Pan-African Alkali Granitoids from the Sør Rondane Mountains, East Antarctica

Alkali granitoids (500-550 Ma) representing a prominent Pan-African magmatic event are widely distributed in the Sør Rondane Mountains, Dronning Maud Land, East Antarctica. Geochemically, they are granitic to syenitic in composition and show an alkaline affinity of A-type granites. They are characterized by high K₂O+Na₂O (7-13 wt%) and K₂O/Na₂O (1-2), low to intermediate Mg#, wide ranges of SiO₂ (45-78 wt%), Sr (20-6500 ppm) and Ba (40-13000 ppm) and have Nb and Ti depletion in the primitive mantle normalized diagram. The granitoids are subdivided into Group I granites, Group II granites, Lunckeryggen Syenitic Complex and Mefjell Plutonic Complex. The Group I granites have higher Mg#, Sr/Ba, Sr/Y, (La/Yb)_N and LREE/HREE, lower A/CNK, SREE and initial ⁸⁷Sr/⁸⁷Sr ratios and lack Eu anomalies compared to those with negative Eu anomalies in the Group II granites. The syenitic rocks from the Mefjell Plutonic Complex are higher in alkali, Ga, Zr, Ba, and have lower Mg#, Rb, Sr, Nb, Y, F and LREE/HREE with positive Eu anomaly, whereas the granites from the Mefjell Plutonic Complex have high LREE/HREE ratios with negative Eu anomaly. The Lunckeryggen syenitic rocks have intermediate Mg#, higher K₂O, P₂O₅, TiO₂, Fe₂O₃/FeO, Ba, Sr/Y and LREE/HREE ratios with lack of Eu anomalies and are lower in Al₂O₃, Ga, Y, Nb and Rb/Sr ratios. Based on chemical characteristics combined with isotopic data, we suggest that the Lunckeryggen syenitic body and Group I granitic bodies may be derived from the mantle-derived hot basic magma by fractional crystallization with minor assimilation. We also suggest that the Group II granites may be derived from assimilation with crustal rocks to varing degrees and then fractional crystallization in higher crustal levels (ACF model). The Mefjell Plutonic Complex seems to be derived from a heterogenetic magma source compared with other granitoids from the Sør Rondane Mountains. The syenitic rocks in the Mefjell Plutonic complex have a unique source (iron-enriched) and have a chemical affinity with the charnockites in Gjelsvikjella and western Mühlig-Hofmannfjella, but not like the Yamato syenites in adjacent areas.     

High‐resolution geochemical record of fluid–rock interaction in a mid‐crustal shear zone: a comparative study of major element and oxygen isotope transport in garnet

Garnet grains from an intensely metasomatized mid‐crustal shear zone in the Reynolds Range, central Australia, exhibit a diverse assortment of textural and compositional characteristics that provide important insights into the geochemical effects of fluid–rock interaction. Electron microprobe X‐ray maps and major element profiles, in situ secondary ion mass spectrometry oxygen isotope analyses, and U–Pb and Sm–Nd geochronology are used to reconstruct their thermal, temporal and fluid evolution. These techniques reveal a detailed sequence of garnet growth, re‐equilibration and dissolution during intracontinental reworking associated with the Ordovician–Carboniferous (450–300 Ma) Alice Springs Orogeny. A euhedral garnet porphyroblast displays bell‐shaped major element profiles diagnostic of prograde growth zoning during shear zone burial. Coexisting granulitic garnet porphyroclasts inherited from precursor wall rocks show extensive cation re‐equilibration assisted by fracturing and fragmentation. Oxygen isotope variations in the former are inversely correlated with the molar proportion of grossular, suggesting that isotopic fractionation is linked to Ca substitution. The latter generally show close correspondence to the isotopic composition of their precursor, indicating slow intergranular diffusion of O relative to Fe²⁺, Mg and Mn. Peak metamorphism associated with shearing (～550 °C; 5.0–6.5 kbar) occurred at c. 360 Ma, followed by rapid exhumation and cooling. Progressive Mn enrichment in rim domains indicates that the retrograde evolution caused partial garnet dissolution. Accompanying intra‐mineral porosity production then stimulated limited oxygen isotope exchange between relict granulitic garnet grains and adjacent metasomatic biotite, resulting in increased garnet δ¹⁸O values over length scales <200 μm. Spatially restricted oxygen interdiffusion was thus facilitated by increased fluid access to reaction interfaces. The concentration of Ca in channelled fracture networks suggests that its mobility was enhanced by a similar mechanism. In contrast, the intergranular diffusion of Fe²⁺, Mg and Mn was rock‐wide under the same P–T regime, as demonstrated by a lack of local spatial variations in the re‐equilibration of these components. The extraction of detailed reaction histories from garnet must therefore take into account the variable length‐ and time‐scales of elemental and isotopic exchange, particularly where the involvement of a fluid phase enhances the possibility of measureable resetting profiles being generated for slowly diffusing components such as Ca and O, even at low ambient temperatures and relatively fast cooling rates.     

