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     

The Ni-S System and Related Minerals

The system Ni-S has been studied systematically from 200^? to1, 030^? C by means of evacuated, sealed silica-glass tube experimentsand differential thermal analyses. Compounds in the system areNi₃S₂ (and a high temperature, non-quenchable Ni_3?S₂ phase),Ni₇S₆, Ni₁–S₄ Ni₃S₄, and NiS₂. The geologic occurrenceof the minerals heazlewoodite (Ni₂S₂), millerite (ßSNi₁-₂S),polydymite (Ni₃S₄), and vaesite (NiS₂) can now be describedin terms of the stability ranges of their synthetic equivalents. Hexagonal heazlewoodite, which is stoichiometric within thelimit of error of the experiments, inverts on heating to a tetragonalor pseudotetragonal phase at 556^? C. This high-temperature phase(Ni₃ has a wide field of stability, from 23.5 to 30.5 wt percent sulfur at 600^? C, and melts incongruently at 806^??3^? C.The ßNi₇S₆ phase inverts to Ni₇₈ at 397^? C6 when inequilibrium with Ni₃S₂, and at 400^? C when in equilibrium withNiS. Crystals of Ni₇S₆ break down to Ni₃-S₂+NiS at 573^??3^?C.The low-temperature form of Ni₁-S₁ corresponding to the mineralmillerite, is rhombohedral, and the high-temperature form hasthe hexagonal NiAs structure. Stoichiometric NiS inverts at379^??3^?C, whereas Ni₁-S with the maximum nickel deficiency invertsat 282^??5OC. The Ni₁-alphS-NiS₂ solvus was determined to 985^??3^?C,the eutectic temperature of these phases. Stoichiometric NiSis stable at 600^?C but breaks down to Ni₂-S₂ and Ni₁-S below797^?C, whereas Ni₁-S with 38.2 wt per cent sulfur melts congruentlyat 992^??3^?C. Vaesite does not vary measurably from stoichiometricNiS₂ composition, and melts congruently at 1.007?5^?C. Polydymitebreaks down to aNi-S? vaesite at 356^??3^?C. Differential thermalanalyses showed the existence of a two-liquid field in the sulfur-richportion of the system above 991^?C and over a wide compositionalrange.     

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     

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.     

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     

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.     

Oxygen Isotope Evidence for Chemical Interaction of Ki lauea Historical Magmas with Basement Rocks

Klauea historical summit lavas have a wide range in matrix ¹⁸O_VSMOWvalues (4·9–5·6) with lower values in rockserupted following a major summit collapse or eruptive hiatus.In contrast, ¹⁸O values for olivines in most of these lavasare nearly constant (5·1 ± 0·1). The disequilibriumbetween matrix and olivine ¹⁸O values in many samples indicatesthat the lower matrix values were acquired by the magma afterolivine growth, probably just before or during eruption. BothMauna Loa and Klauea basement rocks are the likely sources ofthe contamination, based on O, Pb and Sr isotope data. However,the extent of crustal contamination of Klauea historical magmasis probably minor (< 12%, depending on the assumed contaminant)and it is superimposed on a longer-term, cyclic geochemicalvariation that reflects source heterogeneity. Klauea's heterogeneoussource, which is well represented by the historical summit lavas,probably has magma ¹⁸O values within the normal mid-ocean ridgebasalt mantle range (5·4–5·8) based on thenew olivine ¹⁸O values. KEY WORDS: Hawaii; Klauea; basalt; oxygen isotopes; crustal contamination     

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     

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     

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.     

