Metamorphism of Calcic Pelitic Schists, Strafford Dome, Vermont: Compositional Zoning and Reaction History

Four assemblages from calcic pelitic schists from South Strafford,Vermont, have been studied in detail to determine the relationshipbetween reaction history and compositional zoning of minerals.The lowest-grade assemblage is garnet + biotite + chlorite +plagioclase + epidote + quartz + muscovite + graphite + fluid.Along a path of isobaric heating, the net reaction is Chl +Ms + Ep + Gr = Grt + Bt + Pl + fluid. Garnet grows with decreasingFe/(Fe + Mg) and X_Spa, (from 0•2 to 0•05), X_Gra staysnearly constant between 0•20 and 0•25, and plagioclasegrows with X_An increasing from peristerite to 0•2–0•5. The subsequent evolution depends on whether chlorite or epidotereacts out first. If chlorite is removed from the assemblagefirst, the net reaction along an isobaric heating path becomesGrt + Ms + Ep + Qtz + Gr = Bt + Pl + fluid. X_An of plagioclaseincreases to 0•20–0•70, depending on the bulk-rockcomposition and changes in pressure and temperature. If epidoteis removed first, the assemblage becomes a simple pelite andthe net reaction becomes Chl + Pl + Ms + Qtz = Grt + Bt + H₂O.Plagioclase is consumed to provide Ca for growing garnet, andX_An, Fe/(Fe + Mg) of garnet, X_Gra, and X_Spa all decrease. Afterboth chlorite and epidote are removed, continued heating upto the metamorphic peak of {small tilde}600C produces littleprogress of the reaction Grt + Ms = Bt + Pl; and X_An increases. The four assemblages have been numerically modeled using theGibbs method starting with measured compositions. The modelssuccessfully predict the observed compositional zoning and trendsof mineral growth and consumption along the computed P–Tpaths. The models also predict the compositional mineral zoningthat would have resulted from other P–T paths. ^* Present address: Department of Geology, University of Alabama, Tuscaloosa, Alabama 35487     

Melting of a Hydrous Mantle: I. Phase Relations of Natural Peridotite at High Pressures and Temperatures with Controlled Activities of Water, Carbon Dioxide, and Hydrogen

Four natural peridotite nodules ranging from chemically depletedto Fe-rich, alkaline and calcic (SiO₂ = 43.7–45.7 wt.per cent, A1₂O₃ = 1.6O–8.21 wt. per cent, CaO = 0.70–8.12wt. per cent, alk = 0.10–0.90 wt. per cent and Mg/(Mg+Fe²⁺)= 0.94–0.85) have been investigated in the hypersolidusregion from 800? to 1250?C with variable activities of H₂O,CO₂, and H₂. The vapor-saturated peridotite solidi are 50–200?Cbelow those previously published. The temperature of the beginningof melting of peridotite decreases markedly with decreasingMg/(Mg+SFe) of the starting material at constant CaO/Al₂O₃.Conversely, lowering CaO/Al₂O₃ reduces the temperature at constantMg/(Mg+Fe) of the starting material. Temperature differencesbetween the solidi up to 200?C are observed. All solidi displaya temperature minimum reflecting the appearance of garnet. Thisminimum shifts to lower pressure with decreasing Mg/(Mg + Fe)of the starting material. The temperature of the beginning ofmelting decreases isobarically as approximately a linear functionof the mol fraction of H₂O in the vapor (X_H2O^v). The data alsoshow that some CO₂ may dissolve in silicate melts formed bypartial melting of peridotite. Amphibole (pargasitic hornblende) is a hypersolidus mineralin all compositions, although its P/T stability field dependson bulk rock chemistry. The upper pressure stability of amphiboleis marked by the appearance of garnet. The vapor-saturated (H₂O) liquidus curve for one peridotiteis between 1250? and 1300?C between 10 and 30 kb. Olivine, spinel,and orthopyroxene are either liquidus phases or co-exist immediatelybelow the temperature of the peridotite liquidus. The data suggest considerable mineralogical heterogeneity inthe oceanic upper mantle because the oceanic geotherm passesthrough the P/T band covering the appearance of garnet in variousperidotites. The variable depth to the low-velocity zone is explained byvariable aHjo conditions in the upper mantle and possibly alsoby variations in the composition of the peridotite itself. Itis suggested that komatiite in Precambrian terrane could formby direct melting of hydrous peridotite. Such melting requiresabout 1250?C compared with 1600?C which is required for drymelting. The genesis of kimberlite can be related to partial meltingof peridotite under conditions of X_H2O^v = 0.5–0.25 (X_CO2^v= 0.5–0.75). Such activities of H₂O result in meltingat depths ranging between 125 and 175 km in the mantle. Thisrange is within the minimum depth generally accepted for theformation of kimberlite.     

