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1.
Raman microspectroscopy on carbonaceous material (RSCM) from the eastern Tauern Window indicates contrasting peak‐temperature patterns in three different fabric domains, each of which underwent a poly‐metamorphic orogenic evolution: Domain 1 in the northeastern Tauern Window preserves oceanic units (Glockner Nappe System, Matrei Zone) that attained peak temperatures (Tp) of 350–480 °C following Late Cretaceous to Palaeogene nappe stacking in an accretionary wedge. Domain 2 in the central Tauern Window experienced Tp of 500–535 °C that was attained either within an exhumed Palaeogene subduction channel or during Oligocene Barrovian‐type thermal overprinting within the Alpine collisional orogen. Domain 3 in the Eastern Tauern Subdome has a peak‐temperature pattern that resulted from Eo‐Oligocene nappe stacking of continental units derived from the distal European margin. This pattern acquired its presently concentric pattern in Miocene time due to post‐nappe doming and extensional shearing along the Katschberg Shear Zone System (KSZS). Tp values in the largest (Hochalm) dome range from 612 °C in its core to 440 °C at its rim. The maximum peak‐temperature gradient (≤70 °C km?1) occurs along the eastern margin of this dome where mylonitic shearing of the Katschberg Normal Fault (KNF) significantly thinned the Subpenninic‐ and Penninic nappe pile, including the pre‐existing peak‐temperature gradient.  相似文献   

2.
Subsolidus and melting relations for the CaCO3-MgCO3 join at 30 kb have been determined using piston-cylinder apparatus. Data are also presented for the melting curve of CaCO3 to 30 kb, the decomposition and melting curves of MgCO3 to 36 kb, and the calcite-aragonite transition at 800°C, 950°C and 1100°C. At 30kb, the melting loop for the CaCO3-MgCO3 join extends from 1610°C (CaCO3) to 1585°C (MgCO3) through a liquidus minimum at 1290°C (near 42 mole% MgCO3). The dolomite-magnesite solvus barely intersects the 30 kb melting loop to produce a peritectic reaction at 1385°C. Integration of the new experimental data with other published data permits construction of a complete P-T projection and a sequence of isobars for the CaCO3-MgCO3 join for pressures between 5 and 30 kb. The phase relations for this join provide part of the essential framework of the model peridotite system CaO-MgO-SiO8-CO2-H2O, which has particular application to the origin of carbonatitic and kimberlitic magmas. In light of the accumulating evidence for CO2 in various forms within the upper mantle and of its effect on magmatic processes, analysis of the melting relations in this system is of considerable importance.  相似文献   

3.
Mixtures of pure dry CaCO3 and MgCO3 were reacted at 10 kbar in a piston-cylinder apparatus. Solidus and liquidus boundaries were delineated by interpretation of quenched textures. X-ray determined compositions of quenched carbonates are not a reliable guide to the phase relations. The binary melting loop for CaCO3-MgCO3 extends from CaCO3 at 1460°C through a liquidus minimum near 30 wt% MgCO3 and 1075°C, and it is terminated at the incongruent melting reaction for dolomite solid solution at 1125° C (liquid with 32 wt% MgCO3) Magnesite solid solution dissociates at 1090°C to produce dolomite + periclase + CO2, truncating the dolomite-magnesite solvus. The 10 kb liquidus minimum at 1075°C and 30 wt% MgCO3 occurs at lower temperature and higher CaMg ratio than the 27 kbar liquidus minimum at 1290°C and 38 wt% MgCO3. This relationship suggests that the first liquid produced by melting of a carbonate-bearing peridotite has increasing MgCa ratio with increasing pressure. These phase relations provide part of the framework required to trace paths of crystallization of kimberlite and carbonatite magmas.  相似文献   

4.
Limited solid solution of Mg and Fe2+ occurs in calcite coexisting with dolomiteankerite. This substitution is strongly temperature-dependent. Experimentally determined calcite compositions co-existing with a dolomite phase are available in the binary system CaCO3-MgCO3 between 500° C and 900° C (Harker and Tuttle, 1955). This information is extrapolated to lower temperatures and is combined with three synthetic calcite-ankerite pairs determined at 400° C, 450° C and 500° C (Rosenberg, 1967).The compositions of six naturally occurring calcites coexisting with ankerites from Sofala, N.S.W. are not accurately known, but X-ray determined compositional limits assuming firstly Mg substitution then Fe substitution yield maximum and minimum values for both possibilities. These limits are plotted on the ternary CaCO3-MgCO3-FeCO3 together with the experimentally derived solvus isotherms. Assuming equilibration at constant temperature, actual compositions of these natural calcites plot along the 415° C isotherm.  相似文献   

