Prograde garnet growth along complex <Emphasis Type="Italic">P–T–t</Emphasis> paths: results from numerical experiments on polyphase garnet from the Wölz Complex (Austroalpine basement)     

F. Gaidies  C. de Capitani  R. Abart  R. Schuster 《Contributions to Mineralogy and Petrology》2008,155(6):673-688

Garnet in metapelites from the Wölz Complex of the Austroalpine crystalline basement east of the Tauern Window characteristically consists of two growth phases, which preserve a comprehensive record of the geothermal history during polymetamorphism. From numerical modelling of garnet formation, detailed information on the pressure–temperature–time (P–T–t) evolution during prograde metamorphism is obtained. In that respect, the combined influences of chemical fractionation associated with garnet growth, modification of the original growth zoning through intragranular diffusion and the nucleation history on the chemical zoning of garnet as P and T change during growth are considered. The concentric chemical zoning observed in garnet and the homogenous rock matrix, which is devoid of chemical segregation, render the simulation of garnet growth through successive equilibrium states reliable. Whereas the first growth phase of garnet was formed at isobaric conditions of ～3.8 kbar at low heating/cooling rates, the second growth phase grew along a Barrovian P–T path marked with a thermal peak of ～625°C at ～10 kbar and a maximum in P of ～10.4 kbar at ～610°C. For the heating rate during the growth of the second phase of garnet, average rates faster than 50°C Ma^?1 are obtained. From geochronological investigations the first growth phase of garnet from the Wölz Complex pertains to the Permian metamorphic event. The second growth phase grew in the course of Eo-Alpine metamorphism during the Cretaceous.  相似文献   

Anaerobic decomposition of tropical soils and plant material: Implication for the CO₂ and CH₄ budget of the Petit Saut Reservoir   总被引：1，自引：0，他引：1  

Frdric Gurin  Gwenaël Abril  Alexis de Junet  Marie-Paule Bonnet 《Applied Geochemistry》2008,23(8):2272-2283

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Geochemistry of the Mamil Choique granitoids at Rio Chico, Río Negro, Argentina: Late Paleozoic crustal melting in the North Patagonian Massif     

Mnica G. Lpez de Luchi  María Elena Cerredo 《Journal of South American Earth Sciences》2008,25(4):526-546

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Metalliferous paragenesis of the San José mine, Oruro, Bolivia     

F. Keutsch  M.K. de Brodtkorb   《Journal of South American Earth Sciences》2008,25(4):485-491

The San José silver–tin deposit, Oruro, is located in the Cordillera Oriental, which contains most of the metalliferous mineralizations of Bolivia and is related to a quartz-latite dome of Miocene age. The mineral paragenesis encountered in this study is composed of cassiterite, stannite, miargyrite, pyrargyrite, andorite and Bi-rich andorite, jamesonite, pavonite/benjaminite, boulangerite, owyheeite, ramdohrite and Bi-rich ramdohrite, bismuthinite, besides pyrite, chalcopyrite, Ag-rich tetrahedrite, galena and sphalerite, all of which are analyzed by electron microprobe analysis. With semi-quantitative SEM/EDS and XRD analyses, rhodostannite and kësterite/ferrokësterite were found in association with andorite, and chalcostibite was determined by XRD, in association with zinkenite, tetrahedrite and pyrite.  相似文献   

International viewpoint and news     

Anne Marie de Grosbois 《Environmental Geology》2008,54(6):1353-1354

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Upcoming international events     

Anne Marie de Grosbois 《Environmental Geology》2008,55(3):699-703

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Biogeochemical behavior of Ampelozizyphus amazonicus Ducke in the Pitinga mining district, Amazon, Brazil     

M. C. Lima e Cunha  V. P. Pereira  E. Menegotto  A. C. Bastos Neto  L. D. Oliveira  M. L. L. Formoso 《Environmental Geology》2008,55(6):1355-1362

The vegetal species Ampelozizyphus amazonicus Ducke (Rhamnaceae Family) was chosen as a sampling medium for the lateritic surfaces of the Pitinga Mine in the Amazon region, in order to study the biogeochemical behavior of this species and compare it with the chemical composition of a reference plant. The Pitinga mining district is one of the largest producers of tin in the world. This district contains unique deposits of cryolite and rare metals such as Zr, Nb, Ta, Y and REEs related to granitic bodies that intrude into the volcanic and acid pyroclastic rocks. The results showed that the species A. amazonicus predominantly concentrates significant levels of Zr, Nb, Ta, Th, Be, Sc over U, Hf, Ga and In. These elements are characteristic of the mineral paragenesis for the region, suggesting that this plant can provide a representative sampling medium future geochemical exploration programs in the region.  相似文献   

Garnet growth at high- and ultra-high pressure conditions and the effect of element fractionation on mineral modes and composition   总被引：4，自引：0，他引：4  

Matthias Konrad-Schmolke  Patrick J. O'Brien  Christian de Capitani  Dennis A. Carswell   《Lithos》2008,103(3-4):309-332