Quartz–garnet isotope thermometry in the southern Adirondack Highlands (Grenville Province, New York)

Quartz–garnet oxygen isotope thermometry of quartz‐rich metasedimentary rocks from the southern Adirondack Highlands (Grenville Province, New York) yields metamorphic temperatures of 700–800 °C, consistent with granulite facies mineral assemblages. Samples from the Irving Pond quartzite record Δ¹⁸O(Qtz–Grt) = 2.68 ± 0.21‰ (1 s.d. , n = 15), corresponding to peak metamorphic conditions of 734 ± 38 °C. This agrees well with the estimates from garnet–biotite exchange thermometry. Similar temperature estimates are obtained from Swede Pond (682 ± 47 °C, n = 3) and King's Station (c. 700 °C). The Whitehall area records higher temperatures (798 ± 25 °C, n = 3). All of these temperatures are higher than previous regional temperature estimates. The c. 800 °C temperatures near Whitehall are consistent with preservation of pre‐granulite contact temperatures adjacent to anorthosite. The preservation of peak metamorphic temperatures in garnet of all sizes is consistent with slow oxygen diffusion in garnet, and closure temperatures of at least 730 °C. Peak metamorphic fractionations are preserved in rocks with varying quartz:feldspar ratios, indicating that the modal percentage of feldspar does not affect retrograde oxygen exchange in these rocks. The lack of this correlation suggests slow rates of oxygen diffusion in quartz and feldspar, consistent with the results of anhydrous oxygen diffusion experiments.     

Stable isotopic signatures of superposed fluid events in granulite facies marbles of the Rauer Group, East Antarctica

The role of volatiles in the stabilization of the lower (granulite facies) crust is contentious. Opposing models invoke infiltration of CO₂-rich fluids or generally vapour-absent conditions during granulite facies metamorphism. Stable isotope and petrological studies of granulite facies metacarbonates can provide constraints on these models. In this study data are presented from metre-scale forsteritic marble boudins within Archaean intermediate to felsic orthogneisses from the Rauer Group, East Antarctica. Forsteritic marble layers and associated calcsilicates preserve a range of ¹³C- and ¹⁸O-depleted calcite isotope values (δ¹³C= -9.9 to -3.0% PDB, δ¹⁸O = 4.0 to 12.1% SMOW). A coupled trend of ¹³C and ¹⁸O depletion (～2%, ～5%, respectively) from core to rim across one marble layer is inconsistent with pervasive CO₂ infiltration during granulite facies metamorphism, but does indicate localized fluid-rock interaction. At another locality, more pervasive fluid infiltration has resulted in calcite having uniformly low, carbonatite-like δ¹⁸O and δ¹³C values. A favoured mechanism for the low δ¹⁸O and δ¹³C values of the marbles is infiltration by fluids that were derived from, or equilibrated with, a magmatic source. It is likely that this fluid-rock interaction occurred prior to high-grade metamorphism; other fluid-rock histories are not, however, ruled out by the available data. Coupled trends of ¹³C and ¹⁸O depletion are modified to even lower values by the superposed development of small-scale metasomatic reaction zones between marbles and internally folded mafic (?) interlayers. The timing of development of these layers is uncertain, but may be related to Archaean high-temperature (>1000d?C) granulite facies metamorphism.     

Olivine‐bearing symplectites in fractured garnet from eclogite,Moldanubian Zone (Bohemian Massif) – a short‐lived,granulite facies event

Recent petrological studies on high‐pressure (HP)–ultrahigh‐pressure (UHP) metamorphic rocks in the Moldanubian Zone, mainly utilizing compositional zoning and solid phase inclusions in garnet from a variety of lithologies, have established a prograde history involving subduction and subsequent granulite facies metamorphism during the Variscan Orogeny. Two temporally separate metamorphic events are developed rather than a single P–T loop for the HP–UHP metamorphism and amphibolite–granulite facies overprint in the Moldanubian Zone. Here further evidence is presented that the granulite facies metamorphism occurred after the HP–UHP rocks had been exhumed to different levels of the middle or upper crust. A medium‐temperature eclogite that is part of a series of tectonic blocks and lenses within migmatites contains a well‐preserved eclogite facies assemblage with omphacite and prograde zoned garnet. Omphacite is partly replaced by a symplectite of diopside + plagioclase + amphibole. Garnet and omphacite equilibria and pseudosection calculations indicate that the HP metamorphism occurred at relatively low temperature conditions of ~600 °C at 2.0–2.2 GPa. The striking feature of the rocks is the presence of garnet porphyroblasts with veins filled by a granulite facies assemblage of olivine, spinel and Ca‐rich plagioclase. These minerals occur as a symplectite forming symmetric zones, a central zone rich in olivine that is separated from the host garnet by two marginal zones consisting of plagioclase with small amounts of spinel. Mineral textures in the veins show that they were first filled mostly by calcic amphibole, which was later transformed into granulite facies assemblages. The olivine‐spinel equilibria and pseudosection calculations indicate temperatures of ~850–900 °C at pressure below 0.7 GPa. The preservation of eclogite facies assemblages implies that the granulite facies overprint was a short‐lived process. The new results point to a geodynamic model where HP–UHP rocks are exhumed to amphibolite facies conditions with subsequent granulite facies heating by mantle‐derived magma in the middle and upper crust.     

