Reaction Textures in a Suite of Spinel Granulites from the Eastern Ghats Belt, India: Evidence for Polymetamorphism, a Partial Petrogenetic Grid in the System KFMASH and the Roles of ZnO and Fe2O3

Spinel granulites, with or without sapphirine, occur as lensesin garnetiferous quartzofeldspathic gneisses (leptynites) nearGokavaram in the Eastern Ghats Belt, India. Spinel granulitesare mineralogically heterogeneous and six mineral associationsoccur in closely spaced domains. These are (I) spinel–quartz–cordierite,(II) spinel–quartz–cordierite–garnet–orthopyroxene–sillimanite,(III) spinel–cordierite–orthopyroxene–sillimanite,(IV) spinel–quartz–sapphirine–sillimanite–garnet,(V) spinel–quartz-sapphirine–garnet and (IV) rhombohedral(Fe–Ti) oxide–cordierite–orthopyroxene–sillimanite.Common to all the associations are a porphyroblastic garnet(containing an internal schistosify defined by biotite, sillimaniteand quartz), perthite and plagioclase. Spinel contains variableamounts of exsolved magnetite and is distinctly Zn rich in thesapphirine-absent associations. X_Mg in the coexisting phasesdecreases in the order cordierite–biotite–sapphirine–orthopyroxene–spinel–garnet–(Fe–Ti)oxides. Textural criteria and compositional characteristicsof the phases document several retrograde mineral reactionswhich occurred subsequent to prograde dehydration melting reactionsinvolving biotite, sillimanite, quartz, plagioclase and spinel.The following retrograde mineral reactions are deduced: (1)spinel + quartz cordierite, (2) spinel + quartz garnet + sillimanite,(3) garnet + quartz cordierite + orthopyroxene, (4) garnet+ quartz + sillimanite cordierite, (5) spinel + cordierite orthopyroxene + sillimanite, (6) spinel + sillimanite + quartz sapphirine, (7) spinel + sapphirine + quartz garnet + sillimanite,and (8) spinel + quartz sapphirine + garnet. A partial petrogeneticgrid for the system FeO–MgO–Al₂O₃–SiO₂–K₂O–H₂Oat high fo₂, has been constructed and the effects of ZnO andFe₂O₃ on this grid have been explored Combining available experimentaland natural occurrence data, the high fo₂ invariant points inthe partial grid have been located in P–T space. Geothermobarometricdata and consideration of the deduced mineral reactions in thepetrogenetic grid show that the spinel granulites evolved throughan anticlockwise P–T trajectory reaching peak metamorphicconditions >9 kbar and 950C, followed by near-isobaric cooling(dT/dP = 150C/kbar). This was superimposed by an event of near-isothermaldecompression (dT/dP = 15C/kbar). The studied spinel granulites,therefore, preserve relic prograde mineral associations andreaction textures despite being metamorphosed at very high temperatures,and bear evidence of polymetamorphism. KEY WORDS: spinel granulite; Eastern Ghats; India; polymetamorphism; geothermometry; geobarometry Corresponding author     

Phase Equilibria of Amphibolites from the Post Pond Volcanics, Mt. Cube Quadrangle, Vermont