5.
Abstract The Pennine rocks exposed in the south-east Tauern Window, Austria, contain mineral assemblages which crystallized in the mid-Tertiary ‘late Alpine’regional metamorphism. The pressure and temperature conditions at the thermal peak of this event have been estimated for rocks at four different structural levels using a variety of published and thermochemically derived geobarometers and geothermometers. The results are: (a) In the garnet+chlorite zone, 2–5 km structurally above the staurolite+biotite isograd: T= 490.50°C, P= 7° 1 kbar; (b) Within 0.5 km of the staurolite+biotite isograd: T= 560±300C, P=7.1 kbar; (c) In the staurolite+biotite zone, c. 2.5 km structurally below the staurolite+biotite isograd: T= 610±30°C, P=7.6±1.2 kbar; (d) In the staurolite+biotite zone, 3–4 km structurally below the staurolite+biotite isograd: T= 630±40°C, P= 6.6±1.2 kbar. The pressure estimates imply that the total thickness of overburden above the basement-cover interface in the mid-Tertiary was c. 26.4 km. This overburden can only be accounted for by the Austro-Alpine units currently exposed in the vicinity of the Tauern Window, if the Altkristallin (the ‘Middle Austro-Alpine’nappe) was itself buried beneath an ‘Upper Austro-Alpine’nappe or nappe-pile which was 7.4 km thick at that time. The occurrence of epidote + margarite + quartz pseudomorphs after lawsonite in garnet, indicates that part of the Mesozoic Pennine cover sequence in the south-east Tauern experienced blueschist-facies conditions (T<450°C, P<12 kbar) in early Alpine times. Evidence from the central Tauern is used to argue that the blueschist-facies imprint post-dated the main phase of tectonic thickening (D1A) and was thus a direct consequence of continental collision. Combined oxygen-isotope and fluid-inclusion studies on late-stage veins, thought to have been at lithostatic pressure and in thermal equilibrium with their host rocks during formation, suggest that they crystallized from aqueous fluids at 1.1±0.4 kbar and 420.20°C. Early Alpine, late Alpine and vein-formation P–T constraints have been used to construct a P–T path for the base of the Mesozoic cover sequence in the south-east Tauern Window. The prograde part of the P–T path, between early and late Alpine metamorphic imprints, is unlikely to have been a smooth curve and may well have had a low dP/dT overall; the decompression (presumably due to erosion) which occurred immediately before the thermal peak and possibly also earlier in the Tertiary, was probably partly or completely cancelled by the effects of early- to mid-Tertiary (D2A) tectonic thickening. The thermal peak of metamorphism was followed by a phase of almost isothermal decompression, which implies a period of rapid uplift in the middle Tertiary. The peak metamorphic P–T estimates are compared with the solutions of England's (1978) one-dimensional conductive thermal model of the Eastern Alps, and are shown to be consistent with the idea that the late Alpine metamorphism was caused by tectonic burial of the Pennine Zone beneath the Austro-Alpine nappes in the absence of extraneous heat sources, such as large intrusions, at depth.  相似文献   

6.
Monometamorphic metasediments of Paleozoic or Mesozoic age constituting Schneeberg and Radenthein Complex experienced coherent deformation and metamorphism during Late Cretaceous times. Both complexes are part of the Eoalpine high-pressure wedge that formed an intracontinental suture and occur between the polymetamorphosed Ötztal–Bundschuh nappe system on top and the Texel–Millstatt Complex below. During Eoalpine orogeny Schneeberg and Radenthein Complexes were south-dipping and they experienced a common tectonometamorphic history from ca. 115 Ma onwards until unroofing of the Tauern Window in Miocene times. This evolution is subdivided into four distinct tectonometamorphic phases. Deformation stage D1 is characterized by WNW-directed shearing at high temperature conditions (550–600°C) and related to the initial exhumation of the high-pressure wedge. D2 and D3 are largely coaxial and evolved during high- to medium-temperature conditions (ca. 450 to ≥550°C). These stages are related to advanced exhumation and associated with large-scale folding of the high-pressure wedge including the Ötztal-Bundschuh nappe system above and the Texel–Millstatt Complex below. For the area west of the Tauern Window, F2/F3 fold interference results in the formation of large-scale sheath-folds in the frontal part of the nappe stack (formerly called “Schlingentektonik” by previous authors). Earlier thrusts were reactivated during Late Cretaceous normal faulting at the base of the Ötztal–Bundschuh nappe system and its cover. Deformation stage D4 is of Oligo-Miocene age and accounted for tilting of individual basement blocks along large-scale strike-slip shear zones. This tilting phase resulted from indentation of the Southern Alps accompanied by the formation of the Tauern Window.  相似文献   

