Metasomatism Research Papers - Academia.edu (original) (raw)

The igneous rocks of the Katzenbuckel, Southwest Germany, represent a unique and unusual alkaline to peralkaline association within the European Volcanic Province. The magmatic activity can be subdivided into two main phases. Phase I... more

The igneous rocks of the Katzenbuckel, Southwest Germany, represent a unique and unusual alkaline to peralkaline association within the European Volcanic Province. The magmatic activity can be subdivided into two main phases. Phase I comprises the main rock bodies of phonolite and nepheline syenite, which were later intruded by different peralkaline dyke rocks (tinguaites and alkali feldspar syenite dykes) of phase II. The dyke assemblage was accompanied by magnetite and apatite veins and was followed by a late-stage pneumatolytic activity causing autometasomatic alterations.As is typical for alkaline to peralkaline igneous rocks, early mafic minerals of phase I rocks comprise olivine, augite and Fe–Ti oxides, which are substituted in the course of fractionation by Na-amphibole and Na-pyroxene. For the early magmatic stage, calculated temperatures range between 880 and 780 °C with low silica activities (0.4 to 0.6) but high relative oxygen fugacities between 0.5 and 1.9 log units above the FMQ buffer. Even higher oxygen fugacities (above the HM buffer) are indicated for the autometasomatic alteration, which occurred at temperatures between 585 and 780 °C and resulted in the formation of pseudobrookite and hematite.The unusually high oxygen fugacities (even during the early magmatic stage) are recorded by the major element compositions of the mafic minerals (forsterite content in olivine between 68 and 78 mol%, up to 6.2 wt.% ZrO2 and 8.5 wt.% TiO2 in clinopyroxene), the unusual mineral assemblages (pseudobrookite, freudenbergite) and by the enrichment of Fe3+ in the felsic minerals (up to 2.8 wt.% Fe2O3 in alkali feldspar and up to 2.6 wt.% Fe2O3 in nepheline). These observations point to a metasomatically enriched and highly oxidized lithospheric mantle as a major source for the Katzenbuckel melts.

The Kletno deposit in the Śnieżnik Massif (Central Sudetes, SW Poland), mined for Fe, U, Ag, Cu, fluorite, and marble through the ages, developed at the contact of marbles and orthogneiss. Here, we present a new Fe-Ti-V-ore (containing up... more

The Kletno deposit in the Śnieżnik Massif (Central Sudetes, SW Poland), mined for Fe, U, Ag, Cu, fluorite, and marble through the ages, developed at the contact of marbles and orthogneiss. Here, we present a new Fe-Ti-V-ore (containing up to 14.07 wt.% Fe, 2.05 wt.% Ti, and 2055 ppm V in bulk rock) and ornamental- to gem-quality talc prospect at the southwest margin of this deposit. This newly documented Fe-Ti-V mineralization is hosted in hornblendites, dolomite veins, and chlorite schists, which, along with talc, envelopes a tectonic slice of serpentinite. Hornblendites are interpreted as metamorphosed ferrogabbros, derived from the same mafic melts as adjacent barren metagabbros. The oxygen and carbon isotope compositions of metabasites and dolomite veins (amphibole δ18O values = 8.8-9.3‰; carbonate δ18O values = 12.8-16.0‰, and δ13C values = −8.3‰ to −7.2‰), in combination with those of the country marbles (carbonate δ18O and δ13C values = 23.2‰ and +0.1‰, respectively), suggest that mineralization-bearing hornblendites formed due to interaction of the mafic magma with CO2 released during the decarbonation of the sediments. The CO2-bearing fluid interaction with gabbros likely caused carbonation of the gabbros and formation of the dolomite veins, whereas talc formed due to Si-rich fluids, possibly derived from a mafic intrusion, interaction with serpentinite, or due to the metasomatism of the serpentinite-gabbro assemblage. Moreover, fluids leaching Fe and Ti from the adjacent sediments can mix with the mafic magma causing enrichment of the magma in Fe and Ti. Consequently, the mineralization-bearing ferrogabbros became even more enriched in Fe and Ti, which can be linked with the formation of Fe-Ti-V ore bodies.

The Mesoarchean Nuasahi chromite deposits of the Singhbhum Craton in eastern India consist of a lower chromite-bearing ultramafic unit and an upper magnetite-bearing gabbroic unit. The ultramafic unit is a ∼5 km long and ∼400 m wide... more