Amphibolites of the Post Pond Volcanics, south-west corner ofthe Mt. Cube Quadrangle, Vermont, are characterized by a greatdiversity of bulk rock types that give rise to a wide varietyof low-variance mineral assemblges. Original rock types arebelieved to have been intrusive and extrusive volcanics, hydrothermallyaltered volcanics and volcanogenic sediments with or withoutadmixtures of sedimentary detritus. Metamorphism was of staurolite-kyanitegrade. Geothermometry yields a temperature of 535 ± 20°C at pressures of 5–6 kb. Partitioning of Fe and Mg between coexisting phases is systematic,indicating a close approach to chemical equilibrium was attained.Relative enrichment of Fe/Mg is garnet > staurolite >gedrite > anthophyllite cummingtonite hornblende > biotite> chlorite > wonesite > cordierite dolomite > talc;relative enrichment in Mn/Mg is garnet > dolomite > gedrite> staurolite cummingtonite > hornblende > anthophyllite> cordierite > biotite > wonesite > chlorite >talc. between coexisting amphiboles varies as a function ofbulk Fe/Mg, which is inconsistent with an ideal molecular solutionmodel for amphiboles. Mineral assemblages are conveniently divided into carbonate+ hornblende-bearing, hornblende-bearing (carbonate-absent)and hornblende-absent. The carbonate-bearing assemblages allcontain hornblende + dolomite+ calcite + plagioclase (andesineand/or anorthite) + quartz with the additional phases garnetand epidote (in Fe-rich rocks) and chlorite ± cummingtonite(in magnesian rocks). Carbonate-bearing assemblages are restrictedto the most calcic bulk compositions. Hornblende-bearing (carbonate absent) assemblages occur in rocksof lower CaO content than the carbonate-bearing assemblages.All of these assemblages contain hornblende + andesine ±quartz + Fe-Ti oxide (rutile in magnesian rocks and ilmenitein Fe-rich rocks). In rocks of low Al content, cummingtoniteand two orthoamphiboles (gedrite and anthophyllite) are common.In addition, garnet is found in Fe-rich rocks and chlorite isfound in Mg-rich rocks. Several samples were found that containhornblende + cummingtonite + gedrite + anthophyllite ±garnet +chlorite + andesine + quartz + Fe-Ti oxide ±biotite. Aluminous assemblages contain hornblende + staurolite+ garnet ± anorthite/bytownite (coexisting with andesine)± gedrite ± biotite ± chlorite ±andesine ± quartz ± ilmenite. Hornblende-absentassemblages are restricted to Mg-rich, Ca-poor bulk compositions.These rocks contain chlorite ± cordierite ± staurolite± talc ± gedrite ± anthophyllite ±cummingtonite ± garnet ± biotite ± rutile± quartz ± andesine. The actual assemblage observeddepends strongly on Fe/Mg, Ca/Na and Al/Al + Fe + Mg. The chemistry of these rocks can be represented, to a firstapproximation, by the model system SiO₂–Al₂O₃–MgO–FeO–CaO–Na₂O–H₂O–CO₂;graphical representation is thus achieved by projection fromquartz, andesine, H₂O and CO₂ into the tetrahedron Fe–Ca–Mg–Al.The volumes defined by compositions of coexisting phases filla large portion of this tetrahedron. In general, the distributionof these phase volumes is quite regular, although in detailthere are a large number of phase volumes that overlap otherphase volumes, especially with respect to Fe/Mg ratios. Algebraicand graphical analysis of numerous different assemblages indicatethat every one of the phase volumes should shift to more magnesiancompositions with decreasing µ_H2O. It is therefore suggestedthat the overlapping phase volumes are the result of differentassemblages having crystallized in equilibrium with differentvalues of µ_H2O or µ_CO2 and that the different valuesmay have been inherited from the original H₂O and CO₂ contentof the volcanic prototype. If true, this implies that eithera fluid phase was not present during metamorphism, or that fluidflow between rocks was very restricted.     

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. percent, Al₂O3=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 hypersolidus region from 800?to 1250?C with variable activities of H₂O, CO₂, and H₂. Thevapor-saturated peridotite solidi are 50–200?C below thosepreviously published. The temperature of the beginning of meltingof peridotite decreases markedly with decreasing Mg/(Mg+Fe)of the starting material at constant CaO/Al₂O₃. Conversely,lowering CaO/Al₂O₃ reduces the temperature at constant Mg/(Mg+Fe)of the starting material. Temperature differences between thesolidi up to 200?C are observed. All solidi display a temperatureminimum reflecting the appearance of garnet. This minimum shiftsto lower pressure with decreasing Mg/(Mg+Fe) of the startingmaterial. The temperature of the beginning of melting decreasesisobarically as approximately a linear function of the mol fractionof H₂O in the vapor (X_H2O). The data also show that some CO₂may dissolve in silicate melts formed by partial melting ofperidotite. 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 coexist 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 a_H2O conditions in the upper mantle and possibly alsoby variations in the composition of the peridotite itself. It is suggested that komatiite in Precambrian terrane couldform by 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 (). Such activities of H₂Oresult in melting at depths ranging between 125 and 175 km inthe mantle. This range is within the minimum depth generallyaccepted for the formation of kimberlite.     

Sapphirine+Forsterite and Sapphirine+Humite-Group Minerals in an Ultra-Magnesian Lens from Kuhi-lal, SW Pamirs, Tajikistan: Are these Assemblages Forbidden?

Sapphirine occurs with humite-group minerals and forsteritein Precambrian amphibole-facies rocks at Kuhi-lal, SW PamirMountains, Tajikistan, a locality also for talc+kyanite magnesiohornblendewhiteschist. Most of these sapphirine-bearing rocks are graphiticand sulfidic (pyrite and pyrrhotite) and contain enstatite,clinohumite or chondrodite, spinel, rutile, gedrite, and phlogopite.A phlogopite schist has the assemblage with X_Fe = Fe/(Fe+Mg)increasing as follows: chlorite (0-003)<phlogopite (0.004–0.005)sapphirine (0.004–0.006) enstatite (0-006)forsterite (0-006–0-007)<spinel (0-014). This assemblage includes the incompatiblepair sapphirine+forsterite, but there is no textural evidencefor reaction. In one rock with clinohumite, X_Fe increases asfollows: clinohumite (0-002) <sapphirine (0-003) <enstatite(0-004–0-006) <spinel (0-010). Ion microprobe and wet-chemicalanalyses give 0-57–0-73 wt.% F in phlogopite and 0-27wt.% F in chlorite in the phlogopite schist; 0-04, 1.5–1.9,and 4.4 wt.% F in forsterite, clinohumite, and chondrodite,respectively; and 0-0-09 wt.% BeO and 0-05–0-21 wt.% B₂O₃in sapphirine. Stabilization of sapphirine+clinohumite or sapphirine+chondroditeinstead of sapphirine+phlogopite is possible at high F contentsin K-poor rocks, but minor element contents appear to be toolow to stabilize sapphirine as an additional phase with forsterite+enstatite+spinel.Although sapphirine+forsterite is metastable relative to spinel+enstatitein experiments conducted at a_H2O=1 in the MgO-Al₂O₃-SiO₂-H₂Osystem, it might be stabilized at a_H2O0.5, P4 kbar, T650–700C.Textures in the Kuhi-lal whiteschists suggest a polymetamorphicevolution in which the rocks were originally metamorphosed atT650C, P 7 kbar, conditions under which sapphirine+clinohumiteand sapphirine+chondrodite are inferred to have formed, andsubsequently affected by a later event at lower P, similar T,and lower a_H2O. The latter conditions were favorable for sapphirine+forsteriteto form in a rock originally containing chlorite+forsterite+spinel+enstatite.     