Partial Melting of Spinel Lherzolite in the System CaO-MgO-Al2O3-SiO2 {+/-} K2O at 1{middle dot}1 GPa

The compositions of multiply saturated partial melts are valuablefor the thermodynamic information that they contain, but aredifficult to determine experimentally because they exist onlyover a narrow temperature range at a given pressure. Here wetry a new approach for determining the composition of the partialmelt in equilibrium with olivine, orthopyroxene, clinopyroxeneand spinel (Ol + Opx + Cpx + Sp + Melt) in the system CaO–MgO–Al₂O₃–SiO₂(CMAS) at 1·1 GPa: various amounts of K₂O are added tothe system, and the resulting melt compositions and temperatureare extrapolated to zero K₂O. The ‘sandwich’ experimentalmethod was used to minimize problems caused by quench modification,and Opx and Cpx were previously synthesized at conditions nearthose of the melting experiments to ensure they had appropriatecompositions. Results were then checked by reversal crystallizationexperiments. The results are in good agreement with previouswork, and establish the anhydrous solidus in CMAS to be at 1320± 10°C at 1·1 GPa. The effect of K₂O is todepress the solidus by 5·8°C/wt %, while the meltcomposition becomes increasingly enriched in SiO₂, being quartz-normativeabove 4 wt % K₂O. Compared with Na₂O, K₂O has a stronger effectin depressing the solidus and modifying melt compositions. Theisobaric invariant point in the system CMAS–K₂O at whichOl + Opx + Cpx + Sp + Melt is joined by sanidine (San) is at1240 ± 10°C. During the course of the study severalother isobaric invariant points were identified and their crystaland melt compositions determined in unreversed experiments:Opx + Cpx + Sp + An + Melt in the system CMAS at 1315 ±10°C; in CMAS–K₂O, Opx + Cpx + Sp + An + San + Meltat 1230 ± 10°C and Opx + Sp + An + San + Sapph +Melt at 1230 ± 10°C, where An is anorthite and Sapphis sapphirine. Coexisting San plus An in three experiments helpdefine the An–San solvus at 1230–1250°C. KEY WORDS: feldspar solvus; igneous sapphirine; mantle solidus; partial melting; systems CMAS and CMAS–K₂O     

Experimental Reversal of the Na-K Exchange Reaction Between Muscovite-Paragonite Crystalline Solutions and a 2 Molal Aqueous (Na,K)Cl Fluid

Single-phase 2M₁ muscovite-paragonite crystalline solutionsin the range 0?00–0?10 and 0?70–1?00 X_ms have beensynthesized by hydrothermal treatment of gels of appropriatecompositions at 600–700?C, and 7 to 18 kb P_H2O. The molarvolumes of these micas may be expressed as V(J/b?mol) = 13?1845+1?463X_ms+0?0160 X_ms²–0?1679 X_ms³ (?0?005), which translateto a substantial positive excess molar volume of mixing. Na-K ion exchange experiments between presynthesized 2M₁ micacrystalline solutions and 2 molal aqueous (Na,K)Cl fluids failedto proceed to completion despite 98 day runs at 500–600?C,6 kb P_total. Results of analogous exchange experiments provedencouraging however, when a much finer-grained 1M mica was usedas starting material. Applying the tie line rotation technique,reversal of ion exchange experiments could be achieved in the2-phase fields, not, however, in the 3-phase field of the ms-pg-NaCl-KClreciprocal ternary. Using gels as starting material, reversalexperiments were eventually successful both in the 2-phase andthe 3-phase fields; the results of reversal experiments withinthe two-phase fields being identical to those obtained earlierusing 1M micas. Four isobaric-isothermal sections through the ms-pg-NaCl-KClternary were reversibly determined at 450?C/5 kb, 550?C/6 kb,550?C/15 kb, and 620?C/7 kb. At 450?C, the coexisting mica compositionsin the 3-phase field (2 micas plus 1 fluid) are 0?10 and 0?77X_ms, at 550?C they are 0?10 and 0?60 X_ms, and finally, at 620?Cthese are 0?12 and 0?51 X_ms. To the extent that internal equilibriumwas accomplished between the coexisting micas, these data wouldindicate a wide solvus at 450?C, narrowing gradually with increasingtemperature to 620?C. The critical temperature will be wellin excess of 620?C, although the mica at the critical conditionwill prove to be metastable with respect to the assemblage alkalifeldspars+corundum+H₂O. The companion paper by Chatterjee & Flux (1986) presentsa thermodynamic analysis of the above experimental data.     