The Mesoarchean Nuasahi chromite deposits of the Singhbhum Craton in eastern India consist of a lower chromite-bearing ultramafic unit and an upper magnetite-bearing gabbroic unit. The ultramafic unit is a ∼5 km long and ∼400 m wide linear belt trending NNW-SSE with a general north-easterly dip. The chromitite ore bodies are hosted in the dunite that is flanked by the orthopyroxenite. The rocks of the ultramafic unit including the chromitite crystallized from a primitive boninitic magma, whereas the gabbro unit formed from an evolved boninitic magma. A shear zone (10–75 m wide) is present at the upper contact of the ultramafic unit. This shear zone consists of a breccia comprising millimeter- to meter-sized fragments of chromitite and serpentinized rocks of the ultramafic unit enclosed in a pegmatitic and hybridized gabbroic matrix. The shear zone was formed late synkinematically with respect to the main gabbroic intrusion and intruded by a hydrous mafic magma comagmatic with the evolved boninitic magma that formed the gabbro unit. Both sulfide-free and sulfide-bearing zones with platinum group element (PGE) enrichment are present in the breccia zone. The PGE mineralogy in sulfide-rich assemblages is dominated by minerals containing Pd, Pt, Sb, Bi, Te, S, and/or As. Samples from the gabbro unit and the breccia zone have total PGE concentrations ranging from 3 to 116 ppb and 258 to 24,100 ppb, respectively. The sulfide-rich assemblages of the breccia zone are Pd-rich and have Pd/Ir ratios of 13–1,750 and Pd/Pt ratios of 1–73. The PGE-enriched sulfide-bearing assemblages of the breccia zone are characterized by (1) extensive development of secondary hydrous minerals in the altered parts of fragments and in the matrix of the breccia, (2) coarsening of grain size in the altered parts of the chromitite fragments, and (3) extensive alteration of primary chromite to more Fe-rich chromite with inclusions of chlorite, rutile, ilmenite, magnetite, chalcopyrite, and PGE-bearing chalcogenides. Unaltered parts of the massive chromitite fragments from the breccia zone show PGE ratios (Pd/Ir = 2.5) similar to massive chromitite (Pd/Ir = 0.4–6.6) of the ultramafic unit. The Ir-group PGE (IPGE: Ir, Os, Ru) of the sulfide-rich breccia assemblages were contributed from the ultramafic–chromitite breccia. Samples of the gabbro unit have fractionated primitive mantle-normalized patterns, IPGE depletion (Pd/Ir = 24–1,227) and Ni-depletion due to early removal of olivine and chromite from the primitive boninitic magma that formed the ultramafic unit. Samples of the gabbro and the breccia zone have negative Nb, Th, Zr, and Hf anomalies, indicating derivation from a depleted mantle source. The Cu/Pd ratios of the PGE-mineralized samples of the breccia zone (2.0 × 103–3.2 × 103) are lower than mantle (6.2 × 103) suggesting that the parental boninitic magma (Archean high-Mg lava: Cu/Pd ratio ∼1.3 × 103; komatiite: Cu/Pd ratio ∼8 × 103) was sulfur-undersaturated. Samples of the ultramafic unit, gabbro and the mineralized breccia zone, have a narrow range of incompatible trace element ratios indicating a cogenetic relationship. The ultramafic rocks and the gabbros have relatively constant subchondritic Nb/Ta ratios (ultramafic rocks: Nb/Ta = 4.1–8.8; gabbro unit: Nb/Ta = 11.5–13.2), whereas samples of the breccia zone are characterized by highly variable Nb/Ta ratios (Nb/Ta = 2.5–16.6) and show evidence of metasomatism. The enrichment of light rare earth element and mobile incompatible elements in the mineralized samples provides supporting evidence for metasomatism. The interaction of the ultramafic fragments with the evolved fluid-rich mafic magma was key to the formation of the PGE mineralization in the Nuasahi massif.

Subduction is a key component of Earth's long-term sulfur cycle; however, the mechanisms that drive sulfur from subducting slabs remain elusive. Isotopes are a sensitive indicator of the speciation of sulfur in fluids, sulfide... more

Subduction is a key component of Earth's long-term sulfur cycle; however, the mechanisms that drive sulfur from subducting slabs remain elusive. Isotopes are a sensitive indicator of the speciation of sulfur in fluids, sulfide dissolution-precipitation reactions, and inferring fluid sources. To investigate these processes, we report δ 34 S values determined by secondary ion mass spectroscopy in sulfides from a global suite of exhumed high-pressure rocks. Sulfides are classified into two petrogenetic groups: (1) metamorphic, which represent closed-system (re)crystallization from protolith-inherited sulfur, and (2) metasomatic, which formed during open system processes, such as an influx of oxidized sulfur. The δ 34 S values for metamorphic sulfides tend to reflect their precursor compositions: −4.3 ‰ to +13.5 ‰ for metabasic rocks, and −32.4 ‰ to −11.0 ‰ for metasediments. Metasomatic sulfides exhibit a range of δ 34 S from −21.7 ‰ to +13.9 ‰. We suggest that sluggish sulfur self-diffusion prevents isotopic fractionation during sulfide breakdown and that slab fluids inherit the isotopic composition of their source. We estimate a composition of −11 ‰ to +8 ‰ for slab fluids, a significantly smaller range than observed for metasomatic sulfides. Large fractionations during metasomatic sulfide precipitation from sulfate-bearing fluids, and an evolving fluid composition during reactive transport may account for the entire ~36 ‰ range of metasomatic sulfide compositions. Thus, we suggest that sulfates are likely the dominant sulfur species in slab-derived fluids. Plain Language Summary Sulfur is one of the key ingredients for life and drives many biochemical and geochemical reactions in Earth systems. The exchange of sulfur between Earth's exterior and interior during subduction is an important long-term component of the global sulfur cycle. In our study, we use stable isotopes of sulfur as a tracer of sulfur loss and migration from subducting oceanic plates. We demonstrate the utility of sulfur isotopes as a tracer by identifying potential sources of sulfur in the subducting plate. We suggest that the isotopic composition is unaffected by the dissolution of sulfur-bearing minerals and infer that the large compositional range of sulfides formed from fluids expelled from the subducting plate reflect the speciation of sulfur in the fluid. This study represents the first global overview of sulfur isotopes in subducted metamorphic rocks. These data may be compared with sulfur isotope measurements in volcanic arcs overlying subduction zones to trace sulfur from the subducting plate through the overriding plate.