7.
Experimental runs were made in cold-seal pressure vessels using synthetic CaCO8·6H2O, calcite and aragonite as starting materials. The reaction CaCO3·6H2O (ikaite) ? CaCO3 (calcite I) + 6H2O was reversed across its metastable extension into the aragonite stability field and the phase boundary is defined by brackets at 4.14kb, 14.3°C and 2.96 kb, ?3.0°C. An invariant point for CaCO3·6H2O, calcite I, aragonite and water thus occurs at about 3.02 kb and ?2.0°C. No other reaction could be reversed. Calculations based on the equilibrium phase boundary between calcite and ikaite and the available thermochemical data for calcite and water yield the stadard free energy of formation, standard enthalpy of formation and third law entropy of CaCO3·6H2O at 25°C and 1 bar total pressure; ?607.3 kcal/mole, ?705.8 kcal/mole, and 88.4 cal/deg mole, respectively.  相似文献   

8.
The hornblende garbenschist horizon of the Lower Schieferhulleseries (LSH) in the SW Tauern Window, Austria, contains theassemblage hornblende + kyanite + staurolite + garnet + biotite+ epidote + plagioclase + ankerite + quartz + rutile + ilmenite,with either chlorite or paragonite present in all samples. Theseassemblages are divariant in the system SiO2-Al2O3-TiO2-Fe2O3-MgO-FeO-MnO-CaO-Na2O-K2O-H2O-CO2.Garnet-biotite geothermometry yields temperatures of final equilibrationof {small tilde}550 °C, and garnet-plagioclase-kyanite-quartzgeobarometry indicates pressures of 6–8 kb for the matrixassemblage and 9–10 kb for plagioclase inclusions in garnet.Quantitative modelling of zoned garnet, hornblende, and plagioclaseindicates growth and equilibration along a decompression pathfrom {small tilde}530 °C, 10 kb to {small tilde}550 °C,7 kb. Fluid inclusion data constrain the uplift path to havepassed through a point at {small tilde} 375 °C, 1.5 kb. These data permit the construction of a relatively completeP-T loop for metamorphism associated with the Alpine orogeniccycle in the LSH of the SW Tauern Window. The maximum pressureconditions ({small tilde}10 kb at 530 °C) recorded alongthis loop are considerably higher than previous estimates of5–7 kb for the region. Simple overthrust models developedfor the Tauern Window cannot account for pressures of this magnitude;a more likely scenario involves partial subduction of the rocksto a depth of {small tilde}35 km, followed by prolonged heatingin response to decay of the subduction isotherms. Initial upliftappears to have been rapid and occurred along a nearly isothermalpath. Significant cooling did not occur until the rocks werewithin {small tilde}5 km of the surface. Detailed tectonic modelsfor the evolution of the Tauern Window must be able to accountfor the quantitative features of the P-T loop.  相似文献   

9.
The Ordovician Kellerjochgneiss (Schwaz Augengneiss) is a polymetamorphic orthogneiss-bearing unit and is part of the Austroalpine basement nappes north of the Tauern Window. Within the Kellerjochgneiss a small, strongly deformed metapegmatite dike occurs. The pegmatite crosscuts the gneiss discordantly and contains the mineral assemblage muscovite 1,2+plagioclase+K-feldspar+chlorite+quartz+garnet 1 (Alm67–76Andr0.9–2Sps17–28Prp0.4–5)+garnet 2 (Grs36–46Alm24–32Andr8–21Sps15–17Prp0–1)±stilpnomelane±biotite±clinozoisite. The magmatic protolith assemblage is comprised of relict K-feldspar, quartz and garnet 1. Textural observations indicate that biotite and muscovite cores (muscovite 1) are either part of the magmatic- or an earlier (Variscan?) metamorphic assemblage. Geothermobarometry of the metapegmatite was done on the latest-stage (Eo-Alpine) mineral assemblage garnet 2+muscovite 2+chlorite+stilpnomelane+plagioclase+quartz. Calculations of H2O-absent intersections in the system [KCNFMAS] with the multi-equilibrium program THERMOCALC v.3.1 yielded P–T estimates of 4.4 to 6.7 kbar and 321°C to 376°C. Calculations of the P–T conditions by using the assemblage muscovite 2+chlorite+stilpnomelane+quartz yielded slightly higher pressures of 6.4 to 7.2 kbar at temperatures of 310–325°C. Correlating these P–T data with geochronological data from the neighbouring lithologies (Kellerjochgneiss, Innsbruck Quartzphyllite, Wildschönau Schists) and with structural investigations from these units indicate that the P–T estimates obtained in this investigation represent the Eo-Alpine metamorphic overprint. Hence, these unusual rocks provide important information on the Eo-Alpine P–T conditions since most samples studied from the investigated Austroalpine basement nappes north of the Tauern Window rarely contain mineral assemblages suitable for geothermobarometry.  相似文献   