Relict sapphirine + kyanite and spinel + kyanite associations in pyropic garnet from the eastern Sør Rondane Mountains,East Antarctica

Sapphirine, spinel, orthoamphibole ± quartz and kyanite are included in porphyroblastic garnet in biotitic gneiss enclosed in a lens of metamorphosed ultramafic rocks in the Cambrian granulite-facies metamorphic complex of the eastern Sør Rondane Mountains, Queen Maud Land, East Antarctica. A bulk analysis of the biotitic gneiss reveals features characteristic both of ultramafic rocks, e.g., high contents of Cr and Ni, and of metasomatism associated with fluids having a crustal source, e.g., relatively elevated contents of Li, Rb, Mo, Cs, Ba, Tl, and Pb. This trace element signature is consistent with the biotitic gneiss being a slice of blackwall skarn that developed between harzburgite and the enclosing biotite–hornblende ± garnet ± orthopyroxene ± clinopyroxene gneiss and was subsequently infolded or inserted by faulting. The matrix assemblage of the biotitic gneiss is garnet + corundum + hercynite + biotite +plagioclase +allanite + zircon. The included associations (all with biotite and rutile) are (1) sapphirine + kyanite, (2) spinel + kyanite, (3) sapphirine +spinel, (4) kyanite, and (5) orthoamphibole + plagioclase ± quartz. The garnet porphyroblasts are compositionally zoned with broad pyropic cores (X_Mg(=Mg / (total Fe + Mg)) = 0.45–0.55) surrounded by Fe-richer rims (X_Mg ≈ 0.3 at the outermost part). The garnet cores preserve compositions homogenized under peak conditions of the granulite-facies metamorphism (760–800 °C and 7–8 kb), whereas the Fe-enriched rims are attributed to an amphibolite-facies overprint at 500–600 °C. Theoretical calculations of garnet + corundum + spinel ± sapphirine + kyanite equilibria in the FMAS system constrain possible P–T conditions for a sapphirine + spinel + kyanite + garnet (X_Mg ≈ 0.55) assemblage to form near 450 °C and 4 kb on the prograde path. In contrast, a modified calibration of the Das et al. (Das, K., Fujino, K., Tomioka, N., Miura, H., 2006. Experimental data on Fe and Mg partitioning between coexisting sapphirine and spinel: an empirical geothermometer and its application. Eur. J. Mineral., 18, 49–58). sapphirine–spinel thermometer gives 860–895 °C for the included associations; pressures would have to be at least 12 kb to stabilize kyanite at these temperatures. Neither estimate is satisfactory and the stability range of kyanite + spinel–hercynite ± sapphirine assemblages remains an unresolved question. The Sør Rondane Mountains constitute the third region for kyanite, sapphirine and spinel–hercynite inclusions in garnet in granulite-facies rocks of the Neoproterozoic–Cambrian orogen extending from the Sør Rondane Mountains to Lützow–Holm Bay and onward to Sri Lanka, southern India and southern Madagascar, and thus determining the stability range of kyanite + spinel–hercynite ± sapphirine is critical for deducing the tectonic evolution of this orogen.     

Remnants of premetamorphic fluid and oxygen isotopic signatures in eclogites and garnet clinopyroxenite from the Dabie-Sulu terranes, eastern China

A combined oxygen‐isotope and fluid‐inclusion study has been carried out on high‐ and ultrahigh‐pressure metamorphic (HP/UHPM) eclogites and garnet clinopyroxenite from the Dabie‐Sulu terranes in eastern China. Coesite‐bearing eclogites/garnet clinopyroxenite and quartz eclogites have a wide range in whole‐rock δ¹⁸O_VSMOW, from 0 to 11‰. The high‐T oxygen‐isotope fractionations preserved between quartz and garnet preclude significant retrograde isotope exchange during exhumation, and the wide range in whole‐rock oxygen‐isotope composition is thought to be a presubduction signature of the precursors. Aqueous fluids with variable salinities and gas species (N₂‐, CO₂‐, or CH₄‐rich), are trapped as primary inclusions in garnet, omphacite and epidote, and in quartz blebs enclosed within eclogitic minerals. In high‐δ¹⁸O HP/UHPM rocks from Hujialin and Shima, high‐salinity brine and/or N₂ inclusions occur in garnet porphyroblasts, which also contain inclusions of coesite, Cl‐rich blue amphibole and dolomite. In contrast, in low‐δ¹⁸O eclogites from Qinglongshan and Huangzhen, the Cl concentrations in amphibole are very low, < 0.2 wt.%, and low‐salinity aqueous inclusions occur in quartz inclusions in epidote porphyroblasts and in epidote cores. These low‐salinity fluid inclusions are believed to be remnants of meteoric water, although the fluid composition was modified during pre‐ and syn‐peak HP/UHPM. Eclogites at Houshuichegou and Hetang contain CH₄‐rich fluid inclusions, coexisting with high‐salinity brine inclusions. Methane was probably formed under the influence of CO₂‐rich aqueous fluids during serpentinisation of mantle‐derived peridotites prior to or during plate subduction. Remnants of premetamorphic low‐ to high‐salinity aqueous fluid with minor N₂ and/or other gas species preserved in the Dabie‐Sulu HP/UHPM eclogites and garnet clinopyroxenite indicate a great diversity of initial fluid composition in the precursors, implying very limited fluid–rock interaction during syn‐ and post‐peak HP/UHPM.     

