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     

The Gronnedal-Ika Carbonatite-Syenite Complex, South Greenland: Carbonatite Formation by Liquid Immiscibility

The Grønnedal-Ika complex is dominated by layered nephelinesyenites which were intruded by a xenolithic syenite and a centralplug of calcite to calcite–siderite carbonatite. Aegirine–augite,alkali feldspar and nepheline are the major mineral phases inthe syenites, along with rare calcite. Temperatures of 680–910°Cand silica activities of 0·28–0·43 weredetermined for the crystallization of the syenites on the basisof mineral equilibria. Oxygen fugacities, estimated using titanomagnetitecompositions, were between 2 and 5 log units above the fayalite–magnetite–quartzbuffer during the magmatic stage. Chondrite-normalized REE patternsof magmatic calcite in both carbonatites and syenites are characterizedby REE enrichment (La_CN–Yb_CN = 10–70). Calcite fromthe carbonatites has higher Ba (5490 ppm) and lower HREE concentrationsthan calcite from the syenites (54–106 ppm Ba). This isconsistent with the behavior of these elements during separationof immiscible silicate–carbonate liquid pairs. _Nd(T =1·30 Ga) values of clinopyroxenes from the syenites varybetween +1·8 and +2·8, and _Nd(T) values of whole-rockcarbonatites range from +2·4 to +2·8. Calcitefrom the carbonatites has ¹⁸O values of 7·8 to 8·6and ¹³C values of –3·9 to –4·6. ¹⁸Ovalues of clinopyroxene separates from the nepheline syenitesrange between 4·2 and 4·9. The average oxygenisotopic composition of the nepheline syenitic melt was calculatedbased on known rock–water and mineral–water isotopefractionation to be 5·7 ± 0·4. Nd and C–Oisotope compositions are typical for mantle-derived rocks anddo not indicate significant crustal assimilation for eithersyenite or carbonatite magmas. The difference in ¹⁸O betweencalculated syenitic melts and carbonatites, and the overlapin _Nd values between carbonatites and syenites, are consistentwith derivation of the carbonatites from the syenites via liquidimmiscibility. KEY WORDS: alkaline magmatism; carbonatite; Gardar Province; liquid immiscibility; nepheline syenite     

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.     

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.     

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.     

The 1982-83 Eruption at Galunggung Volcano, Java (Indonesia): Oxygen Isotope Geochemistry of a Chemically Zoned Magma Chamber

Mount Galunggung is a historically active volcano in southwesternJava that has erupted four times in the last two centuries.During the most recent event, which occurred during a 9–monthinterval in 1982– 83, some 305 10⁶ m³ of medium–K,calc–alkaline magma was erupted. This eruption was unusualbecause of its duration, the diversity of eruption dynamicsand products, and the range of lava compositions produced. Thecomposition of juvenile material changed gradually during thecourse of the eruption from initial plagioclase (An_60–75)and two–pyrozene bearing andesites with 58% SiO₂ to finalplagioclase (An_85–90), diopside, and olivine (Fo_85–90)bearing primitive magnesium basalts with 47% SiO₂ Mineralogicaland compositional relationships indicate a magmatic evolutioninvolving differentitation of high–Mg parental melt. Theeruptive volumes of 35 10⁶ m³ andesite, 120 10⁶ m³ maficandesite, and 150 10⁶ m³ basalt are consistent with the ideathat the 1982– 83 eruption progressively tapped and draineda magma chamber that had become chemically stratified throughextensive crystal fractionation. Separates of plagioclase and pyroxene have ¹⁸O( SMO W) rangesof + 5. 6 to + 6.0 and + 5.3 to + 5.6, respectively, with ¹⁸O_plag–pxvalues of + 0.4 to + 0.6o, indicating internal O–isotopeequiliburium at temperature of 1100–850 C. The magenesianbasalts have magmatic ¹⁸O/ ¹⁶O ratios similar to those of mid–oceanridge basalt, and the O–isotope ratios of compositionallyevolved derivative melts show no evidence for contaminationof the galunggung magmas by ¹⁸O–rich crust during differentiation.Andesites and transitional mafic and sites have a more variableO–isotope character, with laves and phenocrysts havingboth higher and lower ¹⁸O values than observed in the parentalmagnesium basalts. These features are interpreted to reflectintramagma chamber processes affecting the upper portions ofthe differentiating Galunggung magma body before the 1982–83eruption.     