10.
New Hornblende K-Ar and 39Ar-40Ar and mica Rb-Sr and K-Ar ages are used to place specific timemarks on a well-constrained pressure-temperature path for the late Alpine metamorphism in the Western Tauern Window. After identification of excess 40Ar, the closure behavior of Ar in hornblende is compared with that of Sr and Ar in phengite and biotite. Samples were collected in three locations, whose maximum temperatures were 570° C (Zemmgrund), 550° C (Pfitscher Joch), and 500–540° C (Landshuter Hütte).The average undisturbed age sequence found is: Phengite Rb-Sr (20 Ma)>hornblende K-Ar (18 Ma)>phengite K-Ar (15 Ma)>biotite Rb-Sr, K-Ar (13.3 Ma)>apatite FT (7 Ma). Except for the phengite Rb-Sr age, the significance of which is debatable, all ages are cooling ages. No compositional effects are seen for closure in biotite. Additionally, Rb-Sr phengite ages from shearzones possibly indicate continuous shearing from 20 to 15 Ma, with reservations regarding the validity of the initial Sr correction and possible variations of the closure temperatures. The obviously lower closure temperature (T c) for Ar in these hornblendes than for Sr in the unsheared phengites indicates that the T c sequence in the Western Tauern Window is different from those observed in other terrains. In spite of this discrepancy, valuable geological conclusions can be drawn if the application of closure temperatures is limited to this restricted area with similar T, P and : (1) All ages of samples located on equal metamorphic isotherms decrease from east to west by about 1 Ma which is the result of a westward tilting of the Tauern Window during uplift. (2) In a PT-path, the undisturbed cooling ages yield constantly decreasing uplift rates from 3.6 mm/a to 0.1 mm/a. (3) Use of recently published diffusion data for Ar in hornblende (T c=520° C) and biotite (T c=320° C) suggests an extrapolated phengite closure temperature for Sr at 550° C. This suggests that the prograde thermal metamorphism at this tectonic level of the Tauern Window lasted until some 20 Ma ago.  相似文献   

11.
Fourteen cogenetic quartz-biotite pairs from gneissic wall rocks, and 22 quartz, 16 calcite, and 8 biotite samples and 1 sample of albite from fissure-filling veins in the Western Tauern Window were analyzed for their oxygen isotope composition. The δ18O values show the following ranges: (a) quartz, +6.0 in fissure in amphibolite to +10.3 in fissures in granite gneisses; (b) biotite, +2.5 to +6.7; and (c) calcite, +7.0 to +8.9. The δ18O value of albite is +7.1. Only a small variation in the hydrogen isotope composition of biotite was detected. δD values of 7 biotites from gneisses and fissure fillings varied from −54 to −59. There is no significant difference in the hydrogen isotope composition of fissure biotite and biotite from the host rock. This indicates that a common water source of probably deep-seated origin existed, with no detectable contribution from isotopically light meteoric water. Oxygen isotope fractionations between coexisting quartz and biotite of 3.5 to 7.0‰ indicate equilibrium temperatures of 640 ° to 450 ° C, respectively, using the fractionation curve of Hoernes and Friedrichsen (1978). The highest temperatures of equilibration are for the rocks at the Alpenhauptkamm, i.e., the central part of the Tauern Window. Successively lower temperatures are found to the north and to the south of the Alpenhauptkamm along a traverse through Penninic units of the Tauern Window. The metamorphism of the host rocks and the filling of fissures has occurred at the same temperature in a given sample locality.  相似文献   