Melting Behaviour of Model Lherzolite in the System CaO-MgO-Al2O3-SiO2-FeO at 0{middle dot}7-2{middle dot}8 GPa

Fe–Mg exchange is the most important solid solution involvedin partial melting of spinel lherzolite, and the system CaO–MgO–Al₂O₃–SiO₂–FeO(CMASF) is ideally suited to explore this type of exchange duringmantle melting. Also, if primary mid-ocean ridge basalts arelargely generated in the spinel lherzolite stability field bynear-fractional fusion, then Na and other highly incompatibleelements will early on become depleted in the source, and themelting behaviour of mantle lherzolite should resemble the meltingbehaviour of simplified lherzolite in the CMASF system. We havedetermined the isobarically univariant melting relations ofthe lherzolite phase assemblage in the CMASF system in the 0·7–2·8GPa pressure range. Isobarically, for every 1 wt % increasein the FeO content of the melt in equilibrium with the lherzolitephase assemblage, the equilibrium temperature is lower by about3–5°C. Relative to the solidus of model lherzolitein the CaO–MgO–Al₂O₃–SiO₂ system, melt compositionsin the CMASF system are displaced slightly towards the alkalicside of the basalt tetrahedron. The transition on the solidusfrom spinel to plagioclase lherzolite has a positive Clapeyronslope with the spinel lherzolite assemblage on the high-temperatureside, and has an almost identical position in P–T spaceto the comparable transition in the CaO–MgO–Al₂O₃–SiO₂–Na₂O(CMASN) system. When the compositions of all phases are describedmathematically and used to model the generation of primary basalts,temperature and melt composition changes are small as percentmelting increases. More specifically, 10% melting takes placeover 1·5–2°C, melt compositions are relativelyinsensitive to the degree of melting and bulk composition, andequilibrium and near-fractional melting yield similar melt compositions.FeO and MgO are the oxides that exhibit the greatest changein the melt with degree of melting and bulk composition. Theamount of FeO decreases with increasing degree of melting, whereasthe amount of MgO increases. The coefficients for Fe–Mgexchange between the coexisting crystalline phases and melt,K_dFe–Mg^xl–liq, show a relatively simple and predictablebehaviour with pressure and temperature: the coefficients forolivine and spinel do not show significant dependence on temperature,whereas the coefficients for orthopyroxene and clinopyroxeneincrease with pressure and temperature. When melting of lherzoliteis modeled in the CMASF system, a strong linear correlationis observed between the mg-number of the lherzolite and themg-number of the near-solidus melts. Comparison with meltingin the CMASN system indicates that Na₂O has a strong effecton lherzolite melting behaviour only at small degrees of melting. KEY WORDS: CMASF; lherzolite solidus; mantle melting     

Phase Relations on the Actinolite-Pargasite Join

Phase relations along the join Ca₂Mg₄Fe²⁺Si₈O₂₂ (OH)₂ (Actinolite)-NaCa₂Mg_3?2Fe_0?8²⁺AlSi₆Al₂O₂₂(OH)₂ (Pargasite) have been studied at P_H2O = 1 kb andthe oxygen fugacities defined by the iron-wustite(IW) buffer. Actinolite and bornblende are separated by a solvus and thefield of actinolite+hornblende+vapor is present in the regionbetween Ac₈₅Pa₁₅ and Ac₅₅ Pa₄₅ at 680 ?C. Complete miscibilityis achieved at 720 ?C. At temperatures higher than the solvusthere is a continuous solid solution series between the twoend members. The stability field of amphibole solid solutiongradually increases with increasing pargasite content in actinolite.The phase assemblages at temperatures higher than those of asolid solution series between the two end members change withincreasing pargasite content in the bulk composition as follows;Act+Cpx+Qz+V, ActHbl+Cpx+Opx+Qz+V, Hbl+Cpx+Opx+Pl+V and Hbl+Cpx+Pl+Ol+V. In comparison with the Fe-free system, the extent of the miscibilitygap between actinolite and hornblende is reduced by an increasein the Fe²⁺ content. The present study should provide an adequatebasis for the interpretation of actinolite-hornblende pairsin metamorphic rocks.     

Experimental Petrology of the Kiglapait Intrusion: Cotectic Trace for the Lower Zone at 5 kbar in Graphite

