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Reaction textures and fluid behaviour in very high pressure calc-silicate rocks of the M�nchberg gneiss complex, Bavaria, Germany

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Abstract

Calc-silicate rocks occur as elliptical bands and boudins intimately interlayered with eclogites and high-pressure gneisses in the Munchberg gneiss complex of NE Bavaria. Core assemblages of the boudins consist of grossular-rich garnet, diopside, quartz, zoisite, clinozoisite, calcite, rutile and titanite. The polygonal granoblastic texture commonly displays mineral relics and reaction textures such as postkinematic grossular-rich garnet coronas. Reactions between these mineral phases have been modelled in the CaO-Al203-Si02-C02-H2 0 system with an internally consistent thermodynamic data base. High-pressure metamorphism in the calc-silicate rocks has been estimated at a minimum pressure of 31 kbar at a temperature of 630°C with X^oSQ.Gi. Small volumes of a C02-N2-rich fluid whose composition was buffered on a local scale were present at peak-metamorphic conditions. The P-T conditions for the onset of the amphibolite facies overprint are about 10 kbar at the same temperature. A'co., of the H20-rich fluid phase is regarded to have been <0.03 during amphibolite facies conditions. These P-T estimates are interpreted as representing different stages of recrystallization during isothermal decompression. The presence of multiple generations of mineral phases and the preservation of very high-pressure relics in single thin sections preclude pervasive post-peak metamorphic fluid flow as a cause of a re-equilibration within the calc-silicates. The preservation of eclogite facies, very high-pressure relics as well as amphibolite facies reactions textures in the presence of a fluid phase is in agreement with fast, tectonically driven unroofing of these rocks.

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... The MMC is regarded as a nappe pile transported on top of autochthonous, anchi-metamorphic Palaeozoic sequences of the Thuringian lithofacies, as exposed in the southeastern Frankenwald, in the Lower Carboniferous (Franke, 1984;Klemd et al., 1994;Klemd, 2010). This nappe pile is subdivided from the bottom to the top into the following units: Prasinit-Phyllit, Rand-Amphibolit, Liegendserie, and Hangendserie (e.g. ...
... The Hangendserie unit is dominated by various types of acidic to basic amphibolite facies rocks, but also eclogite bodies like the one outcropping at the Weißenstein hill top (Fig. 1, see, e.g. Stosch & Lugmair, 1990;O'Brien, 1993;Klemd et al., 1994) are part of this unit. ...
... The peak P-T conditions of these eclogites were estimated by several studies (Franz et al., 1986;Klemd, 1989;Klemd et al., 1991Klemd et al., , 1994O'Brien, 1993;Liebscher et al., 2007). These conditions cluster in the P-T range of 20-34 kbar and 550-750°C. ...
Article
Small oval-shaped, unshielded monazite grains found in a Variscan garnet-muscovite-bearing mylonitic paragneiss from the Liegendserie unit of the Münchberg Metamorphic Complex (MMC) in the northwestern Bohemian Massif, central Europe, yield only pre-Variscan ages. These ages, determined with the electron microprobe, have maxima at about 545, 520 and 495 Ma and two side-maxima at 455 and 575 Ma, and are comparable with previously determined ages of detrital zircon reported from paragneisses elsewhere in the NW Bohemian Massif. The pressure (P) - temperature (T) history of this mylonitic paragneiss, determined from contoured P-T pseudosections, involved an initial stage at 6 kbar/600 °C, reaching peak P-T conditions of 12.5 kbar/670 °C with partial melting, followed by mylonitization and retrogression to 9 kbar/610 °C. The monazite, representing detrital grains derived from igneous rocks of a Cadomian provenance between 575–455 Ma, has survived these Variscan metamorphic/deformational events unchanged because this mineral has probably never been outside its P-T stability field during metamorphism. This article is protected by copyright. All rights reserved.
... 1b). Calc-silicate boudins, which are intimately interlayered with the eclogites, indicate minimum pressures of 3Á1GPa at 6308C (Klemd et al., 1994;Fig. 1b). ...
... The Hangendseries records minimum peak metamorphic conditions of 2Á0^2Á5 GPa/ 600^7008C, a re-equilibration stage at 1Á4^1Á5 GPa/600^7008C, and an intense amphibolite-facies overprint at 0Á8^1Á0 GPa/600^6508C. F, Franz et al. (1986); Kl, Klemd (1989); Kl et al. (a), Klemd et al. (1991);Kl et al. (b), Klemd et al. (1994); M, Massonne (1991);O'B, O'Brien (1993); water-saturated MORB solidus redrawn after Vielzeuf & Schmidt (2001); water-saturated trondhjemite solidus redrawn after Schmidt & Poli (2004) (for age data see text). ...
... Water-saturated MORB and trondhjemite solidi are redrawn after Vielzeuf & Schmidt (2001) and Schmidt & Poli (2004), 'am-out' and 'zo-out' are redrawn after Vielzeuf & Schmidt (2001). P^T data for the Hangendseries: F, Franz et al. (1986); Kl, Klemd (1989) Klemd et al. (1994); M, Massonne (1991);O'B, O'Brien (1993). Question marks indicate perturbation of the system between crystallization Stages 1 and 2 and crystallization Stages 2 and 3. Reheating as an explanation for these perturbations, as suggested by the P^T path, is reasonable albeit speculative (see text). ...
Article
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Zoisite-bearing high-pressure pegmatites from the Münchberg Massif, Germany, provide an excellent example of the characteristics of the onset of metabasite melting at eclogite-facies conditions. The pegmatites were derived by partial melting of a mid-ocean ridge basalt (MORB)-like eclogite at T ≥680°C/2.3GPa to 750°C/3.1GPa, which produced small amounts of tonalitic to trondhjemitic melt. The melt concentrated locally in isolated, small melt pockets and crystallized primary zoisite as liquidus phase at P ≥2.3GPa/680°C to 2.1GPa/750°C. Compositional zoning of pegmatite zoisite records an ensuing multi-stage uplift history with successive, discrete crystallization events at 14±0.2GPa/650-700°C and 1.0±0.1GPa/620-650°C. Resorption textures indicate reheating and thermal perturbation of the whole system prior to each successive crystallization event. Final solidification of zoisitepegmatites occurred at 0.9± 0.1GPa/620-650°C.The data suggest that isolated melt + zoisite crystal mush pockets formed an integral part of the eclogite throughout uplift from melt formation at T ≥680°C/ 2.3 GPa to 750°C/3.1GPa to final solidification at ∼0.9 GPa/ 620-630°C; that is, over a depth range of 45-60 km. The entire pegmatite-forming process was probably fluid conserving: fluid present during melt formation was trapped by fully or nearly water-saturated siliceous melts, whereas fluid liberated during pegmatite crystallization interacted with dehydrated eclogite-facies assemblages to form amphibolite-facies hydrous minerals. A set of empirical Dmelt/eclogite values based on mean zoisite-pegmatite and eclogite composition were used to model the onset of partial highpressure melting of metabasites.
... 1b). Calc-silicate boudins, which are intimately interlayered with the eclogites, indicate minimum pressures of 3Á1GPa at 6308C (Klemd et al., 1994;Fig. 1b). ...
... The Hangendseries records minimum peak metamorphic conditions of 2Á0^2Á5 GPa/ 600^7008C, a re-equilibration stage at 1Á4^1Á5 GPa/600^7008C, and an intense amphibolite-facies overprint at 0Á8^1Á0 GPa/600^6508C. F, Franz et al. (1986); Kl, Klemd (1989); Kl et al. Klemd et al. (1994); M, Massonne (1991);O'B, O'Brien (1993); water-saturated MORB solidus redrawn after Vielzeuf & Schmidt (2001); water-saturated trondhjemite solidus redrawn after Schmidt & Poli (2004) (for age data see text). ...
... Water-saturated MORB and trondhjemite solidi are redrawn after Vielzeuf & Schmidt (2001) and Schmidt & Poli (2004), 'am-out' and 'zo-out' are redrawn after Vielzeuf & Schmidt (2001). P^T data for the Hangendseries: F, Franz et al. (1986); Kl, Klemd (1989) Klemd et al. (1994); M, Massonne (1991);O'B, O'Brien (1993). Question marks indicate perturbation of the system between crystallization Stages 1 and 2 and crystallization Stages 2 and 3. Reheating as an explanation for these perturbations, as suggested by the P^T path, is reasonable albeit speculative (see text). ...
Article
Field relations, mineral textures, and mineral compositions for pegmatite segregations in eclogites from Weissenstein (Munchberg Massif, Germany) indicate that partial melting has occurred during isothermal decompression of the eclogite from 25 kbar to about 10 kbar. The pegmatites have a leuco-tonalitic bulk composition. The matrix of the pegmatite is rich in quartz and plagioclase with abundant graphic intergrowths. The complex intergrowth textures and compositional relationships record at least four stages of mineral reaction and growth in the pegmatites during rapid uplift of the eclogites. The P-T path for pegmatite formation has been constrained by experimental data (from the literature) on melting in tonalitic systems and zoisite-clinozoisite phase relations. The results suggest that partial melting during decompression may be a common process in eclogite terranes, leading to high-pressure pegmatites, with low-Fe epidote minerals as magmatic phases. -from Authors
... Author Conditions Locality/setting CLOSED SYSTEM UNFRACTURED (no mesoscale fractures, local fluid) Mora & Valley (1989) med-P amphibolite Idaho batholith Oliver et al. ( 1992a) low-P amphibolite Proterozoic shortening, NW Queensland Goodge & Holdaway (1995) low-P amphibolite Proterozoic shortening, New Mexico Buick & Holland (1991 (in part)* med-to high-P amphibolite Extension, Naxos Getty & Selverstone (1994) eclogite Alpine subduction/collision Klemd et al. (1994) eclogite Bavarian gneiss complex Valley & O'Neil (1984), Valley et al. ( 1990), Cartwright & med-to high-P granulite Adirondacks, Grenville shortening , Eiler & Valley (1994) Baker (1990) (in part) med-to high-P granulite Ivrea Zone collision CLOSED SYSTEM FRACTURED (fluid and mass locally derived) Craw (1992) (in part) low-greenschist Shortening, Otago Schist, N.Z. Banks et al. (1991) low-greenschist Alpine thrusts low-to mid-greenschist Thrusts, central Australia Burkhard & Kerrich (1988), low-to mid-greenschist Thrusts, External Swiss Alps, upper plate Marquer & Burkhard (1992) Rumble ( 1977) med-P amphibolite Acadian nappes Yardley & Bottrell ( 1992) low-to med-P amphibolite shortening, Connemara Raith & Srikantappa (1993)* granulite charnockites, Southern India Selverstone et al. (1992) eclogite Alpine subduction Barnicoat ( 1988) eclogite to amphibolite Alpine thrusts, nappes Philippot & Selverstone (1991) eclogite (1994) low-to mid-greenschist crustal shortening, Hilton deposit, Australia Burkhard & Kerrich (1988), Marquer & Burkhard (1992 low-to mid-greenschist Thrusts, External Swiss Alps Fricke et al. ( 1992) low-P, mid-greenschist Extension/core complex, Nevada, upper plate Poutiainen ( 1990) low-P amphibolite to granulite shortening then extension, SE Finland OPEN SYSTEM FRACTURED, PARTLY OPEN WALLROCKS (fluid and mass multi-sourced, fluid interacts with local wallrock at cm-100 m scales) Craw (1992) (in part) greenschist to prehnite-pumpellyite Uplift/transpression of Otago Schist, N.Z. ...
... At terrane to outcrop scales, rocks with low by reactions occurring in the immediate rock (e.g. Mora & Valley, 1989;Valley et al., 1990; intrinsic permeabilities or those shielded from fluid flow due to heterogeneous deformation may show 1992a; Getty & Selverstone, 1994;Klemd et al., 1994;Goodge & Holdaway, 1995). This definition can be closed-system fluid behaviour. ...
Article
Mechanisms for kilometre-scale, open-system fluid flow during regional metamorphism remain problematic. Debate also continues over the degree of fluid flow channellization during regional metamorphism, and the mechanisms for pervasive fluid flow at depth. The requirements for pervasive long-distance fluid flow are an interconnected porosity and a large regional gradient in fluid pressure and hydraulic head (thermally or structurally controlled) that dominates over local perturbations in hydraulic head due to deformation. In contrast, dynamic or transient porosity interconnection and fluid flow accompanying deformation of heterogeneous rock suites should result in moderately to strongly channellized flow at a range of scales, of which there are many examples in the literature.