Garnet re‐equilibration by coupled dissolution–reprecipitation: evidence from textural,major element and oxygen isotope zoning of ‘cloudy’ garnet

The analysis of texture, major element and oxygen isotope compositions of cloudy garnet crystals from a metapelite sampled on Ikaria Island (Greece) is used to assess the model of growth and re‐equilibration of these garnet crystals and to reconstruct the pressure–temperature–fluid history of the sample. Garnet crystals show complex textural and chemical zoning. Garnet cores (100–200 μm) are devoid of fluid inclusions. They are characterized by growth zoning demonstrated by a bell‐shaped profile of spessartine component (7–3 mol.%), an increase in grossular from 14 to 22 mol.% and δ¹⁸O values between 9.5 ± 0.3‰ and 10.4 ± 0.2‰. Garnet inner rims (90–130 μm) are fluid inclusion‐rich and show a decreasing grossular component from 22 to 5 mol.%. The trend of the spessartine component observed in the inner rim allows two domains to be distinguished. In contrast to domain I, where the spessartine content shows the same trend as in the core, the spessartine content of domain II increases outwards from 2 to 14 mol.%. The δ¹⁸O values decrease towards the margins of the crystals to a lowest value of 7.4 ± 0.2‰. The outer rims (<10 μm) are devoid of fluid inclusions and have the same chemical composition as the outermost part of domain II of the inner rim. Garnet crystals underwent a four‐stage history. Stage 1: garnet growth during the prograde path in a closed system for oxygen. Garnet cores are remnants of this growth stage. Stage 2: garnet re‐equilibration by coupled dissolution–reprecipitation at the temperature peak (630 < T < 650 °C). This causes the creation of porosity as the coupled dissolution–reprecipitation process allows chemical (Ca) and isotopic (O) exchange between garnet inner rims and the matrix. The formation of the outer rim is related to the closure of porosity. Stage 3: garnet mode decreases during the early retrograde path, but garnet is still a stable phase. The resulting garnet composition is characterized by an increasing Mn content in the inner rim’s domain II caused by intracrystalline diffusion. Stage 4: dissolution of garnet during the late retrograde path as garnet is not a stable phase anymore. This last stage forms corroded garnet. This study shows that coupled dissolution–reprecipitation is a possible re‐equilibration process for garnet in metamorphic rocks and that intra‐mineral porosity is an efficient pathway for chemical and isotopic exchange between garnet and the matrix, even for otherwise slow diffusing elements.     

Epitaxial quartz inclusions in corundum from a sapphirine–garnet boudin,Bamble Sector,SE Norway: SiO2–Al2O3 miscibility at high P–T dry granulite facies conditions

An Al‐rich, SiO₂‐deficient sapphirine–garnet‐bearing rock occurs as a metapelitic boudin within granulite facies Proterozoic charnockitic gneisses and migmatites on the island of Hisøy, Bamble Sector, SE Norway. The boudin is made up of peraluminous sapphirine, garnet, corundum, spinel, orthopyroxene, sillimanite, cordierite, staurolite and biotite in a variety of assemblages. Thermobarometric calculations based on coexisting sapphirine–spinel, garnet–corundum–spinel–sillimanite, sapphirine–orthopyroxene, and garnet–orthopyroxene indicate peak‐metamorphic conditions near to 930 °C at 10 kbar. Corundum occurs as single 200 to 3000 micron sized skeletal crystal intergrowths in cores of optically continuous pristine garnet porphyroblasts. Quartz occurs as 5–60 micron‐sized euhedral to lobate inclusions in the corundum where it is in direct contact with the corundum with no evidence of a reaction texture. Some crystal inclusions exhibit growth zoning, which indicates that textural equilibrium was achieved. Electron Back‐Scatter Diffraction (EBSD) studies reveal that the quartz inclusions share a common c‐axis with the host corundum crystal. The origin of the quartz inclusions in corundum is enigmatic as recent experimental studies have confirmed the instability of quartz–corundum over geologically realistic P–T ranges. The combined EBSD and textural observations suggest the presence of a former silica‐bearing proto‐corundum, which underwent exsolution during post‐peak‐metamorphic uplift and cooling. Exsolution of quartz in corundum is probably confined to fluid‐absent conditions where phase transitions by coupled dissolution–precipitation mechanisms are prevented.     

Crustal contamination and fluid/rock interaction in the carbonatites of Fuerteventura (Canary Islands, Spain): a C, O, H isotope study

Fuerteventura—the second largest of the Canary Islands consists of Mesozoic sediments, submarine volcanic rocks, dike swarms and plutons of the Basal Complex, and younger subaerial basaltic and trachytic series. Carbonatites are found in two Basal Complex exposures: the Betancuria Massif in the central part of the island and the Esquinzo area in the north. values of the carbonatites increase progressively from south to north of the island. This phenomenon is attributed to different degrees of assimilation of sedimentary carbonate. Homogeneous, typically magmatic values for carbonatites which have preserved primary igneous textures and minerals suggest a well-mixed reservoir where changes in values result from the storage of carbonate magmas at different structural levels. The magma storage allowed assimilation of sediment to varying degrees before final emplacement of carbonatites. Shifts in towards more positive and negative values from presumed primary compositions are observed in the carbonatites. On the basis of the oxygen isotope compositions of calcite, mica and K-feldspar, and the hydrogen isotope compositions of micas, the changes in the values of the carbonatites can be related to fluid/rock interactions.     