Late Miocene to Quaternary intraplate basaltic volcanism in the Al Ghab and Homs regions, northwestern Syria (the Al Ghab volcanic group and the Homs volcanic group), occurred roughly synchronously with the propagation of the Dead Sea... more

Late Miocene to Quaternary intraplate basaltic volcanism in the Al Ghab and Homs regions, northwestern Syria (the Al Ghab volcanic group and the Homs volcanic group), occurred roughly synchronously with the propagation of the Dead Sea Fault System in these regions. Petrographic evidence as well as major and trace element variations suggest that the basalts have undergone varying amounts of crystal fractionation of olivine ± clinopyroxene ± spinel, with feldspar fractionation only in the most evolved samples. The major and trace element chemistry and Nd–Sr–Pb isotopic variations of the basalts reflect both mantle source heterogeneity and relatively minor crustal contamination. Semi-quantitative assimilation–fractional crystallization modelling suggests that some samples may reflect assimilation of no more than 6% of upper continental crust, probably with Late Proterozoic Arabian Shield characteristics. Amongst the least crustally contaminated and relatively primitive samples, basanites are characterized by marked depletion of K, Rb and Zr relative to other neighbouring incompatible elements on primitive mantle normalized trace element diagrams. This, together with their low SiO2 and high TiO2 and Dy/Yb, is consistent with magma genesis involving a large proportion of garnet-bearing hornblendite or similar amphibole-rich metasomatic veins. Associated alkali and tholeiitic basalts with the higher SiO2, lower TiO2, less negative K, Rb and Zr anomalies, and moderately high Dy/Yb are consistent with melt extraction from a largely peridotitic mantle source. It is suggested that the compositional spectrum from basanite via alkali basalt to tholeiitic basalt can be explained by increasing degrees of metasomatic vein–wall-rock interaction, plus asthenospheric melt assimilation. In agreement with this is the identification of three distinct isotopic and chemical characteristics within the spectrum of mafic lavas, each of which can be referred to a unique mantle source (metasomatic vein, lithospheric wall-rock peridotite mantle and asthenospheric peridotite mantle). A decrease in eruption volume and increase in Si-undersaturation of the lavas from south (Homs) to north (Al Ghab) along the northern Dead Sea Fault System from latest Miocene to Quaternary times suggest a diminishing thermal perturbation and increasing importance of the amphibole-rich veins in magma genesis over time. It is proposed that the genesis of the oldest lavas reflects the arrival of asthenospheric melts beneath the Homs region, which with assimilation of lithospheric metasomatic veins and their wall-rocks produced the parental magmas of the Homs volcanic group. Subsequently, upwelling asthenospheric material could have been channelled northwards at the base of the lithosphere, presumably related to the northward propagation of the Dead Sea Fault System in the Pliocene. Cooling of this channelled asthenospheric material, which did not penetrate the lithosphere during this later period of magma genesis, provided the minimal thermal perturbation necessary for melting of amphibole-rich metasomatic veins and wall-rock peridotite within the lithosphere.

