Metasomatism in the Ultrahigh-pressure Svartberget Garnet-peridotite (Western Gneiss Region, Norway): Implications for the Transport of Crust-derived Fluids within the Mantle

Garnet-peridotites often contain veins or layers of pyroxenite and eclogite of uncertain origin. We investigate the Svartberget garnet-peridotite from the northernmost ultrahigh-pressure domain in the Western Gneiss Region (WGR) in Norway and show that the observed layering represents a sequence of metasomatic reaction zones developed along a fracture system. From the garnet-peridotite wall-rock to the fractures the following sequential reaction zones are recognized: clinohumite bearing garnet-peridotite, olivine-garnet-websterite, garnet-websterite, orthopyroxene-phlogopite-garnet-websterite, coarse-grained phlogopite-garnet-websterite, phlogopite-garnet-websterite, phlogopite-free garnet-websterite, inclusion-rich garnetite, garnetite, eclogite, retrograde omphacitite and felsic amphibole-pegmatite. The MgO, FeO and CaO contents generally decrease from the pristine peridotite towards the most metasomatized samples, with an associated increase in SiO2 and Al2O3. Concentrations of fluid-mobile elements increase from the most pristine peridotite towards the garnetite, whereas Ni and Cr decrease from ∼700 to ∼10 ppm and ∼2600 to ∼25 ppm, respectively. Changes in mineral mode are accompanied by changes in mineral chemistry. All minerals display decreasing Mg# and Cr content with degree of metasomatism, whereas Na2O concentrations in amphibole, and most notably in clinopyroxene, increase from 0·2 to 3·0 and from 0·2 to 8 wt %, respectively. The trivalent ions Cr and Al display complex intra-granular vein-like or patchy zoning in garnet and pyroxenes that may be characteristic of metasomatized peridotites. Dating by the U-Pb method suggests metamorphic growth of zircon in the garnetite at 397·2 ± 1·2 Ma, formation of leucosomes in host-rock gneiss at 391·2 ± 0·8 Ma, and amphibole-pegmatite in the core of a garnetite vein at 390·1 ± 0·9 Ma. Initial 87Sr/86Sr values calculated at 397 Ma are elevated (∼0·723) in the most pristine peridotites and increase to ∼0·743 in the most metasomatized samples. The initial 87Sr/86Sr values of both the host gneiss and its leucosomes are also elevated (0·734-0·776), which suggests that the leucosomes found in the gneisses are the most likely, now solidified, remnants of the reactive agent that metasomatized the Svartberget peridotite. A scenario is envisaged in which material derived from the country rock gneiss was the source of the metasomatic addition of elements to the peridotites and the gneisses acted as the host for all elements removed from the peridotite. The Svartberget peridotite may provide an important analogue of how felsic, slab-derived material interacts with the overlying mantle wedge peridotite in regions of arc magma generatio

Metasomatism in the Ultrahigh-pressure Svartberget Garnet-peridotite (Western Gneiss Region, Norway): Implications for the Transport of Crust-derived Fluids within the Mantle

Garnet-peridotites often contain veins or layers of pyroxenite and eclogite of uncertain origin. We investigate the Svartberget garnet-peridotite from the northernmost ultrahigh-pressure domain in the Western Gneiss Region (WGR) in Norway and show that the observed layering represents a sequence of metasomatic reaction zones developed along a fracture system. From the garnet-peridotite wall-rock to the fractures the following sequential reaction zones are recognized: clinohumite bearing garnet-peridotite, olivine–garnet-websterite, garnet-websterite, orthopyroxene–phlogopite–garnet-websterite, coarse-grained phlogopite–garnet-websterite, phlogopite–garnet-websterite, phlogopite-free garnet-websterite, inclusion-rich garnetite, garnetite, eclogite, retrograde omphacitite and felsic amphibole-pegmatite. The MgO, FeO and CaO contents generally decrease from the pristine peridotite towards the most metasomatized samples, with an associated increase in SiO2 and Al2O3. Concentrations of fluid-mobile elements increase from the most pristine peridotite towards the garnetite, whereas Ni and Cr decrease from ∼700 to ∼10 ppm and ∼2600 to ∼25 ppm, respectively. Changes in mineral mode are accompanied by changes in mineral chemistry. All minerals display decreasing Mg# and Cr content with degree of metasomatism, whereas Na2O concentrations in amphibole, and most notably in clinopyroxene, increase from 0·2 to 3·0 and from 0·2 to 8 wt %, respectively. The trivalent ions Cr and Al display complex intra-granular vein-like or patchy zoning in garnet and pyroxenes that may be characteristic of metasomatized peridotites. Dating by the U–Pb method suggests metamorphic growth of zircon in the garnetite at 397·2 ± 1·2 Ma, formation of leucosomes in host-rock gneiss at 391·2 ± 0·8 Ma, and amphibole-pegmatite in the core of a garnetite vein at 390·1 ± 0·9 Ma. Initial 87Sr/86Sr values calculated at 397 Ma are elevated (∼0·723) in the most pristine peridotites and increase to ∼0·743 in the most metasomatized samples. The initial 87Sr/86Sr values of both the host gneiss and its leucosomes are also elevated (0·734–0·776), which suggests that the leucosomes found in the gneisses are the most likely, now solidified, remnants of the reactive agent that metasomatized the Svartberget peridotite. A scenario is envisaged in which material derived from the country rock gneiss was the source of the metasomatic addition of elements to the peridotites and the gneisses acted as the host for all elements removed from the peridotite. The Svartberget peridotite may provide an important analogue of how felsic, slab-derived material interacts with the overlying mantle wedge peridotite in regions of arc magma generation

