排序方式： 共有16条查询结果，搜索用时 15 毫秒
1.
Polymetamorphism in garnet micaschists of the Saualpe Eclogite Unit (Eastern Alps,Austria), resolved by automated SEM methods and EMP–Th–U–Pb monazite dating 
下载免费PDF全文
下载免费PDF全文 B. Schulz 《Journal of Metamorphic Geology》2017,35(2):141-163
Polymetamorphic garnet micaschists from the Austroalpine Saualpe Eclogite Unit (Kärnten, Austria, Eastern Alps) display complex microstructural and mineral–chemical relationships. Automated scanning electron microscopy routines with energy dispersive X‐ray (EDX) spectral mapping were applied for monazite detection and garnet mineral–chemical characterization. When the Fe, Mg, Mn and Ca element wt% compositions are used as generic labels for garnet EDX spectra, complex zonations and porphyroblast generations can be resolved in complete thin sections for selective electron‐microprobe analyses. Two garnet porphyroblast generations and diverse monazite age populations have been revealed in low‐Ca and high‐Al‐metapelites. Garnet 1 has decreasing Mn, constant Ca and significantly increasing Mg from cores to rims. Geothermobarometry of garnet 1 assemblages signals a crystallization along a M1 prograde metamorphism at ~650 °C/6–8 kbar. Sporadic monazite 1 crystallization started at c. 320 Ma. Subsequent pervasive 300–250 Ma high‐Y and high‐Gd monazite 1 formation during decompression coincided with the intrusion of Permian and Early Triassic pegmatites. Monazite 1 crystallized along the margin of garnet 1. Coronas of apatite and allanite around the large 320–250 Ma monazite signal a retrogressive stage. These microstructures suggest a Carboniferous‐to‐Early‐Permian age for the prograde M1 event with garnet 1. Such a M1 event at an intermediate‐P/T gradient has not yet been described from the Saualpe, and preceded a Permo‐Triassic low‐P stage. The M2 event with garnet 2 postdates the corona formation around Permian monazite. Garnet 2 displays first increasing XCa at decreasing XMg, then increasing XCa and XMg, and finally decreasing XCa with increasing XMg, always at high Ca and Mg, and low Mn. This records a P–T evolution which passed through eclogite facies conditions and reached maximum temperatures at ~750 °C/14 kbar during decompression‐heating. A monazite 2 population (94–86 Ma) with lower Y and Gd contents crystallized at decreasing pressure during the Cretaceous (Eo‐Alpine) metamorphism M2 at a high‐P/T gradient. The Saualpe Eclogite Unit underwent two distinct clockwise metamorphic cycles at different P–T conditions, related to continental collisions under different thermal regimes. This led to a characteristic distribution pattern of monazite ages in this unit which is different from other Austroalpine basement areas. 相似文献
2.
Results from forward modelling of garnet growth and U–Th–Pb chemical dating suggest three periods of metamorphism that affected metapelitic rocks of the Rappold Complex (Eastern European Alps). Garnet first grew during Barrovian-type metamorphism, possibly during the Carboniferous Variscan orogeny. The second period of metamorphism produced monazite and resulted in minor garnet growth in some samples. Variable garnet growth was controlled by changes to the effective bulk rock composition resulting from resorption of older garnet porphyroblasts. Monazite crystals have variable morphology, textures and composition, but all yield Permian ages (267 ± 12 to 274 ± 17 Ma). In samples in which there was Permian garnet growth, monazite forms isolated and randomly distributed grains. In other samples, monazite formed pseudomorphous clusters after allanite. This difference is attributed to higher transport rates of monazite-forming elements in samples which underwent dehydration reactions during renewed garnet growth. The third and final period of garnet growth took place during Eo-Alpine (Cretaceous) metamorphism. Garnet of this age displays a wart-like texture. This may reflect transport-limited growth, possibly as a result of repeated dehydration during polyphase metamorphism. 相似文献
3.
A numerical modelling approach is used to validate the physical and geological reliability of the ablative subduction mechanism during Alpine convergence in order to interpret the tectonic and metamorphic evolution of an inner portion of the Alpine belt: the Austroalpine Domain. The model predictions and the natural data for the Austroalpine of the Western Alps agree very well in terms of P–T peak conditions, relative chronology of peak and exhumation events, P–T–t paths, thermal gradients and the tectonic evolution of the continental rocks. These findings suggest that a pre‐collisional evolution of this domain, with the burial of the continental rocks (induced by ablative subduction of the overriding Adria plate) and their exhumation (driven by an upwelling flow generated in a hydrated mantle wedge) could be a valid mechanism that reproduces the actual tectono‐metamorphic configuration of this part of the Alps. There is less agreement between the model predictions and the natural data for the Austroalpine of the Central‐Eastern Alps. Based on the natural data available in the literature, a critical discussion of the other proposed mechanisms is presented, and additional geological factors that should be considered within the numerical model are suggested to improve the fitting to the numerical results; these factors include variations in the continental and/or oceanic thickness, variation of the subduction rate and/or slab dip, the initial thermal state of the passive margin, the occurrence of continental collision and an oblique convergence. 相似文献
4.