Alteration reactions associated with gold mineralisation can be used to elucidate the nature of the fluid that transported gold into a deposit. At the Junction gold deposit, Kambalda, Western Australia, gold is hosted in a metamorphosed... more

Alteration reactions associated with gold mineralisation can be used to elucidate the nature of the fluid that transported gold into a deposit. At the Junction gold deposit, Kambalda, Western Australia, gold is hosted in a metamorphosed and hydrothermally altered dolerite. Mineralisation at the deposit scale is associated with zones of K, CO2 and S metasomatism, as is common in many greenstone hosted gold deposits. However, at the thin-section scale gold is not closely associated with sulphide minerals but within zones of carbonate metasomatism and K-loss where pre-existing biotite has reacted to produce chlorite, muscovite and Fe–Mg carbonates. Gold precipitation is intimately associated with biotite breakdown where calcite is locally absent. Quantified mineral modes from detailed microstructural mapping are used to balance reactions describing the breakdown of biotite in the presence and absence of calcite. Using the basic assumption that Al is immobile during metasomatism the reactions are successfully balanced, even in a manifestly open system. Modelling of fluid–rock reactions using HCh constrains the fluid composition (0.11 < X(CO2) < 0.13) and fluid–rock ratios (b12:1) that can produce the observed mineral assemblage. Additional modelling of solid solution mineral phases using THERMOCALC estimates alteration conditions of 390 °C, 4 kbar and also suggests a fluid X(CO2) ~ 0.1. Both these models show that the observed muscovite and chlorite compositions can be produced primarily through the removal of K from the measured precursor biotite. We show that it is not possible to transport and deposit all the gold observed in the alteration zone with the low fluid–rock ratios obtained from modelling of silicate alteration and inferred gold concentrations in these fluids. We suggest that this is typical of greenstone hosted gold deposits and that mechanisms other than aqueous solution, which can transport higher gold concentrations, must be considered.

The Olserum mineralisation is situated in the metasediments of the Västervik formation, which repre-sents the southernmost continental margin of the Svecokarelian Domain. After deposition between 1.88 – 1.85 Ga these sediments were... more

The Olserum mineralisation is situated in the metasediments of the Västervik formation, which repre-sents the southernmost continental margin of the Svecokarelian Domain. After deposition between 1.88 – 1.85 Ga these sediments were subsided to a depth of about 10 km and intruded by a series of granitoid melts. The resulting HT/LP metamorphism, with a peak at upper amphibolite facies, transformed the sediments to quartzites, quartzitic gneiss and gneiss. REE mineral-rich magnetite bands and veins occur in the mica-rich quartzite and have been in-terpreted as former heavy mineral-rich alluvial beds that now form placer deposits.
Infiltration of enriched hydrothermal fluids into the Västervik metasediments resulted in assimilation of primary sedimentary minerals, extensive metasomatic alteration and the subsequent reprecipitation of enriched material. At Olserum this process resulted in the formation of REE-bearing, magnetite-rich biotite veins, with monazite and xenotime constituting the main REE-bearing minerals. Subsequent magmatism-associated fluid events resulted in extensive metasomatic alteration, including the enrichment/depletion of REE in monazite/xenotime and the forma-tion of secondary uranium minerals. Electron microprobe analysis suggests that efficient leaching and transport processes, as well as coupled/uncoupled substitution mechanisms, were integral in the alteration of monazite, xeno-time and uraninite.
Textural evidence and chemical analysis suggests a close relationship between REE-bearing minerals and U-Th-minerals at Olserum. The close relationship was reflected in the electron microprobe analysis results in this study, with some monazite and xenotime showing significant U and/or Th concentrations and some U-minerals having substantial REE content.
Based on electron microprobe dating of monazite and xenotime, metasomatic alteration at Olserum is interpreted to have been largely associated with intrusion of granites belonging to the 1.85-1.65 Ga Transscandinavian Igneous Belt (TIB), and in particular the 1.76-1.81 Ga Småland granites. However observations also indicate a later thermal event, interpreted as being possibly associated with the 1.46-1.42 Ga Hallandian orogeny.

The Mount Painter and Mount Babbage Inliers (South Australia) are largely composed of Mesoproterozoic A-type granitoids that intruded marginally older metasediments. Metasomatic activity has had a pronounced influence on the granites in... more

The Mount Painter and Mount Babbage Inliers (South Australia) are largely composed of Mesoproterozoic A-type granitoids that intruded marginally older metasediments. Metasomatic activity has had a pronounced influence on the granites in the southerly Mt Painter Inlier. U–Pb dating and Hf isotope ratios of zircons from granites and hyperaluminous rocks show the latter to be heavily metasomatised equivalents of the granitoids. Similar metasomatic processes are likely to have been responsible for the formation of Fe-oxide–U–REE ores. These ores formed more than 1100 Ma after intrusion of the Mesoproterozoic A-type granites, and elemental remobilisation may have been associated with a new phase of granitoid magmatism around 455 Ma. The ferroan and incompatible element-rich nature of A-type granites makes them a suitable source for ores that can be tapped whenever thermal and fluid conditions are favourable.► A-type granites are prospective sources for mineralisation, even a billion years after emplacement. ► Metasomatism can obscure the identity of igneous rocks beyond recognition. ► Highly resistant minerals like zircons can be used to establish the protolith of metasomatised rocks.