12.
New petrologic and 40Ar/39Ar geochronologic data constrain conditions of Alpine metamorphism along the northwestern border of the Tauern Window. The P-T estimations based on phengite barometry were determined for samples from units of the Lower Austroalpine nappe complex exposed above the Southpenninic interior of the Tauern Window, and from upper parts of the Southpenninic “Bündner Schiefer” sequence. Results suggest that both Mesozoic metasedimentary nappe units (Reckner and Hippold Nappes) and an ophiolitic nappe (Reckner Complex) of the Lower Austroalpine nappe complex have been metamorphosed at pressures between 8 and 10.5 kbar and temperatures around 350 °C. The structurally highest Lower Austroalpine unit (Quartzphyllite Nappe) was not affected by high-pressure metamorphism and records maximum P-T conditions of approximately 4 kbar and 400 °C. Highest parts of the structurally underlying Southpenninic Bündner Schiefer sequence were metamorphosed at intermediate pressures (6–7 kbar). Temperatures increased in all structural units during decompression. Whole-rock 40Ar/39Ar plateau ages of silicic phyllites and cherts with abundant high-Si phengites record ages around 50 Ma in the Reckner Nappe, and 44–37 Ma in the Hippold Nappe and Southpenninic Bündner Schiefer sequence. These ages are interpreted to date closely the high-pressure metamorphism. The Lower Austroalpine-Southpenninic border area in the NW Tauern Window appears to have evolved along an indented, fragmented active continental margin where the Reckner Complex represents one of the oldest sections of the Southpenninic (Piemontais) Oceanic tract that was originally situated close to, or even within, the Lower Austroalpine continent. During closure of the Piemontais Ocean, the resultant subduction zone did not entrain components of the Reckner Complex or its cover sequences (Reckner and Hippold Nappes): therefore “Eoalpine” high-pressure metamorphism did not occur. Sequences exposed within the study area were subducted to relatively shallow depths during the last stage of consumption of oceanic crust and immediately prior to final continental collision. Received: 30 July 1996 / Accepted: 7 April 1997  相似文献   

13.
The paired chemical reactions, Ca2+ + 2HCO3 ? ? CaCO3 + CO2 + H2O, overestimate the ratio of CO2 flux to CaCO3 flux during the precipitation or dissolution of CaCO3 in seawater. This ratio, which has been termed ??, is about 0.6 in surface seawater at 25°C and at equilibrium with contemporary atmospheric CO2 and increases towards 1.0 as seawater cools and pCO2 increases. These conclusions are based on field observations, laboratory experiments, and equilibrium calculations for the seawater carbonate system. Yet global geochemical modeling indicates that small departures of ?? from 1.0 would cause dramatic, rapid, and unrealistic change in atmospheric CO2. ?? can be meaningfully calculated for a water sample whether or not it is in equilibrium with the atmosphere. The analysis presented here demonstrates that the atmospheric CO2 balance can be maintained constant with respect to seawater CaCO3 reactions if one considers the difference between CaCO3 precipitation and burial and differing values for ?? (both <1.0) in regions of precipitation and dissolution within the ocean.  相似文献   

14.
The interior of the Tauern Window exposes underplated Penninic continental lithosphere and the overlying obducted Penninic oceanic crust within a large antiformal dome in the internal zone of the Eastern Alps. These units have been affected by a polyphase deformation history. Generally, three deformation events are distinguished. D1 is related to underplating of, and top-to-the-N nappe stacking within, the Penninic continental units of the Tauern Window. Deformation stage D2 is interpreted to reflect the subsequent continent collision between the Penninic continental units and the European foreland, D3 is related to the formation of the dome structure within the Tauern Window. During thickening of continental lithosphere and nappe stacking (D1), and subsequent intracontinental shortening (D2), these tectonic units have been ductilely deformed close to a plane strain geometry. Conditions for the plastic deformation of the main rock-forming mineral phases (quartz, feldspar, dolomite, calcite) have prevailed during all three phases of crustal deformation. Generally, two types of quartz microstructures that are related to D1 are distinguished within the Tauern Window: (a) Equilibrated and annealed fabrics without crystallographic preferred orientations (CPO) have only been observed in the central part of the southeastern Tauern Window, corresponding with amphibolite-grade metamorphic conditions. (b) In the northeastern and central part of the Tauern Window microstructures are characterized by quartz grains that show equilibrated shape fabrics, but well preserved CPO with type-I cross girdle distributions, indicating a deformation geometry close to plane strain. During D2, two types of quartz microstructures are distinguished, too: (a) Quartz grains that show equilibrated shape fabrics, but well-preserved CPO. The c-axes distributions generally are characterized by type-I cross girdles, locally by type-II cross girdles, and in places, oblique single girdle distributions. (b) A second type of quartz microstructure is characterized by highly elongated grains and fabrics typical for dislocation creep and grain-boundary migration, and strong CPO. This type is restricted to the southern sections of the western and eastern Tauern Window. The c-axis distributions show type-I cross girdles in the western part of the Tauern Window and single girdles in the southeastern part. In the western part of the Tauern Window, a continuous transition from type (b) microstructures in the south to type (a) microstructures in the north is documented. The microstructural evolution also documents that the dome formation in the southeastern and western Tauern Window has already started during D2 and has continued subsequent to the equilibration during amphibolite to greenschist facies metamorphism. D3 is restricted to distinct zones of localized deformation. D3-related quartz fabrics are characterized by the formation of ribbon grains; the c-axes show small-circle distributions around the Z-axis of the finite-strain ellipsoid. During exhumation and doming (D3), deformation occurred under continuously decreasing temperatures.  相似文献   