The inferred crystallization history of the troctolitic LowerZone of the Kiglapait Intrusion in Labrador is tested by meltingmineral mixtures from the intrusion, made to yield the observedcrystal compositions on the cotectic trace of liquid, plagioclase,and olivine. Melting experiments were made in a piston-cylinderapparatus, using graphite capsules at 5 kbar. Lower Zone assemblagescrystallized from 1245°C, 5% normative augite in the liquid,to 1203°C, 24% normative augite in the liquid at saturationwith augite crystals. This transit is consistent with modaldata and the large volume of the Lower Zone. The 1245°Ccotectic composition matches the average Inner Border Zone composition.Quenched troctolitic liquid from the Upper Border Zone, andothers from nearby Newark Island, plot on or near our experimentalcotectic, supporting a common fractionation history. Olivine–plagioclaseintergrowths from cotectic troctolitic melt show mosaic texturesreflecting the differing barriers to nucleation of these twophases. The linear partitioning of X_Ab in plagioclase–meltyields an intercept constant K_D = 0·524 for these maficmelts. Observed subsolidus exchange of Ca between plagioclaseand olivine elucidates the loss of Ca from plutonic olivines.The bulk composition of the intrusion is revised downward inFo and An. KEY WORDS: experimental; olivine; plagioclase; Kiglapait; partitioningAbbreviations: AP, MT, IL, OR, AB, AN, DI, HY, OL, FO, NE, Q, FSP, AUG: (Oxygen) Normative components; Ap, Aug, Ilm, Ol, Pl: Phases; Ab, An, Di, Fa, Fo, Or, Wo: Phase components; also ternary endmembers; BSE: Back-scattered electron; CaTs: Calcium Tschermak's component, CaAlAlSiO₆; D: Partition coefficient; f: Fugacity; F_L: Fraction of the system present as liquid = 1 – (PCS/100); FMQ: Fayalite = magnetite + quartz buffer; IBZ: Inner Border Zone; IW: Iron = wüstite buffer; kbar: kilobar, 10⁸ pascal; K_D: Exchange coefficient; KI: Kiglapait Intrusion; L: Liquid phase; LLD: Liquid line of descent; Ma: Mega-annum, age; Myr: Mega-year, time; OLHY: Normative OL + HY; OLRAT: The ratio OLHY/(OLHY + AUG); P: Pressure; P: Phosphorus; PCS: Percent solidified (volume); SMAR: South Margin average composition; T: Temperature, °C; UBZ: Upper Border Zone; WM: Wüstite = magnetite buffer; Wo: Wollastonite component of pyroxene; X: Mole fraction; X_Mg: Molar ratio Mg/(Mg + Fe²⁺); , X_Mg(0): Initial X_Mg before MT is formed in the norm calculation; X: Coordinate, horizontal axis; Y: Coordinate, vertical axis     

The Effect of Cr2O3 on the Partial Melting of Spinel Lherzolite in the System CaO-MgO-Al2O3-SiO2-Cr2O3 at 1{middle dot}1 GPa

Chromium as Cr³⁺ substitutes for octahedrally coordinated Alin upper-mantle minerals, thereby reducing the activity of Al₂O₃in the system and hence the concentration of Al₂O₃ in partialmelts. The effect of Cr₂O₃ on melt compositions multiply saturatedwith the spinel lherzolite phase assemblage has been quantifiedin the system CaO–MgO–Al₂O₃–SiO₂–Cr₂O₃at 1·1 GPa as a function of 100 Cr/(Cr + Al) in the spinel(Cr#_sp). The decrease of Al₂O₃ in the melt with increasing Cr#_spis accompanied by increasing MgO and SiO₂, whereas CaO remainsalmost constant. Consequently, the CaO/Al₂O₃ ratio of the meltincreases with Cr#_sp, and the melt becomes richer in normativediopside, hypersthene and quartz. The effect may explain certainmantle melts with unusually high CaO/Al₂O₃ ratios. The concentrationof Cr₂O₃ in the melt remains low even at high Cr#_sp, which meansthat the strong effect of Cr₂O₃ on partial melting equilibriais not readily apparent from its concentration in the melt itself.The existence of a highly refractory major component such asCr₂O₃ nullifies simplified conclusions from the ‘inverseapproach’ in the experimental study of basalt petrogenesis,as there is insufficient information in the composition of thepartial melt to reconstruct the conditions of melting. KEY WORDS: basalt petrogenesis; partial melting; reversal experiment; spinel lherzolite; system CMAS–Cr₂O₃; CaO/Al₂O₃ of melt; effect of Cr₂O₃     

Phase Equilibria of Margarite-Bearing Schists and Chloritoid + Hornblende Rocks from Western New Hampshire, USA

A variety of uncommon garnet-grade assemblages have been foundin rocks from three outcrops in the western part of centralNew Hampshire, and include the associations Grt+MrgCld, Grt+Bt+CldMrg,and Mrg+Cld+HblGrt (all rocks contain Ms, Chl, Ilm, and Qtz).These unusual rocks coexist with more typical Grt+Bt+Chl+Plmetapelites and amphibolites. Rim P–T conditions are {smalltilde}49035C and 5•751•25 kbar. Projection of the assemblages from Qtz, H₂O, and Ilm into theCa–Al'–Na–(Fe+Mg) tetrahedron, and from Qtz,Ilm, H₂O, and Chl into the Ca–Al'–Fe'–Mn tetrahedronindicates that Ca/(Ca+Na) and Mn differ among the assemblagesin a systematic fashion. Common Grt+Bt+Chl+Pl assemblages arerestricted to relatively high Mn and low Ca/(Ca+Na) values,whereas Cld+Bt+Mrg and Cld+Hbl+Mrg assemblages are stable atlow Mn and high Ca/(Ca+Na). These data suggest that at thisgrade Cld+Bt is more stable than Grt+Chl in the KFMASH system,whereas in the Ca—KFMASH system, Hbl+Cld assemblages arestable. Composition space analysis using the singular value decompositionmethod indicates that compositions of minerals from individualsamples are consistent with local equilibrium, but that differentoutcrops may not have all equilibrated at the same P–T–a_H2Oconditions. Thermodynamic analysis suggests that a garnet-zoneprograde sequence of ferromagnesian associations for these bulkcompositions would be Hbl+Cld+Grt+ChlBt+Cld+Grt+ChlBt+Grt+Chl. Staurolite-grade rocks from the same stratigraphic units areexposed across strike, and contain the assemblage Grt+StBtPl(all rocks contain Ms, Qtz, Chl, and Ilm). Margarite is commonlypresent as inclusions in the cores of garnets, but is absentas inclusions near garnet rims and from the matrix; conversely,staurolite inclusions are present towards the rims of the garnets,but are absent from the cores. These inclusion relations suggestthat margarite may react to form staurolite and garnet withincreasing grade via a reaction such as chlorite+margarite=staurolite+garnet+H₂O. Biotite is common in the matrix but is not typically abundant,and appears to have been the last phase to join the assemblage.Biotite is inferred to have joined the Grt+St+Chl assemblagesafter margarite breakdown through the reaction Grt+Chl+Ms=St+Bt+H₂O. Thus, uncommon margarite assemblages may evolve into commonGrt+Bt+St+Chl assemblages. ^* Present address: Department of Geology and Geophysics, University of Wisconsin-Madison, Madison, Wisconsin 53706.     