Fluid--Rock Interaction during Low-Pressure Polymetamorphism of the Reynolds Range Group, Central Australia

Marbles and metapelites from the Reynolds Range Group (centralAustralia) were regionally metamorphosed at low pressure duringM₂ at 1.6 Ga, M₂ ranged in grade from greenschist to granulitefacies along the length of the Reynolds Range, and overprinted1.78 Ga granites and their contact aureoles in the ReynoldsRange Group metasediments. At all M₂ grades the marbles andmetapelites have highly variable oxygen isotope ratios [marbles:¹⁸O(carb) 14–20%; metapelites: ¹⁸O 6–14%). Similarly, 1.78 Ga granites have highly variable oxygen isotope ratios(¹⁸O 5–13%), with the lowest values occurring at thegranite margins. In all rock types, the lowest oxygen isotopevalues are consistent with the infiltration of channelled magmaticand/or meteoric fluids. The variable lowering of oxygen isotopevalues resulted from pre-M₂ contact metamorphism and fluid—rockinteraction around the 1.78 Ga granites. In contrast, mineralassemblages in the marbles define a trend of increasing X_CO2with increasing grade from <0.05 (greenschist facies) to0.7–1.0 (granulite facies). This, together with the lackof regionally systematic resetting of oxygen isotope ratios,implies that there was little fluid—rock interaction duringprograde regional metamorphism. KEY WORDS: low pressure; polymetamorphism; fluids; stable isotopes; petrology ^*Corresponding author Fax: 61–3–94791272. e-mail: geoisb{at}lure.latrobe.edu.au     

Talc-Phengite: a Widespread Assemblage in High-Grade Pelitic Blueschists of the Western Alps

Talc-phengite, an assemblage hitherto believed to be rare, isfound in regional distribution in the Gran Paradiso area, whereit occurs in the characteristic mineral association chloritoid-talc-phengite(Si_3·4–_3·5). Talc contains up to 15 moleper cent minnesotaite, and chloritoid up to 45 mole per centof the magnesium end member. The talc-phengite stability resultsbasically from the disappearance of chlorite + quartz in rockswith low and moderate MgO/FeO ratios through the divariant reactionsfirst recognized here: Fe-Mg-Chlorite+quartz talc + garnet + H₂O and Fe-Mg-chlorite + quartz talc + Chloritoid + H₂O These reactions imply the disappearance of the join biotite-chloritein the presence of quartz and thus open a talc-phengite stabilityfield (±garnet or chloritoid or Mg-chlorite) which extends,with increasing P and T, toward Mg-richer compositions. Whetheror not it reaches the magnesian subsystem in the Gran Paradisoarea cannot be ascertained. However, the sporadic occurrenceof the high-pressure assemblage talc-kyanite-chloritoid 50 to70 km further northeast in the vicinity of the Monte Rosa massifwithin the same lithological unit (Zermatt-Saas Fee zone s.l.)indicates the instability of any chlorite in quartz-bearingrocks, and implies that talc-phengite must also be stable forpurely magnesian compositions in that area. This progressivestabilization of talc-phengite with increasing metamorphic gradesupports Abraham & Schreyer's (1976) hypothesis of a high-pressurefield for this assemblage, and rules out Chernosky's construction(1978) implying a low-pressure field. The following paragenetic sequence is proposed for pelitic compositionswith intermediate Mg/Fe ratios and excess quartz subjected tohigh-pressure metamorphism with maximum temperatures near 400–500°C: chlorite-illite chlorite-phengite chloritoid-talc-phengite.The absence of biotite is a compositional effect due to thehigh degree of phengite substitution in the white mica. ^*Present address: Institut fr Mineralogic, Ruhr-Universitt, Postfach 10 21 48, D-4630 Bochum 1, Federal Republic of Germany.     