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 Oxygen and Hydrogen Isotope Study of the Skaergaard Intrusion and its Country Rocks: a Description of a 55 M.Y. Old Fossil Hydrothermal System

Oxygen isotope analyses have been obtained on rocks and coexistingminerals, principally plagioclase and clinopyroxene, from about400 samples of the Skaergaard layered gabbro intrusion and itscountry rocks. The ¹⁸O values of plagioclase decrease upwardin the intrusion, from ‘normal’ values of about+6.0 to +6.4 in the Lower Zone and parts of the Middle Zone,to values as low as –2.4 in the Upper Border Group. The¹⁸O depletions of the plagioclase all took place under subsolidusconditions, and were produced by the Eocene meteoric-hydrothermalsystem established by this pluton. Clinopyroxene, which is moreresistant to ¹⁸O exchange than is plagioclase, also underwentdepletion in ¹⁸O, but to a lesser degree (¹⁸O = +5.2 to +3.5).The ¹⁸O-depleted rocks typically show reversed ¹⁸Oplag–pxfractionations, except at the top of the Upper Zone, where thepyroxenes are very fine-grained aggregates pseudomorphous afterferrowollastonite; these inverted pyroxenes were much more susceptibleto subsolidus ¹⁸O exchange (¹⁸O = +3–9 to +0.7). D/H analysesof the chloritized basalt country rocks and of the minor quantitiesof alteration minerals in the pluton (D = –116 to –149)confirm these interpretations, indicating that the rocks interactedwith meteoric groundwaters having an original D –100.and ¹⁸O –14. Low D values ( –125) were also foundthroughout the biotites of the Precambrian basement gneiss,requiring that small amounts of water penetrated downward todepths of at least 6 to 10 km. These values, together with thelack of ¹⁸O depletion of the gneiss, imply that the overallwater/rock ratios were very small in that unit (<0.01), andthus that convective circulation of these waters was much morevigorous in the overlying highly jointed plateau basalts (¹⁸O –4.0 to +4–0) than in the relatively impermeablegneiss (¹⁸O +7–3 to +7–7). This contrast in permeabilitiesof the country rocks is also reflected in the distribution of¹⁸O values in the pluton; the plagioclases with ‘normal’¹⁸O values all lie stratigraphically beneath the projectionof the basalt-gneiss unconformity through the pluton. Elsewhere,the ¹⁸O depletions are correlated with abundance of fracturesand faults, particularly in the NE portion of the intrusion,where the Layered Series is very shallow-dipping and permeablebasalts underlie the gabbro. The transgressive granophyres in the lower part of the intrusivehave ¹⁸O values identical to those of the basement gneiss, indicatingthey were probably formed by partial melting of stoped blocksof gneiss. In the upper part of the intrusion these granophyredikes have ¹⁸O values similar to the adjacent host gabbro; thissuggests that much of the hydrothermal alteration occurred aftertheir emplacement. However, because of the rarity of low-temperaturehydrous alteration minerals, it is also clear that most of theinflux of H₂O into the layered gabbro occurred at very hightemperatures (>400–500 °C). Prior to flowing intothe gabbro, these fluids had exchanged with similar mineralassemblages in the basaltic country rocks, explaining the lackof chemical alteration of the gabbro. Xenoliths of roof rockbasalt and of Upper Border Group leucogabbro were strongly depletedin ¹⁸O by the hydrothermal system prior to their falling tothe bottom of the magma chamber and being incorporated in thelayered series. This proves that the hydrothermal system wasestablished very early, at the time of emplacement of the Skaergaardintrusion. However, no measurable ¹⁸O depletion of the gabbromagma could be detected, indicating that very little H₂O penetrateddirectly into the liquid magma, in spite of the fact that ahydrothermal circulation system totally enveloped the magmachamber for at least 100, 000 years during its entire periodof crystallization. Only as crystallization proceeded was thehydrothermal system able to collapse inward and interact withthe solidified and fractured portions of the gabbro. Neverthelesssome H₂O was clearly added directly to the magma by dehydrationof the stoped blocks of altered roof rock. It is also plausiblethat small amounts of meteoric water diffused directly intothe magma, most logically in the vicinity of major fracturezones that penetrated close to, or were underneath, the late-stagesheet of differentiated ferrodiorite magma. It is suggestedthat such influx of meteoric waters was responsible for manyof the gabbro pegmatite bodies that are common in the MarginalBorder Group; also, such H₂O might have produced local increasesin Fe⁺³/Fe⁺² in the magma that in turn could explain some ofthe asymmetric crystallization effects in the magma chamber.Local lowering of the liquidus temperature would also occur,perhaps leading to topographic irregularities on the floor ofthe magma chamber (e.g. the trough bands?).     