... In the competent eclogites, which often escape deformation, interface kinetics and rates of diffusion are too sluggish to enhance (retrograde) metamorphic reactions occurring in the less competent metasediments (e.g., Rubie and Thompson, 1985). However, several studies revealed that some metasediments also experienced HP to ultra-high-pressure (UHP) metamorphism and shared a common metamorphic history with the enclosed/interlayered eclogites (e.g., Compagnoni et al., 2012; Gross et al., 2008; Hirajima et al., 1988; Klemd et al., 1991 Klemd et al., , 1994 Liu et al., 2001; Spear and Franz, 1986), suggesting that they may be subducted and exhumed as a coherent unit. Thus, the study of the metamorphic evolution of both (U)HP eclogite lenses/interlayers and associated host metasediments may provide important information in understanding HP–UHP metamorphism during subduction and exhumation processes and the respective geodynamic setting. ...
... It is not possible to obtain an absolute pressure estimate from the marbles . However, the lithostratigraphic sequence of cm-scale interlayered marble and quartz–mica schist and the absence of tectonic contacts suggest that they experienced the same HP metamorphic evolution as the eclogite and quartz–mica schist (cf., Klemd et al., 1991Klemd et al., , 1994). This has been further confirmed by the thermodynamic modeling of interlayered marble and quartz–mica schist from a different location in the Chinese Tianshan (U)HP/LT belt (Lü et al., 2013). . ...
Article
In the (ultra-)high-P-low-T metamorphic terrane of the Chinese South Tianshan, discontinuous mafic blocks and boudins (former upper oceanic crust) are now embedded in voluminous (mainly metasedimentary) host rocks. Two different models were proposed and relate the occurrence of both high-P and ultra-high-P mafic and metasedimentary rocks to either (i) a tectonic mélange style exhumation, with no exhumation of coherent units, but different lithologies derived from different depths juxtaposed and intermingled during exhumation in the subduction channel, or (ii) the evolution of two coherent metamorphic belts: one with high-P and the other with ultra-high-P conditions. In contrast to most previous studies in the Chinese South Tianshan which focused either on single eclogites or metasedimentary rocks (assumed as representative), this study concentrates on the systematic investigation of both mafic boudins and their immediate sedimentary host rocks, because the investigation of both lithologies and the comparison of their metamorphic evolution is crucial to reconstruct the geodynamical context of the whole (ultra-)high-P-low-T metamorphic complex. Several sample pairs consisting of both lithologies were geochemically investigated and their respective metamorphic evolution was reconstructed using geothermobarometry and thermodynamic modelling. The latter approach considers changes in the mineral assemblage during the metamorphic evolution, as well as changes in mineral composition, which may help to determine the metamorphic history of a rock despite the preservation of critical mineral assemblages. All samples experienced a clockwise P-T path with overall maximum P-T conditions of 540-550 °C and 1.9-2.25 GPa for the host rocks, and 555-575 °C and 2.2-2.5 GPa for the eclogites. Peak-metamorphic temperatures of ~525-540 °C of the metasedimentary host rocks were also confirmed by Raman spectroscopy of carbonaceous material. Results from thermobarometry and thermodynamic modelling are consistent with the observation that none of the samples contains mineral relicts indicating UHP conditions (like coesite in garnet) and neither conventional thermobarometry, nor thermodynamical modelling resulted in P-T conditions in the stability field of coesite. Thus, no evidence of ultra-high-P conditions was found. Given that the whole sampled river valley lies within the proposed 'ultra-high-P sub-belt' and considering former studies, which showed that at other places within this 'unit' both ultra-high-P and high-P rocks are now juxtaposed on a small scale, the formation of the whole (ultra-)high-P-low-T metamorphic belt in the Chinese South Tianshan as a tectonic mélange style exhumation is more convincing than the formation and juxtaposition of two coherent metamorphic units with high-P and ultra-high-P conditions respectively.
... In the competent eclogites, which often escape deformation, interface kinetics and rates of diffusion are too sluggish to enhance (retrograde) metamorphic reactions occurring in the less competent metasediments (e.g., Rubie and Thompson, 1985). However, several studies revealed that some metasediments also experienced HP to ultra-high-pressure (UHP) metamorphism and shared a common metamorphic history with the enclosed/interlayered eclogites (e.g., Compagnoni et al., 2012;Gross et al., 2008;Hirajima et al., 1988;Klemd et al., 1991Klemd et al., , 1994Liu et al., 2001;Spear and Franz, 1986), suggesting that they may be subducted and exhumed as a coherent unit. Thus, the study of the metamorphic evolution of both (U)HP eclogite lenses/interlayers and associated host metasediments may provide important information in understanding HP-UHP metamorphism during subduction and exhumation processes and the respective geodynamic setting. ...
... It is not possible to obtain an absolute pressure estimate from the marbles. However, the lithostratigraphic sequence of cm-scale interlayered marble and quartz-mica schist and the absence of tectonic contacts suggest that they experienced the same HP metamorphic evolution as the eclogite and quartz-mica schist (cf., Klemd et al., 1991Klemd et al., , 1994. This has been further confirmed by the thermodynamic modeling of interlayered marble and quartz-mica schist from a different location in the Chinese Tianshan (U)HP/LT belt (Lü et al., 2013). ...
Article
Petrological and mineralogical data of interlayered eclogite, marble and quartz-mica schist from a drill core are used to constrain the metamorphic evolution of metavolcanics and intercalated metasediments in the Tianshan (ultra-)high-pressure/low-temperature [(U)HP/LT] metamorphic belt, NW China. The eclogite mainly consists of varying amounts of garnet, omphacite, quartz and zoisite, the marble of calcite (>95 vol.%) with minor zoisite and phengite, and the schist of quartz and mica with minor calcite, chlorite, albite and garnet. Using garnet isopleth thermobarometry, pseudosection calculations for the eclogite and quartz-mica schist reveal a common metamorphic evolution under HP condition of both rock types that is also consistent with the temperature estimates for the marble using conventional thermometry. The uniform P–T paths of the interlayered eclogite and quartz-mica schist, as well as compatible temperature data of the marble, document that the whole rock suite constitutes a coherent HP unit during peak metamorphic conditions and exhumation. Thus protoliths of eclogite and associated sediments are believed to have undergone the same metamorphic evolution. In addition, the data gained by the present study of the HP rocks, which were collected in the northern part of the Chinese Tianshan (U)HP/LT belt, do not support the recently proposed tectonic scheme that this metamorphic terrane consists of a northern “coherent UHP unit” and a southern “coherent HP unit”.
... Very high-pressure Ca-rich garnet-clinopyroxene rocks including garnetites have been found as layers in ultrahighpressure (UHP) metamorphic belts (Evans et al., 1979;Yang, 1991;Becker & Altherr, 1992;Klemd et al., 1994;Becker, 1996;Vrána & Frýda, 2003;Yang et al., 2005) and xenoliths in kimberlites (O'Hara & Mercy, 1966;Harte & Gurney, 1975;Jerde et al., 1993). Whereas some of those associated with marbles and gneisses may be derived from sedimentary protoliths (Becker & Altherr, 1992;Klemd et al., 1994), and some were derived from rodingites (Evans et al., 1979), the origin and cause of the Ca-rich characteristic of others remain unclear. ...
... Very high-pressure Ca-rich garnet-clinopyroxene rocks including garnetites have been found as layers in ultrahighpressure (UHP) metamorphic belts (Evans et al., 1979;Yang, 1991;Becker & Altherr, 1992;Klemd et al., 1994;Becker, 1996;Vrána & Frýda, 2003;Yang et al., 2005) and xenoliths in kimberlites (O'Hara & Mercy, 1966;Harte & Gurney, 1975;Jerde et al., 1993). Whereas some of those associated with marbles and gneisses may be derived from sedimentary protoliths (Becker & Altherr, 1992;Klemd et al., 1994), and some were derived from rodingites (Evans et al., 1979), the origin and cause of the Ca-rich characteristic of others remain unclear. Study of the nature of the protolith and the metamorphic histories of such rocks is important to understand the previous architecture and evolution of orogenic belts, and may shed some light on the origin of xenoliths in kimberlites. ...
Article
Layers of Ca-rich garnet–clinopyroxene rocks enclosed in a serpent-inite body at Hujialin, in the Su–Lu terrane of eastern China, preserve igneous textures, relict spinel in garnet, and exsolution lamellae of Ca-rich garnet, ilmenite/magnetite, Fe-rich spinel, and also amphibole in clinopyroxene. In terms of their major and trace element compositions, the studied samples form a trend from arc cumulates towards Fe–Ti gabbros. Reconstructed augite composi-tions plot on the trend for clinopyroxene in arc cumulates. These data suggest that the rocks crystallized from mantle-derived magmas differentiated to various extents beneath an arc. The Ca-rich garnet þ diopside assemblage is inferred to have formed by com-pressing Ca-rich augite, whereas the relatively Mg-rich cores of garnet porphyroblasts may have formed at the expense of spinel. The protolith cumulates were subducted from near the crust–mantle boundary (c. 1 GPa) deep into the upper mantle (4Á8 AE 0Á6 GPa and 750 AE 50 C). Negatively sloped P–T paths for the garnet– clinopyroxene rocks and the corollary of corner flow induced sub-duction of mantle wedge peridotite are not supported by the available data. Cooling with, or without, decompression of the cumulates after the igneous stage probably occurred prior to deep subduction.
... Given in ppm. Klemd et al. (1994) investigated calcsilicates interlayered with metabasic rocks in the Münchberg Gneiss Complex, another klippe besides the Zone of Erbendorf-Vohenstrauß in the Oberpfalz both of which are relicts of a former coherent allochthonous nappe complex (Fig. 1). The authors estimated a temperature of 630°C. ...
... The process is frequently associated with the enrichment of tungsten, which is conducive to scheelite mineralization (Shabeer et al., 2003) as encountered in lithotype 2. The question where the calcium came from can only be answered by providing geological circumstantial evidence. There is a predecessor existing in the metabasic rocks of lithotype 4 and in metaultrabasic rocks of the nappe complex (Table 5) (Klemd et al., 1994). These investigations were extended to subduction complex of the Tianshan mountains, western China, where massive blue schist is cross-cut by an eclogite-facies major fluid conduit (Bierlein et al., 2009). ...
Article
In some areas of the Variscan orogen felsic mobilizates (pegmatitic and aplitic rocks) are closely associated with stratiform and stockwork-like bodies enriched in Ca minerals (e.g. wollastonite, diopside-hedenbergite s.s.s., grossular-spessartine s.s.s., siderite..) and bodies aligned to them similar in structure but abundant in quartz, plagioclase and mica. Geological mapping and lithochemical studies are the tools to decipher the nature of these crystalline rocks which are common to the Hagendorf-Pleystein Pegmatite Province, SE Germany, and present in many ensialic orogens elsewhere. Geological and chemical data suggest paired belts of a restite-mobilization system. The Ca and Si metasomatites are different from calcareous metasediments and quartzites elsewhere in the SE German basement devoid of mobilizates (parent rocks: limestones and cherts). Mobilization conducive to this paired belt of metasomatites involved silica mobilized from a deep level of the crust as a result of metamorphic-metasomatic alteration of Precambrian to Early Paleozoic metagreywackes during retrograde metamorphism from HP to LP metamorphism around 680–600 °C. The arrangement of mobilizates and restites in the field has been denominated as metamorphic differentiation sensu lato. The zone of silica mobilization is transitional into a zone of pegmatoids and aploids that overlaps with another one characterized by rocks derived from Ca metasomatism the footwall facies of which developed in the range 750–400 °C while in the hanging wall metamorphic rocks of rare-element pegmatites 570–430 °C occurred. The intensity of Ca metasomatism diminishes from the footwall to the hanging wall rocks and reflects a subcrustal impact. These investigations call attention among exploration geologists and petrologists to an alternative origin of “metasilica” and “metacarbonate” rocks being encountered in a zoned arrangement with felsic mobilizates (pegmatitic and aplitic rocks). The current study also raises the question “Quo vadis” pegmatology? It is an amendment to the mainstream geoscientific handling of pegmatitic rocks as “..texturally complex igneous rocks” genetically linked to granitic plutons (see review of London (2018) in Ore Geology Reviews). Taking a holistic approach can give us a reality check and prevent pegmatology from converting into a one-way street (granites-only) that eventually ends up in a dead-end street. The field evidence is the litmus test for all our models created in the laboratory and on the PC. There is no ore geology without field geology. To get access: Your personalized Share Link: https://authors.elsevier.com/a/1YVxT_3s85-rFt
... These comprise paragneiss and orthogneiss with ophiolitic protolith metamorphosed to eclogite facies at c. 400-380 Ma (Gebauer & Grünnenfelder 1979) with Nd model ages c. 100 Ma older. Pressures .25 kbar (Klemd et al. 1994;O'Brien, pers. comm.) ...