In Situ Oxygen Isotope Determination in Serpentine Minerals by SIMS: Addressing Matrix Effects and Providing New Insights on Serpentinisation at Hole BA1B (Samail ophiolite,Oman)

The ability to constrain the petrogenesis of multiple serpentine generations recorded at the microscale is crucial for estimating the extent and conditions of modern versus fossil serpentinisation in ophiolites. To address matrix bias effects during oxygen isotope analysis by SIMS, we present the first investigation analysing antigorite in the compositional range Mg# = 77.5–99.5 mole %, using a CAMECA IMS‐1280 secondary ion mass spectrometer. Spot‐to‐spot homogeneity is ≤ 0.5‰ (2s) for the new antigorite reference materials. The relative bias between antigorite reference materials with different Mg/Fe ratios is described by a second‐order polynomial, and a maximum difference in bias of ~ 1.8‰ was measured for Mg# ~ 78 to 100. We observed a bias up to ~ 1.0‰ between lizardite and antigorite attributed to their different crystal structures. Orientation effects up to ~ 1‰ were observed in chrysotile. The new analytical protocol allowed the identification of oxygen isotope zoning up to ~ 7‰ in serpentine minerals from two serpentinites recovered from an area of active serpentinisation in the Samail ophiolite. Thus, in situ analysis is capable of resolving isotopic heterogeneity that may directly reflect changes in the physical and chemical conditions of multiple serpentinisation events in the Samail ophiolite.     

Intragrain oxygen isotope zoning in titanite by SIMS: Cooling rates and fluid infiltration along the Carthage‐Colton Mylonite Zone,Adirondack Mountains,NY, USA

Oxygen isotopes are an attractive target for zoning studies because of the ubiquity of oxygen‐bearing minerals and the dependence of mineral ¹⁸O/¹⁶O ratios on temperature and fluid composition. In this study, subtle intragrain oxygen isotope zoning in titanite is resolved at the 10‐μm scale by secondary ion mass spectrometry. The patterns of δ¹⁸O zoning differ depending on microstructural context of individual grains and reflect multiple processes, including diffusive oxygen exchange, partial recrystallization, grain‐size reduction, and grain growth. Using the chronological framework provided by structural relations, these processes can be related to specific events during the Grenville orogeny. Titanite was sampled from two outcrops within the Carthage‐Colton Mylonite Zone (CCMZ), a long‐lived shear zone that ultimately accommodated exhumation of the Adirondack Highlands from beneath the Adirondack Lowlands during the Ottawan phase (1090–1020 Ma) of the Grenville orogeny. Titanite is hosted in the Diana metasyenite complex, which preserves three sequentially developed fabrics: an early NW‐dipping protomylonitic fabric (S₁) is crosscut by near‐vertical ultramylonitic shear zones (S₂), which are locally reoriented by a NNW‐dipping mylonitic fabric (S₃). Texturally early titanite (pre‐S₂) shows diffusion‐dominated δ¹⁸O zoning that records cooling from peak Ottawan, granulite‐facies conditions. Numerical diffusion models in the program Fast Grain Boundary yield good fits to observed δ¹⁸O profiles for cooling rates of 50 ± 20 °C Ma^?1, which are considerably faster than the 1–5 °C Ma^?1 cooling rates previously inferred for the Adirondack Highlands from regional thermochronology. High cooling rates are consistent with an episode of rapid shearing and exhumation along the CCMZ during gravitational collapse of the Ottawan orogen at c. 1050 Ma. Texturally later titanite (syn‐S₂) has higher overall δ¹⁸O and shows a transition from diffusion‐dominated to recrystallization‐dominated δ¹⁸O zoning, indicating infiltration of elevated‐δ¹⁸O fluids along S₂ shear zones and continued shearing below the blocking temperature for oxygen (~≤500 °C for grain sizes at the study site). The texturally latest titanite (post‐S₃) has growth‐dominated δ¹⁸O zoning, consistent with porphyroblastic grain growth following cessation of shearing along the Harrisville segment of the CCMZ.     

Metamorphic petrology of a high‐T/low‐P granulite terrane (Damara belt,Namibia) – Constraints from pseudosection modelling and high‐precision Lu–Hf garnet‐whole rock dating

Migmatites comprise a minor volume of the high‐grade part of the Damara orogen of Namibia that is dominated by granite complexes and intercalated metasedimentary units. Migmatites of the Southern Central Zone of the Damara orogen consist of melanosomes with garnet+cordierite+biotite+K‐feldspar, and leucosomes, which are sometimes garnet‐ and cordierite‐bearing. Field evidence, petrographic observations, and pseudosection modelling suggest that, in contrast to other areas where intrusion of granitic magmas is more important, in situ partial melting of metasedimentary units was the main migmatite generation processes. Pseudosection modelling and thermobarometric calculations consistently indicate that the peak‐metamorphic grade throughout the area is in the granulite facies (~5 kbar at ~800°C). Cordierite coronas around garnet suggest some decompression from peak‐metamorphic conditions and rare andalusite records late, near‐isobaric cooling to <650°C at low pressures of ~3 kbar. The inferred clockwise P–T path is consistent with minor crustal thickening through continent–continent collision followed by limited post‐collisional exhumation and suggests that the granulite facies terrane of the Southern Central Zone of the Damara orogen formed initially in a metamorphic field gradient of ~35–40°C/km at medium pressures. New high‐precision Lu–Hf garnet‐whole rock dates are 530 ± 13 Ma, 522.0 ± 0.8 Ma, 520.8 ± 3.6 Ma, and 500.3 ± 4.3 Ma for the migmatites that record temperatures of ~800°C. This indicates that high‐grade metamorphism lasted for c. 20–30 Ma, which is compatible with previous estimates using Sm–Nd garnet‐whole rock systematics. In previous studies on Damara orogen migmatites where both Sm–Nd and Lu–Hf chronometers have been applied, the dates (c. 520–510 Ma) agree within their small uncertainties (0.6–0.8% for Sm–Nd and 0.1–0.2% for Lu–Hf). This implies rapid cooling after high‐grade conditions and, by implication, rapid exhumation at that time. The cause of the high geothermal gradient inferred from the metamorphic conditions is unknown but likely requires some extra heat that was probably added by intrusion of magmas from the lithospheric mantle, i.e., syenites that have been recently re‐dated at c. 545 Ma. Some granites derived from the lower crust at c. 545 Ma are the outcome rather than the cause of high‐T metamorphism. In addition, high contents of heat‐producing elements K, Th, and U may have raised peak temperatures by 150–200°C at the base of the crust, resulting in the widespread melting of fertile crustal rocks. The continuous gradation from centimetre‐scale leucosomes to decametre‐scale leucogranite sheets within the high‐grade metamorphic zone suggests that leucosome lenses coalesced to form larger bodies of anatectic leucogranites, thereby documenting a link between high‐grade regional metamorphism and Pan‐African magmatism. In view of the close association of the studied high‐T migmatites with hundreds of synmetamorphic high‐T granites that invaded the terrane as metre‐ to decametre‐wide sills and dykes, we postulate that crystallization of felsic lower crustal magma is, at least partly, responsible for heat supply. Late‐stage isobaric cooling of these granites may explain the occurrence of andalusite in some samples.     