In this paper we show that thermodynamic forward modelling, using Gibbs energy minimisation with consideration of element fractionation into refractory phases and/or liberated fluids, is able to extract information about the complex physical and chemical evolution of a deeply subducted rock volume. By comparing complex compositional growth zonations in garnets from high-and ultra-high pressure samples with those derived from thermodynamic forward modelling, we yield an insight into the effects of element fractionation on composition and modes of the co-genetic metamorphic phase assemblage. Our results demonstrate that fractionation effects cause discontinuous growth and re-crystallisation of metamorphic minerals in high pressure rocks. Reduced or hindered mineral growth at UHP conditions can control the inclusion and preservation of minerals indicative for UHP metamorphism, such as coesite, thus masking peak pressure conditions reached in subducted rocks.Further, our results demonstrate that fractional garnet crystallisation leads to strong compositional gradients and step-like zonation patterns in garnet, a feature often observed in high-and ultra-high pressure rocks. Thermodynamic forward modelling allows the interpretation of commonly observed garnet growth zonation patterns in terms of garnet forming reactions and the relative timing of garnet growth with respect to the rock's pressure–temperature path. Such a correlation is essential for the determination of tectonic and metamorphic rates in subduction zones as well as for the understanding of trace element signatures in subduction related rocks. It therefore should be commonplace in the investigation of metamorphic processes in subduction zones.  相似文献   

Origin of the Tongshankou porphyry–skarn Cu–Mo deposit,eastern Yangtze craton,Eastern China: geochronological,geochemical, and Sr–Nd–Hf isotopic constraints     

Jian-Wei Li  Xin-Fu Zhao  Mei-Fu Zhou  Paulo Vasconcelos  Chang-Qian Ma  Xiao-Dong Deng  Zorano Sérgio de Souza  Yong-Xin Zhao  Gang Wu 《Mineralium Deposita》2008,43(3):315-336

The Tongshankou Cu–Mo deposit, located in the westernmost Daye district of the Late Mesozoic Metallogenic Belt along the Middle-Lower reaches of the Yangtze River, eastern China, consists mainly of porphyry and skarn ores hosted in the Tongshankou granodiorite and along the contact with the Lower Triassic marine carbonates, respectively. Sensitive high-resolution ion microprobe zircon U–Pb dating constrains the crystallization of the granodiorite at 140.6 ± 2.4 Ma (1σ). Six molybdenite samples from the porphyry ores yield Re–Os isochron age of 143.8 ± 2.6 Ma (2σ), while a phlogopite sample from the skarn ores yields an ⁴⁰Ar/³⁹Ar plateau age of 143.0 ± 0.3 Ma and an isochron age of 143.8 ± 0.8 Ma (2σ), indicating an earliest Cretaceous mineralization event. The Tongshankou granodiorite has geochemical features resembling slab-derived adakites, such as high Sr (740–1,300 ppm) and enrichment in light rare earth elements (REE), low Sc (<10 ppm), Y (<13.3 ppm), and depletion in heavy REE (<1.2 ppm Yb), and resultant high Sr/Y (60–92) and La/Yb (26–75) ratios. However, they differ from typical subduction-related adakites by high K, low MgO and Mg#, and radiogenic Sr–Nd–Hf isotopic compositions, with (⁸⁷Sr/⁸⁶Sr) _t  = 0.7062–0.7067, ɛ _Nd(t) = −4.37 to −4.63, (¹⁷⁶Hf/¹⁷⁷Hf) _t  = 0.282469–0.282590, and ɛ _Hf(t) = −3.3 to −7.6. The geochemical and isotopic data, coupled with geological analysis, indicate that the Tongshankou granodiorite was most likely generated by partial melting of enriched lithospheric mantle that was previously metasomitized by slab melts related to an ancient subduction system. Magmas derived from such a source could have acquired a high oxidation state, as indicated by the assemblage of quartz–magnetite–titanite–amphibole–Mg-rich biotite in the Tongshankou granodiorite and the compositions of magmatic biotite that fall in the field between the NiNiO and magnetite–hematite buffers in the Fe³⁺–Fe²⁺–Mg diagram. Sulfur would have been present as sulfates in such highly oxidized magmas, so that chalcophile elements Cu and Mo were retained as incompatible elements in the melt, contributing to subsequent mineralization. A compilation of existing data reveals that porphyry and porphyry-related Cu–Fe–Au–Mo mineralization from Daye and other districts of the Metallogenic Belt along the Middle-Lower reaches of the Yangtze River took place coevally in the Early Cretaceous and was related to an intracontinental extensional environment, distinctly different from the arc-compressive setting of the Cenozoic age that has been responsible for the emplacement of most porphyry Cu deposits of the Pacific Rim.  相似文献   

Time dependent structural architecture of subsidiary fracturing and stress pattern in the tip region of an extensional growth fault system, Tarquinia basin, Italy     

F. Balsamo  F. Storti  B. Piovano  F. Salvini  F. Cifelli  C. Lima 《Tectonophysics》2008,454(1-4):54-69

Fault tip regions, relay ramps and accommodation zones in between major segments of extensional fault systems provide zones of additional structural and stratigraphic complexities and also significantly affect their hydraulic behaviour. The great interest for both academic and industrial purposes encouraged specific studies of fault tip regions that, in some cases, produced controversial results. We approached the study of fault tip regions by integrating structural, AMS and stratigraphic analyses of the tip of an extensional growth fault system in the Tarquinia basin, on the Tyrrhenian side of the Northern Apennines. Detailed structural mapping indicates the occurrence of systematic relationships between the orientation of the main subsidiary fault zones, the orientation and position of the two main joint sets developed in the fault damage zones, and the overprinting relationships between the two main joint sets themselves. Microstructural analysis of fault core rocks indicates a progression of deformation from soft-sediment to brittle conditions. The AMS study supports the evolution of deformation under a constantly oriented stress field. By combining this multidisciplinary information we propose an evolutionary model for the tip of the extensional growth fault system that accounts for the progressively changing sediment rheological properties, and for the time dependent subsidiary deformation pattern by invoking the interplay between the regional stress field and the local, kinematically-derived one by fault activity. We also speculate on the overall implications for fluid flow of the proposed evolutionary model.  相似文献