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Multidisciplinary investigations of the western margin of the East European Craton (EEC) by EUROPROBE projects since 1992 have confirmed that the Trans-European Suture Zone (TESZ) is the most fundamental lithospheric boundary in Europe, extending 2000 km from the North Sea to the Black Sea-Crimean region. The crust of the EEC is thicker and denser than that of Phanerozoic-accreted Europe, and the base of the lithospheric mantle significantly deeper. These characteristics persist throughout the length of the TESZ, despite the variation in age of the accreted crust along strike. Geological studies of key deep borehole cores and the limited outcrop data confirm that the crust of Phanerozoic-accreted Central Europe comprises a number of terranes, each thought to be derived from Gondwana during several episodes of rifting, ocean formation, ocean destruction and sequential accretion to the EEC throughout Palaeozoic time. There is still much discussion about the identity, provenance and history of these orogenic terranes. The process of accretion led to the formation of terrane-bounding orogenic sutures, which may be marked in outcrop by ophiolitic and eclogitic relics. Recognition of concealed satures is obviously more difficult, and relies on a variety of geophysical techniques, used in an integrated way by multidisciplinary teams; the evidence from deep seismic reflection and refraction surveys, teleseismic tomography, magnetotelluric experiments and from geophysical potential-field modelling is crucial for such studies. Since the European Geotraverse, much has been learnt about the geometry of the Thor, Iapetus, Rheic, Saxo-Thuringian and Moldanubian oceanic sutures, through the crust and sometimes into the mantle. This has led to a much better understanding of the 3D crustal structure of the Western Accretionary Margin of the EEC, and the lithospheric processes that have shaped it. From this, the influence of tectonic heterogeneities within the orogenic crust on the development of post-orogenic structures and basins can be much better constrained.
... The Bohemian Massif is known as one of the regions with ultrahigh-pressure (UHP) rocks and mineral assemblages, e.g., Coleman & Wang (1995). UHP calc-silicate rocks (P > 31 kbar) have been described in association with eclogites in the Münchberg gneiss complex (Klemd et al., 1994), coesite eclogites occur in the Saxothuringian Zone, Erzgebirge (Massonne, 2001) and microdiamonds have been identified in quartz-feldspathic gneisses of the same unit (Nasdala & Massonne, 2000). Since the Variscan structure of the Bohemian Massif consists of several terranes (Matte et al., 1990) with distinct structures and metamorphic histories, it is not possible to discuss UHP rocks from the Moldanubian Zone together with UHP rocks from other units. ...
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A grossular-rich garnetite composed of similar to 80 vol. % garnet (Grs(63)Alm(25)Prp(7)And(4)) and 15 vol. % hercynite+anorthite symplectite after kyanite occurs as a boudin in migmatitic gneiss of the Gfohl unit in the easternmost part of the Moldanubian Zone, next to its overthrust on the Cadomian foreland in Moravia, Czech Republic. Garnet cores contain 0.10 to 0.18 wt. % Na2O and up to 0.5 wt. % TiO2. Both rutile and ilmenite are present as early inclusions in garnet. The UHP assemblage underwent partial decompression breakdown under granulite/amphibolite facies conditions, including the reaction garnet + kyanite --> hercynite + anorthite (An(99)) and localised reaction of garnet to anorthite+aluminous augite+ferropargasite+titanite veinlets. A Ba-Al-Fe silicate loosely related to bafertisite is a minor phase tied to anorthite aggregates. The widespread preservation of Na-rich garnet indicates that Na diffusion was controlled by the limited extent of decompression reactions. The elevated sodium content in garnet, comparable to those of diamond-bearing UHP crustal rocks and diamondiferous eclogites, indicates imprint of sub-lithospheric high pressures, P approximate to 4 GPa. The garnetite can be interpreted as a calc-silicate rock subducted to mantle depths and subsequently obducted with mantle peridotite and pyroxenites. This scenario is supported by published isotopic evidence on a Palaeozoic crustal component in some mantle-derived Variscan garnet pyroxenites and eclogites in the Gfohl unit. Very close similarity of garnetite composition to epidote (except Fe oxidation state and H2O content) indicates that a nearly monomineralic epidosite is the most probable parent rock.
... Typically, the derived PT conditions for specific HP-UHP rocks scatter as different geothermobarometric methods were used by these researchers. For instance, the eclogites and related rocks of the Münchberg complex were studied by Franz et al. (1986), Klemd (1989), Stosch andLugmair (1990), O'Brien (1993), Massonne (1993) and Klemd et al. (1994). On the basis of a summary of these works by Massonne and O'Brien (2003), an approximate mean PT condition of 670 °C and 27 kbar for the Münchberg eclogites is quoted in Table 1. ...
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An overview of the presently known high-pressure (HP) and ultrahigh-pressure (UHP) occurrences in the Bohemian Massif is given. This overview refers exclusively to rocks, in which indicator minerals and mineral assemblages were detected and/or for which PT conditions were reasonably deduced, pointing to the envisaged pressure regime. So far, HP and UHP rocks are only known from the Münchberg complex, the Góry Sowie, the Tepla-Barrandian unit, the Granulitgebirge, the Saxonian Erzgebirge, the Śnieżnik and the Gföhl terrane sensu lato. Two metamorphic events, leading to HP and UHP metamorphism, occurred in Early to Middle Devonian and Early Carboniferous times. The first event can probably be related to the beginning of the collision of Gondwana and Laurussia leading to the formation of the Variscides. KEY WORDS: Bohemian Massif, Variscan orogeny, eclogite, garnet peridotite, high-pressure granulite, ultrahigh-pressure meta-morphism.
... For example, in marbles of high pressure and ultrahigh pressure metamorphism from the Dabie Shan, the X CO 2 < 0.03 (Wang & Liou, 1993) and Su-Lu terrane the X CO 2 ¼ 0.001 (Kato et al., 1997). High-pressure calc-silicates from Bavaria yield X CO 2 £ 0.03 (Klemd et al., 1994). Very low X CO 2 values have also been reported for high-pressure eclogite facies metamorphic rocks in the Alps (e.g. ...
Article
Eclogite facies carbonate rocks have been discovered associated with the granulite–eclogite transitional rocks within Bergen Arc system, Caledonian Orogen of western Norway. The local occurrences of marbles and calc-silicates are found subparallel to the mafic eclogite facies shear zones on Holsnøy Island. Marbles contain the assemblage calcite (Ca0.99Sr0.01CO3), calcian strontianite (Ca0.18−0.44Sr0.53−0.84CO3), clinopyroxene (Jd7−32), epidote/allanite (Ps0−33), titanite, garnet (Alm52−56Grs28−33Pyp11−16), barite (Ba0.90−0.99Sr0.01−0.10SO4), celestine (Sr0.67−0.98Ba0.01−0.23Ca0.01−0.11SO4), and one apparently homogeneous grain of intermediate composition (Ba0.49Ca0.01Sr0.50SO4). Adjacent eclogites have clinopyroxene with similar jadeite contents (Jd14−34) and similar garnet (Alm51−60Grs26−36Pyp8−14) compositions. The marbles have high contents of Sr (9500–11000 p.p.m) and Y (115–130 p.p.m). However, low concentrations of some key trace elements (110–160 p.p.m. Ba and <5 p.p.m. Nb) appear to indicate that the marble is not a metamorphosed carbonatite. The ⁸⁷Sr/⁸⁶Sr ratios range from 0.7051 to 0.7059. Field and petrological relationships suggest that metasomatic reactions and fluids played a significant role in producing and/or modifying the marbles. The breakdown of scapolite in the granulite into carbonates and sulphates during eclogite facies metamorphism may have contributed to the metasomatic formation of the marbles along shear zones.
... Marbles and carbonate-bearing HP metabasalts and metapelites are commonly observed in the Tianshan (U)HP/LT metamorphic belt. The occurrence of magnesite and calcite inclusions in dolomite in eclogites and metapelites was considered as evidence for UHP metamorphism (Zhang et al. 2002, but the petrological interpretation of carbonate/silicate mineral assemblages in (U)HP rocks is often highly controversial due to disequilibrium, crystallographical, or textural considerations (e.g., Klemd et al. 1994;Klemd 2003;Smit et al. 2008;Hammouda et al. 2011). More recently, interlayered dolomite-and magnesitebearing eclogite and blueschist in the Tianshan were interpreted to have coexisted at the same peak metamorphic HP conditions due to different bulk-rock compositions (Li et al. 2012). ...
Article
Dolomite with compositional zoning was discovered in carbonate-lawsonite-bearing eclogites in the Tianshan (ultra-)high-pressure/low-temperature metamorphic belt, northwestern China. The eclogite-facies dolomite occurs as matrix porphyroblast and as inclusion in garnet, both of which display the same chemical zoning pattern. The dolomite contains inclusions of calcite (probably after aragonite), magnesite, glaucophane, lawsonite (and its pseudomorphs), allanite, epidote, paragonite, phengite, and omphacite. The chemical zoning in dolomite is well defined by a continuous core-to-rim Mg increase and Fe-Mn decrease. The concentrations of transition metal elements, REE, and Y also decrease from core to rim of the dolomite. Thermodynamic modeling demonstrates that the Fe-Mg zoning of dolomite is largely temperature dependent and, thus, is interpreted as prograde growth zoning, which developed during subduction of carbonate-bearing oceanic crust. It is suggested that dolomite in equilibrium with garnet formed as a result of changing matrix compositions due to increasing temperatures. In addition, thermodynamic modeling demonstrates that during subduction at high-pressure conditions prograde-formed aragonite and dolomite were transformed to dolomite and magnesite. Furthermore, Fe-rich magnesite inclusions in matrix dolomite and in dolomite inclusions in garnet are shown to have formed during high-pressure conditions prior to peak metamorphic conditions and, therefore caution is warranted using Fe-bearing magnesite occurrences in eclogite-facies rocks as an unambiguous ultrahigh pressure indicator as previously suggested.
... However, up to now previous studies on¯uid composition and¯uid¯ow within subduction zones show no consensus. Studies done so far on eclogite using dierent data sets draw contrasting conclusions about the role of the¯uid: (1) high-pressure/low-temperature rocks formed by subduction of oceanic crust display textural, stable isotope or Nd-isotope evidence for the in¯ux of relatively large quantities of low salinity aqueous¯uids in an open system on kilometer-scale (Bebout 1991;Bebout and Barton 1993;Giaramita and Sorensen 1994;Nelson 1995); and (2) detailed¯uid inclusion, geochemical and isotopic studies of eclogites and related high-pressure veins (veins containing high-pressure minerals) demonstrate that¯uids were retained within subducted slabs and¯uid¯ow was limited and restricted to a millimeter to centimeter scale (e.g., Philippot and Selverstone 1991;Klemd et al. 1992Klemd et al. , 1994Selverstone et al. 1992;Andersen et al. 1993;Nadeau et al. 1993;Philippot 1993;Getty and Selverstone 1994;Vallis and Scambelluri 1996;El-Shazly and Sisson 1999;Svensen et al. 1999;Xiao et al. 2000). ...