Contact Metamorphosed Ultramafic Rocks in the Western Sierra Nevada Foothills, California

Within the western Sierra Nevada metamorphic belt, linear bodiesof alpine-type ultramafic rock, now composed largely of serpentineminerals, parallel the regional strike and commonly coincidewith major fault zones. Within this metamorphic belt, east ofSacramento, California, ultramafic rocks near a large maficintrusion, the Pine Hill Intrusive Complex, have been emplacedduring at least two separate episodes. Those ultramafic rocks,evidently unaffected by the Pine Hill Intrusive Complex andcomposed largely of serpentine minerals, were emplaced alonga major fault zone after emplacement of the Pine Hill IntrusiveComplex. Those ultramafic rocks, contact metamorphosed by thePine Hill Intrusive Complex, show a zonation of mineral assemblagesas the igneous contact is approached: olivine+antigorite+chlorite+tremolite+Fe-Cr spinel olivine+talc+chlorite+tremolite+Fe-Crspinel olivine+anthophyllite+chlorite+tremolite+Fe-Cr spinel olivine+orthopyroxene+aluminous spinel+hornblende+Fe-Cr spinel.Superimposed on these mineral assemblages are abundant secondaryminerals (serpentine minerals, talc, chlorite, magnetite) whichformed after contact metamorphism. Correlation of observed mineralassemblages with the experimental systems, MgO-SiO₂-H₂O andMgO-Al₂O₃-SiO₂-H₂O suggests an initial contact temperature of775±25 °C for the Pine Hill Intrusive Complex assumingPtotal P_fluid PH₂O. The pressure acting on the metamorphic rockduring emplacement of the intrusion is estimated to be a minimumof 1.5 kb.     

A Method for Thermodynamical Calculation of Basicity Indicators of Rocks and Minerals

A new method has been suggested for evaluating the overall basicityof minerals and rocks by using ionization reactions involvingone proton: (sum of cations) + H₂O = mineral + H⁺, (sum of cations) + H₂O = (sum of normative minerals of a rock)+ H⁺. The basicity indicators are expressed as standard free energychanges of these reactions (). At standard water pressure (logP_H2O = 0) and chemical activity of the metal ions ( log _Mⁿ⁺= 0), the relationship between and alkalinity of solutions(pH) becomes: = –2.303 RTlog _H⁺ = 2.303 RT pH. The overall basicities of rock-forming oxides, minerals andmajor rocks were calculated from the thermodynamic data on ionsin water solutions and solid compounds.     

Genesis of Ultramafic Lamprophyres and Carbonatites at Aillik Bay, Labrador: a Consequence of Incipient Lithospheric Thinning beneath the North Atlantic Craton

Numerous dykes of ultramafic lamprophyre (aillikite, mela-aillikite,damtjernite) and subordinate dolomite-bearing carbonatite withU–Pb perovskite emplacement ages of 590–555 Ma occurin the vicinity of Aillik Bay, coastal Labrador. The ultramaficlamprophyres principally consist of olivine and phlogopite phenocrystsin a carbonate- or clinopyroxene-dominated groundmass. Ti-richprimary garnet (kimzeyite and Ti-andradite) typically occursat the aillikite type locality and is considered diagnosticfor ultramafic lamprophyre–carbonatite suites. Titanianaluminous phlogopite and clinopyroxene, as well as comparativelyAl-enriched but Cr–Mg-poor spinel (Cr-number < 0.85),are compositionally distinct from analogous minerals in kimberlites,orangeites and olivine lamproites, indicating different magmageneses. The Aillik Bay ultramafic lamprophyres and carbonatiteshave variable but overlapping ⁸⁷Sr/⁸⁶Sr_i ratios (0·70369–0·70662)and show a narrow range in initial _Nd (+0·1 to +1·9)implying that they are related to a common type of parentalmagma with variable isotopic characteristics. Aillikite is closestto this primary magma composition in terms of MgO (15–20wt %) and Ni (200–574 ppm) content; the abundant groundmasscarbonate has ¹³C_PDB between –5·7 and –5,similar to primary mantle-derived carbonates, and ¹⁸O_SMOW from9·4 to 11·6. Extensive melting of a garnet peridotitesource region containing carbonate- and phlogopite-rich veinsat 4–7 GPa triggered by enhanced lithospheric extensioncan account for the volatile-bearing, potassic, incompatibleelement enriched and MgO-rich nature of the proto-aillikitemagma. It is argued that low-degree potassic silicate to carbonatiticmelts from upwelling asthenosphere infiltrated the cold baseof the stretched lithosphere and solidified as veins, therebycrystallizing calcite and phlogopite that were not in equilibriumwith peridotite. Continued Late Neoproterozoic lithosphericthinning, with progressive upwelling of the asthenosphere beneatha developing rift branch in this part of the North Atlanticcraton, caused further veining and successive remelting of veinsplus volatile-fluxed melting of the host fertile garnet peridotite,giving rise to long-lasting hybrid ultramafic lamprophyre magmaproduction in conjunction with the break-up of the Rodinia supercontinent.Proto-aillikite magma reached the surface only after coatingthe uppermost mantle conduits with glimmeritic material, whichcaused minor alkali loss. At intrusion level, carbonate separationfrom this aillikite magma resulted in fractionated dolomite-bearingcarbonatites (¹³C_PDB –3·7 to –2·7)and carbonate-poor mela-aillikite residues. Damtjernites maybe explained by liquid exsolution from alkali-rich proto-aillikitemagma batches that moved through previously reaction-lined conduitsat uppermost mantle depths. KEY WORDS: liquid immiscibility; mantle-derived magmas; metasomatism, Sr–Nd isotopes; U–Pb geochronology     