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.     

Sr Isotope and Trace Element Studies of the Great Dyke and Bushveld Mafic Phase and their Relation to Early Proterozoic Magma Genesis in Southern Africa

New Rb-Sr and trace element data are reported for the GreatDyke and Bushveld Mafic Phase layered intrusions. It is arguedthat geochemical characteristics, such as ⁸⁷Sr/⁸⁶Sr ratios andR.E.E. distribution patterns have been little modified by crustalcontamination. Rb-Sr data for whole-rocks of the Great Dyke yield an age of2514±16 m.y. and an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.70261±4.Mineral data are consistent with these results. The low errorson the results indicate no significant variation of ⁸⁷Sr/⁸⁶Srratios of successive magmatic influxes emplaced in differentmagma chambers. Earlier Great Dyke magmas were highly Mg-richand represent extensive partial melts of the source material.One such influx is shown to have a high Rb/Sr ratio (0.25) anda fractionated R.E.E. pattern (Ce_N/YB_N 12). These ratios areconsidered to approximate those of the source region. The Bushveld Mafic Phase has been dated accurately for the firsttime and has a Rb-Sr age of 2095±24 m.y. Initial ⁸⁷Sr/⁸⁶Srratios increase in a stepwise manner upwards in the intrusionfrom 0.70563±2 to 0.70769±6. Each increase isabrupt and occurs at a horizon also characterized by a suddenirregularity in cryptic variation. The Mafic Phase was emplacedas a succession of magmatic influxes each of which had higher⁸⁷Sr/⁸⁶Sr ratio than its predecessor. The first magma was both Mg-rich (MgO 21.5 per cent) and SiO₂-rich(50–55 per cent SiO₂) and was derived by extensive partialmelting of a shallow level upper mantle source. This sourcewas characterized by trace element abundance ratios (e.g. Rb/Sr 0.25; K/Rb 90; Ce_N/Yb_N 11), similar to those of kimberlitesand some potassic lavas and comparable with those deduced forthe Great Dyke source region. It is postulated that when the Rhodesian and Kaapvaal cratonsstabilized, underlying refractory mantle became fixed theretoto form a proto-lithosphere. Shortly afterwards, at about 2800m.y. ago, this proto-lithospheric mantle was enriched by passagethrough it of fluids with kimberlitic trace element chemistry.This sub-cratonic mantle thereafter evolved with a relativelyhigh Rb/Sr ratio. Magmas derived from it have anomalous chemicalcharacteristics with respect to those of ocean-floor basalts,reflecting major differences in the evolution of their respectivesource regions.     

Early-Middle Jurassic Dolerite Dykes from Western Dronning Maud Land (Antarctica): Identifying Mantle Sources in the Karoo Large Igneous Province