15.
Summary ?The low-pressure eutectic for the coprecipitation of calcite, portlandite, and periclase/brucite (with H2O-rich vapor) has served as a model for the existence and crystallization of carbonatite magmas. Attempts to determine conditions for the appearance of dolomite at this eutectic have been unsuccessful. We have discovered a second low-temperature eutectic for more magnesian liquids which excludes portlandite and includes dolomite (all results are vapor-saturated). Addition of Ca(OH)2-Mg(OH)2 to CaCO3-MgCO3 at 0.2 GPa depresses the liquidus to temperatures below the crest of the calcite-dolomite solvus; the vapor-saturated liquidus surface falls steeply, and the field boundary for liquids coexisting with calcite and periclase reaches a peritectic at 880 °C, where a narrow field for liquidus dolomite begins, extending down to the eutectic at 659 °C for the coprecipitation of calcite, dolomite and periclase (brucite should replace periclase at slightly higher pressures). The calcite liquidus is very large. The field boundary for coexistence of calcite and dolomite extends approximately in the direction from CaMg(CO3)2 towards Mg(OH)2. The results illustrate conditions for the formation of mineral-specific cumulates from variable magma compositions. Hydrous (or sodic) carbonate-rich liquids with compositions from CaCO3 to CaMg(CO3)2 will precipitate calcite-carbonatites first, followed by calcite-dolomite-carbonatites, with the prospect of precipitating dolomite-carbonatite alone through a limited temperature interval, and with periclase joining the assemblage in the closing stages. Periclase in the Fe-free system may represent the ubiquitous occurrence of magnetite in natural carbonatites. The restricted range for the precipitation of dolomite-carbonatites adds credibility to the evidence for primary magnesiocarbonatite (near-dolomite composition) magmas. Magnesiocarbonatite magmas can precipitate much calcite-carbonatite rock.
Zusammenfassung ?Calciokarbonatitische und magnesiokarbonatitische Gesteine und Magmen im System CaO-MgO-CO 2 -H 2 O bei 0.2 GPa Das Niedrigdruck-Eutektikum der gemeinsamen Ausscheidung von Calcit, Portlandit und Periklas/Brucit (mit H2O-reicher Fluidphase) diente als Modell um die Existenz und Kristallisation karbonatitischer Magmen zu erkl?ren. Versuche die Bedingungen des Auftretens von Dolomit an diesem Eutektikum zu bestimmen blieben bisher ergebnislos. Wir entdeckten ein zweites Niedrigtemperatur-Eutektikum für magnesiumreichere Schmelzen, das Portlandit ausschlie?t, aber Dolomit inkludiert (alle Ergebnisse bei Fluids?ttigung). Die Zugabe von Ca(OH)2-Mg(OH)2 zu CaCO3-MgCO3 bei 0.2 GPa senkt den Liquidus auf Temperaturen unter die Solvus-Schwelle von Calcit-Dolomit. Die fluidges?ttigte Liquidusfl?che verl?uft steil und die Grenzfl?che von Schmelze, die mit Calcit und Periklas koexistiert erreicht ein Peritektikum bei 880 °C. Dort ?ffnet sich ein schmales Feld für Liquidus-Dolomit, das bis zum Eutektikum bei 659 °C reicht, an dem Calcit, Dolomit und Periklas (Brucit sollte Periklas bei geringfügig h?heren Drucken ersetzen) gemeinsam ausgeschieden werden. Der Calcit- Liquidus ist sehr gro?. Die Linie an der Calcit und Dolomit koexistieren erstreckt sich ungef?hr von CaMg(CO3)2 zu Mg(OH)2. Die Ergebnisse zeigen die Bildungsbedingungen für die Bildung mineralspezifischer Kumulate aus unterschiedlichen Magmenzusammensetzungen. Aus w?ssrigen (oder Na-reichen) karbonatreichen Schmelzen mit Zusammensetzungen zwischen CaCO3 und CaMg(CO3)2 werden sich zuerst Calcitkarbonatite und dann Calcit-Dolomitkarbonatite ausscheiden, mit der M?glichkeit Dolomitkarbonatite über ein sehr eingeschr?nktes Temperaturintervall zu bilden und mit Periklas, der zu dieser Vergesellschaftung im Endstadium hinzukommt. Periklas im Fe-freien System k?nnte das weitverbreitete Analog zu Magnetit in natürlichen Karbonatiten sein. Der enge Bereich für die Ausscheidung von Dolomitkarbonatiten untermauert die Existenz prim?rer magnesiokarbonatitischer Magmen (nahe der Zusammensetzung von Dolomit). Magnesiokarbonatitische Magmen k?nnen daher entsprechende Mengen an calcitkarbonatitischen Gesteinen ausscheiden.