A New Interpretation of Centimetre-scale Variations in the Progress of Infiltration-driven Metamorphic Reactions: Case Study of Carbonated Metaperidotite, Val d'Efra, Central Alps, Switzerland

Progress () of the infiltration-driven reaction, 4olivine +5CO₂ + H₂O = talc + 5magnesite, that occurred during Barrovianregional metamorphism, varies at the cm-scale by a factor of3·5 within an 3 m³ volume of rock. Mineral and stableisotope compositions record that X_CO2, ¹⁸O_fluid, and ¹³C_fluidwere uniform within error of measurement in the same rock volume.The conventional interpretation of small-scale variations in in terms of channelized fluid flow cannot explain the uniformityin fluid composition. Small-scale variations in resulted insteadbecause (a) reactant olivine was a solid solution, (b) initiallythere were small-scale variations in the amount and compositionof olivine, and (c) fluid composition was completely homogenizedover the same scale by diffusion–dispersion during infiltrationand subsequent reaction. Assuming isochemical reaction, spatialvariations in image variations in the (Mg + Fe)/Si of the parentrock rather than the geometry of metamorphic fluid flow. Ifinfiltration-driven reactions involve minerals fixed in composition,on the other hand, spatial variations in do directly imagefluid flow paths. The geometry of fluid flow can never be determinedfrom geochemical tracers over a distance smaller than the oneover which fluid composition is completely homogenized by diffusion–dispersion. KEY WORDS: Alpine Barrovian metamorphism; diffusion; metamorphic fluid composition; metamorphic fluid flow; reaction progress     

Element Mobility During Incipient Granulite Formation at Kabbaldurga, Southern India

At Kabbaldurga, infiltration of carbonic fluids along a systemof ductile shears and foliation planes has led to partial transformationof Archaean grey biotite–hornblende gneiss to coarse-grainedmassive charnockite at about 2.5 b.y. ago. The dehydration ofthe gneiss assemblage was induced by a marked metasomatic changeof the reacting system from granodioritic to granitic, and obviouslytook place under conditions of an open system at 700–750?C and 5–7 kb. Extensive replacement of plagioclase (An_16–30)by K-feldspar through Na, Ca–K exchange reactions withthe ascending carbonic fluids led to strong enrichment in K,Rb, Ba, and SiO₂, and to a depletion in Ca. Progressive dissolutionof hornblende, biotite, magnetite, and the accessory mineralsapatite and zircon resulted in a marked depletion in Fe, Mg,Ti, Zn, V, P, and Zr. Most important is the recognition of REEmobility: with advancing charnockitization, the moderately fractionatedREE distribution patterns of the grey gneisses (La_N270; La_N/Yb_N= 5–20; Eu_N27; Eu/Eu^* = 0.6–0.3) give way to stronglyfractionated REE patterns with a positive Eu-anomaly (La_N200;La_N/Yb_N = 20–80; Eu_N22; Eu/Eu^* = 0.6–1.8). The systematicdepletion especially in the HREE is due to the progressive dissolutionof zircon, apatite (and monazite), which strongly concentratethe REE. Stable isotope data (¹⁸O of 6.9–8.0 per mille for gneissesand charnockites; ¹³C of –8.5 and –6.5 per millefor late carbonate) indicate a magmatogenic source for the carbonicfluids. In contrast to the currently favoured derivation ofcarbonic fluids by decarbonation of the upper mantle or degassingof underplated basaltic intrusions, it is discussed here thatabundant fluid inclusions in lower crustal charnockites providedan extensive reservoir of ‘fossil’ carbonic fluids.Shear deformation has tapped this reservoir and generated thechannel-ways for fluid ascent. Charnockitization of the Kabbaldurgatypethus appears to be a metasomatic process which is tectonicallycontrolled and restricted to the crustal level of the amphiboliteto granulite transition.     