Article
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The Tianshan is the first locality worldwide where primary fluids at blueschist to eclogite transition have been documented. Veins containing high-pressure minerals in massive host eclogites transitional to blueschists, or eclogite boudins surrounded by blueschists, indicate that a free fluid phase was present at the time of eclogitization. The high-pressure veins are predominantly composed of omphacite fibers with minor quartz or calcite. The transition from blueschist- to eclogite-facies parageneses occurs as "dehydration" halos around these veins. Clinozoisite, paragonite, glaucophane, and omphacite inclusions preserved in garnet porphyroblasts in wall eclogites document the transformation of blueschist to eclogite. C-axis-parallel, non-planar populations of fluid inclusions were trapped during the growth of omphacite in high-pressure veins and dehydrated wall rocks. Low salinity H2O + NaCl - solid-bearing inclusions are preserved in omphacite fibers in veins and matrix omphacite of wall rocks. None of the isochores of these low salinity aqueous fluid inclusions intersect peak eclogite-facies metamorphic conditions, suggesting that, although the textural evidence constrains the entrapment of fluid inclusions to peak metamorphic conditions, their densities must have been modified during exhumation. The fluids are interpreted to have been derived from the host blueschist as a result of dehydration reactions such as 13 Gln+5 Czo=9 Prp+26 Jd+12 Di+19 Qtz+15 H2O and Gln+Pg=Prp+3 Jd+2 Otz+2 H2O. The similarity of vein and wall rock mineral compositions, fluid inclusion characteristics and O-isotope data also favor an internal source for the fluids. The major element composition of veins indicate that Si, Na, and Ca-rich aqueous fluids were released during dehydration at a depth of 50ᆞ km within a Paleozoic subduction zone.
... Both textural varieties of titanite are clearly part of the retrograde amphibolite-facies assemblage, which is linked to shearing and partial exhumation of the eclogite. Framboidaltextured titanites are always developed around a prismatic rod of rutile or ilmenite and composed of numerous tiny subhedral prisms of titanite (Fig. 2b), this type of texture being a common decompression texture in high-P rocks (Klemd et al. 1994; Frost et al. 2000; Bingen et al. 2001). This distinctive texture is heterogeneously developed and primarily reflects the distribution of host rutile or ilmenite crystals. ...
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The Glenelg–Attadale Inlier is the largest basement inlier within the Caledonian Moine nappe of NW Scotland. In the eastern part of the inlier amphibolite-facies retrogression of the eclogites is associated with tectonic fabrics, and P–T estimates indicate significant decompression (c. 20 km). Previous Sm–Nd mineral–whole-rock dates indicated that peak eclogite-facies metamorphism occurred around c. 1.08 Ga, which was correlated with the Grenvillian orogeny. However, the middle REE enrichment of the analysed garnets suggests the influence of apatite inclusions. It is therefore likely that the interpretation of the c. 1.08 Ga age is complex, possibly reflecting re-equilibration at lower temperatures. Sampled eclogites contain zircon in a number of distinct textural forms that are mainly associated with pargasite and plagioclase, and are part of the retrograde amphibolite-facies assemblages. Titanite extensively replaces rutile, and is clearly associated with the retrograde amphibolite-facies event. A second textural type of titanite forms anhedral grains with plagioclase and pargasite, which is developed where the retrograde amphibolite-facies assemblage overprints the eclogite mineralogy. U–Pb dating has yielded the following ages: zircon age of 995 AE 8 Ma, and variably discordant rutile ages between 416 and 480 Ma. U–Pb and Pb–Pb isochrons on titanite and plagioclase/quartz separates yielded ages of 971 AE 65 Ma and 945 AE 57 Ma, respectively, in agreement with the zircon age. Analysed zircons and titanites are texturally part of the amphibolite-facies assemblage. The new zircon age demonstrates that amphibolite-facies metamorphism during exhumation occurred at 995 AE 8 Ma; the titanites could have closed with respect to Pb at this time or alternatively at some time between c. 1000 and 900 Ma. These data clearly demonstrate that parts of the Scottish basement underwent major thick-skinned tectonics during the Grenvillian orogeny. Rutile is part of the eclogite-facies paragenesis, and yet has young ages; these data are best explained by reheating producing near-total Pb loss related to emplacement of the late-to post-tectonic Ratagain Granite Complex at c. 425 Ma, at the end of the Caledonian orogeny.
... Being among the oldest eclogitization ages in the European Variscides, this age coincides with an early subduction of the Rheic Ocean at the onset of the Variscan Orogeny (e.g., Kroner and Romer, 2013). Numerous studies (compiled in Massonne andO'Brien, 2003, andLiebscher et al., 2007;Waizenhöfer, 2017) estimated pressure-temperature conditions of eclogite facies metamorphism in the range of 20-34 kbar and 550-750 • C. The adjacent paragneisses contain relics of this HP metamorphism, but were subjected to amphibolite facies retrogression (Klemd et al., , 1994. Retrogression had a variable effect on the eclogites: observations range from essentially unaffected eclogites, and local amphibole/diopside + plagioclase and biotite + plagioclase symplectites around omphacite and phengite, to almost complete retrogression, whereby garnet, often partially replaced by hornblende or chlorite, is generally better preserved than omphacite and phengite (Matthes et al., 1974;Franz et al., 1986). ...
Article
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Eclogites, metagabbros, and paragneisses from the Variscan Münchberg Massif record a complex succession of igneous, hydrothermal and metamorphic processes. The geodynamic setting related to the protolith formation and the impact of different types of fluid-rock interactions have been uncertain up to now. We use major and trace element chemistry as well as oxygen isotopes to disentangle the geochemical signatures related to the different stages of the rocks' history. In the Münchberg Massif, dark eclogites (kyanite-free; Fe-Ti-MORB signature) are distinguished from light eclogites (kyanite-bearing; higher Mg#, Al2O3, and Cr; lower incompatible element contents; positive Eu anomalies; MORB to arc basalt signature). The δ18O values for both types (+5.0 to +10.8‰) are equal to, or higher than those of MORB. Amphibolite facies metagabbros have a more enriched, almost OIB-like trace element signature and high δ18O values (+9.4 to +10.3‰). Good linear correlations between fluid-immobile elements throughout the eclogite types confirm their derivation from a common, N-MORB to E-MORB-like parental magma. We interpret the light eclogites as former plagioclase-rich cumulates and the dark eclogites as their complementary differentiates. This relationship is partly obscured by variable degrees of magma contamination by sediments, which also affected the metagabbros. However, the metagabbros originated from a more enriched mantle source than the eclogites. Following intrusion, the eclogites were subjected to hydrothermal alteration under the influence of seawater, as indicated by positive correlations between Li, B, Sb, and δ18O. Metamorphic fluid-rock interactions appear to be mostly of limited extent, probably due to the lack of lawsonite dehydration as a fluid source. Nevertheless, the contents at least of some fluid-mobile elements, such as LILE, Li, and Pb, were probably modified during the subduction-exhumation cycle of the eclogites. The crustal contamination of the protolith magmas argues against derivation of the eclogites and metagabbros from typical oceanic crust. Instead, a rift-drift transition setting related to the opening of the Rheic or Saxothuringian Ocean seems most likely. The eclogites and metagabbros, alongside with similar rocks in the Mariánské Lázně complex and other resembling high-pressure massifs, may record different stages of this rift-drift transition.
... No chemical difference between matrix paragonite and paragonite inclusions in garnet was observed.Table 6 ). The large compositional difference in the pistacite component in clinozoisite and zoisite within a single sample was also reported by Franz & Selverstone (1992) and Klemd et al. (1994) for eclogite facies rocks. This seems to indicate that large compositional differences can exist in these minerals under high pressure conditions. ...
Article
The late Palaeozoic western Tianshan high-pressure /low-temperature belt extends for about 200 km along the south-central Tianshan suture zone and is composed mainly of blueschist, eclogite and epidote amphibolite/greenschist facies rocks. P±T conditions of ma®c garnet omphacite and garnet±omphacite blueschist, which are interlayered with eclogite, were investigated in order to establish an exhumation path for these high-pressure rocks. Maximum pressure conditions are represented by the assemblage garnet±omphacite±paragonite±phengite±glaucophane±quartz±rutile. Estimated maximum pressures range between 18 and 21 kbar at temperatures between 490 and 570 uC. Decompression caused the destabilization of omphacite, garnet and glaucophane to albite, Ca-amphibole and chlorite. The post-eclogite facies metamorphic conditions between 9 and 14 kbar at 480±570 uC suggest an almost iso-thermal decompression from eclogite to epidote±amphibolite facies conditions. Prograde growth zoning and mineral inclusions in garnet as well as post-eclogite facies conditions are evidence for a clockwise P±T path. Analysis of phase diagrams constrains the P±T path to more or less isothermal cooling which is well corroborated by the results of geothermobarometry and mineral textures. This implies that the high-pressure rocks from the western Tianshan Orogen formed in a tectonic regime similar tòAlpine-type' tectonics. This contradicts previous models which favouFranciscan-type' tectonics for the southern Tianshan high-pressure rocks.
... It is therefore likely that partial Ti-Si substitution may also be observed in natural rocks of high-pressure origin, provided the activities of SiO 2 and CaSiO 3 are high. This combination of conditions exists: For instance, Klemd et al. (1994) described the paragenesis of titanite with quartz and diopside formed near 630 8C and a minimum pressure of 3.1 GPa. ...
Article
Phase relations on the join CaTiSiO5-CaSi2O5 were determined at 1350 °C over the pressure range 3.5-12 GPa by a combination of synthesis and reversal experiments in a piston cylinder and a multi-anvil press. Titanite-like phases were recovered from all experiments in this pressure range. At 3.5 GPa the maximum Si(VI) content of titanite is 3.0 ± 0.6 mol%, whereas bulk compositions with higher Si content yield a mixture of titanite solid solution plus coesite and walstromite-structured CaSiO3. The maximum Si(VI) content of the titanite increases with pressure to 21 ± 2 mol% at 7 GPa and 46 ± 2 mol% at 7.5 GPa. At pressures of 8.5 to 12 GPa all intermediate compositions yield a single titanite phase. X-ray and TEM analysis show that these have the A2/a symmetry of the titanite aristotype. The variations of the room-pressure unit-cell parameters of the A2/a phases with composition can be described by the equations a [Å] = 7.040(9) - 0.492(15) X(Si(VI)); b [Å] = 8.713(7) - 0.316(11) X(Si(VI)); c [Å] = 6.564(4) - 0.220(7) X(Si(VI)); β [°] = 113.721(6) - 0.537(12) X2(Si(VI)); V (cell) [Å3] = 367.8(9) - 47.5(1.6) X(Si(VI)). For the CaSi2O5 composition the recovered material has I1̄ symmetry but is known to transform back to A2/a titanite structure at 0.2 GPa at room temperature. Similarly, with increasing pressure the P21/a CaTiSiO5 transforms to A2/a symmetry at 3.6 GPa at room temperature. The conclusion is that at 1350 °C and pressures in excess of 8.5 GPa there is complete solid solution between CaTiSiO5 and CaSi2O5 based upon the isovalent exchange of Si for Ti in the octahedral site of the A2/a structure. Rietveld structure analysis of intermediate compositions reveals no evidence for ordering or intermediate phases. Preliminary experiments at pressures between 13.5 GPa and 16 GPa yielded mixtures of titanite solid solution plus perovskite and stishovite. From these data and information on the phase relations for the CaSiO3-CaTiO3 join the topology of the phase relations between ~3 and ~13 GPa in the central part of the CaO-TiO2-SiO2 ternary have been deduced.
... Aranovich et al. 1987;Newton 1995;Aranovich and Newton 1998;Fonarev et al. 1998). On the other hand, uid inclusion studies of eclogite-facies rocks indicate a dierent¯uid regime consisting of aqueous¯uids with variable salinities, and N 2 CO 2 (Andersen et al. 1989Klemd 1989;Klemd et al. 1992Klemd et al. , 1994Selverstone et al. 1992;Giaramita and Sorensen 1994;Philippot et al. 1995;Perchuk 1995). This apparent contrast in uid regimes of granulite-and eclogite-facies rocks is supported by recent studies in the Western Gneiss Region (WGR) of Norway, where Grenvillian granulites were subjected to eclogite-facies metamorphism during the Caledonian orogeny 600 Ma later (Austrheim 1990). ...
Article
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Small tectonic slices of undeformed eclogites and ultrahigh-pressure granulites occur in three tectonic units of the Śnieżnik Mts. (SW Poland). Ultrahigh-pressure granulite/eclogite transitions with peak metamorphic conditions between 21 and 28 kbar at 800 to 1000 °C occur only in the Złote unit. Conventional U-Pb multigrain analyses of zircons from a mafic granulite provided 207Pb/206Pb ages between 360 to 369 Ma which are interpreted to approximate timing of original crystallisation from a melt. Diffusion kinetics and the restricted availability of a fluid phase mainly controlled the conversion from granulite to eclogite, although some bulk-chemical differences were also recognised. The ultrahigh-pressure granulites from the Złote unit exclusively contain H2O-rich inclusions with variable salinities which distinguishes them from high-temperature (HT)-granulites world-wide. This is also in contrast to the fluid regime (H2O-N2-CO2) recognised in the lower-temperature eclogites (600–800 °C) from the closely associated Międzygórze and Śnieżnik units. The variation in fluid composition between the lower-temperature eclogites and ultrahigh-pressure granulites on the one hand and ultrahigh-pressure granulites and HT-granulites on the other hand probably indicates contrasting P-T-t paths as a result of different tectonic environments.
