Calculated Phase Relations in High-Pressure Metapelites in the System NKFMASH (Na2O-K2O-FeO-MgO-Al2O3-SiO2-H2O)

Using an internally consistent thermodynamic dataset and updatedmodels of activity–composition relation for solid solutions,petrogenetic grids in the system NKFMASH (Na₂O–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O)and the subsystems NKMASH and NKFASH have been calculated withthe software THERMOCALC 3.1 in the P–T range 5–36kbar and 400–810°C, involving garnet, chloritoid,biotite, carpholite, talc, chlorite, kyanite/sillimanite, staurolite,phengite, paragonite, albite, glaucophane, jadeite, with quartz/coesiteand H₂O in excess. These grids, together with calculated AFMcompatibility diagrams and P–T pseudosections, are shownto be powerful tools for delineating the phase equilibria andP–T conditions of Na-bearing pelitic assemblages for avariety of bulk compositions from high-P terranes around theworld. These calculated equilibria are in good agreement withpetrological studies. Moreover, contours of the calculated phengiteSi isopleths in P–T pseudosections for different bulkcompositions confirm that phengite barometry is highly dependenton mineral assemblage. KEY WORDS: phase relations; HP metapelite; NKFMASH; THERMOCALC; phengite geobarometry     

Blueschist and Eclogite transformations in Fe-Ti Gabbros: A Case from the Western Alps Ophiolites

The study of the alpine metamorphism of three suites of Fe-Timetagabbros occurring in the western Alps ophiolites has showna set of reactions governed by T, P, X_H2O, and diffusion. T-Pestimates point to 350?50?C at a minimum of 9–10 kb forthe Queyras blueschist rocks and to 450–500 ?C at a minimumof 12–13 kb for the Lanzo and Rocciavr` eclogitized rocks.These variations are the result of different T-P-time trajectoriesduring subduction/obduction events of alpine age. In the Fe-Timetagabbros, little-deformed volumes showing a crystallizationhistory controlled by local equilibrium are bounded by mylonites.Water-poor and water-rich volumes alternate during eclogitizationof the Rocciavr? suite. The persistence in the little-deformedrocks of prealpine metastable relics, of corona structures andof chemical gradients, demonstrates that a complete high pressureequilibration is inhibited by slow reaction kinetics and slowdiffusion. Only in the mylonites has the catalytic effect ofdeformation favoured an approach to bulk-rock metamorphic equilibration.In the eclogitized coronitic rocks the apparent O₂, releasefrom the alteration of the magmatic opaques plays an importantrole in reaction rates; increasing extent of eclogitizationmay be enhanced either by the release of free O₂ from the rocks,or by a process in which new H₂O formed by the combination ofoxygen with hydrogen introduced into the system.     

Lawsonite-Omphacite-Bearing Metabasites of the Pam Peninsula, NE New Caledonia: Evidence for Disrupted Blueschist- to Eclogite-Facies Conditions

The Diahot terrane of NE New Caledonia contains an interbeddedsequence of Cretaceous to Eocene metasediments, felsic and maficmetavolcanics that experienced c. 40 Ma high-P/T metamorphism.Metabasaltic assemblages define two prograde events (M₁ andM₂) and a tectonically disrupted crustal profile that extendsfrom lawsonite–blueschist conditions in the SW to paragonite–eclogiteconditions in the NE. Weakly deformed metabasalts from lowest-gradeparts of the Diahot terrane contain M₁ omphacite, chlorite,lawsonite and glaucophane-bearing assemblages that partiallypseudomorph igneous plagioclase and augite, and reflect P =0·7–1·0 GPa and T = 350–400°C.M₁ assemblages are enveloped by a steeply SW-dipping S₂ foliationthat becomes progressively more intense towards the NE overa distance of c. 15 km. S₂ assemblages are divided into fourzones: (1) lawsonite–omphacite; (2) lawsonite–clinozoisite–spessartine;(3) clinozoisite–hornblende–almandine; (4) almandine–omphacite.S₂ assemblages reflect a P–T gradient that spans the exposed15 km of the Diahot terrane from P = 0·8–1·0GPa and T = 350–400°C (Zone 1) to P = 1·6–1·7GPa and T = 550–600°C (Zone 4). The systematic mineralogicalchanges reflect parts of a P–T array between 1·0and 1·7 GPa that was extensively disrupted by tectonicthinning during exhumation. KEY WORDS: blueschist; eclogite; New Caledonia; CNFMASH; pseudosection     

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.     