Self-subduction of the Pangaean global plate

One of the most striking and rare occurrences in the Earth's history is the amalgamation of most of the continental lithosphere into one supercontinent. The most recent supercontinent, Pangaea, lasted from 320 to 200 million years ago. Here, we show that after the continental collisions that led to the formation of Pangaea, plate convergence continued in a large, wedge-shaped oceanic tract. We súggest that plate strain at the periphery of the supercontinent eventually resulted in self-subduction of the Pangaean global plate, when the ocean margin of the continent subducted beneath the continental edge at the other end of the same plate. Our scenario results in a stress regime within Pangaea that explains the development of a large fold structure near the apex of the Palaeotethys Ocean, extensive lower crustal heating and continental magmatism at the core of the continent as well as the development of radially arranged continental rifts in more peripheral regions of the plate

Caracterização geoquímica e isotópica de depósitos sedimentares vulcanogénicos do Ordovícico Inferior do Sinclinal de Moncorvo, Nordeste de Portugal

Os ambientes geodinâmicos extensionais na Zona Centro Ibérica, durante o Ordovícico Inferior, favoreceram a ocorrência de episódios vulcânicos. No sinclinal de Moncorvo há evidências desses episódios, preservados sob a forma de depósitos sedimentares vulcanogénicos intercalados em metassedimentos da Formação Marão. Estes depósitos são essencialmente constituídos por quartzo, mas também contêm moscovite, fosfatos da série lazulite - scorzalite, zircão, monazite, raros opacos e turmalina. Embora os depósitos se apresentem recristalizados e com foliação metamórfica penetrativa, a sua granulometria fina, espessura (< 40 cm) e mineralogia sugerem que corresponderiam a depósitos de cinzas distais, com afinidade riodacítica, envolvendo a mistura com materiais detríticos. A datação U-Pb, obtida por ID-TIMS em zircão, indica uma idade preliminar de formação de 484.5 ± 3.0 Ma. Os elevados teores em alguns elementos incompatíveis, a razão CaO/Na2O de 0.33 a 1.77, perfis de REE com (La/Lu)N = 3.37 a 7.96 e discreta anomalia negativa de Eu, valores de (87Sr/86Sr)i t variáveis entre 0.71156 e 0.71304 e -10.8 e -10.0, respectivamente, indicam que os materiais vulcânicos teriam resultado da fusão parcial de metassedimentos semelhantes aos do Grupo do Douro e equivalentes do n orte da Zona Centro Ibérica. Porém, o envolvimento de processos sedimentares terá tido uma influência significativa na geoquímica elementar e isotópica destas rochas.The extensional geodynamic conditions in the Central Iberian Zone, during Lower Ordovician, were favorable to the occurrence of volcanic episodes. At the syncline of Moncorvo there are evidences of those episodes, which occur in the form of volcanogenic sedimentary deposits interbedded in the metasediments of Marão Formation. These deposits are mainly composed of quartz, but also contain muscovite, phosphates of the lazulite-scorzalite series, zircon, monazite, rare opaques and tourmaline. Despite the recrystallization and metamorphic foliation overprint, the fine -grained texture, the thickness (< 40 cm) and mineralogy of the deposits suggest that they correspond to distal ash -fall layers, with rhyodacitic to dacitic affinity, involving mixing with detrital materials. The U-Pb zircon dating, obtained by ID-TIMS, indicates a preliminary formation age of 484.5 ± 3.0 Ma. The high contents of some incompatible elements, the CaO/Na2O ratio between 0.33 and 1.77, the REE pattern with (La / Lu)N = 3.37 to 7.96 and slight negative Eu anomaly,(87Sr/86Sr)i and Ndt values variable between 0.71156 and 0.71304 and -10.8 and -10.0, respectively, indicate that the volcanogenic materials were derived from partial melting of metasediments similar to those of Douro Group and equivalents from northern Central Iberian Zone. However, the involvement of sedimentary processes may have had significant influence on elemental and isotopic geochemistry of these rocks