Age constraints on the evolution of the Austroalpine basement to the south of the Tauern Window 总被引：1，自引：0，他引：1
Siegfried Siegesmund Till Heinrichs Rolf L. Romer Daniel Doman 《International Journal of Earth Sciences》2007,96(3):415-432
The Austroalpine basement to the south of the Tauern Window once was part of the northern margin of Gondwana. It includes
the “Altkristallin” and the phyllitic Thurntaler Complex. In the Altkristallin (AMU, MPU), suites of arc-related metamafic
sequences occur together with calc-alkaline metagranite. SHRIMP U–Pb dating of zircon from calc-alkaline metagranite associated
with an eclogitic amphibolite give an age of 470 ± 3 Ma interpreted as the age of protolith emplacement. In the Thurntaler
Complex, metaporphyroids occur together with tholeiitic as well as alkaline within-plate basalt-type metabasite. The metaryholites
of this association give a crystallization SHRIMP age of 477 ± 4 Ma, which suggests contemporaneity of arc-related and extensional
settings in the Austroalpine basement units. The age data demonstrate widespread magmatic activity associated with the Early-Ordovician
amalgamation at the end of the 550–470 Ma subduction–accretion–collision cycle. The Pb–Pb and U–Pb systematics of step-wise
leached staurolite and kyanite from the peak-metamorphic assemblage of the Altkristallin indicate that (1) step-wise leaching
of staurolite and kyanite yields the age of inclusions rather than the host; (2) zircon inclusions in staurolite suggest an
Ordovician or older age for the precursor of the staurolite-schists; (3) the weighted average of the 206Pb/238U data of the various leaching steps yields a Variscan age for the inclusions (ilmenite, biotite, and andesine). Since these
inclusions are part of the metamorphic mineral assemblage, this age provides a minimum estimate for staurolite growth, i.e.,
metamorphism. Thus, the Pb–Pb and U–Pb systematics of staurolite provide evidence for a Variscan metamorphism of the Austroalpine
basement, e.g., MPU, AMU and Thurntaler Complex, to the south of the Tauern Window. 相似文献
5.
Duration of Eo-Alpine metamorphic events obtained from multicomponent diffusion modeling of garnet: a case study from the Eastern Alps 总被引：2，自引：2，他引：0
Shah?Wali?FaryadEmail author Sumit?Chakraborty 《Contributions to Mineralogy and Petrology》2005,150(3):306-318
Rocks from the Micaschist–Marble Complex of the Wölz Tauern, which are part of the middle Austroalpine unit, contain large (up to 2 cm) garnet crystals that show clear evidence of multistage growth. Isotopic dating indicates that a Variscan (～270 Ma) garnet core was overgrown by new garnet formed during Eo-Alpine metamorphism at Cretaceous times. P-T paths for the Eo-Alpine metamorphism were obtained using the method of pseudosections (Powell and Holland in Metam Geol 16:309–343, 1998) and are consistent with earlier results from independent thermobarometry. Due to the large size of the garnets, growth zoning was preserved during amphibolite facies metamorphism at both Variscan and Alpine times. Full multicomponent diffusion modeling of compositional zoning at the interface of the Alpine and Pre-Alpine garnets in conjunction with the retrieved P-T paths allow average subduction/exhumation as well as heating/cooling rates to be retrieved. The modeling suggests that a minimum subduction/exhumation rate of ～4 cm/a and heating/cooling rates on the order of 100–260°C/Ma for a 60°C subduction angle are required to preserve the observed compositional zoning overall while modifying the zoning at the interface between two garnets to the extent observed. Such rapid rates of burial/exhumation are consistent with the results of direct GPS measurements of convergence rates at several orogenic belts as well as with inferred rates from modeling in the Alps and other areas. In combination, this indicates that such rapid rates are commonplace during metamorphism in collisional orogens and places important constraints on the rheological behavior of crustal blocks in such orogens. 相似文献
6.