Experimental Interaction of Granitic and Basaltic Magmas and Implications for Mafic Enclaves

We have performed time series experiments for periods rangingfrom 3 min to 44 h on the interaction of granite melt and partiallymolten basalt at 920C and 10 kbar, in the presence of 5 wt.%water. With time, the assemblage of the basalt domain changesfrom predominantly amphibole+plagioclase to clinopyroxene+garnet;the melt fraction increases from {small tilde}2•5 to 40%;and between the two domains, the melt compositions progressivelyequilibrate. Initially in each run, melts of the basalt domainhave uniform plateau concentrations for SiO₂, Al₂O₃, CaO, MgO,and FeO because the activities of these components are regulatedby the mineral assemblage, but at advanced stages of reaction,no such control is evident. We have derived analytical expressionsto describe and simulate the diffusion profiles. The concentrationprofiles for SiO₂, Al₂O₃, CaO, and Na₂O in the granite, emanatingfrom the basalt–granite interface, have been used to estimateeffective diffusivities. The values from the shorter runs arecompared with those of the experiment of longest duration forwhich we assumed finite couples in our calculations. In thediffusion calculations for K₂O the difference in melt fractionbetween the two domains is accounted for. The resulting values(in cm²/s) are: D_Na2O=6 10^–7, D_K2O=3 10^–7, D_MgO=9 10^–8, D_CaO=(4–6) 10^–8, and D_SiO2 and D_Al2O3=(3–0•6) 10^–8. They are in reasonable agreement with values fromother studies. On the basis of our experiments we calculatethat mafic enclaves of magmatic origin should equilibrate toa large degree with their host magma in slowly cooling non-convectinggranitic plutons. Enclaves approaching complete re-equilibrationretain distinctly higher modal amounts of mafic minerals. Theydo not compositionally resemble binary magma mixtures, but aremore like host magma with accumulated crystals. We show thatthe modal differences between enclave and host are indicativeof the temperature of homogenization and that, in principle,this temperature can be deduced from equilibrium phase diagrams. ^* Present address: Mineralogisch-Petrologisches Institut, Universitt Gttingen, Goldschmidtstrasse 1, 3400 Gttingen, Germany     

Petrology and Geochemistry of Cretaceous Ultramafic Volcanics from Eastern Kamchatka

The origin, evolution and primary melt compositions of lateCretaceous high-K ultramafic volcanics and associated basaltsof Eastern Kamchatka are discussed on the basis of a study ofthe mineralogy and geochemistry of the rocks and magmatic inclusionsin phenocrysts. The exceptionally primitive composition of thephenocryst assemblage [olivine—Fo;_88–95, Cr-spinel—Cr/(Cr + Al) up to 85] provides direct evidence of the mantleorigin of primary melts, which were highly magnesian compositions(MgO 19–24 wt%). The rocks and meltsare characterizedby strong high field strength element (HFSE) depletion in comparisonwith rare earth elements, and high and variable levels of enrichmentin large ion lithophile elements (LILE), P, K and H₂O (0.6–12wt % in picritic to basaltic melts). _Nd values lie in a narrowrange (+107 to +91), typical of N-MORB (mid-ocean ridge basalt),but ⁸⁷Sr/⁸⁶Sr (0.70316–0.70358) is slightly displacedfrom the mantle array. High-K ultramafic melts from Kamchatkaare considered as a new magma type within the island-arc magmaticspectrum; basaltic members of the suite resemble arc shoshonites.The primary melts were produced under high-pressure (30–50kbar) and high-temperature(1500–1700C) conditions bypartial melting of a refractory peridotitic mantle. KEY WORDS: Kamchatka; Late Cretaceous magmatism; ultramafic volcanics; shoshonites ^*Corresponding author. Present address: Department of Geology, University of Tasmania, GPO Box 252C, Hobart, Tas., Australia     