‘Sakenites’ constitute a unique association of corundum-, spinel- and sapphirine-bearing anorthitic to phlogopitic rocks, first described in rocks from an exposure along the beds of the Sakena river to the NW of Ihosy, south Madagascar.... more

‘Sakenites’ constitute a unique association of corundum-, spinel- and sapphirine-bearing anorthitic to phlogopitic rocks, first described in rocks from an exposure along the beds of the Sakena river to the NW of Ihosy, south Madagascar. The exposure has been revisited and subjected to a detailed petrological and geochemical study. The aluminous anorthitic rocks occur as boudinaged bands and lenses, closely associated with corundum-, spinel- and sapphirine-bearing phlogopitites, diverse calcsilicate rocks and marbles within a series of biotite-sillimanite-cordierite gneisses of the Ihosy granulite unit in the NW of the Pan-African Bongolava-Ranotsara shear zone. Bimineralic anorthite + corundum domains preserve the earliest record of a polyphasic evolutionary history that includes two distinct metasomatic episodes. Probable protoliths of these bimineralic rocks were kaolinite-rich sediments or calcareous bauxites that were altered by Ca or Si infiltration-metasomatism prior to or coeval with the development of the anorthite-corundum assemblage. P–T pseudosection modelling of metapelitic gneisses suggests peak-conditions around 800 °C and 6–7 kbar for the regional high-grade metamorphism and deformation in the NW part of the Bongolava-Ranotsara shear zone. The well-annealed granoblastic-polygonal textures indicate complete chemical and textural re-equilibration of the foliated bimineralic rocks during this event. Subsequently, at somewhat lower P–T conditions (750–700 °C, 6 kbar), the influx of Mg-, Si- and K-bearing fluids into the anorthite-corundum rocks caused significant metasomatic changes. In zones infiltrated by ‘primary’ potassic fluids, the bimineralic assemblage was completely replaced by phlogopite and Mg-Al minerals, thereby producing corundum-, spinel- and sapphirine-bearing phlogopitites. Further advance of the resulting ‘residual’ Mg- and Si-bearing fluids into anorthite-corundum domains led to partial to complete replacement of corundum porphyroblasts by spinel, spinel + sapphirine or sapphirine, depending on the activities of the solutes. The static textures developed during this second metasomatic episode suggest fluid influx subsequent to intense ductile deformation in the Bongolava-Ranotsara ductile shear zone c. 530–500 Ma ago.

Abstract The behavior of fluids at the subduction plate interface and their chemical and rheological impacts remain poorly constrained. Based on detailed fieldwork, petrographic and geochemical analyses and thermobarometry, the present... more

Abstract The behavior of fluids at the subduction plate interface and their chemical and rheological impacts remain poorly constrained. Based on detailed fieldwork, petrographic and geochemical analyses and thermobarometry, the present study documents an example of a block-in-matrix ‘melange’ metasomatized along the subduction interface (Kampos-Lia melange zone, Cycladic Blueschists, Syros island). We show that this particular melange zone is a preserved fragment of discontinuous oceanic domain associated with the Pindos basin, which underwent dominant exhumation-related deformation with top to the east shearing. A large part of the ‘melange’ structure is inherited from the initial intraoceanic setting and should not be considered as a tectonic ‘melange’. Through exhaustive, meter-scale mapping of the nature of blocks and matrix we show that metasomatism dominantly occurs at the contact between metavolcanic layers and serpentinite, with diffusion of Ca from the metavolcanics to the matrix and diffusion of Mg in the opposite direction. Most of the metavolcanic layers and blocks (mafic and carbonate) are only partly digested contrary to the ultramafic matrix which has been largely metasomatized and forms a tremolite-chlorite-talc schist, i.e. a ‘hybrid’ rock with intermediate chemical composition. Geochemical data demonstrate that metasomatic element mobilization (e.g. Li, B, U, LILE and REE) varies at the scale of the unit, suggesting a complex evolution of fluid composition at the kilometer scale. We suggest that the fluid which intruded the Kampos subunit was derived from the subducting slab and caused an enrichment in Li, B, U, LILE (Rb, K, Na, Ba, Eu), MREE and HREE. Mineralogical and P-T constraints indicate that the dominant part of metasomatism and rock hybridization occurred during exhumation, at ~ 1 GPa, well after peak burial and detachment from the slab (~ 2 GPa). Due to the absence of major tectonic mixing and the scarce preservation of prograde and peak metasomatic reactions, this metasomatism may be representative of fore-arc hybridization processes at depths of ~ 35 km. Deeper hybridization processes as hypothesized to produce a potential source for arc magmas are not well preserved in the studied examples. However, by changing the mineralogy of the matrix, metasomatism changes the rheological properties of the melange and could thus manipulate the rheology of the subduction interface and influence exhumation processes.