Interdependence of deformation, fluid infiltration and reaction progress recorded in eclogitic metagranitoids (Sesia Zone, Western Alps)

The effects of high-strain deformation and fluid infiltration during Alpine eclogite facies metamorphism have been studied across ductile shear zones in relatively undeformed metagranitoids at Monte Mucrone (Sesia Zone, Western Alps, Italy). Microfabrics together with bulk rock and stable isotope data indicate that the mineralogical and chemical variations are related to the degree of deformation, rather than to changes in P-T conditions or tectonic position. Transformation of meta-quartz diorite to recrystallized eclogitic mylonites involved the breakdown of biotite and plagioclase and required the influx of H₂O. Bulk-rock geochemical data show that ductile deformation to form eclogitic mylonites involved an increase in volume with a weight percent gain in H₂O and Si and variable loss of K, Na, Ca and Al. δ¹⁸O changes systematically across ductile shear zones into the undeformed country rocks. Constant values in shear zone centres indicate advection parallel to the shear zone and within 10 cm of the mylonites. A dominant component of diffusive oxygen exchange perpendicular to the shear zones produced isotopic fronts, evident from a gradual increase in δ¹⁸O values to the reference values of the country rocks. The degree of isotopic shift within the shear zones reflects increasing deformation and degree of reaction progress. Multiple phases of Alpine deformation and mineral growth are recognized in the Monte Mucrone metagranitoids, and in some cases, eclogite facies shear zones were reactivated under greenschist facies conditions. The results of this study suggest that high-strain deformation provided pathways for both synkinematic and post-kinematic metamorphic fluids which were necessary for complete reactions. Relict igneous fabrics, as well as the presence of corona textures around biotite and pseudomorphs after primary igneous plagioclase in the least deformed rocks, indicate a paucity of hydrous fluids and support the conclusion that fluid movement was channelled rather than pervasive.     

Oriented growth of garnet by topotactic reactions and epitaxy in high‐pressure,mafic garnet granulite formed by dehydration melting of metastable hornblende‐gabbronorite (Jijal Complex,Kohistan Complex,north Pakistan)

Garnet growth in high‐pressure, mafic garnet granulites formed by dehydration melting of hornblende‐gabbronorite protoliths in the Jijal complex (Kohistan palaeo‐island arc complex, north Pakistan) was investigated through a microstructural EBSD‐SEM and HRTEM study. Composite samples preserve a sharp transition in which the low‐pressure precursor is replaced by garnet through a millimetre‐sized reaction front. A magmatic foliation in the gabbronorite is defined by mafic‐rich layering, with an associated magmatic lineation defined by the shape‐preferred orientation (SPO) of mafic clusters composed of orthopyroxene (Opx), clinopyroxene (Cpx), amphibole (Amp) and oxides. The shape of the reaction front is convoluted and oblique to the magmatic layering. Opx, Amp and, to a lesser extent, Cpx show a strong lattice‐preferred orientation (LPO) characterized by an alignment of [001] axes parallel to the magmatic lineation in the precursor hornblende‐gabbronorite. Product garnet (Grt) also displays a strong LPO. Two of the four 〈111〉 axes are within the magmatic foliation plane and the density maximum is subparallel to the precursor magmatic lineation. The crystallographic relationship 〈111〉_Grt // [001]_Opx,Cpx,Amp deduced from the LPO was confirmed by TEM observations. The sharp and discontinuous modal and compositional variations observed at the reaction front attest to the kinetic inhibition of prograde solid‐state reactions predicted by equilibrium‐phase diagrams. The P–T field for the equilibration of Jijal garnet granulites shows that the reaction affinities are 5–10 kJ mol.^?1 for the Grt‐in reaction and 0–5 kJ mol.^?1 for the Opx‐out reaction. Petrographic and textural observations indicate that garnet first nucleated on amphibole at the rims of mafic clusters; this topotactic replacement resulted in a strong LPO of garnet. Once the amphibole was consumed in the reaction, the parallelism of [001] axes of the mafic‐phase reactants favoured the growth of garnet crystals with similar orientations over a pyroxene substrate. These aggregates eventually sintered into single‐crystal garnet. In the absence of deformation, the orientation of mafic precursor phases conditioned the nucleation site and the crystallographic orientation of garnet because of topotaxial transformation reactions and homoepitaxial growth of garnet during the formation of high‐pressure, mafic garnet‐granulite after low‐pressure mafic protoliths.     