An Experimental Determination of Iron Biotite-Alkali Feldspar Equilibria

Experiments in the system KAlSi0₄-NaAlSi0₄-SiOj-Fe-0-H indicatethat approximately 20 mole per cent of sodium annite [NaFe₃AlSiO₁₀(OH)₂]can enter into solid solution with potassium annite. This substitutionreduces the biotite stability field. With excess alkali feldsparand no free quartz, the following reactions limit the biotitestability: Feldspar(s)+biotite feldspar(s)+magnetite+vapor. (1) Feldspar+biotite feldspar+fayalite+leucite+vapor. (2) Feldspar+biotite feldspar+fayalite+liquid+vapor. (3) Biotite feldspar+fayalite+nepheline+vapor. (4) On the QFM buffer (P_Total = 2 Kb), reaction 1 occurs at 685°C in the sodium free system and at 625 °C when twofeldspars are present; on NNO, it occurs at 630 and 590 °C.On the G-CH buffer, reaction 2 is stable at high temperatures(795–830 °Q and potassic compositions (), reaction3 at intermediate conditions (T = 740–95 °C; ) andreaction 4 at lower temperatures (710–40 °C) and sodiccompositions (). In the presence of excess quartz, reaction1 remains stable below the QFM buffer, but the reaction Feldspars)+biotite+quartz feldspars)+fayalite+vapor (5) is stable at higher hydrogenfugacities. On the G-CH buffer, reaction 5 occurs at 610 °Cin the sodium free system and at 595 °C when two alkalifeldspars are present. On the MW buffer, the temperatures are585 and 515 °C (M.I.). The experimental data presented suggest that biotite will notbe stable in the presence of granitic liquids at total pressuresbelow 4 kb, providing the fluorine and titanium content of thebiotite is low. They also suggest that gradients in the a£1S,l0 in a rock could produce variations in the biotite Fe/Fe+Mgratio and in extreme cases could result in the complete breakdown(decreased ) of biotite or its crystallization (increased )where previously there was none.     

Intersecting Isograds in the Whetstone Lake Area, Ontario

Pelitic and calcareous rocks in the Whetstone Lake area havean unusually wide range of chemical composition. Metamorphicreactions have been deduced that represent the observed ‘discontinuities’in compatible mineral assemblages, and by plotting the reactantand the product assemblage of each reaction on a map, metamorphicisograds have been delincated ‘from both sides’.For the pelitic rocks, successively higher-grade isograds arebased on the following reactions: (1)chlorite+muscovite+garnetstaurolite+biotite+quartz+water; (2) chlorite+muscovite+staurolite+quartz kyanite+biotite+water; (3) kyanitesillimanite; (4)staurolite+museovite+quartzsillimanite+garnet+biotite+water. A fifth isograd, based on the reaction (5) biotite+calcite+quartzCa-amphibole+K-feldspar+carbon dioxide+water intersects the isograds based on reactions (2), (3), and (4)in such a manner as to indicate that the H₂O/CO₂ fugacity ratiowas significantly higher in the vicinity of a granite plutonthan in the metasedimentary rocks remote from the pluton. Chemicalanalyses of the coexisting minerals in reaction (5) indicatethat the real reaction may involve plagioclase, epidote, sphene,and Fe-Ti oxides as well.     

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

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

Infiltration-Driven Metamorphism in Northern New England, USA

Mineral reactions at the biotite isograd were investigated inpelitic schists, micaceous sandstones, micaceous limestones,and metaigneous rocks from three stratigraphic units over anarea of 10000 km² in north-central New England. The biotiteisograd in north-central New England represents a metamorphicdecarbonation front that affected all major rock types in eachstratigraphic unit. Pressure at the isograd was near 3500 bat the northern end of the study area and near 5500 b in thesouth. Temperature was in the range 400–450?C. Equilibriummetamorphic fluids were approximately CO₂-H₂O mixtures withX_CO2=0?04–0?07. Volumetric fluid]-rock ratios were calculatedfor more than 70 samples of all major rock types from each formationusing measured progress of the prograde reactions and the estimatedP-T-X_CO2, conditions of metamorphism. Regardless of stratigraphicunit, limestones record low values of 0–0?2, pelites andmetaigneous rocks generally record high values of 1–3,and standstones record intermediate values of 0?2–1. With exception of the limestones, all samples from the biotitezone record fluid-rock ratios significantly greater than likelyrock porosity during metamorphism. The prograde decarbonationreactions therefore were driven by infiltration of rock by reactiveaqueous fluids. The observed correlations between fluid-rockratio and rock type indicate that significant permeability contrastsoccurred during low-grade metamorphsim with permeability increasingin the order: limestones<sandstones<pelites rocks. Asa corollary, reactive fluid flow must have been channelizedwith enhanced flow in pelites and metaigneous rocks relativeto sandstones and limestones. Results of this study in north-centralNew England taken together with studies of the biotite isogradin south-central Maine (Ferry, 1984, 1986a, 1988) demonstratethat low-grade metamorphism over much of the northern Appalachianorogen was infiltration-driven.     