The Solubility of Alumina in Orthopyroxene Coexisting with Garnet in FeO-MgO--Al2O3--SiO2 and CaO--FeO--MgO--Al2O3--SiO2

The pressure-temperature-compositional (P-T-X) dependence ofthe solubility of Al₂O₃ in orthopyroxene coexisting with garnethas been experimentally determined in the P-T range 5–30kilobars and 800–1200 ?C in the system FeO—MgO—Al₂O₃—SiO₂(FMAS). These results have been extended into the CaO—FeO—MgO—Al₂O₃—SiO₂(CFMAS) system in a further set of experiments designed to determinethe effect of the calcium content of garnet on the Al₂O₃ contentsof coexisting orthopyroxene at near-constant Mg/(Mg + Fe). Startingmaterials were mainly glasses of differing Mg/(Mg + Fe) or Ca/(Ca+ Mg + Fe) values, seeded with garnet and orthopyroxene of knowncomposition, but mineral mixes were also used to demonstratereversible equilibrium. Experiments were performed in a piston-cylinderapparatus using a talc/pyrex medium. Measured orthopyroxene and corrected garnet compositions werefitted by multiple and stepwise regression techniques to anequilibrium relation in the FMAS system, yielding best-fit,model-dependent parameters G^o_y= –5436 + 2.45T cal mol^–1,and W^M1_FeA1= –920 cal mol^–1. The volume change ofreaction, V^o, the entropy change, S^o₉₇₀ and the enthalpy changeH^o_1,970, were calculated from the MAS system data of Perkinset al. (1981) and available heat capacity data for the phases.Data from CFMAS experiments were fitted to an expanded equilibriumrelation to give an estimate of the term W^ga_CaMg = 1900 ? 400cal/mole cation, using the other parametric values already obtainedin FMAS. The experimental data allow the development of a arnet-orthopyroxenegeobarometer applicable in FMAS and CFMAS: where This geobarometer is applicable to both pelitic and metabasicgranulites containing garnet orthopyroxene, and to garnet peridoditeand garnet pyroxenite assemblages found as xenoliths in diatremesor in peridotite massifs. It is limited, however, by the necessityof an independent temperature estimate, by errors associatedwith analysis of low Al₂O₃ contents in orthopyroxenes in high-pressureor low-temperature parageneses, and by uncertainties in thecomposition of garnet in equilibrium with orthopyroxene. Ananalysis of errors associated with this formulation of the geobarometersuggests that it is subject to great uncertainty at low pressuresand for Fe-rich compositions. The results of application ofthis geobarometer to natural assemblages are presented in acompanion paper.     

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.     

Mineralogic Study of Eclogitic Rocks from Alpe Arami, Lepontine Alps, Southern Switzerland

Certain ultramafic-mafic lenses exposed in Ticino along thecontact zone between the underlying Simano and overlying Adulanappes display relatively high-pressure phase assemblages. AtAlpe Arami, metabasaltic layers associated with pyropic garnet-bearingIherzolite consist mainly of an early eclogitic assemblage characterizedby Alm₃₉Py₃₇Gross₂₃Spess₀₁+ Di₄₉Hd₀₈Jd₄₃+rutile±kyanite.Iron-magnesium fractionation between garnet+omphacite pairsyields a K_D, (Fe²⁺/Mg)_garnet/(Fe²⁺/Mg)_{cllnopyroxence}, of about6. This earlier assemblage has been replaced by a later, somewhatpargasitic hornblende+oligoclase+clinozoisite phase compatibility.Associated primary garnet peridotites contain Ca-rich clinopyroxeneand Al₂O₃-poor orthopyroxene. Both rock types have been affectedby a still later period of incipient chloritization. Available phase equilibrium and element partitioning data arecompatible with an inferred P-T condition of origin for theAlpe Arami mafic-ultramafic complex of 965–1000 °C,30–50 kilobars, indicating deep upper mantle generation.Amphibolites could have been produced during depressurizationaccompanying ascent of the mass through the upper mantle, butinasmuch as plagioclase accompanies the hornblende, the assemblageprobably crystallized after emplacement of the complex in theLepontine terrane prior to the termination of the Late Alpineregional metamorphism. Incipient production of high-rank greenschistphases certainly reflects a crustal event.     