The present article is a result of the study of the altered granitoid rocks cropping out in the northeastern periphery of the Sakar batholith. The investigated rocks are enriched by amphibolite xenoliths and additionally, they are... more

Alkaline igneous rocks host many global high-field-strength element (HFSE) and rare-earth element (REE) deposits. While HFSEs are commonly assumed to be immobile in hydrothermal systems, transport by late-stage hydrothermal fluids... more

Alkaline igneous rocks host many global high-field-strength element (HFSE) and rare-earth element (REE) deposits. While HFSEs are commonly assumed to be immobile in hydrothermal systems, transport by late-stage hydrothermal fluids associated with alkaline magmas is reported. However, the magnitude of the flux and the conditions are poorly constrained and yet essential to understanding the formation of REE-HFSE ores. We examined the alteration of country rocks (“fenitization”) accompanying the emplacement of a syenite magma at Illerfissalik in Greenland, through analysis of changes in rock chemistry, mineralogy, and texture. Our novel geochemical maps show a 400-m-wide intrusion aureole, within which we observed typically tenfold increases in the concentrations of many elements, including HFSEs. Textures suggest both pervasive and structurally hosted fluid flow, with initial reaction occurring with the protolith’s quartz cement, leading to increased permeability and enhancing chemica...

The behavior of fluids at the subduction plate interface and their chemical and rheological impacts remain poorly constrained. Based on detailed fieldwork, petrographic and geochemical analyses and thermobarometry, the present study... more

The behavior of fluids at the subduction plate interface and their chemical and rheological impacts remain poorly constrained. Based on detailed fieldwork, petrographic and geochemical analyses and thermobarometry, the present study documents an example of a block-in-matrix 'mélange' metasomatized along the subduction interface (Kampos-Lia mélange zone, Cycladic Blueschists, Syros island). We show that this particular mélange zone is a preserved fragment of discontinuous oceanic domain associated with the Pindos basin, which underwent dominant exhumation-related deformation with top to the east shearing. A large part of the 'mélange' structure is inherited from the initial intraoceanic setting and should not be considered as a tectonic 'mélange'. Through exhaustive , meter-scale mapping of the nature of blocks and matrix we show that metasomatism dominantly occurs at the contact between metavolcanic layers and serpentinite, with diffusion of Ca from the metavolcanics to the matrix and diffusion of Mg in the opposite direction. Most of the metavolcanic layers and blocks (mafic and carbonate) are only partly digested contrary to the ultramafic matrix which has been largely metasomatized and forms a tremolite-chlorite-talc schist, i.e. a 'hybrid' rock with intermediate chemical composition. Geochem-ical data demonstrate that metasomatic element mobilization (e.g. Li, B, U, LILE and REE) varies at the scale of the unit, suggesting a complex evolution of fluid composition at the kilometer scale. We suggest that the fluid which intruded the Kampos subunit was derived from the subducting slab and caused an enrichment in Li, B, U, LILE (Rb, K, Na, Ba, Eu), MREE and HREE. Mineralogical and P-T constraints indicate that the dominant part of metasoma-tism and rock hybridization occurred during exhumation, at~1 GPa, well after peak burial and detachment from the slab (~2 GPa). Due to the absence of major tectonic mixing and the scarce preservation of prograde and peak metasomatic reactions, this metasomatism may be representative of fore-arc hybridization processes at depths of~35 km. Deeper hybridization processes as hypothesized to produce a potential source for arc magmas are not well preserved in the studied examples. However, by changing the mineralogy of the matrix, metasomatism changes the rheological properties of the mélange and could thus manipulate the rheology of the subduction interface and influence exhumation processes.

Concentrations of boron in whole rocks and minerals of the peralkaline, 1.16 Ga Ilímaussaq intrusion and its granitic country rocks (South Greenland) were analysed using secondary ion mass spectrometry (SIMS) and prompt gamma neutron... more

