Prehnite-forming reactions in scapolite-bearing calc-silicate granulites from Rundvågshetta, Lützow-Holm Complex, East Antarctica

ABSTRACTNine metacarbonate layers from the regionally metamorphosed terrane of the Sør Rondane Mountains in the Eastern Dronning Maud Land in East Antarctica were examined in detail for constraining the thermal events using carbon isotope exchange between dolomite/calcite and graphite. Equilibrium carbon isotope fractionation between dolomite and graphite suggested peak metamorphic temperature conditions reaching up to 802°C ± 29°C were estimated at the Balchenfjella locality, where multiple samples from six thick layers of metacarbonate rocks were examined. However, some of the samples exhibit lower carbon isotope fractionation reflecting the possibility of ultrahigh‐temperature metamorphic conditions, which is consistent with recent reports. Furthermore, several metacarbonate rock samples display large variations in δ13CVPDB values for graphite grains, despite dolomite and calcite showing homogeneous carbon and oxygen isotopic composition indicating signatures of retrograde metamorphism and fluid infiltration events. Detailed textural observation suggested alteration of δ13CVPDB values of graphite during retrograde metamorphism might have resulted due to the overgrowth of graphite crystals by the infiltration of low δ13CVPDB‐bearing fluids, the extent of alteration being a direct function of the fluid–rock ratio. Field evidence indicates the presence of carbonate veins cutting across the metacarbonate rocks suggesting that carbon isotope thermometry can also be utilised to understand the effect of external fluid infiltration. At Perlebandet locality the metamorphic temperature conditions were estimated to be around 915°C, whereas those from Tanngarden and Menipa gave lower temperature estimates. Detailed textural analysis of graphite in combination with isotopic composition provided clear evidence for retrograde events. Thus, our results provide tight constraints of peak and post‐peak metamorphic temperature conditions and a regional thermal structure for the Sør Rondane Mountains and further testify the usefulness of carbon isotope thermometry in polymetamorphic terrains.

Spinel- and orthopyroxene-bearing metapelitic granulites exposed in the Bunger Hills, East Antarctica, have been intensively studied in recent years because they are supposed to record evidence for UHT metamorphism. Detailed petrographic observations, as well as whole rock and mineral chemistry, together with SIMS trace element data on quartz, garnet, and orthopyroxene, are presented for these rocks. Mineral thermobarometry, including Al-in-orthopyroxene, ternary feldspar, Ti-in-quartz, and Fe-Ti oxide solvus, has been used to quantify the UHT conditions. Based on phase equilibrium modeling, a tight clockwise P-T path has been deduced, which involves near-isobaric heating at 6–7 kbar to ~950 °C followed by near-isobaric to slightly up-pressure cooling at 5–6 kbar to ~750 °C. It is concluded that the outlined metamorphic history is characteristic of an extensional crustal regime which is also evidenced by the correlation of prograde and retrograde metamorphism with the extensional and compressional phases of major ductile deformations recognized in the region. In order to constrain the tectonic setting of the granulites, this result is discussed in the context of current views on the Mesoproterozoic evolution of the Albany-Fraser Orogen, the westernmost part of which the Bunger Hills are considered to be.

Temperature dependence of the Fe 2+ –Mg exchange between orthopyroxene (Opx) and spinel (Spl), 1 / 2 Fe 2 Si 2 O 6 + MgAl 2 O 4 = 1 / 2 Mg 2 Si 2 O 6 + FeAl 2 O 4 Opx Spl Opx Spl was experimentally determined at 9–13 kbar and 900–1200 °C using an ultrahigh-temperature (UHT) granulite collected from the Napier Complex in Enderby Land, East Antarctica. The Fe 2+ –Mg distribution coefficient, K D = X Fe Spl X Mg Opx X Mg Spl X Fe Opx is empirically obtained as ln K D = − 1.134 + [ 2341 + 7.5 ( P − 11 ) ] / T where X is the cationic mole fraction, and pressure P and temperature T are in kbar and Kelvin, respectively. The new geothermometer was applied to various natural UHT and associated high-grade metamorphic rocks from East Antarctica and other regions of the world. The results indicate temperatures between 735 and 902 °C at pressures in the range of 5–14 kbar. This geothermometer utilizing spinel does not give peak metamorphic condition, because it is relatively easy for spinel to re-equilibrate during the cooling stage of metamorphism. Hence, we conclude that the geothermometer is suitable for evaluating the closure temperature for the K D between aluminous orthopyroxene and spinel during retrograde metamorphism rather than the thermal peak.