A suite of dolerite dykes from the Ahlmannryggen region of westernDronning Maud Land (Antarctica) forms part of the much moreextensive Karoo igneous province of southern Africa. The dykecompositions include both low- and high-Ti magma types, includingpicrites and ferropicrites. New ⁴⁰Ar/³⁹Ar age determinationsfor the Ahlmannryggen intrusions indicate two ages of emplacementat 178 and 190 Ma. Four geochemical groups of dykes have beenidentified in the Ahlmannryggen region based on analyses of60 dykes. The groups are defined on the basis of whole-rockTiO₂ and Zr contents, and reinforced by rare earth element (REE),⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd isotope data. Group 1 were intrudedat 190 Ma and have low TiO₂ and Zr contents and a significantArchaean crustal component, but also evidence of hydrothermalalteration. Group 2 dykes were intruded at 178 Ma; they havelow to moderate TiO₂ and Zr contents and are interpreted tobe the result of mixing of melts derived from an isotopicallydepleted source with small melt fractions of an enriched lithosphericmantle source. Group 3 dyke were intruded at 190 Ma and formthe most distinct magma group; these are largely picritic withsuperficially mid-ocean ridge basalt (MORB)-like chemistry (flatREE patterns, ⁸⁷Sr/⁸⁶Sr_i 0·7035, Nd_i 9). However, theyhave very high TiO₂ (4 wt %) and Zr (500 ppm) contents, whichis not consistent with melting of MORB-source mantle. The Group3 magmas are inferred to be derived by partial melting of astrongly depleted mantle source in the garnet stability field.This group includes several high Mg–Fe dykes (ferropicrites),which are interpreted as high-temperature melts. Some Group3 dykes also show evidence of contamination by continental crust.Group 4 dykes are low-K picrites intruded at 178 Ma; they havevery high TiO₂–Zr contents and are the most enriched magmagroup of the Karoo–Antarctic province, with ocean-islandbasalt (OIB)-like chemistry. Dykes of Group 1 and Group 3 aresub-parallel (ENE–WSW) and both groups were emplaced at190 Ma in response to the same regional stress field, whichhad changed by 178 Ma, when Group 2 and Group 4 dykes were intrudedalong a dominantly NNE–SSW strike. KEY WORDS: flood basalt; depleted mantle; enriched mantle; Ahlmannryggen; Karoo dyke     

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     

Geochemical Constraints on the Origin of the Late Archean Skjoldungen Alkaline Igneous Province, SE Greenland

The present work reports the first broad geochemical investigationof the recently discovered late Archean (2700 Ma) Skjoldungenalkaline igneous province (SAP) in southeast Greenland. Therocks studied range in composition from ultramafic to felsicand comprise pyroxenites, hornblendites, hornblende noritesand diorites, monzonites, syenites, and nephelinitic rocks andcarbonatites. Various lithologic units from the host Archeangneissic basement are also investigated. The magmatic rocksshow remarkably coherent major element, trace element, rareearth element (REE), and Sr and Nd isotope systematics, suggestinga petrogenetic relationship. The most important geochemicalfeatures are high normative proportions of nepheline, forsteriteand albite, low TiO₂ (<15 wt %) and moderate FeO (total)(<12 wt %) contents, enrichments in large ion lithophileelements (LILE) and light rare earth elements both absoluteand relative to high field strength elements (HFSE) that displaylarge negative anomalies, and generally low to moderate abundancesof compatible elements. Field relations and REE and compatibleelement systematics among Skjoldungen rocks suggest that maficand ultramafic hornblende-rich samples may represent cumulatelithologies of the regional parental magma. On the basis ofmineral data, this is deduced to have had mg-number of 064,shoshonitic affinities (K₂O15 wt %), been close to silica saturationand volatile rich. Major element, trace element and REE systematicsfurther suggest that felsic intrusions are related to the maficregional parental magma through extensive olivine, hyperstheneand hornblende fractionation. Lack of correlation between La/Yband other critical trace and REE ratios indicates that apatite,zircon and titaniferous minerals were not important cumulusphases at advanced stages of evolution. The measured Sm–Ndwhole-rock isochron age is 2716 23 Ma (2 error) [mean squareof weighted deviates (MSWD) = 14], whereas linear regressionof the Sr isotope data yields an age of 26047 Ma (2 error)(MSWD = 22•2). The age obtained by Nd isotopes is corroboratedby U–Pb zircon results (2698 7 Ma), suggesting thatthe Sm–Nd system remained closed since crystallization.By contrast, the 100 Ma younger age obtained by Sr isotopessuggests that the Rb–Sr system has been disturbed. Initial¹⁴³Nd/¹⁴⁴ Nd ratios span a narrow range corresponding to _Nd(27Ga) =+074 to –109, whereas initial _Sr values at 27Ga cover a comparatively larger interval from –10 to +20.Neither initial _Nd nor initial _Sr values conform to previouslysuggested mantle depletion curves and no meaningful correlationexists between Nd and Sr isotopes for the Skjoldungen magmaticrocks as a whole. Although compositionally heterogeneous, theanalyzed suite of samples from the host agmatitic basement isextremely homogeneous with respect to age, with T^CHUR crustalresidence times around 2700–2800 Ma confirming limitedavailable isotopic evidence. Large-scale assimilation of Archeancrust or recycling of sediments derived from the local basementinto the mantle source fails to explain adequately negativeNb anomalies and low ^Nd signatures characteristic of the Skjoldungenintrusions. Rather, the nearchondritic isotopic compositionof Nd in the Skjoldungen samples together with the decoupledLILE and HFSE enrichment and slightly positive _Sr values areconsidered to reflect characteristics of the mantle source ina subduction zone environment. The geodynamic site hosting theSkjoldungen province thus may be an early manifestation of modern-styleplate tectonics. KEY WORDS: Skjoldungen province; Greenland; Archean; alkaline igneous rocks; geochronology; geochemistry ^*Corresponding author. Present address: Ecole Normale Suprieure de Lyon, 46 AlLe d'Italie, 69364 Lyon Cedex 07, France     