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

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

Metamorphic Evolution of the Ribeira Belt: Evidence from Outcrops in the Rio de Janeiro Area, Brazil

Migmatitic granulites and arc-related felsic intrusives of Pan-Africanage form the bedrock in the Rio de Janeiro area, SE Brazil.These rocks preserve a partial record of three parageneses.The earliest assemblage (M₁) grew during fabric formation inthe rocks (D₁) and is characterized by the mineral assemblagePl + Bt + Sil + Kfs + Qtz. Peak metamorphic conditions (M₂)are characterized by the assemblage Bt + Crd + Kfs + Pl + Grt+ liq + Qtz and are inferred to have developed during D₂ foldingof the rocks at T = 750–800°C and P = 7 kbar. M₃ reactiontextures overprint the M₂ assemblage and comprise symplectiticintergrowth of cordierite(II) and quartz that formed after garnet,whereas secondary biotite formed as a result of reactions betweengarnet and K-feldspar. By comparing the observed modal abundanceswith modal contours of garnet, cordierite and quartz on therelevant pseudosection a post M₂ P–T vector indicatingcontemporaneous cooling and decompression can be deduced. Theinferred equilibrium assemblage and reaction textures are interpretedto reflect a clockwise P–T path involving heating followedby post-peak decompression and associated cooling. We inferthat metamorphism occurred in response to advective heatingby the abundant syn-collisional (arc-related) I-type granitoidsin the region, consistent with the unusually high peak T/P ratio. KEY WORDS: advective heating; Ribeira belt; granulite; partial melting; P–T pseudosection     

The Mineralogy and Geochemistry of the Metamorphosed Basic and Ultrabasic Rocks of the Jijal Complex, Kohistan, NW Pakistan

The Jijal complex, covering more than 150 sq. km in the extremenorth of Pakistan, is a tectonic wedge of garnet granulitesintruded in the south by a 10 x 4 km slab of ultramafic rocks.The granulites are divisible into plagioclase-bearing (basicto intermediate) and plagioclase-free (ultrabasic to basic)types, the two types reflecting differences in bulk chemistry.Garnet + plagioclase + clinopyroxene + quartz + rutile ±hornblende ± epidote is the most common assemblage. Theplagioclase-free rocks are composed mainly of two or three ofthe minerals garnet, amphibole, clinopyroxene and epidote. Orthopyroxeneoccurs in websteritic rocks devoid of epidote. Much of the amphiboleand some epidote appear to be prograde products. Although variationdiagrams do not reveal a genetic link between the two typesof granulite, it is considered that they are comagmatic ratherthan the products of two or more unrelated magmas. The compositions of garnet (Py_28–46 Alm _27–43Gro_16–28),clinopyroxene (Mg_44–34Fe_5–17Ca_51–49, Al₂O₃3·0–9·9 per cent), orthopyroxene (with upto 5·5 per cent Al₂O₃), amphibole (with up to 16·3per cent Al₂O₃ and high Al^vi/Al^iv), and the abundance of garnetsuggest a high-pressure origin for the granulites. The rocksappear to have differentiated from a tholeiitic magma of oceanicaffinity or they may be genetically related to the pyroxenegranulites of Swat considered to have originally crystallizedfrom a calc-alkaline magma of island arc or continental marginaffinity. They probably crystallized in the ancient Tethyancrust/upper mantle (or less likely in a continental margin),later to be metamorphosed to granulites (670–790 °C,12–14 kb) during the collision of the Indian-Asian landmasses,and carried upwards during later Himalayan orogenic episodes. The ultramafic rocks are alpine-type in nature and devoid ofgarnet. They are dominated by diopsidites; dunites, peridotites,and harzburgites together form <50 per cent of the area ofoutcrop. The chemistry of the rocks, and their olivines (Fo_92–89)and clinopyroxenes (Mg_49.5–48Fe_2.8–5.2Ca_47.4–46.8)are similar to those of alpine complexes of the harzburgitesubtype. It is not clear whether they represent a faulted slabof suboceanic crust/upper mantle, mantle diapirs in deep orogenicroots, or dismembered ultramafic rocks differentiated from abasaltic magma. They seem to have a complex history; their presentmineralogy is suggestive of high grade metamorphism (800–850°C, 8–12 kb). They are magmatically unrelated to thegarnet granulites and were probably intruded into the latteras plastic crystalline material after both had been independentlymetamorphosed, but before the entire complex was carried tectonicallyinto its present surroundings. The abundances of the diopsiditesis in marked contrast to other alpine-type complexes and thepossibility of Ca and Si metasomatism during or before theirmetamorphism should not be totally ruled out.     

Peralkalinity, Al{rightleftharpoons}Si Substitution, and Solubility Mechanisms of H2O in Aluminosilicate Melts