Concentrations of boron in whole rocks and minerals of the peralkaline, 1.16 Ga Ilímaussaq intrusion and its granitic country rocks (South Greenland) were analysed using secondary ion mass spectrometry (SIMS) and prompt gamma neutron activation (PGNAA) analysis. The intrusion consists of an early augite–syenite shell, a later alkali-granite sheet and still later nepheline syenites, which dominate the Complex. Boron concentrations are high (250–280 μg/g) in all rocks containing fresh sodalite, whereas boron is constantly low in the sodalite-free augite syenites (4–6 μg/g) and in the alkali granites (7–22 μg/g). Rocks with sodalite altered to analcime contain only low amounts of boron (2–7 μg/g), which records boron extraction by late-magmatic fluids.Concentration profiles of B in the analysed minerals (olivine, amphibole, clinopyroxene, aenigmatite, eudialyte, biotite, feldspar, nepheline and sodalite) record magmatic fractionation to various extents, late-magmatic to hydrothermal fluid/rock interaction, and sub-solidus diffusion. Whole-rock concentration data cannot be directly translated into the geochemical evolution of the peralkaline melts, since they are largely affected by cumulate fractionation of sodalite and amphibole and furthermore by late-stage hydrothermal alteration processes resulting in B loss. However, trace-element concentrations of mineral zones representing equilibrium fractionation from magmatic liquids can be used in combination with mineral–melt partition coefficients to unravel the enrichment processes of elements in the melt.Boron isotope values of minerals from the intrusion and the country rocks resemble the trend observed for Li isotopes in an earlier study. Amphibole and feldspar display a clear trend from light boron in the inner nepheline syenitic part of the intrusion (δ11B = −20‰ and −17‰ for amphibole and feldspar, respectively) through intermediate values in the outer augite syenites (δ11B = −10‰ and −6‰ for amphibole and feldspar, respectively) to heavy boron with δ11B = + 2‰ for amphibole and + 4‰ for feldspar in the country rock granites close to the contact with the intrusion. The values are interpreted to reflect the entry of meteoric fluids with heavy B along the intrusive contact.► Fresh-sodalite rocks show high [B], whereas [B] is low in the sodalite-free samples. ► Melt [B] evolution is estimated from mineral [B] combined with partition coefficients. ► Magmatic fractionation, fluid/rock interaction and sub-solidus diffusion are recorded. ► The parental melt's B isotopic composition (−17 ‰) reveals a low-δ11B mantle source. ► Heavy B entered along the intrusive contact via meteoric fluids.

The April 1906 eruption of Vesuvius is the type-example of the “final eruptions” that close the short cycles of semi-persistent activity that characterized the volcano in the 1631–1944 period. The eruption had a marked explosive character... more

The April 1906 eruption of Vesuvius is the type-example of the “final eruptions” that close the short cycles of semi-persistent activity that characterized the volcano in the 1631–1944 period. The eruption had a marked explosive character that accompanied the emission of lava from several vents on the southern slopes of the volcano. The observed sequence of events was characterized by repeated fluctuations of the magma level within the conduit, by large lava fountains, by conduit partial collapses, and by the final explosive decapitation of the summit cone. Contemporary chronicles, although frequently contradictory, allow reconstruction of the eruption, which can be divided into four main phases: (1) lateral lava effusions; (2) lava fountains; (3) gas-pyroclasts column; (4) low dense clouds. Pyroclastic deposits of the Monte Somma ridge and northeastern slope can be related to observed and described events and mainly refer to the 2nd and 3rd phases. The increase in the degree of fragmentation of the juvenile component together with the marked increase of the lithic component and morphologic evidence emphasize the repeated occurrence of magmawater interaction. This was most spectacular in the 3rd phase of the eruption in which, after the decapitation of the cone, a high gas-pyroclasts eruption column was formed. Because of the nature of the lithic fragments (mainly hydrothermally altered and metasomatic rocks), the huge amount of steam, and the high lithic/juvenile ratio, it is unlikely that the largest part of the energy in play was related to the contact between magma and cold phreatic water. We suggest that most of the steam involved in this phase of the eruption came from flashing of the hydrothermal system connected to the very shallow feeding system of the volcano and formed as a consequence of repeated subsurface intrusions between 1872 and 1906. Juvenile products were ejected through the eruption, and represent (at least) two different magma bodies: the first (“older”) was erupted during the initial phase of the eruption and was exhausted at the beginning of the lava fountains phase, when fresh magma was involved in the eruption.

The Puy Beaunit maar presents a large variety of mantle xenoliths (spinel peridotites, pyroxenites and layered rocks). A detailed study of the textures and mineral equilibria shows the unusual character of this occurrence and the local... more

The Puy Beaunit maar presents a large variety of mantle xenoliths (spinel peridotites, pyroxenites and layered rocks). A detailed study of the textures and mineral equilibria shows the unusual character of this occurrence and the local complexity of the upper mantle beneath the French Massif Central. Ultramafic nodules have a metamorphic, magmatic or pyrometamorphic origin; they display different stages of

A suite of fifteen peridotite xenoliths from the Bakony-Balaton Highland Volcanic Field (BBHVF, Pannonian Basin, Central Europe) that show abundant petrographic evidence of fluid and melt percolation were studied in order to decipher the... more