Fingerprinting a multistage metamorphic fluid–rock history: Evidence from grain scale Sr, O and C isotopic and trace element variations in high-grade marbles from East Antarctica

This study investigates the microstructures, geochemistry, and hydrothermal evolution of gold-bearing pyrite and arsenopyrite from six orogenic gold deposits in the Archean Eastern Goldfields Province, Western Australia. Scanning electron microscope (SEM), electron microprobe (EMP) and laser ablation-inductively coupled plasma-mass spectroscopy (LA-ICP-MS) analyses show that the gold-bearing minerals possess a number of similar textural features, including the occurrence of invisible gold within initial phases of growth, and later-stage visible gold associated with alteration rims. The alteration rims are characterized by a higher-than-average atomic mass (mainly owing to arsenic enrichment) and are preferentially located along fractures and grain boundaries in the pyrite and arsenopyrite. These observations suggest that visible gold formation is associated with hydrothermal alteration of preexisting pyrite and arsenopyrite. Textural observations and LA-ICP-MS data suggest that some invisible gold was remobilized from early-formed pyrite and arsenopyrite to form visible gold during development of these alteration rims. Gold may also have been added by hydrothermal fluids during a later stage of mineralization. In situ geochemistry and phase relationships of alteration rims are used to further constrain the hydrothermal process responsible for formation of alteration rims and visible gold in fractures. Based on sulfide stability relations, our data indicate that development of arsenopyrite alteration rims associated with late-stage visible gold formation was related to an increase in temperature (maximum increase from 310° to 415°C) and up to of sic orders of magnitude increase in sulfur fugacity, whereas changes in oxygen fugacity were less important. LA-ICP-MS analyses show that the relative and absolute variations in selected trace element (Au, Ag, Sb, Bi, Ba, Te, Pb, Co, and Mo) concentrations can also be used to distinguish between unaltered and altered pyrite and arsenopyrite. In general, trace elements within pyrite and arsenopyrite have a relatively uniform distribution, whereas later-stage alteration rims have more variable trace element distributions. Although the observed textures are typical of prograde metamorphic coronae, we suggest that they are the consequence of variations in fluid conditions and chemistry, and that mineralization occurred in response to syn- and/or postpeak metamorphic fluid infiltration.

The alkali-feldspar and biotite in the sillimanite-biotite-garnet gneiss from East Antarctica preserves characteristic microstructural evidence of multi-stage H2O supplement during the retrograde metamorphism. The first microstructural evidence is the “zoned feldspar,” in which the mesoperthitic zone, the anti-perthitic zone, and lamella-free plagioclase zone coexist within a single crystal. They are occasionally found next to biotite, and are always depleted in orthoclase (Or) component toward the biotite. The formation process of this microstructure could be explained by the diffusion that oversteps the solvus. The second microstructural evidence is the serrate boundary between alkali-feldspar and biotite. The projections of biotite are selectively developed next to Or lamellae of alkali-feldspar every 3–5 μm. These two microstructures would have formed as the biotite grew by consuming potash in alkali-feldspar when H2O-bearing fluid locally passed through the grain boundaries. The former microstructure was formed at 825–900 °C before lamella formation, and the latter microstructure was formed after the lamella formation. These microstructures are the indicators of fluid pathways formed under two different temperature conditions. The common coexistence of these microstructures implies that the fluid used similar pathways during the retrograde metamorphism.

The iron stable isotope compositions (δ56Fe) and iron valence states of ultrahigh‐pressure eclogites from Bixiling in the Dabie orogen belt, China, were measured to trace the changes of geochemical conditions during vertical transportation of earth materials, for example, oxygen fugacity. The bulk Fe3+/ΣFe ratios of retrograde eclogites, determined by Mössbauer spectroscopy, are consistently higher than those of fresh eclogites, suggesting oxidation during retrograde metamorphism and fluid infiltration. The studied eclogites (five samples) display limited mid‐ocean ridge basalts (MORB)‐like (~0.10‰) δ56Fe values, which are indistinguishable from their protoliths, that is, gabbro cumulates formed through differentiation of mantle‐derived basaltic magma. This suggests that Fe isotope fractionation during continental subduction is limited. Garnet separates display limited δ56Fe variation ranging from −0.08 ± 0.07 ‰ to 0.02 ± 0.07‰, whereas coexisting omphacite displays a large variation of δ56Fe values from 0.15 ± 0.07‰ to 0.47 ± 0.07‰. Omphacite also has highly variable Fe3+/ΣFe ratios from 0.367 ± 0.025 to 0.598 ± 0.024, indicating modification after peak metamorphism. Omphacite from retrograde eclogites has elevated Fe3+/ΣFe ratios (0.54–0.60) compared to that from fresh eclogites (~0.37), whereas garnet displays a narrow range of ferric iron content with Fe3+/ΣFe ratios from 0.039 ± 0.013 to 0.065 ± 0.022. The homogenous δ56Fe values and Fe3+/ΣFe ratios of garnet suggest that it survived the retrograde metamorphism and preserved its Fe‐isotopic features and ferric contents of peak metamorphism. Because of similar diffusion rates of Fe and Mg in garnet and omphacite, and constant Δ26Mgomphacite‐garnet values (1.14 ± 0.04‰), equilibrium iron isotope fractionation between garnet and omphacite was probably achieved during peak metamorphism. Elevated Fe3+/ΣFe ratios of omphacite from retrograde eclogites and variant Δ56Feomphacite‐garnet values of the studied eclogites (0.13 ± 0.10‰ to 0.48 ± 0.10‰) indicate that oxidized geofluid infiltration resulted in the elevation of δ56Fe values of omphacite during retrograde metamorphism.