The solubility mechanisms of H₂O in peralkaline sodium aluminosilicatequenched melts (anhydrous NBO/T = 0.5) have been studied withRaman spectroscopy as a function of Al/(Al + Si) (0–0–3)and H²O content (0–7.5 wt.%). The coexisting structuralunits in the anhydrous quenched melts are TO2 (Q4), T2O5(Q3),and TO3 (Q2). In Al-free Na₂Si₄O₉ (NS4) melt, H₂O forms complexes with Na⁺(Na–OH bonds) and with Si⁴⁺ (Si–OH bonds). MolecularH₂O is also detected. TO3 structural units are not detectedin this composition. In the H₂O concentration range between0 and 4 wt.%, there is an approximately 20% increase in NBO/Tresulting from the increased abundance ratio, T₂O₅/TO2. Withfurther increments in water activity, the NBO/T of hydrous NS4melt is reduced. The depolymerization results from hydroxylationof the silica tetrahedra, whereas polymerization is due to formationof complexes with Na–OH bonding. In Al-bearing compositions on the Na₂Si₄O₉–Na₂(NaAl)₄O₉–join, there is evidence for Al–OH bonding in additionto Na–OH and Si–OH bonds. Among these complexes,the relative abundance of those with Si–OH bonds diminisheswith increasing Al/(A1 + Si), whereas complexes with Al–OHand Na–OH bonds become more important. Complexes withNa–OH bonds dominate for H₂O4 wt.%, whereas complexeswith Al–OH dominate at higher water content. The threestructural units, TO₃, T₂O₅, and TO², were observed in bothanhydrous and hydrous peralkaline sodium aluminosilicate melts.Their abundance varies, however, with the H₂O concentrationin the melts. The NBO/T decreases to a minimum (a 30–50%lowering of NBO/T relative to anhydrous materials) for low H₂Ocontents (3–4 wt.% H₂O), and increases as the H₂O contentis increased further.     

Evidence from Xenoliths for a Dynamic Lower Crust, Eastern Mojave Desert, California

Garnet-rich xenoliths in a Tertiary dike in the eastern MojaveDesert, California, preserve information about the nature andhistory of the lower crust. These xenoliths record pressuresof 10–12 kbar and temperatures of 750–800C. Approximately25% have mafic compositions and bear hornblende + plagioclase+ clinopyroxene + quartz in addition to garnet. The remainder,all of which contain quartz, include quartzose, quartzofeldspathic,and aluminous (kyanitesillimanite-bearing) varieties. Mostxenoliths have identifiable protoliths—mafic from intermediateor mafic igneous rocks, quartzose from quartz-rich sedimentaryrocks, aluminous from Al-rich graywackes or pelites, and quartzofeldspathicfrom feldspathic sediments and/or intermediate to felsic igneousrocks. However, many have unusual chemical compositions characterizedby high FeO(t), FeO(t)/MgO, Al₂O₃, and Al₂O₃/CaO, which correspondto high garnet abundance. The mineralogy and major-and trace-elementcompositions are consistent with the interpretation that thexenoliths are the garnet-rich residues of high-pressure crustalmelting, from which granitic melt was extracted. High ⁸⁷Sr/⁸⁶Srand low ¹⁴³Nd/¹⁴⁴Nd, together with highly discordant zirconsfrom a single sample with Pb/Pb ages of 1.7 Ga, demonstratethat the crustal material represented by the xenoliths is atleast as old as Early Proterozoic. This supracrustal-bearinglithologic assemblage may have been emplaced in the lower crustduring either Proterozoic or Mesozoic orogenesis, but Sr andNd model ages> 4 Ga require late Phanerozoic modificationof parent/daughter ratios, presumably during the anatectic event.Pressures of equilibration indicate that peak metamorphism andmelting occurred before the Mojave crust had thinned to itscurrent thickness of <30 km. The compositions of the xenolithssuggest that the lower crust here is grossly similar to estimatedworld-wide lower-crustal compositions in terms of silica andmafic content; however, it is considerably more peraluminous,has a lower mg-number, and is distinctive in some trace elementconcentrations, reflecting its strong metasedimentary and restiticheritage. ^* Author to whom correspondence should be addressed. Present address: Rensselaer Polytechnic Institute, Department of Earth and Environmental Sciences, Troy, New York 12180, USA. Fax: 518–276–8627; email: hanchj{at}rpi.edu.     

High-Pressure Granulites (Retrograded Eclogites) from the Hengshan Complex, North China Craton: Petrology and Tectonic Implications

Both high- and medium-pressure granulites have been found asenclaves and boudins in tonalitic–trondhjemitic–granodioriticgneisses in the Hengshan Complex. Petrological evidence fromthese rocks indicates four distinct metamorphic assemblages.The early prograde assemblage (M₁) is preserved only in thehigh-pressure granulites and represented by quartz and rutileinclusions within the cores of garnet porphyroblasts, and omphacitepseudomorphs that are indicated by clinopyroxene + sodic plagioclasesymplectic intergrowths. The peak assemblage (M₂) consists ofclinopyroxene + garnet + sodic plagioclase + quartz ±hornblende in the high-pressure granulites and orthopyroxene+ clinopyroxene + garnet + plagioclase + quartz in the medium-pressuregranulites. Peak metamorphism was followed by near-isothermaldecompression (M₃), which resulted in the development of orthopyroxene+ clinopyroxene + plagioclase symplectites and coronas surroundingembayed garnet grains, and decompression-cooling (M₄), representedby hornblende + plagioclase symplectites on garnet. The THERMOCALCprogram yielded peak (M₂) P–T conditions of 13·4–15·5kbar and 770–840°C for the high-pressure granulitesand 9–11 kbar and 820–870°C for the medium-pressuregranulites, based on the core compositions of garnet, matrixpyroxene and plagioclase. The P–T conditions of pyroxene+ plagioclase symplectite and corona (M₃) were estimated at