... The VHP conditions deduced here for the Podolsko Complex rock are consistent and overlap with pressure conditions derived for these Saxothuringan Zone eclogites (Schmädicke et al., 1992). They also overlap with pressure conditions obtained for metabasite eclogites and associated calc-silicate rocks from the Miinchberg Massif (O'Brien, 1993;Klemd, 1989;Klemd et al, 1994) and for the Orlica-Snieznik region (Brocker & Klemd, 1995), and they are close to those of Saxothuringian eclogites in Bohemia (Klápová & Schulmann, 1993). This suggests a common depth of initial Variscan VHP-UHP metamorphism involving not only subducted oceanic crust and adjacent mantle but also, at least on a small scale, slivers or relicts of continental crust comparable to the situation envisaged for other VHP-UHP terranes (Hacker & Peacock, 1995;Platt, 1993). ...
Article
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A unique Mg-Al rich rock from the Podolsko Complex of the South Bohemian Moldanubicum contains a relic assemblage of pyrope-rich garnet + low-Al orthopyroxene + quartz that implies very high-pressure (VHP) conditions of metamorphism. Primary garnet (70 mol% Prp; XMg = 0.70-0.73, Grs < 1 mol%) and coarse orthopyroxene (XMg = 0.88-0.90) are both very magnesian, and the initial orthopyroxene also preserves very low alumina contents (Al2O3 = 0.8-1.2 wt%). Garnet-orthopyroxene thermobarometry yields relatively precise P-T estimates of 26 ± 3 kbar and 830 ± 30 °C using the Harley (1984a) thermometer and Harley (1984b) barometer, or 28 ± 3 kbar using the Harley & Green (1982) thermobarometer. These pressures are consistent with the absence of coesite relics or palisade quartz after coesite, which places an upper pressure limit of about 29 kbar at the estimated temperature. This VHP assemblage is extensively overprinted by symplectites and coronas. Orthopyroxene (XMg = 0.74-0.78, 5-7 wt% Al2O3) and plagioclase (An45) form layered coronas between quartz and garnet. Garnet is now further replaced by fingerprint and filamentous symplectites involving sapphirine (XMg = 0.80-0.81), orthopyroxene (XMg = 0.72-0.76, 6-9 wt% Al2O3), cordierite (XMg = 0.90), and locally spinel. Highly resorbed garnet is zoned to much lower XMg (0.62-0.55) within 50-100 μm of these symplectites, which formed at granulitefacies pressure-temperature conditions of 8 ± 1.5 kbar and 800-900 °C. Despite its demonstrably continental crustal origin, the investigated Mg-Al-rich rock documents an initial phase of VHP metamorphism at pressures exceeding any estimates obtained so far for the crustal eclogites present in the Moldanubian Zone of the Bohemian Massif, which are generally nearer 20 kbar. The pressures estimated here are more consistent with > 30 kbar pressure estimates obtained on mantle-derived garnet peridotites and garnet pyroxenites, as well as eclogites with reported occurrence of palisade quartz after coesite in the Saxothuringian Zone. It remains uncertain whether the lower pressure (< 10 kbar) granulite-facies overprint recorded in the reaction textures reflects simple decompression, with higher-pressure reactions impeded by sluggish kinetics, or the effects of a distinct heating event superimposed on the VHT assemblage subsequent to decompression.
... However, we also know several examples of quartzofeldspathic and related rocks from the Bohemian Massif that experienced (near-)UHP conditions similar to those of eclogites and ultramafic rocks. Klemd et al. (1994) reported minimum pressures of 31 kbar at 630 °C for calc-silicate rocks from the Münchberg Massif. Nearby, Gayk et al. (1995b) found quartz laths as exsolution from clinopyroxene in an intermediate granulite. ...
... However, we also know several examples of quartzofeldspathic and related rocks from the Bohemian Massif that experienced (near-)UHP conditions similar to those of eclogites and ultramafic rocks. Klemd et al. (1994) reported minimum pressures of 31 kbar at 630 °C for calc-silicate rocks from the Münchberg Massif. Nearby, Gayk et al. (1995b) found quartz laths as exsolution from clinopyroxene in an intermediate granulite. ...
... Jednalo se spíše o metamorfované karbonátové výplně mezi polštářovými lávami nebo o karbonatizovaná hyaloklastika (např. Sobolev et al. 2006, Klemd et al. 1994 ...
Article
The northern part of the Mariánské Lázně Metaophiolite Complex, consist of inhomogeneous amphibolites (sometimes with a garnet), locally with small layers of calc-silicate rocks. Mineral assemblage of calc-silicate rocks is relatively simple and includes garnet, clinopyroxene and plagioclase. Chemical composition of garnet from surrounding amphibolites (Alm45-54 Grs33-35 Prp8-14 Sps2-4 Adr0-4) differs from the chemical composition of garnets from calc-silicate rocks (Grs84-88 Adr11-15 Sps1). Microstructural relations indicate clinozoisite-consuming reaction for formation of garnets in calc-silicate rocks. Mineral assemblage preserved as inclusions in garnet in amphibolites indicate PT conditions for the formation of garnet are ~ 660 °C and 10 kbar. The main metamorphic event in the adjacent Teplá Crystalline Complex, indicates significantly lower temperature and pressure setting (~ 600 °C and 7 kbar).
... In most cases, titanite 1 and its overgrowths are metamorphic with characteristic low (b 2.2) 232 Th/ 238 U ratios (Abraham et al., 1994). Titanite 1 and titanite 2 developed at the expense of ilmenite (Fig. 5a, b) illustrate prograde metamorphism, and titanite 2 overgrowth on rutile (Fig. 5d) is a typical decompression reaction (Ghent et al., 1993;Klemd et al., 1994). The fabric-titanite textural relationships (Fig. 3c, d) together with geochemical and petrological characteristics thus described indicate that metamorphic titanite 1 and titanite 2 developed principally during D 1 and D 2 deformation events, respectively. ...
Article
The Elu Link connecting the Neoarchean Hope Bay and Elu granite-greenstone belts in the Bathurst block of the northeast Slave craton consists of volcano-sedimentary and gabbro–granite rocks. Laser ablation ICP-MS zircon and titanite U-Pb dating was combined with mineral ⁴⁰Ar-³⁹Ar age data to date the Elu Link rocks and D1–D3 tectono-metamorphic events, as well as characterize the response to the Thelon Orogeny in the area. The volcanic rocks are correlated with the ca. 2716 Ma Flake Lake suite in the adjacent Hope Bay belt, whereas the sedimentary units have a minimum depositional age of 2684 ± 11 Ma nearly coinciding with the onset of D1 deformation. Most gabbro–granite bodies were emplaced between 2651 ± 14 Ma and 2577 ± 13 Ma coeval with the D2 deformation whose accompanying metamorphism culminated at 2632 ± 7 Ma. Older ages (up to 3042 ± 22 Ma) retained by either the gabbro–granite or their hosts are indicative of crustal contamination. A few gabbro–granites were also emplaced during the 2577 ± 13 Ma to 2494 ± 21 Ma D3 deformation event. However, the 2494 ± 21 Ma intrusions are unusual and mark the Archean–Paleoproterozoic transition. The D3 deformation was followed by erosional exhumation and localized thermal pulses associated with the emplacement of diabase dikes at 2228 ± 8 Ma, 2128 ± 11 Ma, and 802 ± 75 Ma. The Thelon Orogeny imprint, inferred from the 2054 ± 11 Ma to 1919 ± 5 Ma biotite ages, corresponds to nearly isobaric, low-temperature (< 350 °C) metamorphic re-equilibration.
... Studies of other HP-UHP belts revealed that some metasedimentary rocks also experienced HP to UHP metamorphism and shared a common metamorphic history with the enclosed/interlayered eclogites (e.g. Compagnoni, Rolfo, & Castelli, 2012;Gross, Burchard, Schertl, & Maresch, 2008;Klemd, Matthes, & Okrusch, 1991;Klemd, Matthes, & Sch€ ussler, 1994;Klemd et al.,1991;Spear & Franz, 1986). ...
Article
The Trans-European Suture Zone (TESZ) is the most fundamental lithospheric boundary in Europe, separating the ancient crust of the Fennoscandian Shield–East European Craton from the younger crust of central Europe, and extending deep into the mantle. Geophysical potential field images provide an overview of the entire Palaeozoic orogenic system of northern and central Europe for the first time. The TESZ is largely concealed by sedimentary basins of Permian–Cenozoic age; geological observations are largely restricted to local basement highs and deep boreholes, and the coverage of deep seismic surveys is widely spaced, despite experiments recently acquired within the EUROPROBE programme. By contrast, the potential field data offer a relatively detailed coverage of standardised observations throughout the TESZ. While some features of the images may be sourced in the near surface, particularly in the gravity image, much of their content reflects the structure of the underlying Palaeozoic basement. At the scale presented, the images highlight the most fundamental features of the crustal structure of the TESZ. These include the strong contrast between the highly magnetic crust of the East European Craton and the less magnetic Palaeozoic-accreted terranes of central Europe; the lateral continuity of terranes and their internal structure, particularly where arc-magmatic complexes are involved; and the location and geometry of the terrane boundaries (oceanic sutures and strike-slip zones) that separate them.
Article
Useful U–Pb isotopic data may be obtained from sphene (or titanite, CaTiSiO5) because: (1) it is a widespread accessory mineral, (2) it can incorporate uranium in its structure, and (3) it has a high closure temperature. In igneous rocks, sphene is most abundant in relatively oxidized rocks, such as metaluminous rocks of intermediate composition. These rocks have the high Ca/Al ratios wherein sphene is stabilized relative to ilmenite+quartz or ilmenite+anorthite. In metamorphic rocks, sphene is stable to the highest temperatures in mafic and calc-silicate rocks. It is found mostly in greenschist, blueschist, and amphibolite facies, although in calcic rocks its stability may extend into granulite facies. Recent studies show that the closure temperature for sphene lies at the upper limit of amphibolite facies. Because sphene reacts readily during metamorphism, U–Pb sphene ages are likely to yield the age of metamorphic crystallization, rather than resetting by simple diffusion. For this reason, metamorphic sphene may yield complex U–Pb systematics that contain information on the whole metamorphic history of the rock.