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.     

Open-System Magma Chamber Evolution: an Energy-constrained Geochemical Model Incorporating the Effects of Concurrent Eruption, Recharge, Variable Assimilation and Fractional Crystallization (EC-E'RA{chi}FC)

Significant petrogenetic processes governing the geochemicalevolution of magma bodies include magma Recharge (includingformation of ‘quenched inclusions’ or enclaves),heating and concomitant partial melting of country rock withpossible ‘contamination’ of the evolving magma body(Assimilation), and formation and separation of cumulates byFractional Crystallization (RAFC). Although the importance ofmodeling such open-system magma chambers subject to energy conservationhas been demonstrated, the effects of concurrent removal ofmagma by eruption and/or variable assimilation (involving imperfectextraction of anatectic melt from wall rock) have not been considered.In this study, we extend the EC-RAFC model to include the effectsof Eruption and variable amounts of assimilation, A. This model,called EC-E'RAFC, tracks the compositions (trace elements andisotopes), temperatures, and masses of magma body liquid (melt),eruptive magma, cumulates and enclaves within a composite magmaticsystem undergoing simultaneous eruption, recharge, assimilationand fractional crystallization. The model is formulated as aset of 4 + t + i + s coupled nonlinear differential equations,where the number of trace elements, radiogenic and stable isotoperatios modeled are t, i and s, respectively. Solution of theEC-E'RAFC equations provides values for the average temperatureof wall rock (T_a), mass of melt within the magma body (M_m),masses of cumulates (M_ct), enclaves (M_en) and wall rock () and the masses of anatectic melt generated () and assimilated (). In addition, t trace element concentrations and i + s isotopic ratios inmelt and eruptive magma (C_m, _m, _m), cumulates (C_ct, _m, _m), enclaves(C_en, , ) and anatectic melt (C_a, , ) as a function of magma temperature (T_m) are also computed. Input parametersinclude the (user-defined) equilibration temperature (T_eq),a factor describing the efficiency of addition of anatecticmelt () from country rock to host magma, the initial temperatureand composition of pristine host melt (, , , ), recharge melt (, , , ) and wall rock (, , , ), distribution coefficients (D_m, D_r, D_a) and their temperaturedependences (H_m, H_r, H_a), latent heats of transition (meltingor crystallization) for wall rock (h_a), pristine magma (h_m)and recharge magma (h_r) as well as the isobaric specific heatcapacity of assimilant (C_p,a), pristine (C_p,m) and recharge(C_p,r) melts. The magma recharge mass and eruptive magma massfunctions, M_r(T_m) and M_e(T_m), respectively, are specified apriori. M_r(T_m) and M_e(T_m) are modeled as either continuous orepisodic (step-like) processes. Melt productivity functions,which prescribe the relationship between melt mass fractionand temperature, are defined for end-member bulk compositionscharacterizing the local geologic site. EC-E'RAFC has potentialfor addressing fundamental questions in igneous petrology suchas: What are intrusive to extrusive ratios (I/E) for particularmagmatic systems, and how does this factor relate to rates ofcrustal growth? How does I/E vary temporally at single, long-livedmagmatic centers? What system characteristics are most profoundlyinfluenced by eruption? What is the quantitative relationshipbetween recharge and assimilation? In cases where the extractionefficiency can be shown to be less than unity, what geologiccriteria are important and can these criteria be linked to fieldobservations? A critical aspect of the energy-constrained approachis that it requires integration of field, geochronological,petrologic, and geochemical data, and, thus, the EC-ERAFC ‘systems’approach provides a means for answering broad questions whileunifying observations from a number of disciplines relevantto the study of igneous rocks. KEY WORDS: assimilation; energy conservation; eruption; open system; recharge     