Petrology and Petrogenesis of Cumulate Peridotites and Gabbros from the Marum Ophiolite Complex, Northern Papua New Guinea

The Marum ophiolite complex in northern Papua New Guinea includesa thick (3–4 km) sequence of ultramafic and mafic cumulates,which are layered on a gross scale from dunite at the base upwardsthrough wehrlite, lherzolite, plagioclase lherzolite, pyroxenite,olivine norite-gabbro and norite-gabbro to anorthositic gabbroand ferrogabbro at the top. Igneous layering and structures,and cumulus textures indicate an origin by magmatic crystallizationin a large magma chamber(s) from magma(s) of evolving composition.Most rocks however show textural and mineralogical evidenceof subsolidus re-equilibration. The cumulate sequence is olivine and chrome spinel followedby clinopyroxene, orthopyroxene and plagioclase, and the layeredsequence is similar to that of the Troodos and Papuan ophiolites.These sequences differ from ophiolites such as Vourinos by thepresence of cumulus magnesian orthopyroxene, and are not consistentwith accumulation of low pressure liquidus phases of mid-oceanridge-type olivine tholeiite basalts. The cumulus phases show cryptic variation from Mg- and Ca-richearly cumulates to lower temperature end-members, e.g. olivineMg_93–78, plagioclase An_94–63. Co-existing pyroxenesdefine a high temperature solidus with a narrower miscibilitygap than that of pyroxenes from stratiform intrusions. Re-equilibratedpyroxene pairs define a low-temperature, subsolidus solvus.Various geothermometers and geobarometers, together with thermodynamiccalculations involving silica buffers, suggest the pyroxene-bearingcumulates crystallized at 1200 °C and 1–2 kb pressureunder low f_O2. The underlying dunites and chromitites crystallizedat higher temperature, 1300–1350 °C. The bulk of thecumulates have re-equilibrated under subsolidus conditions:co-existing pyroxenes record equilibration temperatures of 850–900°C whereas olivine-spinel and magnetite-ilmenite pairs indicatefinal equilibration at very low temperatures (600 °C). Magmas parental to the cumulate sequence are considered to havebeen of magnesian olivine-poor tholeiite composition (>50per cent SiO₂, 15 per cent MgO, 100 Mg/(Mg + Fe²⁺) 78) richin Ni and Cr, and poor in TiO₂ and alkalies. Fractionated examplesof this magma type occur at a number of other ophiolites withsimilar cumulate sequences. Experimental studies show that suchlavas may result from ial melting of depleted mantle lherzoliteat shallow depth. The tectonic environment in which the complexformed might have been either a mid-ocean ridge or a back-arebasin.     

Thermobarometry, Geochronology and the Interpretation of P-T-t Data in the Britt Domain, Ontario Grenville Orogen, Canada

Textural evidence, thermobarometry, and geochronology were usedto constrain the pressure-temperature-time (P—T—t)history of the southern portion of the Britt domain in the CentralGneiss Belt, Ontario Grenville Province. Typical metapeliticassemblages are quartz+plagioclase+ biotite + garnet + kyanite alkali feldspar sillimanite rutile ilmenite staurolite gahnite muscovite. Metatonalitic assemblages have quartz+ plagioclase + garnet biotite + hornblende + rutile + ilmenite.Metagabbroic rocks contain plagioclase + garnet + clinopyroxene+ biotite + ilmenite hornblende rutile quartz. Notabletextural features include overgrowths of sillimanite on kyaniteand of spinel on staurolite. The spinel overgrowths can be modeledby the breakdown of staurolite via the reaction Fe-staurolite= hercynite +kyanite + quartz + H₂O. The decomposition of stauroliteto her-cynite has a steep dP/dT slope and constrains the lateprograde path of a staurolite metapelite. Garnet—Al₂SiO₅—plagioclase—quartz(GASP) barometry applied to metapelitic garnets that preservecalcium zoning reveals a pressure decrease from 11 to 6 kbat an assumed temperature of 700 C. Garnet—plagioclase—ilmenite—rutile—quartzand garnet—clinopyroxene—plagioclase—quartzbarometry is in good agreement with pressures obtained withthe GASP barometer. Geochronologic data from garnet, allanite,and monazite in metapelitic rocks give ages that fall into twogroups, 1–4 Ga and 1.1 Ga, suggesting the presence ofat least two metamorphic events in the area. It is most reasonableto assign the 1.4 Ga age to the high-pressure data and the 1.1Ga age to the lower-pressure data. Collectively the P—T—tdata indicate a complex and protracted history rather than asingle cycle of burial and uplift for this part of the GrenvilleProvince.     

Hafnium Isotope and Trace Element Constraints on the Nature of Mantle Heterogeneity beneath the Central Southwest Indian Ridge (13{degrees}E to 47{degrees}E)