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     

Petrographic, Chemical and B-Isotopic Insights into the Origin of Tourmaline-Rich Rocks and Boron Recycling in the Martinamor Antiform (Central Iberian Zone, Salamanca, Spain)

Tourmaline in the Martinamor antiform occurs in tourmalinites(rocks with >15–20% tourmaline by volume), clasticmetasedimentary rocks of the Upper Proterozoic Monterrubio formation,quartz veins, pre-Variscan orthogneisses and Variscan graniticrocks. Petrographic observations, back-scattered electron (BSE)images, and microprobe data document a multistaged developmentof tourmaline. Overall, variations in the Mg/(Mg + Fe) ratiosdecrease from tourmalinites (0·36–0·75),through veins (0·38–0·66) to granitic rocks(0·23–0·46), whereas Al increases in thesame order from 5·84–6·65 to 6·22–6·88apfu. The incorporation of Al into tourmaline is consistentwith combinations of ^xAl(NaR)_–1 and AlO(R(OH))_–1exchange vectors, where ^x represents X-site vacancy and R is(Mg + Fe²⁺ + Mn). Variations in ^x/(^x + Na) ratios are similarin all the types of tourmaline occurrences, from 0·10to 0·53, with low Ca-contents (mostly <0·10apfu). Based on field and textural criteria, two groups of tourmaline-richrocks are distinguished: (1) pre-Variscan tourmalinites (probablyCadomian), affected by both deformation and regional metamorphismduring the Variscan orogeny; (2) tourmalinites related to thesynkinematic granitic complex of Martinamor. Textural and geochemicaldata are consistent with a psammopelitic parentage for the protolithof the tourmalinites. Boron isotope analyses of tourmaline havea total range of ¹¹B values from –15·6 to 6·8;the lowest corresponding to granitic tourmalines (–15·6to –11·7) and the highest to veins (1·9to 6·8). Tourmalines from tourmalinites have intermediate¹¹B values of –8·0 to +2·0. The observedvariations in ¹¹B support an important crustal recycling ofboron in the Martinamor area, in which pre-Variscan tourmaliniteswere remobilized by a combination of mechanical and chemicalprocesses during Variscan deformation, metamorphism and anatexis,leading to the formation of multiple tourmaline-bearing veinsand a new stage of boron metasomatism. KEY WORDS: tourmalinites; metamorphic and granitic rocks; mineral chemistry; whole-rock chemistry; boron isotopes     

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     

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     

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.     

Oxygen and Hydrogen Isotope Geochemistry of the Mourne Mountains Tertiary Granites, Northern Ireland

An oxygen and hydrogen isotopic study of minerals and wholerocks from the granites of the Mourne Mountains Tertiary complex,and related rocks, shows that whereas a significant circulationof meteoric water was associated with the complex, it had onlyminor and localized effects on the granites themselves. TheSilurian slate and greywacke country rocks, which would havehad ¹⁸O_(SMOW) values of +10 to +20 before the Tertiary igneousevents, have been depicted ¹⁸O to values of –40 to –05Tertiary acid minor intrusions outside the main granite massesare also ¹⁸O depleted. ^l8O whole-rock data on the granites showa range of +6.0 to +9.5, and include values significantly higherthan most of those obtained for the granites of the Tertiarycentral complexes of Skye, Mull, and Ardnamurchan. Many of thelowest whole-rock ¹⁸O values are found in samples where theminerals are not in isotopic equilibrium. The mineral oxygenisotopic data can be explained in terms of localized interactionwith meteoric water, resulting in preferential ¹⁸O depletionin feldspar(s) and biotite, with quartz being much less affected.The granites all show low values of D_(SMOW) for biotite andamphibole separates (–137 to –104). The lowest valuesoccur close to the margins of the plutons, near internal contactsor near greisen localities, and these probably reflect limitedinteraction with meteoric water. The higher D values are fromsamples which show evidence of chloritization. This processappears to have occurred both during interaction with meteoricwater, and also during autometasomatism by an exsolved magmaticfluid in other parts of the plutons, including central locationswhere there is little or no evidence for the penetration ofmeteoric water. Granite samples which exhibit near-equilibriumoxygen isotope fractionations for constituent minerals are characterizedby magmatic O-isotopic compositions. The G2 granite, the largestpluton of the eastern centre, has a magmatic ¹⁸O_(SMOW) valueof {small tilde}+95; intrusions G3 (eastern centre) and G4(western centre) both have ¹⁸O_(SMOW) values of {small tilde}+90.The other two main intrusive phases have distinctly lower ¹⁸O_(SMOW)values: {small tilde}+75 for Gl (the least fractionated graniteof the Mourne Mountains central complex), and from +75 to +85for G5. The oxygen isotopic data rule out simple partial meltingof the country rocks as the origin of the granites and alsopreclude an origin by closed-system fractional crystallizationof basaltic magma typical of that represented by Tertiary basicigneous rocks of the region. ^* Present address: NERC Isotope Geosciences Laboratory, Keyworth, Nottingham BG12 5GG, UK Present address: School of Engineering Technology, Georgian College, Barrie, Ontario, L4M 3X9, Canada     

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.     