Carbon isotopic equilibrium between calcite and graphite in Skallen Marbles, East Antarctica: evidence for the preservation of peak metamorphic temperatures

ABSTRACTCalc‐silicate boudins within Proterozoic granulite facies gneisses of the northern Prince Charles Mountains, East Antarctica, preserve a number of reaction textures including garnet coronas between calcite and scapolite; garnet‐quartz coronas between scapolite and wollastonite and between plagioclase and wollastonite; calcite‐quartz intergrowths in wollastonite; and calcite‐plagioclase symplectites in scapolite. These textures have been modelled using petrogenetic grids for reactions in the CaO‐Al2,O3‐SiO2‐CO2 system, but with reduced mineral activities to account for additional components in real mineral compositions. Such fixed‐composition reduced‐activity grids are strictly valid only at the point in P‐T‐aCO2 space where an assemblage last equilibrated, and do not show the true positions of reactions away from this point because mineral compositions change with reaction progress. In this case, however, mineral compositions close to end‐member values and low extents of reaction progress mean that compositional change was limited and the grids are good approximations to true pseudosections over the entire P‐T‐aco2 range of interest.The grids show that the textures are consistent with near‐isobaric cooling from about 850 to 700d̀ C at 7 kbar, a P‐T path compatible with thermobarometric studies of other lithologies from the area. Phase relationships indicate that CO2 activities were buffered by the local mineral assemblage during peak and retrograde metamorphism, either under fluid‐absent conditions or within a non‐pervasive fluid phase. Previous studies of garnet coronas in scapolite‐wollastonite calc‐silicates have used qualitative grids based on limited experimental data to invoke garnet growth during water infiltration at high temperature, but the grids used here show that garnet coronas can form on cooling, without any need for water influx.

Fluid evolution of partially retrogressed pelitic granulite from the Southern Marginal Zone of the Neoarchean Limpopo Complex, South Africa: Evidence from phase equilibrium modelling

The tectonic significance of (U,Th)/Pb ages of monazite inclusions in garnet from the Himalaya of central Nepal

The Elu Link in the northeast Bathurst Block of the Slave Craton, Canada, is composed of variably D1–D3-deformed, low- to medium-grade supracrustal and intrusive rocks connecting the Neoarchean Hope Bay and Elu granite-greenstone belts (∼2.7 Ga). Textural observations and pseudosection modeling show that the primary metamorphic reactions in the supracrustal rocks derived from D1-assisted hydrothermal alteration in albite-epidote hornfels-facies to greenschist-facies conditions. This was followed by one main cycle of prograde, peak, and retrograde metamorphism that variably accompanied D1 and D2 events along a clockwise pressure-temperature (PT) path. The pseudosection for a garnet-bearing orthogneiss whose protolith emplacement occurred before peak metamorphism indicates maximum upper-amphibolite conditions of 6.7 kbar and 710°C, reached during D2 deformation. Thereafter, the orthogneiss followed a counterclockwise PT path showing an increase in pressure (up to 8.7 kbar) during the late stages of D2, which turned into D3 during near-isothermal decompression characterized by retrogression in the greenschist-facies conditions. This evolution of the orthogneiss is evidence that part of the heat budget used for metamorphism in the Elu Link derived from intrusive bodies. However, most pressure and temperature estimates are in the range of 3.5–9 kbar and 500°–660°C, respectively. These PT intervals are indicative of orogenic rather than low-P/T metamorphism. Thermobarometric signatures for low-P/T metamorphism are scarce and likely relate to intrusions postdating peak conditions in the belt.

High-pressure metapelites with the assemblage kyanite-staurolite-garnet ′ chloritoid-biotite-muscovite-plagioclase-quartz and crosscutting gneiss veins were studied in the Sopron Mountains near Obrennberg (Hungary, Eastern Alps). Different garnet generations were investigated by textural analysis and study of garnet zoning from different textural domains from both the micaschist and the gneiss veins. Thermobarometry of subsequent garnet generations and the surrounding mineral assemblage was used to reconstruct the Alpine P-T path of the metamorphic rocks during the Alpine orogenesis. Peak pressure conditions (1390-1470 MPa) are recorded by high-Si phengites (Si = 7.03 a.p.f.u.) and temperatures by white mica-garnet-albite-quartz assemblage at about 550-600 °C. This is in good agreement with the peak pressures obtained from the massive orthogneisses of the area. Gneiss veins crosscutting the micaschist show slightly lower peak pressures between 1230 and 1280 MPa at similar temperatures (575-590 °C). Subsequent garnet generations and associated mineral assemblages show decreasing pressure with slightly decreasing temperature. Textural observations, white mica chemistry and comparison of garnet zoning trends provided additional evidence on the formation of leucophyllites during retrograde metamorphism.

Mode of occurrence of fracture-filling microcrystalline quartz and Al2SiO5 mineral assemblage in the Sør Rondane Mountains, East Antarctica: Implication for fluid infiltration during retrograde metamorphism

Two-stage fluid infiltration during retrograde metamorphism: an example from Perlebandet, Sør Rondane Mountains, East Antarctica