Pressure, Temperature and C-O-H Fluid Fugacities across the Amphibolite-Granulite Transition, Northwest Adirondack Mountains, New York

Pressures, temperatures, water activities (a_H2O) and fugacitiesof the other C-O-H fluid species have been estimated on a traverseacross the amphibolite-granulite facies boundary in the MajorParagneiss, northwest Adirondacks, N.Y. Two-feldspar pairs givetemperatures ranging from 650?C in the central portion of theunit to 760?C towards the northeast. Biotite-garnet pairs giveerratic temperatures compared to two-feldspar temperatures.This discrepancy appears to be due to retrograde resetting asdetermined from compositional zoning patterns in biotites andgarnets. Some of the discrepancy may also be due to non-idealityof pyrope-almandine mixing or to non-ideality from other components.Pressures ranging from 5?4 kb for the southwestern portion ofthe unit to 8?0 kb in the northeast were determined from anorthite-grossular-sillimanite-quartzbarometry. Minimum pressures of 5?8 kb were also determinedfrom coexisting garnet + rutile. Values of a_H2O of 0?08-0?5estimated from biotite and muscovite dehydration reactions showno correlation with grade. The variability in a_H2O suggeststhat it is locally controlled and that a homogeneous, pervasivefluid was not present during high grade metamorphism. Graphiteequilibria indicate that f_O2 was less than 0?5 log units belowQFM and that if a fluid was present, it was rich in CO₂ andH₂O. P-T-a_H2O values suggest that partial melting did not occurduring metamorphism. Pervasive flooding with CO₂ does not appearto have occurred. The amphibolite-granulite transition at thislocality is characterized by increasing temperature and pressure.     

Mesoproterozoic Reworking of Palaeoproterozoic Ultrahigh-temperature Granulites in the Central Indian Tectonic Zone and its Implications

In the southern periphery of the Sausar Mobile Belt (SMB), thesouthern component of the Central Indian Tectonic Zone (CITZ),a suite of felsic and aluminous granulites, intruded by gabbro,noritic gabbro, norite and orthopyroxenite, records the polymetamorphicevolution of the CITZ. Using sequences of prograde, peak andretrograde reaction textures, mineral chemistry, geothermobarometricresults and petrogenetic grid considerations from the felsicand the aluminous granulites and applying metamorphosed maficdyke markers and geochronological constraints, two temporallyunrelated granulite-facies tectonothermal events of Pre-Grenvillianage have been established. The first event caused ultrahigh-temperature(UHT) metamorphism (M₁) (T 950°C) at relatively deepercrustal levels (P 9 kbar) and a subsequent post-peak near-isobariccooling P–T history (M₂). M₁ caused pervasive biotite-dehydrationmelting, producing garnet–orthopyroxene and garnet–rutileand sapphirine–spinel-bearing incongruent solid assemblagesin felsic and aluminous granulites, respectively. During M₂,garnet–corundum and later spinel–sillimanite–biotiteassemblages were produced by reacting sapphirine–spinel–sillimaniteand rehydration of garnet–corundum assemblages, respectively.Applying electron microprobe (EMP) dating techniques to monazitesincluded in M₁ garnet or occurring in low-strain domains inthe felsic granulites, the UHT metamorphism is dated at 2040–2090Ma. Based on the deep crustal heating–cooling P–Ttrajectory, the authors infer an overall counterclockwise P–Tpath for this UHT event. During the second granulite event,the Palaeoproterozoic granulites experienced crustal attenuationto 6·4 kbar at T 675°C during M₃ and subsequentnear-isothermal loading to 8 kbar during M₄. In the felsic granulites,the former is marked by decomposition of M₁ garnet to orthopyroxene–plagioclasesymplectites. During M₄, there was renewed growth of garnet–quartzsymplectites in the felsic granulites, replacing the M₃ mineralassemblage and also the appearance of coronal garnet–quartz–clinopyroxeneassemblages in metamorphosed mafic dykes. Using monazites frommetamorphic overgrowths and metamorphic recrystallization domainsfrom the felsic granulite, the M₄ metamorphism is dated at 1525–1450Ma. Using geochronological and metamorphic constraints, theauthors interpret the M₃–M₄ stages to be part of the sameMesoproterozoic tectonothermal event. The result provides thefirst documentation of UHT metamorphism and Palaeo- and Mesoproterozoicmetamorphic processes in the CITZ. On a broader scale, the findingsare also consistent with the current prediction that isobaricallycooled granulites require a separate orogeny for their exhumation. KEY WORDS: Central Indian Tectonic Zone; UHT metamorphism; counterclockwise P–T path; monazite chemical dating     

Fluid Flow During Contact Metamorphism of Ophicarbonate Rocks in the Bergell Aureole, Val Malenco, Italian Alps