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Hydrous high-pressure veins formed during dehydration of eclogites in two paleo-subduction zones (Trescolmen locality in the Adula nappe, central Alps and Münchberg Gneiss Massif, Variscan fold belt, Germany) constrain the major and trace element composition of solutes in fluids liberated during dehydration of eclogites. Similar initial isotopic compositions of veins and host eclogites at the time of metamorphism indicate that the fluids were derived predominantly from the host rocks. Quartz, kyanite, paragonite, phengite, zoisite and omphacite are the dominant minerals in the veins. The major element compositions of the veins are in agreement with experimental evidence indicating that the composition of solutes in such fluids is dominated by SiO2 and Al2O3. Relative to N-MORB, the veins show enrichments of Cs, Rb, Ba, Pb, and K, comparable or slightly lower abundances of Sr, U, and Th, and very low abundances of Nd, Sm, Zr, Nb, Ti and Y. The differential fractionation of highly incompatible elements such as K, U and Th in the veins, as well as the presence of hydrous minerals in the eclogites rule out partial melting as a cause for vein formation. These results confirm previous suggestions that fluids derived from subducted basalt may have low abundances of high field strength elements, rare earth elements and Y. Variable vein-eclogite enrichment factors of incompatible alkalis and to a lesser extent Pb appear to reflect mineralogical controls (phengite, epidote-group minerals) on partitioning of these elements during dehydration of eclogite in subduction zones. However, abundance variations of incompatible elements in minerals from eclogites suggest that the composition of fluids released from eclogites at temperatures
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The Fe isotope systematics of subducted lithologies are crucial for the understanding of redox-dependent mass transfer in subducting slabs, with consequences for the compositions of arc magmas and of the deep mantle. We investigated eclogites, metagabbros, and paragneisses from the Variscan Münchberg Massif to unravel whether their Fe isotope compositions are dominated by the igneous/sedimentary protolith signature, by low-temperature seawater alteration, or by later fluid-rock interactions during the subduction-exhumation cycle. Although the eclogites are thought to be derived from a continental rather than oceanic setting (possibly a rift-drift transition stage), they have mid-ocean ridge basalt (MORB)-like major and trace element compositions. They are often moderately oxidized compared to MORB (Fe³⁺/ΣFe = 0.06 to 0.30). Their δ⁵⁶Fe values (+0.00 to +0.17‰; mean + 0.08 ± 0.01‰) mostly resemble those of MORB (+0.07 to +0.17‰). The metagabbros, which are derived from a more enriched mantle source than the eclogites, yielded heavier δ⁵⁶Fe values (+0.09 to +0.22‰) similar to ocean island basalts, whereas those of the paragneisses (+0.03 to +0.10‰) are typical for pelitic sediments. It appears that the Fe isotope compositions of the igneous protoliths are largely preserved and little if any Fe was mobilized during the diverse fluid-rock interaction stages. The parental magma of the eclogites was probably somewhat isotopically lighter than similarly differentiated MORB magmas, perhaps due to the presence of metasomatized, isotopically light peridotites in the subcontinental lithospheric mantle (SCLM) source. Although it is possible that δ⁵⁶Fe values were slightly modified during seawater alteration and/or metamorphic fluid-rock interactions in some of the eclogites, the impact of fluid-rock interactions on the major element compositions of the eclogites appears to be small. Furthermore, the scarcity of metamorphic veins in the Münchberg Massif argues against significant Fe mobilization in the slab. We suggest that continental eclogites tend to retain their magmatic δ⁵⁶Fe values throughout the subduction-exhumation cycle, whereas δ⁵⁶Fe values of oceanic eclogites may often be dominated by seafloor alteration with potential local modifications in the slab close to fluid channels. The remarkable robustness of the Fe isotope compositions of continental eclogites suggests that they may be used to reconstruct protolith mantle source properties despite the complex post-magmatic history.
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Large ion lithophile element enrichment is a feature of the lithologies affected by mass transfer owing to fluids from the subduction zones (Sorensen et al. 1997), well documented, for example, in mélanges of two subduction complexes: the Franciscan Complex (California) and Samana Metamorphic Complex (Dominican Republic). Here we illustrate a similar style of K–Ba–Rb alteration of various types of eclogites and their host rocks acquired during subduction zone metamorphism and subsequent inclusion in a metamorphosed subduction–accretion mélange complex in the Leaota Massif (South Carpathians). Textural evidence at different scales (knockers-with-rinds structures, phengite and atoll garnet overprints, open cavities) and thermobaric calculations document protracted fluid–rock interaction from near-peak conditions to the retrograde stages during detachment and ascension along the subduction channel. Fluids reverting to the surface along the subduction channel play an important part in the fluid budget of subduction zones.
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The Trans-European Suture Zone (TESZ) is a broad and complex zone of terrane accretion separating ancient lithosphere of the Baltic Shield and East European Craton (EEC) from the younger lithosphere of western and southern Europe. There is debate about the number of terranes involved, and their origins. The most significant terrane boundaries, originally oceanic sutures, are poorly exposed, and are defined using faunal provinciality. Prominent geophysical lineaments may represent sub-terrane boundaries within composite terranes. The Avalonia Composite Terrane was amalgamated to Baltica in latest Ordovician time. The provenance of the Malopolska and Moravo–Silesian terranes, whose Neoproterozoic basement is suspect with respect to Baltica, but with which they nonetheless share faunal affinities, is still debated. The extent of the Variscide Rhenohercynian Zone defines the southern limit of the early Palaeozoic-accreted terranes. Terranes within the Bohemian Massif exhibit HP metamorphism recording subduction-driven collision prior to incorporation in the TESZ collage, itself associated with a HP event. In Romania, the Saxothuringian Zone (and Rheic Suture) are directly juxtaposed with the EEC, and the crustal structure of the TESZ has been much modified by post-Palaeozoic tectonism. Deep seismic reflection data, where available, show that the oceanic sutures are frequently associated with inclined reflectivity zones (IRZ) in the lithospheric mantle, here inferred to mark relict subduction zones. Interpreting the age of subduction is complicated by offset of originally coplanar mantle and crustal segments of the IRZ, resulting from early orogenic lithospheric delamination at the Moho interface and/or post-orogenic 'reordering' processes. The mantle IRZ appear to indicate a N-directed polarity for early Palaeozoic subduction, apparently incompatible with the geological evidence, which indicates subduction of the Tornquist Sea beneath Avalonia, and a large amount of underthrusting of Avalonia by Baltica. Possible explanations for these apparently incompatible observations are evaluated. © 1999 Elsevier Science B.V. All rights reserved.
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The widespread occurrence of epidote minerals in metamorphic and igneous rocks as well as in many ore deposit types makes it a promising candidate for fluid inclusion studies. Apart from high- to very high-temperature and low- to intermediate-pressure conditions, epidote minerals are stable over a wide range of pressure and temperature in the continental and oceanic crust (e.g., Poli and Schmidt 1998). Yet fluid inclusion studies on epidote minerals are surprisingly scarce, even in fluid-saturated environments like certain vein-type deposits or hydrothermal-volcanic vugs and druses. For example, epidote minerals are not mentioned in the subject index of Roedder’s (1984) outstanding summary and review of fluid inclusion studies and occurrences, which lists more than sixty different host minerals for fluid inclusions. Nonetheless, more recent studies showed fluid inclusions in epidote minerals to be the only direct witness of the physiochemical and compositional fluid evolution during certain geodynamic processes mainly found in fossil geothermal systems, ore deposits and high-pressure to ultra-high pressure rocks. The aim of this review is to outline and summarize some aspects and interpretations of geodynamic processes, which are based on temperature ( T ), pressure ( P ), molar volume ( V ) and composition ( X ) data from fluid inclusions in epidote minerals as well as associated host minerals from various geological environments. The review starts with a chapter on some typical mixed volatile solid-fluid equilibria involving epidote minerals, which are relevant for the here discussed environments. This is followed by a short introduction into the basic concepts of fluid inclusion research and the role of epidote minerals. The next section covers fluid inclusion studies on epidote minerals from active and fossil geothermal systems as well as low-grade metamorphic rocks and constraints on the P - T - X properties of the fluids present in these systems. This is followed by a short introduction …
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We report here the mode of occurrence of calc-silicate rocks and marbles from the Lützow-Holm Complex, East Antarctica, and a worked example from Rundvågshetta. Calc-silicate boudins were observed in Cape Hinode, Akarui Point, Byôbu Rock, Skarvsnes, Skallevikshalsen and Rundvågshetta, whereas they were reported earlier from Sinnan Rock, Cape Ryûgû, Akebono Rock, Cape Hinode, Niban Rock, Kasumi Rock, Daruma Rock, Cape Omega, Langhovde, Ytrehovdeholmen and Skarvsnes. They vary in size from decimeters to few meters and are commonly enclosed within pelitic or psammitic gneisses. In addition, extensive layers of marbles and calc-silicate rocks are distributed in Skallevikshalsen. - The calc-silicate megaboudins within the layered pyroxene-gneiss from Rundvågshetta, up to 5 m long and 2 m thick, comprises of coarse to medium grained assemblage of scapolite + anorthite + garnet + clinopyroxene + calcite + quartz + titanite ± wollastonite. Co-existing scapolite and plagioclase suggest a "minimum" estimate of peak metamorphic temperature of ∼830°C. Peak metamorphic mineral assemblages equilibrated at moderate to high X CO2 conditions (0.3-0.7) and temperatures between 850 and 1000°C, consistent with the ultrahigh temperature metamorphic conditions reported in the region. Multistage garnet corona formation preserved in the calc-silicate assemblage suggests a local increase in hydrous fluid activity during retrogression. Preliminary bulk rock geochemistry of different mineralogical zones in the boudin shows chemical potential gradients in some major elements, especially SiO 2, Al 2O 3 and CaO, possibly controlled by the compositional variations in the protolith. Altogether, these results suggest that calc-silicate rocks preserve information on the metamorphic evolution and help us in deducing the geodynamic evolution of high-grade terrains.
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The study of subduction-related high-pressure- and ultra-high-pressure metamorphic rocks exposed in orogenic belts is critical for unraveling the tectonothermal evolution of an orogen. The South Tianshan Orogen of the southwestern Central Asian Orogenic Belt, one of the world's largest accretionary orogens, contains several Paleozoic (ultra-)high-pressure ((U)HP) metamorphic terranes. (U)HP eclogites, blueschists and associated (U)HP metasedimentary rocks of continental and oceanic origin were reported from two localities in the Northern Tianshan (NTS) of Kazakhstan and Kyrgyzstan, namely from the Ordovician Aktyuz and Makbal metamorphic terranes. Furthermore Lower Carboniferous (U)HP eclogites and blueschists of oceanic origin are located in the Southern Tianshan Accretionary Complex (STAC) of southern Kyrgyzstan (Atbashi) and northwestern China (Akeyazi).
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Epidotes in Qinglongshan ultrahigh pressure metamorphic eclogites experienced four metamorphic stages: Early epidote-amphibolite facies prograde metamorphic stage (I), coesite-eclogite facies stage (II), quartz-eclogite facies stages (III) and late amphobolite facies retrograde metamorphic stage (IV). Chemical compositions of the epidotes formed in stage I are characterized by the high Fe2O3 contents (14.796 ∼ 17.84%) and XFe (0.413 ∼ 0.486), and low CaO and Al2O3. Epidotes formed in stage II and III show continuous compositional variation. The core which was mainly formed in stage II is relatively enriched in Fe (Fe2O3 = 11.933 ∼ 12.993%, XFe = 0.322 ∼ 0.358) and depleted in CaO and Al2O3, while the rim which was mainly formed in stage III shows relatively low Fe concentration (Fe2O3 = 9.628 ∼ 10.138%, XFe = 0.275 ∼ 0.286) and high CaO and Al2O3 contents. Epidotes formed in stage IV possess similar chemical composition with those of stage I, and are characterized by high Fe2 O3 concentrations and XFe (0.433). Compositional variation of epidotes reflects the rapid subduction and exhumation process of Su-Lu ultrahigh pressure metamorphic belt. Three kind of Fe2+ and Mg2+ bearing aqueous inclusions have been recognized in the epidote: high-salinity (22.5 to slightly more than 23.2wt% NaCl) inclusions, moderate to high salinity (12.6 ∼ 16.0wt% NaCl) and medium salinity(6.4 ∼ 11.7wt% NaCl) inclusions, which were formed in stage II, III and IV respectively. Studies of fluid inclusions in epidotes confirm that no large scale penetrative fluid flow occurred during the UHP metamorphism, and the metamorphic fluid expelled during exhumation of the Qinglongshan UHP rocks flowed mainly at grain scale.
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Phase relations on the join CaTiSiO5-CaSi2O5 were determined at 1350 degrees C over the pressure range 3.5-12 GPa by a combination of synthesis and reversal experiments in a piston cylinder and a multi-anvil press. Titanite-like phases were recovered from all experiments in this pressure range. At 3.5 GPa the maximum Si-VI content of titanite is 3.0 +/- 0.6 mol%, whereas bulk compositions with higher Si content yield a mixture of titanite solid solution plus coesite and walstromite-structured CaSiO3. The maximum Si-VI content of the titanite increases with pressure to 21 +/- 2 mol% at 7 GPa and 46 +/- 2 mol% at 7.5 GPa. At pressures of 8.5 to 12 GPa all intermediate compositions yield a single titanite phase. X-ray and TEM analysis show that these have the A2/a symmetry of the titanite aristotype. The variations of the room-pressure unit-cell parameters of the A2/a phases with composition can be described by the equations a [Angstrom] = 7.040(9) - 0.492(15) X(Si-VI); b [Angstrom] = 8.713(7) - 0.316(11) X(Si-VI); c [Angstrom] = 6.564(4) - 0.220(7) X(Si-VI); beta [degrees] = 113.721(6) - 0.537(12) X-2(Si-VI); V (cell) [Angstrom(3)] = 367.8(9) - 47.5(1.6) X(Si-VI). For the CaSi2O5 composition the recovered material has I (1) over bar symmetry but is known to transform back to A2/a titanite structure at 0.2 GPa at room temperature. Similarly, with increasing pressure the P2(1)/a CaTiSiO5 transforms to A2/a symmetry at 3.6 GPa at room temperature. The conclusion is that at 1350 degrees C and pressures in excess of 8.5 GPa there is complete solid solution between CaTiSiO5 and CaSi2O5 based upon the isovalent exchange of Si for Ti in the octahedral site of the A2/a structure. Rietveld structure analysis of intermediate compositions reveals no evidence for ordering or intermediate phases. Preliminary experiments at pressures between 13.5 GPa and 16 GPa yielded mixtures of titanite solid solution plus perovskite and stishovite. From these data and information on the phase relations for the CaSiO3-CaTiO3 join the topology of the phase relations between similar to 3 and similar to 13 GPa in the central part of the CaO-TiO2-SiO2 ternary have been deduced.