Zircon M127 - A Homogeneous Reference Material for SIMS U-Pb Geochronology Combined with Hafnium, Oxygen and, Potentially, Lithium Isotope Analysis

In this article, we document a detailed analytical characterisation of zircon M127, a homogeneous 12.7 carat gemstone from Ratnapura, Sri Lanka. Zircon M127 has TIMS-determined mean U-Pb radiogenic isotopic ratios of 0.084743 ± 0.000027 for 206Pb/238U and 0.67676 ± 0.00023 for 207Pb/235U (weighted means, 2s uncertainties). Its 206Pb/238U age of 524.36 ± 0.16 Ma (95% confidence uncertainty) is concordant within the uncertainties of decay constants. The d18O value (determined by laser fluorination) is 8.26 ± 0.06‰ VSMOW (2s), and the mean 176Hf/177Hf ratio (determined by solution ICP-MS) is 0.282396 ± 0.000004 (2s). The SIMS-determined d7Li value is -0.6 ± 0.9‰ (2s), with a mean mass fraction of 1.0 ± 0.1 µg g-1 Li (2s). Zircon M127 contains ~ 923 µg g-1 U. The moderate degree of radiation damage corresponds well with the time-integrated self-irradiation dose of 1.82 × 1018 alpha events per gram. This observation, and the (U-Th)/He age of 426 ± 7 Ma (2s), which is typical of unheated Sri Lankan zircon, enable us to exclude any thermal treatment. Zircon M127 is proposed as a reference material for the determination of zircon U-Pb ages by means of SIMS in combination with hafnium and stable isotope (oxygen and potentially also lithium) determination

Interbedded quartz-muscovite layers in the ferriferous quartzites of the Lower Ordovician deposits of Moncorvo synclinorium (NE Portugal): an example of volcanogenic metasedimentary deposits?