A suite of fifteen peridotite xenoliths from the Bakony-Balaton Highland Volcanic Field (BBHVF, Pannonian Basin, Central Europe) that show abundant petrographic evidence of fluid and melt percolation were studied in order to decipher the formation of their melt pockets and veins. The suite mainly consists of " fertile " lherzolites (5.8–19.9 vol.% clinopyroxene) and a few harzburgites (1.9–5.4 vol.% clinopyroxene) from well-known localities (Szentbékkálla, Szigliget) and two previously unreported localities (Füzes-tó and Mindszentkálla). Major and trace element data indicate that most of the peridotites record variable degrees of partial melt extraction, up to N 15% for the harzburgites. Subsequently, the xenoliths experienced at least two stages of metasomatic modification. The first stage was associated with percolation of a volatile-bearing silicate melt and resulted in crystalliza-tion of amphibole, enrichment in the most incompatible trace elements (Ba, Th, U, Sr), and development of negative Nb-Ta anomalies in clinopyroxene. The second and last metasomatic event, widespread beneath the BBHVF, is associated with the formation of silicate melt pockets, physically connected to a network of melt veins, with large and abundant CO 2 vesicles. The glass in these veins has sub-alkaline trachy-andesitic composition and displays an OIB-like trace element signature. Its composition attests to the migration through a supra-subduction zone mantle wedge of silicic melt highly enriched in volatiles (CO 2 , H 2 O, Cl, F), LILE, REE and HFSE and consistent with compositions of natural and experimental examples of slab melting-derived magma. In the present case, however, melt was likely derived from melting of oceanic crust and carbonated sediments under conditions where Nb-rich mineral phases were not stable in the residue. A likely scenario for the origin such melts involves melting after subduction ceased as the slab thermally equilibrated with the asthenosphere. Melt-rock reactions due to ascent of hot, CO 2-rich, siliceous melt to near-Moho depths triggered destabilization of amphibole and primary clinopyroxene, spinel, and possibly olivine. The resulting andesitic glass in melt pockets evolved to more mafic compositions due to mantle mineral assimilation but has heterogeneous trace element signatures mostly inherited from preexisting amphibole. The present example of melt-rock reactions between highly volatile-enriched siliceous slab-derived melt and peridotite from the upper part of the lithospheric mantle ultimately produced derivative melt with major element composition akin to calc-alkaline basaltic andesite, with generally low trace elements concentrations but selective pronounced enrichments in LILE's such as Ba, Sr, Pb.

Petrographic studies and mineral chemical analyses support the hypothesis that garnet lherzolites and websterites from Oahu, Hawaii were produced by mechanical disintegration (stoping), chemical alteration (metasomatism) and mechanical... more

Petrographic studies and mineral chemical analyses support the hypothesis that garnet lherzolites and websterites from Oahu, Hawaii were produced by mechanical disintegration (stoping), chemical alteration (metasomatism) and mechanical mixing of spinel lherzolite wall rock (i.e., lithosphere) with intrusive clinopyroxenite veins. This conclusion is inconsistent with two prevalent hypotheses concerning the origin of these rocks, one of which proposes that the garnet lherzolites represent the upper mantle source for Hawaiian tholeiitic magmas, and the other that invokes origin of these xenolith lithologies by high-pressure crystallization of Hawaiian alkalic magmas. Although the undepleted mantle source of Hawaiian tholeiitic magmas may be chemically similar to some of the garnet lherzolite xenoliths, it is physically impossible for these xenoliths to represent actual source rocks.

Chemical disparities at the interface between subducting oceanic crustal rocks and the harzburgitic mantle lead to the formation of reaction zones in the mantle above the subducting slabs composed of hybrid rocks that may carry exotic... more

Chemical disparities at the interface between subducting oceanic crustal rocks and the harzburgitic mantle lead to the formation of reaction zones in the mantle above the subducting slabs composed of hybrid rocks that may carry exotic trace-element patterns and isotopic signatures. Subsequent burial of these metasomatised rocks as part of the progressively subducted slab could deliver trace elements and volatiles
to the source region of arc magma.
A natural laboratory to study reactions at the slab–mantle interface maybe found in exhumed high-pressure mélanges, where sedimentary, mafic and ultramafic lithologies are juxtaposed and metamorphosed at high-P/T conditions. A mélange zone of that type is found in northern Syros, where metasomatic reaction zones (“blackwalls”) formed on a metre scale at the contact of metasedimentary blueschists and serpentinite. Five different zones within such a contact display the assemblages (I) glaucophane+garnet+phengite+epidote, (II) glaucophane+epidote+chlorite, (III) chlorite+epidote+omphacite±albite (IV) chlorite±titanite±rutile±apatite and (V) serpentine+chromite. Accessory phases, such as apatite, allanite, rutile, titanite, tourmaline, zircon and monazite are abundant in zones II to IV. The observed succession of assemblages together with whole-rock major and trace-element compositions reflect the two dominant processes that are thought to have operated along the lithological contact: (A) diffusion of chemical components driven by the compositional contrast of the juxtaposed rocks, and (B) flux of hydrous fluids along the contact, which depleted (e.g., LILE, SiO2) or enriched (e.g., B, LREE) certain elements in various zones. Thermodynamic modelling is able to closely predict the succession of mineral assemblages as they are expected from diffusion of Mg and Ca across the contact zone. Employed to various P–T conditions and different juxtaposed rock types, this type of modelling could be used to access and evaluate larger portions of the subduction system.
Our results support existing models that suggest that mixing and redistribution of major and trace elements in subduction zones may be related to the formation of hybrid rocks in mélange zones.