新疆东天山玉海铜(钼)矿床流体包裹体和稳定同位素研究

玉海铜(钼)矿床成矿岩体为石英闪长(玢)岩,矿化呈细脉状、细脉-浸染状和稀疏浸染状。围岩蚀变主要为钾硅酸盐化、石英-绢云母化、青磐岩化和黏土化蚀变。矿床类型为斑岩型。铜(钼)矿化主要发育于钾硅酸盐化阶段、石英-绢云母化阶段和青磐岩化阶段。流体包裹体可划分为气液两相包裹体、含子晶三相包裹体和CO_2包裹体3种类型。钾硅酸盐化阶段的均一温度为307~423℃,盐度w(NaCleq)为4.18%~10.11%,密度0.62~0.77g/cm~3,属于高温、中-低盐度流体;石英-绢云母化阶段均一温度为172~336℃,盐度为w(NaCleq)为3.23%~8.55%,密度0.70~0.93 g/cm~3,属于中温、低盐度流体;晚期青磐岩化阶段均一温度155~296℃,盐度w(NaCleq)为3.71%~9.08%,密度0.80~0.96 g/cm~3,属于中低温、低盐度流体。从早阶段到晚阶段,成矿流体温度逐渐下降,各成矿阶段成矿流体盐度均小于11%,但钾硅酸盐化阶段成矿流体盐度稍高。石英-绢云母化阶段成矿流体δD=-91.6‰~-72.1‰,δ~(18)OH_2O=-1.8‰~6.3‰;青磐岩化阶段成矿流体δD=-97.1‰~-68.3‰,δ~(18)OH_2O=-6.3‰~2.2‰;成矿流体具有岩浆水和大气降水混合特征,但青磐岩化阶段大气降水含量更高。硫化物的δ~(34)S值为-3.5‰~2.8‰,硫来自石英闪长(玢)岩。     

Ultrahigh temperature granulite metamorphism (1050 °C, 12 kbar) and decompression in garnet (Mg70)–orthopyroxene–sillimanite gneisses from the Rauer Group, East Antarctica

Highly magnesian and aluminous migmatitic gneisses from Mather Peninsula in the Rauer Group, Eastern Antarctica, preserve ultrahigh temperature (UHT) metamorphic assemblages that include orthopyroxene+sillimanite±quartz, garnet+sillimanite±quartz and garnet+orthopyroxene±sillimanite. Garnet that ranges up to X_Mg of 71.5 coexists with aluminous orthopyroxene that shows zoning from cores with 7.5–8.5 wt% Al₂O₃ to rims with up to 10.6 wt% Al₂O₃ adjacent to garnet. Peak P–T conditions of 1050 °C and 12 kbar are retrieved from Fe–Mg–Al thermobarometry involving garnet and orthopyroxene, in very good agreement with independent constraints from petrogenetic grids in FeO–MgO–Al₂O₃–SiO₂ and related chemical systems. Sapphirine, orthopyroxene and cordierite form extensive symplectites and coronas on the early phases. The specific reaction textures and assemblages involving these secondary phases correlate with initial garnet X_Mg , with apparent higher-pressure reaction products occurring on the more magnesian garnet, and are interpreted to result from an initial phase of ultrahigh temperature near-isothermal decompression (UHT-ITD) from 12 to 8 kbar at temperatures in excess of 950 °C. Later textures that involved biotite formation and then partial breakdown, along with garnet relics, to symplectites of orthopyroxene+cordierite or cordierite+spinel may reflect hydration through back-reaction with crystallizing melts on cooling below 900–850 °C, followed by ITD from 7 to 8 kbar to c. 5 kbar at temperatures of 750–850 °C. The tectonic significance of this P–T history is ambiguous as the Rauer Group records the effects of Archean tectonothermal events as well as high-grade events at 1000 and 530 Ma. Late-stage biotite formation and subsequent ITD can be correlated with the P–T history preserved in the Proterozoic components of the Rauer Group and hence with either 1000 or 530 Ma collisional orogenesis. However, whether the preceding UHT-ITD history reflects a temporally unrelated event (e.g. Archean) or is simply an early stage of either the late-Proterozoic or Pan-African tectonism, as recently deduced for similar UHT rocks from other areas of the East Antarctica, remains uncertain.     

Juvenile CO2 in enderbites of Tromøy near Arendal, southern Norway: a fluid inclusion and stable isotope study

The enderbites from Tromøy in the central, granulite facies part of the Proterozoic Bamble sector of southern Norway contain dominantly CO₂ and N₂ fluid inclusions. CO₂ from fluid inclusions in quartz segregations in enderbites was extracted by mechanical (crushing) and thermal decrepitation and the δ¹³C measured. Measurement was also made on samples washed in 10% HCl, oxidized with CuO at high temperatures, and step-wise extracted with progressive heating. Results between the different techniques are systematic. The main results show δ¹³C of -4.5±1.5% for crushing and -7±2% for thermal decrepitation. δ¹³C is about constant for CO₂ extracted at different temperatures and points to a homogeneous isotopic composition. Due to the presence of carbonate particles and/or induced contaminations for the extraction by thermal decrepitation, the results for the crushing experiments are assumed the most reliable for fluid-inclusion CO₂. Very low values of δ¹³C have not been found in enderbite samples and δ¹³C combined with δ¹⁸O of the host quartzes (8-11%) indicates juvenile values. In addition, the fluid inclusions were examined by microthermometry and Raman analysis and host quartz by acoustic emission and cathodoluminescence. CO₂ fluid inclusions have varying densities with a frequency maximum of 0.92 g cm^-3 and generally do not concur with trapping densities at granulite conditions. Textures show that CO₂ must have been trapped in fluid inclusions in one early event, but transformed to different extents during late isothermal uplift without important fractionation of isotope compositions. The present data support a model of intrusion and crystallization of a CO₂-rich enderbitic magma at granuiite conditions.     