Received July 20, 1998;/revised version accepted August 18, 1999  相似文献   

16.
Metasediments in the southern Grossvenediger area (Tauern Window, Austria) were studied along a cross-section through rocks of increasing metamorphic grade from the margin of the Tauern Window in the south to the base of the Upper Schieferhülle, including the Eclogite Zone, in the north. In the southern part of the cross-section there is no evidence for a pre-late Alpine metamorphic history in the form of high-pressure relics or pseudomorphs. Mineral assemblages are characterized by the stability of tremolite + calcite, biotite + calcite and biotite + chlorite + calcite. In the northern part a more complete Alpine metamorphic evolution is preserved. Primary high-pressure assemblages are dolomite + quartz, tremolite + zoisite, zoisite + dolomite + quartz + phengite I and probably tremolite + dolomite + phengite I. Secondary, post-kinematic assemblages [tremolite + calcite, talc + calcite, phengite II + chlorite + calcite (+ quartz), biotite + chlorite + calcite, biotite + zoisite + calcite] formed as a result of the dominant late Alpine metamorphic overprint. The occurrence of biotite + zoisite + calcite is confined to the northernmost area and defines a biotite–zoisite–calcite isograd. P–T estimates based on standard thermobarometric techniques and on stability relationships of tremolite + calcite + dolomite + quartz and zoisite give consistent results. P–T conditions of the main Tertiary metamorphic overprint were 525° C, P= 7.5 ± 1 kbar in the northern part of the cross-section. The southern part was metamorphosed at lower temperatures of 430–470° C. The Si-content of phengites from this area is almost as high as that of phengites from the Eclogite Zone (Simax= 3.4 pfu). Pressures > 10 kbar at 420° C are suggested by phengite barometry according to Massone & Schreyer (1987). In the absence of high-pressure relics or pseudomorphs, these phengites, which lack late Alpine re-equilibration, are the only record that rocks of the southern part probably also experienced an early non-eclogitic high-pressure metamorphism.  相似文献   

17.
This study focuses on metapelites of the Polinik complex in the Kreuzeck Mts. southeast of the Tauern Window, Eastern Alps, where kyanite — staurolite — garnet gneisses host eclogites and high pressure (HP) amphibolites of the Austroalpine basement. The stable mineral assemblage is garnet — staurolite — biotite — kyanite — quartz. Estimated metamorphic conditions from conventional geothermobarometry are 654±30 °C and 0.9±0.08 GPa, and Average P-T values calculated by THERMOCALC, are 665±15 °C at 0.77±0.09 GPa. Formation of the present mineral association in gneisses is related to the exhumation (D2) stage of hosted eclogites/HP amphibolites within a lateral strike-slip zone.  相似文献   

18.
Subduction carries atmospheric and crustal carbon hosted in the altered oceanic crystalline basement and in pelagic sediments back into the mantle. Reactions involving complex carbonate solid solutions(s) lead to the transfer of carbon into the mantle, where it may be stored as graphite/diamond, in fluids or melts, or in carbonates. To constrain the thermodynamics and thus reactions of the ternary Ca–Mg–Fe carbonate solid solution, piston cylinder experiments have been performed in the system CaCO3–MgCO3–FeCO3 at a pressure of 3.5 GPa and temperatures of 900–1,100°C. At 900°C, the system has two miscibility gaps: the solvus dolomite–calcite, which closes at X MgCO3 ~0.7, and the solvus dolomite–magnesite, which ranges from the Mg to the Fe side of the ternary. With increasing temperature, the two miscibility gaps become narrower until complete solid solutions between CaCO3–Ca0.5Mg0.5CO3 is reached at 1,100°C and between CaCO3–FeCO3 at 1,000°C. The solvi are characterized by strong compositional asymmetry and by an order–disorder mechanism. To deal with these features, a solid solution model based on the van Laar macroscopic formalism has been calculated for ternary carbonates. This thermodynamic solid solution model is able to reproduce the experimentally constrained phase relations in the system CaCO3–MgCO3–FeCO3 in a broad P–T range. To test our model, calculated phase equilibria were compared with experiments performed in carbonated mafic protolithes, demonstrating the reliability of our solid solution model at pressures up to 6 GPa in complex systems.  相似文献   