Contact-mctamorphic assemblages in ophicarbonate from the Bergellaureole correspond either to model isobaric invariant T-XCO₂points [Atg-Cal-Di-Tr-Fo (6 samples) and Atg-Cal-Tr-Fo-Dol (2)]or to isobaric univariant T-XCO₂, curves [Tr-Cal-Di-Atg (18),Tr-Dol-Atg-Cal (1), Atg-Cal-Fo-Di (1), and Atg-Cal-Tr-Fo (1)].Calcite-dolomite thermometry and mineral-fluid equilibria inthe invariant assemblages record T=440–540C at P=3•5kbar. Equilibrium metamorphic fluids were very H₂O rich withX CO₂,=0•001–0•027. In the invariant assemblagesTr + Fo were produced by prograde decarbonation-dehydrationreactions. In contrast, measured modes and reaction texturesin samples with univariant assemblages indicate thai Tr wasproduced by carbonation reactions. The apparent paradox of simultaneousdecarbonation reactions in the model isobaric invariant assemblagesand carbonation reactions in univariant assemblages is resolvedby local mineral-fluid equilibrium and fluid flow through ophicarbohatesin the direction of decreasing temperature as the aureole heated.Time-integrated flux (q) was computed from measured reactionprogress in 28 samples for models of both horizontal and verticaldown-temperature flow. Results are similar, with q decreasingrapidly from (0•2–5•1) 10⁵ cm³ fluid/cm² rock1•3–1•7 km from the intrusion to 0–0•610⁵cm³/cm² at 1•8–4•0 km. The decrease in q ismore consistent with vertical than horizontal flow. Variationsin time-integrated flux of more than an order of magnitude arerecorded by samples from the same outcrop. The absence of carbonatein adjacent metaperidotite indicates that flow was confinedto the ophicarbonate. Channelized, spatially heterogeneous,vertical flow can be explained by the brecciation and strongvertical foliation of the ophicarbonate relative to surroundingmassive metaperidotite. Generation of metamorphicfluids by decarbonation-dehydrationreactions within the ophicarbonates explains larger averageflux 1–2 km from the intrusion compared with more distalpoints. KEY WORDS: Bergell; contact metamorphism; fluid flow; ophicarbonate ^*Telephone: (410) 516-8121. Fax: (410) 516-7933     

A petrogenetic grid for aluminous granulite facies metapelites in the KFMASH system

Due to the retrograde cation exchange problems experienced by conventional geothermobarometers above their closure temperatures, petrogenetic grids are a potentially powerful alternative to unravelling the P–T evolution of ultrahigh‐T granulite terranes. A new qualitative KFMASH (K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O) petrogenetic grid for Mg–Al rich metapelites containing K‐feldspar, sillimanite and quartzofeldspathic melt that successfully accounts for the majority of assemblages composed of variations of sapphirine, spinel, garnet, orthopyroxene, cordierite, biotite and quartz is developed. Univariant reactions are predicted utilizing a newly derived ‘melt projection’ and these reactions are entirely consistent with algebraically calculated reaction coefficients obtained using a set of standard phase compositions. Based upon observations of commonly associated mineral assemblages in natural lithologies the [Spr, Spl], [Qtz, Spl], [Bt, Spl], [Opx, Spr], [Opx, Qtz] and [Bt, Opx] invariant points are assumed to be stable, whilst the [Grt, Spr], [Grt, Qtz], [Spr, Qtz] and [Crd, Qtz] are assumed to be metastable. Biotite‐bearing assemblages are confined to the lowest temperatures, and sapphirine + quartz to the highest temperatures. Orthopyroxene + sillimanite ± quartz assemblages are confined to the highest pressures, whilst spinel‐bearing assemblages are stabilized by lower pressures. The alternative choice of invariant point stability leads to significant differences between this grid and previously proposed topologies. Spinel cannot be stable along with the orthopyroxene and sillimanite assemblage as previously proposed. Further, more subtle differences in topology result from the treatment of H₂O in the chemographic projection used to deduce univariant reactions, and projecting from a water‐bearing quartzofeldspathic melt does not yield the same reaction coefficients as projection from H₂O. The new grid allows reinterpretation of previously proposed evolutionary P–T paths for Mg–Al rich granulites from the Napier Complex and Rauer Group, East Antarctica, and In Ouzzal, Algeria.     

Petrogenesis of Early Neogene Magmatism in the Northern Puna; Implications for Magma Genesis and Crustal Processes in the Central Andean Plateau

New compositional data and petrogenetic models are presentedfor pre-Upper Miocene volcanism in the northern Puna of Argentina(22°S–24°S). Two phases of volcanism producedsmall dome complexes of mainly silicic andesite to low-SiO₂rhyolite. The Upper Oligocene–Lower Miocene phase (UOLM,20–17 Ma), produced two distinct groups of rocks. TheUOLM-1 group is metaluminous and mainly andesitic, with isotopiccompositions like those of the recent arc (⁸⁷Sr/⁸⁶Sr_T