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This paper presents a new interactive MATLAB-based visualization and calculation tool (TETGAR_C) for assessing the provenance of detrital garnets in a four-component (tetrahedral) plot system (almandine-pyrope-grossular-spessartine). Based on a freely-accessible database and additional electron-microprobe data, the chemistry of more than 2,600 garnet samples was evaluated and used to create various subfields in the tetrahedron that correspond to calc-silicate rocks, felsic igneous rocks (granites and pegmatites) as well as metasedimentary and metaigneous rocks of various metamorphic grades. These subfields act as reference structures facilitating assignments of garnet chemistries to source lithologies. An integrated function calculates whether a point is located in a subfield or not. Moreover, TETGAR_C determines the distance to the closest subfield’s mean value. Compared with conventional ternary garnet discrimination diagrams, this provenance tool enables a more accurate assessment of potential source rocks by reducing the overlap of specific subfields and offering quantitative testing of garnet compositions. In particular, a much clearer distinction between garnets from greenschist-facies rocks, amphibolite-facies rocks, blueschist-facies rocks and felsic igneous rocks is achieved. Moreover, TETGAR_C enables a distinction between garnet grains with metaigneous and metasedimentary provenance. In general, metaigneous garnet tends to have higher grossular content than metasedimentary garnet formed under similar P-T conditions.
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The trace element compositions of eclogites, blueschists and mafic granulites from high-pressure terranes have been analysed to investigate element losses and fractionation that occur during dehydration of oceanic basalt in subduction zones. Abundances of elements that are suggested to be near-immobile (e.g., Nb, Zr, Ti), Sr-Nd isotopic compositions, and major element compositions indicate that most samples had altered MORB protoliths. The samples show only limited retrograde alteration, and cover a range in pressure-temperature conditions (1.2–4 GPa, 300–1000°C). In ratio diagrams, strong depletions (95–98%) of K, Rb and Ba relative to Nb and Th in most samples are obvious when compared with unaltered and altered MORB or ocean island basalts. The largest fraction of K, Rb and Ba appears to be lost at temperatures average arc front magma compositions. The low estimate for Sr confirms previous indications that contributions [page end] from average altered MORB cannot explain the Sr enrichment in arc lavas. Most of the Nd, heavy rare earth elements (REE), Y, high-field strength elements (HFSE) and compatible elements in primitive arc front magmas must be supplied by the depleted mantle wedge and a sedimentary component in arc lavas.
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The southern Dora Maira Massif consists of several continental crustal slices differing in their metamorphic grade, separated and themselves dissected by major low-angle faults. In the entire pile, the main structural imprint is a regional foliation which roughly parallels the lithologic and tectonic boundaries. This mylonitic foliation contains a widespread ENE-WSW stretching lineation, parallel to isoclinal-fold axes and with clear top-to-the-west kinematic indicators in most units. This strong Alpine ductile imprint developed under greenschist-facies conditions and may be related to the piling up of the units; it postdates and overprints earlier Alpine high-pressure (HP) assemblages. -from Authors
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Experimental kinetic studies and related theory for solid-state transformations relevant to metamorphic processes unfortunately lag far behind kinetic investigations of mineral-fluid reactions, related to natural waters and their precipitates, and mineral-melt reactions, related to crystallization of igneous rocks (see volume edited by Lasaga and Kirkpatrick, 1981). While metamorphic rocks have experienced successive temperature (T) and pressure (P) changes during their evolution, only rarely can we constrain the magnitude and direction of these P-T changes as functions of time (t). It is at present difficult to say whether characteristic disequilibrium features, observed petrographically, mean that a certain temperature was never achieved, or was only achieved for a short time. Such features might include the coexistence of two polymorphs or any complete reactant plus product assemblage as well as thermodynamically incompatible phases, irregular grain boundaries, and oriented fabrics. A great variety of environmental factors (presence of fluid, deformation, local chemical catalysts, etc.) can influence the kinetics of most mineral transformations. However, it is not often possible to see to what degree such factors have influenced an observed mineral assemblage from petrographic data alone.
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The eclogite-facies metasedimentary rocks in the Münchberg gneiss complex (T=630±30° C/P=17–24 kbar) locally contain CO2-N2-rich fluid inclusions of extremely low molar volumes (32 cm3/mol) in quartz. These fluid compositions are mainly found in rocks intercalated with calcsilicate bands. Densities were determined from low-temperature phase transitions like stable or metastable homogenization (L+V?L), partial homogenization (S+L+V?S+L) and the transition S+L?L (L = liquid, V = vapour, S = solid). The high fluid densities are in agreement with eclogite-facies pressure and temperature and subsequent amphibolite facies. CO2-N2 inclusions were not observed in adjacent eclogites nor in non-calcareous metasediments. These rock types contain predominantly H2O-rich inclusions correlating with amphibolite-facies conditions. The variation of fluid composition with lithological differences indicates local fluid gradients and speaks against a pervasive fluid flow during eclogite-facies metamorphism.
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Fluid activity ratios calculated between millimeter- to centimeter-scale layers in banded mafic eclogites from the Tauern Window, Austria, indicate that variations in a H 2 O existed between layers during equilibration at P approximately equal to 2GPa and T approximately equal to 625C, whereas a CO 2 was nearly constant between the same layers. Model calculations in the system H2O–CO2–NaCl show that these results are consistent with the existence of different saturated saline brines, carbonic fluids, or immiscible pairs of both in different layers. The data cannot be explained by the exisience of water-rich fluids in all layers. The model fluid compositions agree with fluid inclusion compositions from eclogite-stage veins and segregations that contain (1) saline brines (up to 39 equivalent wt. % NaCl) with up to six silicate, oxide, and carbonate daughter phases, and (2) carbonic fluids. The formation of crystalline segregations from fluid-filled pockets or hydrofractures indicates high fluid pressures at 2 GPa; the record of fluid variability in the banded eclogite host rocks, however, implies that fluid transport was limited to local flow along individual layers and that there was no large-scale mixing of fluids during devolatilization at depths of 60–70 km. The lack of evidence for fluid mixing may, in part, reflect variations in wetting behavior of fluids of different composition; nonwetting fluids (water-rich or carbonic) would be confined to intergranular pore spaces and would be essentially immobile, whereas wetting fluids (saline brines) could migrate more easily along an interconnected fluid network. The heterogeneous distribution of chemically distinct fluids may influence chemical transport processes during subduction by affecting mineral-fluid element partitioning and by altering the migration properties of the fluid phase(s) in the downgoing slab.
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Recently published thermodynamic and experimental data in a variety of chemical systems have been evaluated to derive Gibbs free energies for hedenbergite and pyrope. These were used to calculate the geobarometric equilibria Hedenbergite+Anorthite=Grossular+Almandine +Quartz: “HD barometer”, Diopside+Anorthite=Grossular+Pyrope+Quartz: “DI barometer”. We have compared the pressures obtained from these equilibria for garnet-clinopyroxene-orthopyroxene-plagioclasequartz assemblages with the geobarometer Ferrosilite+Anorthite=Almandine+Grossular+Quartz: “FS barometer”. Pressures calculated for 68 samples containing the above assemblage from a variety of high grade metamorphic terranes indicate that, in general, the HD and DI barometers yield values that are in good agreement with the FS barometer, and that the three barometers are generally consistent with constraints from aluminosilicate occurrences. However, in some samples the HD barometer yields pressures up to 2 kbar greater than constraints imposed by the presence of an aluminosilicate phase. Relative to the FS barometer, the HD barometer overestimates pressure by an average of 0.2±1.0 (1σ) kbar and the DI barometer underestimates pressure by an average of 0.6±1.6 (1σ) kbar. The pressure discrepancies for the HD and DI barometers are likely to be a result of imprecision in thermodynamic data and activity models for silicates, and not a result of resetting of the clinopyroxene equilibria. The relative imprecision of the DI barometer relative to the FS barometer results from overestimates of pressure by the DI and FS barometers in Fe-rich and Mg-rich systems, respectively. Application of the HD and DI barometers to high grade Cpx-Gt-Pg-Qz assemblages yields pressures that are generally consistent with other petrologic constraints and geobarometers. It is concluded that the HD and DI barometers can place reasonable constraints on pressure (±1 kbar relative to the FS barometer) if not extrapolated to mineral assemblages whose compositions are extremely far removed from the end member system for which the barometers were calibrated.
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Both the coarse- and fine-grained varieties of the partly coesite-bearing pyrope-quartzites, their interlayered jadeite-kyanite rocks, and the biotite-phengite gneiss country rock common to all of them were subjected to detailed petrographic and textural studies in order to determine the sequence of crystallisation of their mineral constituents, which were also studied analytically by microprobe. Prior to pyrope and coesite growth, the Mg-rich metapelites were talc-kyanite-chlorite-rutile-ellenbergerite schists which — upon continued prograde metamorphism — developed first pyrope megacrysts in silica-deficient local environments at the expense of chlorite + talc + kyanite, and subsequently the smaller pyrope crystals with coesite inclusions from reacting talc + kyanite. Based on geobarometrically useful mineral inclusions as well as on experimentally determined phase relations, a prograde PT-path — simplified for water activity = 1 — is constructed which passes through the approximate PT-conditions 16 kbar and 560° C, 29 kbar and 720° C, and finally up to 37 kbar at about 800° C, where the Mg-rich metapelite was a pyrope-coesite rock with phengite, kyanite, and talc still present. During the retrograde path, pyrope was altered metasomatically either into phlogopite + kyanite + quartz or, at a later stage, to chlorite + muscovite + quartz. Both assemblages yield PT-constraints, the latter about 7–9 kbar, 500–600° C. The country rock gneisses have also endured high-pressures of at least 15 kbar, but they provide mostly constraints on the lowest portion of the uplift conditions within the greenschist facies (about 5 kbar, 450° C). Microprobe data are presented for the following minerals: pyrope, ellenbergerite, dumortierite (unusually MgTi-rich), jadeite, vermiculite (formed after Na-phlogopite?), paragonite, and for several generations of phengite, chlorite, talc, phlogopite, dravite, and glaucophane in the high-pressure rocks, as well as for biotite, chlorite, phengites, epidote, garnet, albite, and K-feldspar in the country rock gneisses. An outstanding open problem identified in this study is the preservation of minerals as inclusions within kyanite and pyrope beyond their PT-stability limits.
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Phengites from eclogites and pegmatites (3T, 2M1, coarse-grained and recrystallized) of the Mnchberg Massif (Weissenstein and Oberkotzau) have been dated by the 40Ar/39Ar method. 3T-micas from the eclogites yielded plateau and isochron ages of 3657 Ma. 2M1-micas show disturbed degassing spectra. Micas from pegmatites show a slight excess Ar component, with an isochron age of 353 to 3513 Ma. An age component of approximately 300 Ma was also detected. In combination with age values from the literature, the cooling history of the Mnchberg Massif from eclogite-facies conditions (390 Ma) to cooling below 350C (350 Ma) is documented. The age component of 300 Ma is attributed to a low-grade stage of mineral growth accompanied by a transitional ductile-brittle deformation. The petrological effects include formation of pumpellyite-prehnite-facies minerals, frequently precipitated in microcraks and cleavage planes of earlier formed minerals. This stage has to be seen in conjunction with the intrusions of the Fichtelgebirge granite.