Fluid Flow During Contact Metamorphism at Mary Kathleen, Queensland, Australia

Corella marbles in the Mary Kathleen Fold Belt were infiltratedby fluids during low-pressure (200-MPa) contact metamorphismassociated with the intrusion of the Burstall granite at 1730–1740Ma. Fluids emanating from the granite [whole-rock (WR) ¹⁸O=8.1–8.6%]produced Fe-rich massive and banded garnet—clinopyroxeneskarns [¹⁸O(WR)=9.1–11.9%]. Outside the skarn zones, marblemineralogies define an increase in temperature (500 to >575C) and XCO₂ (0.05 to >0.12) towards the granite, andmost marbles contain isobarically univariant or invariant assemblagesin the end-member CaO–MgO–Al₂O₃–SiO₂–H₂O–CO₂system. Marbles have calcite (Cc) ¹⁸O and ¹³C values of 12.3–24.6%and –1.0 to –3.9%, respectively. A lack of down-temperaturemineral reactions in the marbles suggests that pervasive fluidinfiltration did not continue after the thermal peak of contactmetamorphism. The timing of fluid flow probably correspondsto a period of high fluid production and high intrinsic permeabilitiesduring prograde contact metamorphism. The petrology and stableisotope geochemistry of the marbles suggest that these rockswere infiltrated by water-rich fluids. If fluid flow occurredup to the peak of contact metamorphism, the mineralogical andisotopic resetting is best explained by fluids flowing up-temperaturetoward the Burstall granite. However, if fluid flow ceased beforthe peak of regional metamorphism, the fluid flow directioncannot be unambiguously determined. At individual outcrops,marble ¹⁸O(Cc) values vary by several permil over a few squaremetres, suggesting that fluid fluxes varied by at least an orderof magnitude on the metre to tens-of-metre scale. Fluids werefocused across lithological layering; however, mesoscopic fracturesare not recognized. The focusing of fluids was possibly viamicrofractures, and the variation in the degree of resettingmay reflect variations in microcrack density and fracture permeability.The marble—skarn contacts represent a sharp discontinuityin both major element geochemistry and ¹⁸O values, suggestingthat, at least locally, little fluid flow occurred across thesecontacts.     

An Experimental Study of Fe-Al Solubility in the System Corundum-Hematite up to 40 kbar and 1300{degrees}C

The mutual solubility in the system corundum–hematite[-(Al, Fe³⁺)₂O₃] was investigated experimentally using bothsynthetic and natural materials. Mixtures of -Al₂O₃ and -Fe₂O₃(weight ratios of 8:2 and 10:1) were used as starting materialsfor synthesis experiments in air at 800–1300°C withrun times of 7–34 days. Experiments at 8–40 kbarand 490–1100°C were performed in a piston-cylinderapparatus (run times of 0·8–7·4 days) usinga natural diasporite consisting of 60–70 vol. % diasporeand 20–30 vol. % Ti-hematite. During the diasporite–corunditetransformation, the FeTiO₃ component (12–18 mol %) ofTi-hematite only slightly increased, implying that oxygen fugacitywas maintained at high values. Run products were studied byelectron microprobe and X-ray diffraction (Rietveld) techniques.An essentially linear volume of mixing exists in the solid solutionwith a slight positive deviation at the hematite side. Up to1000°C, corundum contains <4 mol % Fe₂O₃ and hematite<10 mol % Al₂O₃; at 1200°C these amounts increase to9·3 and 17·0 mol %, respectively. At 1300°Chematite was no longer stable and coexists with the orthorhombic phase . The present results agree with corundum (solvus) compositions obtained inprevious studies but indicate a larger solubility of Al in hematite.The miscibility gap in the solution can be modelled with anasymmetric Margules equation with interaction parameters (2uncertainties): ; ; ; . Application of the corundum–hematite solution as a solvus geothermometer is limited because of thescarcity of suitable rock compositions. KEY WORDS: corundum; hematite; corundum–hematite miscibility gap; experimental study; Margules model; metabauxite     