Carbonatite melt–CO2 fluid inclusions in mantle xenoliths from Tenerife, Canary Islands: a story of trapping, immiscibility and fluid–rock interaction in the upper mantle

Three types of fluid inclusions have been identified in olivine porphyroclasts in the spinel harzburgite and lherzolite xenoliths from Tenerife: pure CO₂ (Type A); carbonate-rich CO₂–SO₂ mixtures (Type B); and polyphase inclusions dominated by silicate glass±fluid±sp±silicate±sulfide±carbonate (Type C). Type A inclusions commonly exhibit a “coating” (a few microns thick) consisting of an aggregate of a platy, hydrous Mg–Fe–Si phase, most likely talc, together with very small amounts of halite, dolomite and other phases. Larger crystals (e.g. (Na,K)Cl, dolomite, spinel, sulfide and phlogopite) may be found on either side of the “coating”, towards the wall of the host mineral or towards the inclusion center. These different fluids were formed through the immiscible separations and fluid–wall-rock reactions from a common, volatile-rich, siliceous, alkaline carbonatite melt infiltrating the upper mantle beneath the Tenerife. First, the original siliceous carbonatite melt is separated from a mixed CO₂–H₂O–NaCl fluid and a silicate/silicocarbonatite melt (preserved in Type A inclusions). The reaction of the carbonaceous silicate melt with the wall-rock minerals gave rise to large poikilitic orthopyroxene and clinopyroxene grains, and smaller neoblasts. During the metasomatic processes, the consumption of the silicate part of the melt produced carbonate-enriched Type B CO₂–SO₂ fluids which were trapped in exsolved orthopyroxene porphyroclasts. At the later stages, the interstitial silicate/silicocarbonatite fluids were trapped as Type C inclusions. At a temperature above 650 °C, the mixed CO₂–H₂O–NaCl fluid inside the Type A inclusions were separated into CO₂-rich fluid and H₂O–NaCl brine. At T<650 °C, the residual silicate melt reacted with the host olivine, forming a reaction rim or “coating” along the inclusion walls consisting of talc (or possibly serpentine) together with minute crystals of NaCl, KCl, carbonates and sulfides, leaving a residual CO₂ fluid. The homogenization temperatures of +2 to +25 °C obtained from the Type A CO₂ inclusions reflect the densities of the residual CO₂ after its reactions with the olivine host, and are unrelated to the initial fluid density or the external pressure at the time of trapping. The latter are restricted by the estimated crystallization temperatures of 1000–1200 °C, and the spinel lherzolite phase assemblage of the xenolith, which is 0.7–1.7 GPa.     

Assessing the geochemical and tectonic impacts of fluid–rock interaction in mid‐crustal shear zones: a case study from the intracontinental Alice Springs Orogen,central Australia

The Reynolds–Anmatjira Ranges, central Australia, form part of a high‐grade basement terrane dissected by intensely metasomatized transpressional shear zones active during the Ordovician–Carboniferous Alice Springs Orogeny. Unlike typical retrograde structures associated with discrete fluid flow, the mid‐crustal setting and intracontinental nature of these shear systems present significant problems for the source and ingress mechanism of the fluid involved in their rehydration. To address these issues, we describe two detailed traverses across deformed and metasomatized basement rocks in this region, and interrogate their record of fluid–rock interaction from various perspectives. Both traverses combine structural and petrological observations with Zr‐in‐rutile and Ti‐in‐quartz thermobarometry, oxygen and hydrogen stable isotope analysis, and major, trace and rare earth element mobility trends. Each technique is critically evaluated for its utility in this study and its more widespread applicability to alternative field areas, providing a strategic framework for the general investigation of fluid‐affected shear zones. Ultimately, the integrated data sets specify pressure–temperature conditions of ～530 °C and 4–5 kbar, implying average apparent thermal gradients of 29–36 °C km^?1 and depths of 14–18 km. Other characteristic features to emerge include strongly variable element mobilities and pronounced isotopic depletion fronts consistent with the alteration effects of an externally derived, non‐equilibrium fluid. This is confirmed by calculated fluid compositions indicative of contributions from a fluid of meteoric origin, with estimated δ¹⁸O and δD values as low as 2.3‰ and ?59.8‰, respectively. We propose that these surficial fluid signatures are imposed on the mid‐crust by the prograde burial and dehydration of hydrothermally altered fault panels produced during pre‐orogenic basin formation. Progressive fluid release with continued subsidence then leads to the accumulation of increasing fluid volumes in the vicinity of the brittle–ductile transition, promoting extensive hydration, metasomatism and reaction softening at the locus of stress transmission from plate‐boundary sources. The sustained injection of externally derived fluids into refractory crustal material may thus stimulate a critical reduction in the long‐term strength of the lithosphere, providing strong impetus for the initiation and advancement of intracontinental orogenesis.