Nos quartzitos ferríferos do Membro Malhada da Formação Marão aflorantes no sinclinório de Moncorvo (Ordovícico Inferior), encontram-se intercalados níveis de depósitos metassedimentares, de granulometria fina e espessura máxima de 40 cm, que são constituídos essencialmente por quartzo e moscovite, contendo também fosfatos da série lazulite-scorzalite, zircão, turmalina, hematite, localmente com núcleos de magnetite, esfena e, mais raramente, monazite, xenótimo, rútilo, hidróxidos de Fe e Ti e clorite. A geoquímica de elementos tendencialmente imóveis (Ti, Zr, La e Sc) sugere que a sua deposição terá ocorrido próximo de uma margem continental ativa, sendo posteriormente afetados por processos metamórficos de baixo grau e metassomáticos/hidrotermais, relacionados com a Orogenia Varisca, que terão influenciado a sua mineralogia e geoquímica elementar e isotópica. Contudo, o contraste mineralógico, textural e granulométrico existente entre estes depósitos metassedimentares e os quartzitos ferríferos, bem como a ocorrência de episódios vulcânicos/vulcano-sedimentares na Zona Centro-Ibérica, durante o Ordovícico Inferior, sugere que estes depósitos teriam incorporado materiais detríticos resultantes da erosão de rochas vulcânicas/hipabissais com idade anterior à Idade Floiano superior-Dapingiano (471-467 Ma). Com efeito, os elevados teores em alguns elementos incompatíveis, a razão CaO/Na2O de 0,33 a 1,77, perfis de REE com (La/Lu)N = 3,37 a 7,96 e discreta anomalia negativa de Eu, valores de (87Sr/86Sr)i e εNdt variáveis entre 0,71156 e 0,71304 e -10,8 e -10,0, respetivamente, indicam que os materiais vulcânicos primordiais poderiam ter afinidade riodacítica, resultando da fusão parcial de metassedimentos semelhantes aos do Grupo do Douro e equivalentes do norte da Zona Centro Ibérica. A datação U-Pb, obtida por ID-TIMS em zircão, indica uma idade de 484,5 ± 3,0 Ma para a formação desses materiais vulcânicos primordiais.In the Moncorvo synclinorium, ferriferous quartzites of the Malhada Member of the Marão Formation (Lower Ordovician) comprise interbedded layers of fine grained metasedimentary deposits, with a maximum thickness of 40 cm, and mainly composed of quartz and muscovite, but also containing phosphates of the lazulite-scorzalite series, zircon, tourmaline, hematite, locally with magnetite cores, titanite, and, more rarely, monazite, xenotime, rutile, Fe and Ti hydroxides and chlorite. The geochemistry of immobile elements (Ti, Zr, La and Sc) suggests that the metasedimentary rocks have been deposited near an active continental margin, but the subsequent low grade metamorphism and metasomatic/hydrothermal processes, related to the Variscan Orogeny, had a significant influence on their mineralogy and elemental and isotopic geochemistry. The mineralogical, textural and granulometric contrast between these metasedimentary deposits and the ferriferous quartzite, as well as the occurrence of Lower Ordovician volcanic/volcano-sedimentary episodes in the Central Iberian Zone, suggest that these deposits could have incorporated detritus derived from volcanic/hypabyssal rocks, older than the upper Floian-Dapingian age (471-467 Ma). However, the high contents of some incompatible elements, the CaO/Na2O ratio between 0.33 and 1.77, the REE pattern with (La / Lu)N = 3.37 to 7.96 and a slight negative Eu anomaly, (87Sr/86Sr)i varying from 0.71156 to 0.71304 and εNdt values of -10.8 to -10.0 indicate that the primary volcanogenic materials could have had a rhyodacitic to dacitic affinity, deriving from partial melting of metasediments similar to those of the Douro Group and equivalents from the northern Central Iberian Zone. Dating of zircon by U-Pb ID-TIMS indicates a formation age of 484.5 ± 3.0 Ma for the original volcanogenic materials

Characterization of xenotime from Datas (Brazil) as a potential reference material for in situ U-Pb geochronology

CITATION: Vasconcelos, A. D. et al. 2018. Characterization of xenotime from Datas (Brazil) as a potential reference material for in situ U-Pb geochronology. Geochemistry, Geophysics, Geosystems, 19:2262–2282, doi:10.1029/2017GC007412.The original publication is available at https://agupubs.onlinelibrary.wiley.comThis study investigates five megacrysts of xenotime (XN01, XN02, XN03, XN04, and XN05) as potential reference materials (RMs) for U‐Pb geochronology. These crystals belong to a 300 g xenotime assortment, collected from alluvial deposits in SE Brazil. Electron microprobe and Laser Ablation‐Inductively Coupled Plasma‐Mass Spectrometry (LA‐ICP‐MS) analyses show that the selected crystals are internally homogeneous for most rare earth element, (REE, except some light REE) but are relatively heterogeneous for U and Th. The xenotime REE patterns are consistent with an origin from hydrothermal quartz veins in the Datas area that cut greenschist‐facies metasediments and that locally contain other accessory phases such as rutile and monazite. High‐precision U‐Pb Isotope Dilution‐Thermal Ionization Mass Spectrometry (ID‐TIMS) analyses showed slight age heterogeneity for the XN01 crystal not observed in the XN02 sample. The two crystals have slightly different average 206Pb/238U ages of 513.4 ± 0.5 Ma (2 s) and 515.4 ± 0.2 Ma (2 s), respectively. In situ U‐Pb isotope data acquired via LA‐(Q,SF,MC)‐ICP‐MS are within the uncertainty of the ID‐TIMS data, showing homogeneity at the 1% precision of the laser ablation (and probably ion microprobe) technique. U‐Pb LA‐(MC, SF)‐ICP‐MS analyses, using XN01 as a primary RM, reproduced the ages of other established RMs within less than 1% deviation. Other Datas crystals (XN03‐05) also display a reproducibility in Pb/U dates better than 1% on LA‐ICP‐MS, making them good candidates for further testing by ID‐TIMS.https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2017GC007412Publisher's versio