Hafnium isotope and incompatible trace element data are presentedfor a suite of mid-ocean ridge basalts (MORB) from 13 to 47°Eon the Southwest Indian Ridge (SWIR), one of the slowest spreadingand most isotopically heterogeneous mid-ocean ridges. Variationsin Nd–Hf isotope compositions and Lu/Hf ratios clearlydistinguish an Atlantic–Pacific-type MORB source, presentwest of 26°E, characterized by relatively low _Hf valuesfor a given _Nd relative to the regression line through all Nd–Hfisotope data for oceanic basalts (termed the ‘Nd–Hfmantle array line’; the deviation from this line is termed_Hf) and low Lu/Hf ratios, from an Indian Ocean-type MORB signature,present east of 32°E, characterized by relatively high _Hfvalues and Lu/Hf ratios. Additionally, two localized, isotopicallyanomalous areas, at 13–15°E and 39–41°E,are characterized by distinctly low negative and high positive_Hf values, respectively. The low _Hf MORB from 13 to 15°Eappear to reflect contamination by HIMU-type mantle from thenearby Bouvet mantle plume, whereas the trace element and isotopiccompositions of MORB from 39 to 41°E are most consistentwith contamination by metasomatized Archean continental lithosphericmantle. Relatively small source-melt fractionation of Lu/Hfrelative to Sm/Nd, compared with MORB from faster-spreadingridges, argues against a significant role for garnet pyroxenitein the generation of most central SWIR MORB. Correlations between_Hf and Sr and Pb isotopic and trace element ratios clearly delineatea high-_Hf ‘Indian Ocean mantle component’ that canexplain the isotope composition of most Indian Ocean MORB asmixtures between this component and a heterogeneous Atlantic–Pacific-typeMORB source. The Hf, Nd and Sr isotope compositions of IndianOcean MORB appear to be most consistent with the hypothesisthat this component represents fragments of subduction-modifiedlithospheric mantle beneath Proterozoic orogenic belts thatfoundered into the nascent Indian Ocean upper mantle duringthe Mesozoic breakup of Gondwana. KEY WORDS: mid-ocean ridge basalt; isotopes; incompatible elements; Indian Ocean     

Stability and Phase Relations of Trioctahedral Calcium Brittle Micas (Clintonite Group)

On the pseudobinary join CaO:3MgO:Al₂O₃:2SiO₂:_xH2O–CaO:1.25MgO:2.75 Al₂O₃: 0.25SiO₂:_xH₂O clintonite mixed crystals Ca(Mg_{1+ x}Al_{2 – x}) (Al_{4 – x}Si_xO₁₀)(OH)₂ with x rangingfrom 0.6 to 1.4 occur in the temperature range 600–830?C, 2 kb fluid pressure. On the MgSirich side clintonites coexistwith chlorite, forsterite, diopside, and calcite (due to smallamounts of CO₂ in the gas phase) and, at lower temperatures,also with idocrase, hydrogrossularite, and aluminous serpentine.Decomposition of clintonite over a divariant temperature rangeoccurs above 830 ?C, 2 kb; clintonite-free subsolidus assemblagescomprising three or four solid phases are formed in the temperatureranges 890 ?–1120 ?C. The subsolidus assemblages can berepresented in a polyhedron defined by the corners forsterite,diopside, melilite, spinel, anorthite, corundum, and calciumdialuminate. Above 1120 ?C partial melting occurs. The upper thermal stability limits of three selected compositionshave been reversed in the P-T range 0.5–20 kb and 730–1050 ?C, respectively. Below some 4 kb breakdown is dueto the divariant reactions: (1)Ca(Mg_2.25Al_0.75)(Al_2.75)(Si_1.25O₁₀)(OH)₂ spinel+diopside_ss+forsterite+clintonite_ss+vapor, (2)Ca(Mg₂Al)(Al₃SiO₁₀)(OH)₂ spinelx002B;melilite_ss+anorthite+clintonite_ss+vapor, (3)Ca(Mg_1.75Al_1.25)(Al_3.25)(Si_0.75O₁₀)(OH)₂ spinel+melilite_ss+corundum+clintonite_ss+vapor, At the terminations of the divariant temperature ranges (1)melilite_ss, (2) diopside_ss, and (3) anorthite enter those assemblagesand clintonitess disappears completely. The reactions can berepresented by the following equations (1)log,_H2O = 10.2879–8113/T+0.0856(P–1)/T, (2)log = 9.5852–7325/T+0.0794(P–1)/T, (3)log = 7.8358–5250/T+0.077(P–1)/T, with P expressed in bars and Tin ?K. Above 4 kb the upper thermalstability limit of clintonite is defined by incongruent melting,with grossularite participating at pressures above 9 kb. Thesecurves exhibit a very steep, probably even negative slope inthe P-T diagram. There is a close correspondence between natural clintonite-bearingassemblages and thosefound experimentally. The rarity of clintonitein nature is not due to special conditions of pressure and temperaturebut rather due to special bulk compositions of the rocks.     

Properties of Synthetic Triclinic KFeSi3O8, Iron-microcline, with some Observations on the Iron-microcline{rightleftharpoons}Iron-sanidine Transition

Triclinic KFeSi₃O₈, iron-microcline, has been synthesized fromoxide mixes and by complete conversion of monoclinic KFeSi₃O₈,iron-sanidine. Iron-microcline is triclinic, C, a=8?68?0?01?, b=13?10?0?01, c=7?34?0?01, =90? 45'?10', ß=116?03'?10', =86?14'?10'. The optical properties (Na light) are:=1?585?0?002, ß=1?596?0?002, =1?605?0?002, 2V=85?(calc.), Xb, Z c=20??5?. A reversible phase transition betweentriclinic and monoclinic KFeSi₃O₈ occurs at 704??6? C at 2000bars total pressure. Iron-microcline is the low-temperaturepolymorph; no intermediate polymorphs were observed in eitherhydrothermal or dry heating experiments.