19.
High-pressure zoisite- and clinozoisite-bearing segregations are common in garnet- and albite-bearing amphibolites of the Palaeozoic part of the Lower Schieferhülle, south-central Tauern Window, Austria. The zoisite segregations (primary assemblage: Zo+Qtz+Cal) formed during an early to pre-Hercynian high-pressure event (P≫0.6 GPa, T =500–550 °C) by hydrofracturing as a result of protolith dehydration. Zoisite is growth zoned from Fe3+-poor cores (Al2Fe=9 mol%) to Fe3+-rich rims (17 mol%), and has high Sr, Pb and Ga contents and LREE-enriched REE patterns, controlling the trace element budget of the segregations. Hercynian deformation at c. 0.7 GPa/600 °C kinked and cracked primary zoisite and enhanced breakdown into secondary zoisite (13 mol% Al2Fe), clinozoisite (40–55 mol% Al2Fe), albite (an<20), calcite and white mica during an Eoalpine high-pressure event at 0.9–1.2 GPa/400–500 °C. The clinozoisite segregations (primary assemblage: Czo+Qtz+Omp+Ttn+Chl+Cal) are mm- to cm-wide, vein-like bodies, cross-cutting fabric elements of the host garnet amphibolite. They formed during the Eoalpine high-pressure event at 0.9–1.2 GPa/400–500 °C. During Alpine exhumation, omphacite was pseudomorphed by amphibole, albite, quartz and clinozoisite. Oxygen isotope data suggest equilibrium between host metabasite and zoisite segregations and indicate an internal fluid source and fluid buffering by the protolith. Mobility of P, Nb and LREE changed the protolith’s trace element composition in the vicinity of the zoisite segregations: Mobilization of LREE is evidenced by decreasing modal amounts of LREE-rich epidote and decreasing LREE contents in LREE-rich epidote towards the segregations, changing the REE patterns of the host metabasite from LREE-enriched to LREE-depleted. Tectonic discrimination diagrams, based on the trace element content of metabasites, should be applied with extreme caution.  相似文献   

20.
Three monazite generations were observed in garnet-bearing micaschists from the Schobergruppe in the basement to the south of the Tauern Window, Eastern Alps. Low-Y monazite of Variscan age (321?±?14?Ma) and high-Y monazite of Permian age (261?±?18?Ma) are abundant in the mica-rich rock matrix and in the outer domains of large garnet crystals. Pre-Alpine monazite commonly occurs as polyphase grains with low-Y Variscan cores and high-Y Permian rims. Monazite of Eo-Alpine age (112?±?22?Ma) is rarer and was observed as small, partly Y-enriched grains (3?wt. %?Y2O3) in the rock matrix and within garnet. Based on monazite-xenotime thermometry, Y?+?HREE values in monazite indicate minimum crystallization conditions of 500?°C during the Variscan and 650?°C for the Permian and Alpine events, respectively. Garnet zoning and thermobarometric calculations with THERMOCALC 3.21 record an amphibolite facies, high-pressure stage of ~600?°C/13?C16?kbar, followed by a thermal maximum at 650?C700?°C and 6?C9?kbar. The Eo-Alpine age for these two events is supported by inclusions of Cretaceous monazite in the garnet domains used for thermobarometric constraints and through the high growth temperatures of Eo-Alpine monazite, which is consistent with that of the thermal maximum (~700?°C). The age and growth conditions of a few Mn-rich garnet cores, sporadically present within Eo-Alpine garnet, are unclear because inclusions of monazite, plagioclase and biotite necessary for thermobarometric- and age constraints are absent. However, based on monazite thermometry, Permian and Variscan metamorphic conditions were high enough for the growth of pre-Alpine garnet. The formation of Variscan garnet and its later resorption, plus Y-release, would also explain the high Y in Permian monazite, which cannot originate from preexisting Variscan monazite only. Monazite of Variscan, Permian and/or Eo-Alpine ages were also observed in other garnet-bearing micaschists from the Schobergruppe. This suggests that the basement of the Schobergruppe was overprinted by three discrete metamorphic events at conditions of at least lower amphibolite facies. While the Variscan event affected all parts of this basement, the younger events are more pronounced in its structurally lower units.  相似文献   

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