New brachiopods (Dimerelloidea, Rhynchonellida) from Lower Jurassic (?lower Hettangian) hemipelagic sediments of the Swiss National Park in south-eastern Engadine are described: Sulcirostra doesseggeri sp. nov. and Carapezzia engadinensis sp. nov. Sulcirostra doesseggeri is externally similar to S. fuggeri (Frauscher 1883), a dubious species, that could not be included in a comparative study, because relevant samples no longer exist. A single specimen was tentatively assigned to Sulcirostra ?zitteli (Böse 1894) by comparison of its external morphology with S. zitteli from the type locality. The partly silicified brachiopods are associated with sponge spicules, radiolarians and crinoid ossicles. Macrofossils are rare: dictyid sponges, gastropods, bivalves, crustaceans, shark teeth and scales of an actinopterygian fish. The Lower Jurassic sediments (Alpisella beds, a basal member of the Allgäu Formation) preserving the brachiopods belong to the Ortler nappe (Upper Austroalpine nappes). The exact age of the Alpisella beds is not known, as index fossils are lacking. Their stratigraphic position above the Rhaetian Kössen Formation and below the ammonite-dated Trupchun beds suggests a very Early Jurassic, probably early Hettangian age for the new brachiopod fauna. The new species of Sulcirostra and Carapezzia are confined to a very small geographic area, a peculiarity also observed in other Early Jurassic dimerelloid brachiopods. These brachiopods presumably adapted to current-dominated submarine highs, where their shells could not accumulate, except when trapped in submarine cavities or re-deposited in submarine fans. Transport by turbidity currents is suggested for the Early Jurassic dimerelloids from the Engadine. Problems with the generic definition of Sulcirostra and the higher rank classification of Carapezzia are discussed. 相似文献
7.
Flysch and pelagic sedimentation of the Penninic and Austroalpine tectonic units of the Eastern Alps are results of the closure of the Tethyan-Vardar and the Ligurian-Piemontais Oceans as well as of the progressive deformation of the Austroalpine continental margin. The Austroalpine sequences are characterized by Lower Cretaceous pelagic limestones or minor carbonate flysch and various siliciclastic mid- and Upper Cretaceous flysch formations. Chrome spinel is the most characteristic heavy mineral delivered by the southern Vardar suture, the northern obduction belt at the South Penninic-Austroalpine margin and its continuation into the Klippen belt sensu lato of the Carpathians. The South Penninic sequences, e.g. the Arosa zone, the Ybbsitz Klippen zone and some flysch nappes also contain chrome spinel, whereas the sediments of the North Penninic Rhenodanubian flysch zone are characterized by stable minerals and garnet. 相似文献
8.
W. Prochaska W. Pohl R. Belocky H. Kucha 《International Journal of Earth Sciences》1995,84(4):831-842
The specularite deposit at Waldenstein is an epigenetic replacement/vein-type hematite mineralization with distinct alteration zones. It is situated in highly metamorphosed schists and, gneisses of the Koralm Crystalline Complex. Sericitization and chloritization are strictly bound to this alteration and occur within a few meters of the hematite veins. Radiometric dating of the sericitization and the implicity of the mineralization yield a Middle Eocene age. Petrographic evidence proves an early, relatively reduced paragenesis (magnetite, ilmenite) being partly replaced by hematite and chlorite during the main phase of ore formation. Fluid inclusion and stable isotope investigations indicate that formation temperatures were approximately 300°C during the main stage and decreased to 200°C in the latest stage of the hydrothermal event. The H-isotope compositions of chlorites and of fluid inclusions in quartz indicate the influence of seawater. This also explains the high salinities determined by fluid inclusion studies (20–30% NaCl equiv.). According to the genetic model presented in this paper the hydrothermal activity started when the deformation accompanying the uplift of the Koralm Crystalline Complex passed from a ductile to brittle regime (Middle Eocene). The brittle faults acted as channelways for rising, deep fluids, probably of metamorphic origin. Temperature differences between the different uplifting crustal segments supported fluid circulation. Precipitation of the ore occurred at shallow crustal levels where the hydraulic regime was dominated by seawater, and oxidation of the original reducing fluids took place. 相似文献
9.
A palynology and organic matter study has been carried out on samples of the Upper Triassic Raibl beds from an Upper Austroalpine
thrust sheet, preserved in the area of the Iberg Klippen (Laucherenst?ckli). The palynological assemblages indicate an early
Carnian (Julian) age. Comparison with well calibrated successions from the Southern Alps suggests a correlation with the Late
Julian ammonoid zone Austrotrachyceras austriacum.
The lithofacies of the studied sections suggests a substantial similarity to Raibl beds of the Upper Austroalpine of the Northern
Calcareous Alps and those of the Silvretta- and the S-charl nappe of the Grisons. The minute Upper Austroalpine thrust sheet
represents the westward extension of series with similar facies, which are still preserved in the Northern Calcareous Alps
of Liechtenstein and western Austria (Vorarlberg).
According to the Thermal Alteration Scale (TAS) of Batten (1996) the observed medium brown colors of the pollen grains correspond
to a value of 4/5, which is equivalent to vitrinite reflectance (VR) values of 0.7–0.9 %Ro. In comparison with other sites in Liechtenstein, Vorarlberg and the Upper Austroalpine of Central Grisons (Silvretta and
S-charl nappe) with measured VR values of > 2%Ro the organic matter of the studied samples shows only minor thermal alteration, indicating that the Upper Austroalpine thrust
sheets of Iberg represent the highest part of the nappe stack in Central Switzerland, formerly covered by a comparatively
thin overburden.
相似文献
10.