Dehydration--Melting Phenomena in Leptynitic Gneisses and the Generation of Leucogranites: a Case Study from the Kerala Khondalite Belt, Southern India

Pan-African high-grade metamorphism in the Kerala KhondaliteBelt (South India) led to the in situ formation of garnet-bearingleucosomes (L1) in sodic quartz—alkali feldspar—biotitegneisses. Microtextures, mineralogy and the geochemical characteristicsof in situ leucosomes (L1) and gneiss domains (GnD) indicatethat the development of leucosomes was mainly controlled bythe growth of garnet at the expense of biotite. This is documentedby the selective transfer of FeO, MgO, , Sm and the heavy rareearth elements into the L1 domains. P-T constraints (T>800C,P>6kbar, aH₂O0.3) suggest that the leucosomes were formedthrough complete melting of biotite in fluid-absent conditions,following the model reaction Biotite+Alkali feldspar+QuartzlGarnet+Ilmenite+Melt.The fraction of melt generated during this process was low (<10vol.%). The identical size of the leucosomes as well as theirhomogeneous and isotropic distribution at outcrop scale, whichlacks any evidence for melt segregation, suggest that the migmatiteremained a closed system. Subsequent to migmatization, the leptyniticgneisses were intruded by garnet-bearing leucogranitic melts(L2), forming veins parallel and subperpendicular to the foliation.The leucogranites are rich in potassium (K₂O5.5 wt%), (Ba400p.p.m.) and Sr (300 p.p.m.), and exhibit low concentrationsof Zr (40 p.p.m.), Th (<1 p.p.m.) and (<10 p.p.m.). Thechondrite-normalized REE spectra show low abundances (La_N20,Lu_N3) and are moderately fractionated (La_N/Lu_N7). An Eu anomalyis absent or weakly negative. The higher ⁸⁷Sr/⁸⁶Sr ratio at550 Ma (0.7345) compared with the migmatite (0.7164) precludesa direct genetic relationship between leptynitic gneisses andleucogranites at Manali.Nevertheless, the chemical and mineralogicalcompositions of the leuocogranites strongly favour a derivationthrough fluid-absent biotite melting of isotopically distinctbut chemically comparable Manali-type gneisses. The undersaturationof Zr, Th and REE, a typical feature of leucogranitic meltsgenerated during granulite facies anatexis of psammo-peliticlithologies and attributed to disequilibrium melting with incompletedissolution of accessory phases (zircon, monazite), is weaklydeveloped in the leucogranites of Manali.It is concluded thatthis is mainly due to the sluggish migration of the melts instatic conditions, which facilitated equilibration with therestitic gneisses. ^*Fax: 0228-732763; e-mail: ingo.braun{at}uni-bonn.de