235 research outputs found
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 âŒ700 to âŒ10 ppm and âŒ2600 to âŒ25 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 (âŒ0·723) in the most pristine peridotites and increase to âŒ0·743 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, olivineâgarnet-websterite, garnet-websterite, orthopyroxeneâphlogopiteâgarnet-websterite, coarse-grained phlogopiteâgarnet-websterite, phlogopiteâgarnet-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 âŒ700 to âŒ10 ppm and âŒ2600 to âŒ25 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 UâPb 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 (âŒ0·723) in the most pristine peridotites and increase to âŒ0·743 in the most metasomatized samples. The initial 87Sr/86Sr values of both the host gneiss and its leucosomes are also elevated (0·734â0·776), 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 sĂșggest 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 geoquĂmica e isotĂłpica de depĂłsitos sedimentares vulcanogĂ©nicos do OrdovĂcico Inferior do Sinclinal de Moncorvo, Nordeste de Portugal
Os ambientes geodinĂąmicos extensionais na Zona Centro IbĂ©rica, durante o OrdovĂcico Inferior, favoreceram a ocorrĂȘncia de episĂłdios vulcĂąnicos. No sinclinal de Moncorvo hĂĄ evidĂȘncias desses episĂłdios, preservados sob a
forma de depósitos sedimentares vulcanogénicos intercalados em metassedimentos da Formação Marão. Estes
depĂłsitos sĂŁo essencialmente constituĂdos por quartzo, mas tambĂ©m contĂȘm moscovite, fosfatos da sĂ©rie 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 depĂłsitos de cinzas distais, com afinidade riodacĂtica, envolvendo a mistura com materiais
detrĂticos. A datação U-Pb, obtida por ID-TIMS em zircĂŁo, indica uma idade preliminar de formação de 484.5 ± 3.0 Ma. Os elevados teores em alguns elementos incompatĂveis, a razĂŁo 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 variĂĄveis entre 0.71156 e
0.71304 e -10.8 e -10.0, respectivamente, indicam que os materiais vulcĂąnicos teriam resultado da fusĂŁo parcial de
metassedimentos semelhantes aos do Grupo do Douro e equivalentes do n orte da Zona Centro IbĂ©rica. PorĂ©m, o envolvimento de processos sedimentares terĂĄ tido uma influĂȘncia significativa na geoquĂmica elementar e isotĂłpica 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 MarĂŁo 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 ± 0.06ⰠVSMOW (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 ± 0.9Ⱐ(2s), with a mean mass fraction of 1.0 ± 0.1 ”g g-1 Li (2s). Zircon M127 contains ~ 923 ”g g-1 U. The moderate degree of radiation damage corresponds well with the time-integrated self-irradiation dose of 1.82 à 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 ferrĂferos do Membro Malhada da Formação MarĂŁo aflorantes no sinclinĂłrio de Moncorvo (OrdovĂcico Inferior), encontram-se intercalados nĂveis de depĂłsitos metassedimentares, de granulometria fina e espessura mĂĄxima de 40 cm, que sĂŁo constituĂdos essencialmente por quartzo e moscovite, contendo tambĂ©m fosfatos da sĂ©rie lazulite-scorzalite, zircĂŁo, turmalina, hematite, localmente com nĂșcleos de magnetite, esfena e, mais raramente, monazite, xenĂłtimo, rĂștilo, hidrĂłxidos de Fe e Ti e clorite. A geoquĂmica de elementos tendencialmente imĂłveis (Ti, Zr, La e Sc) sugere que a sua deposição terĂĄ ocorrido prĂłximo de uma margem continental ativa, sendo posteriormente afetados por processos metamĂłrficos de baixo grau e metassomĂĄticos/hidrotermais, relacionados com a Orogenia Varisca, que terĂŁo influenciado a sua mineralogia e geoquĂmica elementar e isotĂłpica. Contudo, o contraste mineralĂłgico, textural e granulomĂ©trico existente entre estes depĂłsitos metassedimentares e os quartzitos ferrĂferos, bem como a ocorrĂȘncia de episĂłdios vulcĂąnicos/vulcano-sedimentares na Zona Centro-IbĂ©rica, durante o OrdovĂcico Inferior, sugere que estes depĂłsitos teriam incorporado materiais detrĂticos resultantes da erosĂŁo de rochas vulcĂąnicas/hipabissais com idade anterior Ă Idade Floiano superior-Dapingiano (471-467 Ma). Com efeito, os elevados teores em alguns elementos incompatĂveis, a razĂŁo 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 ΔNdt variĂĄveis entre 0,71156 e 0,71304 e -10,8 e -10,0, respetivamente, indicam que os materiais vulcĂąnicos primordiais poderiam ter afinidade riodacĂtica, resultando da fusĂŁo parcial de metassedimentos semelhantes aos do Grupo do Douro e equivalentes do norte da Zona Centro IbĂ©rica. A datação U-Pb, obtida por ID-TIMS em zircĂŁo, indica uma idade de 484,5 ± 3,0 Ma para a formação desses materiais vulcĂąnicos primordiais.In the Moncorvo synclinorium, ferriferous quartzites of the Malhada Member of the MarĂŁo 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 ΔNdt 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, 19:2262â2282, 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 UâPb geochronology. These crystals belong to a 300 g xenotime assortment, collected from alluvial deposits in SE Brazil. Electron microprobe and Laser AblationâInductively Coupled PlasmaâMass Spectrometry (LAâICPâMS) 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 greenschistâfacies metasediments and that locally contain other accessory phases such as rutile and monazite. Highâprecision UâPb Isotope DilutionâThermal Ionization Mass Spectrometry (IDâTIMS) 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 UâPb isotope data acquired via LAâ(Q,SF,MC)âICPâMS are within the uncertainty of the IDâTIMS data, showing homogeneity at the 1% precision of the laser ablation (and probably ion microprobe) technique. UâPb LAâ(MC, SF)âICPâMS analyses, using XN01 as a primary RM, reproduced the ages of other established RMs within less than 1% deviation. Other Datas crystals (XN03â05) also display a reproducibility in Pb/U dates better than 1% on LAâICPâMS, making them good candidates for further testing by IDâTIMS.https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2017GC007412Publisher's versio
- âŠ