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     

Immiscibility in Ca-amphiboles

The literature data of nine different occurrences of coexistingmineral pairs of Ca-amphibole have been studied and the bulkvectors, spanning the miscibility gap, derived. The additivecomponent is always impure Mg-tremolite accompanied by someglaucophane and cummingtonite component. The four major exchangecomponents required to describe the compositional variationin coexisting mineral pairs are the edenite (ED), tschermak's(TS), FeMg_–1 and Fe³⁺-tschermak's (FeTs) vector. Trivalentiron is postulated on the basis of excess charges in the bulkvector the size of which coincides with residuals in Al^tet,–Si, Fe and –Mg. The four cations have equal sizes,forming the vector Fe³⁺ Al^tet Mg_–1Si_–1. This distributionscheme is consistent for all the different occurrences and setsthe basis for a comparison. Deviations from the scheme wouldradically complicate the proposed exchange pattern. The ratioTS:ED in most mineral samples fluctuates between one and two.Projection of the data points in the vector space TS–EDonto the line 1ED: 2TS (Tr–Hbl) or 1ED:1TS (Tr–Prg)provides the projected tremolite content (= 1–X_Hbl or = 1–X_prg). This parameter,applied to coexisting pairs, and plotted against the ratio Mg/(Mg+ Fe) shows some characteristic features about the miscibilitygap. In the Mg-pure system the solvus is almost symmetric andlocated in the temperature range between 800 and 870C. Smallamounts (0.10 pfu) of Fe²⁺ in the M(4) -sites and replacingCa have a dramatic effect, forcing the solvus to much lowertemperatures of 650C. An increase in the ratio Fe/(Fe + Mg)causes a shift of the solvus towards more tremolitic compositionswith temperatures 500–650C. The maximum asymmetry ofthe solvus is reached where the Al-poor member (tremolite) hasa composition of =1.0 and Mg/ (Mg + Fe) 0.6. The corresponding Al-rich member has =0.5 and Mg/ (Mg + Fe) 0.4. An anomalyof the solous is observed at Mg/ (Mg + Fe)=0.8. It manifestsas a kind of highly asymmetric ‘sub-gap’ in thetremolite-rich composition range. This is explained by the partitioningof Fe²⁺ into the single M(3) -site and is characterized by athermal hump to 650–700C. KEY WORDS: tremolite; hornblende; pargasite; immiscibility; solous     

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.     

Petrology of Mafic Plutons Associated with Calc-Alkaline Granitoids, Chilliwack Batholith, North Cascades, Washington

Small (<5 km²), lithologically diverse gabbro and dioritestocks make up 2% of the 34 to 2 Ma Chilliwack batholith, andoverlap in age with associated calc-alkaline granitoids. Thesemafic plutons are similar to those in other I-type bath-oliths,and represent basaltic magmas present during batholith formation.Objectives of this study are: (1) to examine the origins ofboth interpluton and intrapluton petrologic diversity, and (2)to compare chemical and Sr-Nd isotopic traits of these gabbroswith those of Cascade are basalts. Mafic rocks in the Chilliwackare divided into a medium-K series (MKS) and a low-K series(LKS). The former contain 0.7–2.4 wt% K₂0 and are similarin composition to calc-alkaline basalts and basaltic andesites.Inverse REE modeling supports derivation of the MKS by 9–27%melting of a garnet-free, LREE-enriched source (La/b_N 2). ChilliwackLKS gabbros have chemical characteristics of low-K olivine tho-eiites,including low K₂O (0.3–0.5 wt%) and La/b_N (1.7–3.4),and high CaO (8.8–11.3 wt%) and Na₂O/K₂O (6–22).These traits suggest a source with more clinopyroxene and lowerLa/b_N than the MKS source. Differences in _Nd(O) between MKSand LKS gabbros suggest that lower Nd/Sm is a long-lived LKSsource characteristic. Lithologic variation within compositeplutons of both series resulted primarily from multiple intrusionof related magmas, in some cases differentiates of a commonparent. Two contrasting examples were studied in detail. AtMt Sefrit, MKS variation (gabbronorite-quartz diorite) is modeledby low-pressure fractionation (ol + plag + cpx), accompaniedby 10% wallrock assimilation. In contrast, chemical and Sr-Ndisotopic variation among LKS gabbro-quartz diorite at CopperLake points to crystallization dominated by clinopyroxene+plagioclaseCr-spinel,indicative of differentiation at pressures 10 kbar, althoughthe assimilant in this case is poorly constrained. Chemicaland isotopic similarities between these mafic plutons and QuaternaryCascade lavas indicate that mafic magmas present during theproduction of Chilli-wack granitoids were low-and medium-K arebasalts. KEY WORDS: are magmatism; Cascades; gabbro; granitoid; trace element ^*Present address: Department of Physics, Astronomy, & Geology, Valdosta State University, Valdosta, GA 31698, USA. Telephone: (912) 249–4847 or 333–5752 Fax: (912) 333–7389. e-mail: jtepper{at}valdosta.peachnet.edu     

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.