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The effects of pressure, temperature, and Fe3+ ⇌ Al exchange on the transformation from orthorhombic zoisite to clinozoisite are examined using thermodynamic data and analyses of naturally coexisting mineral pairs. The position of the two-phase zoisite-clinozoisite area in a T-X section varies with pressure. Equilibrium mineral pairs from low- to intermediate-pressure regimes indicate a smaller two-phase area than those from high-pressure regimes. The width of the two-phase region also depends on minor elements. The effect on the phase diagram of a possible low-temperature miscibility gap in the monoclinic series is also discussed. Disequilibrium textures are common features in naturally coexisting clinozoisite-zoisite pairs. Owing to the sluggish reaction behaviour of the epidote minerals, these can serve as important petrogenetic indicators of prepeak or relict metamorphic conditions. -from Authors
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Epitaxial zoisite/clinozoisite crystals hydrothermally synthesized at approx 650oC and 6500 bar were characterized by XRD and electron microprobe techniques and analysed thermodynamically. The results suggest that single-crystal (vs bulk) techniques could be employed to bracket equilibria for phases that are extremely difficult to synthesize with a homogeneous composition.-J.A.Z.
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Two successive generations of mineral assemblages are reported to occur in the siliceous dolomites. The earlier assemblage crystallized at Pfluid approx 18-25 kbar, with an almost pure H2O fluid phase. The later assemblage crystallized at Pfluid approx 8-15 kbar. Results of a study of phase relations are discussed. Their interpretation suggests that, together with associated omphacite-garnet eclogites, the siliceous dolomites were subducted to depths of 60-80 km and subsequently rose to a depth of 30-45 km.-M.S.
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With reference to the general character of the Variscan foldbelt a key problem within the Saxothuringian Zone is the question of the nature of the metamorphic Munchberg Massif in NE Bavaria. Several new lines of evidence are brought to bear on the problem. The Munchberg Massif is regarded as a pile of nappes. In Carboniferous time nappes of Palaeozoic sedimentary and volcanic rocks, either non-metamorphic or of low grade, evolved and were then overriden by the nappes of high-grade crystalline rocks. The probable root zone of the nappes is at the boundary between the Saxothuringian and Moldanubian Zones, now some 50 km away to the S. Such an interpretation probably applies to all the 'Zwischengebirge' present to the NE along the strike and may extend to include the Gory Sowie (Eulengebirge) in W Poland. The evidence would not support interpretation in terms of a conventional B (Benioff) subduction of oceanic crust. A possible mechanism involving A (Ampferer) intraplate subduction is briefly discussed.-from Authors
Article
The recognition of nappes in the Saxothuringian Zone, together with tectonic fabrics and structures in the Moldanubian Zone, combine to suggest a mobilistic concept for the geotectonic developement of these regions. The origin of the Saxothuringian basin can be referred to an Upper Proterozoic to Ordovician rifting process. Metamorphosed sedimentary and basic igneous rocks found along the western margin of the Moldanubian Zone and in the Tepla-Parrandean block, conventionally interpreted as part of a Cadomian cycle, might well have been part of a contiguous Saxothuringian/N-Moldanubian rift basin. This basin unterwent tectonic deformation and metamorphism in Devonian and Lower Carboniferous time. The existence of a Cadomian event is uncertain. Padiometric indications of an Ordovician thermal event can be explained, in the Saxothuringian Zone, as representing rift-metamorphism. Similar data from the southwestern part of the Moldanubian Zone cannot, at present, be reconciled with geological observations. The medium-pressure metamorphic rocks of the Münchberg and related crystalline massifs are the result of Devonian overthrusting and crustal thickening. Later, they were thrust over lower grade Palaeozoic rocks, and this tectonic pile overrode, in Lower Carboniferous time, the area of the later Fichtelgebirge/Erzgebirge antiform, thus effecting regional metamorphism in that area. Certain rock units in the nothwestern part of the Moldanubian Zone and in the Tepla/Barrandean area probably are equivalents of the medium-grade Münchberg rocks. This is suggested by close affinities in their lithology, metamorphic facies, and tectonic fabrics. The arrangement of thrust zones within the Bohemian massif might also be related to tectonic shortening and to the rotation of blocks during the Variscan orogeny.
Article
Eclogitic relics testifying to one or more early events of high-pressure metamorphism are present in different tectonic units of the NE Bavarian crystalline basement forming the north-western margin of the Bohemian Massif. Eclogites and their amphibolized derivatives are most frequent in the Munchberg nappe complex where they form constituents of the uppermost crystalline nappe, the so-called Hangendserie. -from Authors
Article
In the Münchberg Massif in the Variscan foldbelt of southern Germany two varieties of eclogite are known which are intercalated with amphibolite-facies meta-igneous and meta-sedimentary rocks: a dark kyanite-free and a lighter colored kyanite-bearing type. Kyanite-free eclogites, which are discussed here, have a major and trace element composition which suggests derivation from ocean-floor basalts with melt to cumulate compositions. Internal Sm-Nd isochrons (clinopyroxene-amphibole-garnet) and one Rb-Sr isochron (clinopyroxene-amphibole-mica) yield eclogitization ages in the range of 380 to 395 Ma. Thus, the age of eclogitization is only marginally higher ( < 15 Ma) than the age of amphibolite-facies metamorphism in the Münchberg Massif as derived from K-Ar ages of amphiboles and micas from metasediments and meta-igneous rocks. A seven point whole-rock Sm-Nd isochron for one eclogite body results in an age of 480 +/- 23 Ma with an initial εNd of 8.7 +/- 0.6 and is likely to record the age of igneous formation of the eclogite protoliths. Sr isotopic compositions back-calculated to that time are anomalously high and variable if compared to Nd isotopes. This can be explained by alteration with an aqueous or fluid phase with high 87Sr/86Sr, most likely seawater, either during igneous formation in an oceanic rift environment or subduction-related eclogitization. In addition, some eclogites show a marked enrichment of incompatible, immobile elements and plot far below the whole-rock Sm-Nd isochron. These features are ascribed to the presence of an evolved crustal component, probably acquired during extrusion of the basaltic protoliths by mixing with country-rock gneisses.
Article
The gneiss has a complex intrusive and deformational history. Outcrop is dominated by homogeneous felsic orthogneiss, which encloses boudinaged mafic and ultramafic units. These boudins preserve structures (S1, F2) not seen in the host gneiss. Basement lithologies were deformed into a series of flat-lying structures (S3, F4) consistent with progressive horizontal shear, and then into a series of upright structures (F5) culminating in the development of regional synforms, antiforms and monoclines (F6) separated by zones of intense upright fabric (S6). The D3 to D6 time interval was associated with several episodes of partial melting which produced discordant leucogneiss bodies, and with the emplacement of mafic dykes and charnockite plutons correlated with 950 to 1000 Ma charnockite elsewhere in East Antarctica. The stability of granulite assemblages throughout the D3 to D6 interval is attributed to a widespread 1000 Ma metamorphic event. Thermobarometry yields peak conditions of 700-800°C and 0.6-0.7 GPa. Petrogenetic grid constraints on calc-silicate assemblages indicate peak temperatures of 830°C, suggesting that the lower temperatures derived by thermometry have been reset. -from Authors
Article
Highly aluminous titanite with Al2O3 contents up to 14 wt.% (50% of the CaAlSiO4(F,OH) end-member) occurs in siliceous dolomites from the Eclogite Zone, south-central Tauern Window, Austria. 140 chemical analyses of titanites from 13 samples from the Eclogite Zone and adjacent tectonic units reveal that Al is incorporated into titanite primarily by the mechanism Ti + O  Al + (F,OH). There is also a strong positive correlation between Al and F content of these titanites, indicating that the combined substitution is enhanced by the presence of F.Al contents of titanite vary between tectonic units, with the most aluminous varieties occurring in the high-pressure Eclogite Zone. Variations also occur from sample to sample within a single tectonic unit and both between and within individual grains in a single sample. Complex zoning with sharp chemical and optical discontinuities is observed, with Al2O3 variations of up to 6 wt.% within a single sample. Individual grains generally have low-Al cores, mantled by high-Al zones and rimmed by low-Al edges.It is suggested that the incorporation of Al into titanite is favoured by high P, low T and high F activity.
Article
A model for the mixing properties of quaternary Ca-Mg-Fe-Mn garnets is developed by mathematical programming analysis of reversed phase-equilibrium data as well as calorimetric and volumetric data. Excellent support for the derived mixing properties of garnet is demonstrated by 1) convergence of different equilibria involving various garnet components in a single P-T region for a two-pyroxene granulite, 2) calculating presures with the equilibrium 3 anorthite=grossular+2 aluminosilicate+quartz that place the observed aluminosilicate polymorph in natural assemblages within its computed stability field, and 3) computing pressures as above and garnet-biotite temperatures that place rocks with more than one aluminosilicate phase in proximity to the appropriate aluminosilicate phase boundary. -from Author
Article
GEO-CALC is a microcomputer software package consisting of a programme for calculation of phase diagrams, a thermodynamic database for minerals, and auxiliary programmes for plotting and printing of computed phase diagrams. Minimum hardware requirements are an IBM-compatible personal computer with 512 K memory, maths coprocessor, and graphics card (CGA, EGA, or Hercules).-J.A.Z.
Article
Distribution of Al-silicate minerals in pelitic schists indicates that the T and P of metamorphism were approximately those of the Al2SiO5 invariant point. This natural system is used to evaluate the internal consistency of several published geothermometers and geobarometers with the Al2SiO5 invariant point of Holdaway. (M.A. 74-3148)-J.A.Z.
Article
Abstract Eclogites with a wide range in bulk composition are present in the Münchberg Massif, part of the Variscan basement of the Bohemian Massif in north-east Bavaria. New analyses of the primary phases garnet, omphacite, phengite and amphibole, as well as the secondary phases clinopyroxene II, various amphiboles, biotite/phlogopite, plagioclase, margarite, paragonite, prehnite and pumpellyite, reveal a complex uplift history. New discoveries were made of samples with very jadeite-rich primary omphacite as well as a secondary omphacite in a symplectite with albite. Various geothermobarometric techniques, together with thermodynamic databases (incorporating separately determined activity–composition values) and experimental data have clustered the minimum conditions for the primary assemblages to the P–T range 650 ± 60° C, 14.3 ± 1 kbar. However, jadeite (in omphacite)–kyanite–paragonite (in phengite) and zoisite–grossular (in garnet)–kyanite–quartz relationships suggests pressures of 25–28 kbar at the same temperatures. The fact that the secondary omphacite–plagioclase assemblage yields pressures within a few hundred bars of the minimum pressures for the plagioclase-free assemblages strongly suggests that the minimum values are serious underestimates. Zoning, inclusion suites and breakdown reactions of primary phases, in addition to new minerals formed during uplift, define a polyphase metamorphic evolution which, from geochronological evidence, occurred solely within the Variscan cycle. The complex breakdown in other Bohemian Massif eclogites and the distinct variation in their temperatures during uplift suggest a multi-stage thrusting model for the regional evolution of the eclogites. Such an evolution has significance with respect to incorporation of mantle slices into crustal sequences and fluid derivation from successively subducted units, possibly driving the breakdown reactions.
Article
Abstract This paper provides methods and a description of a Pascal computer program, thermocalc, for various thermodynamic calculations using the thermodynamic dataset presented in earlier papers in this series (Holland & Powell, 1985; Powell & Holland, 1985). The dataset involves uncertainties on the thermodynamic parameters and therefore allows uncertainties to be calculated on results, for example in geothermometry and geobarometry. Recommendations are made for the uncertainties on activities to be used in calculations on rocks, particular emphasis being placed on preventing underestimates of these uncertainties at small mole fractions. Apposite examples of phase diagram and rock calculations are presented with ouput from thermocalc, demonstrating the utility of the program. Of the rock calculations, the most valuable are considered to be those involving simultaneous combination ‘least squares’of calculated conditions for a set of reactions applicable to a rock. This set of reactions involves the independent reactions which can be written between the end-members in the minerals in a rock and in the thermodynamic dataset. In contrast to an approach based on specific geothermometers and geobarometers, this approach maximizes the benefit of having an internally consistent thermodynamic dataset. thermocalc is available in IBM PC and Mac versions, from Roger Powell for A$25 or Tim Holland for £10 per version.
Article
We present, as a progress report, a revised and much enlarged version of the thermodynamic dataset given earlier (Holland & Powell, 1985). This new set includes data for 123 mineral and fluid end-members made consistent with over 200 P–T–XCO2–fO2 phase equilibrium experiments. Several improvements and advances have been made, in addition to the increased coverage of mineral phases: the data are now presented in three groups ranked according to reliability; a large number of iron-bearing phases has been included through experimental and, in some cases, natural Fe:Mg partitioning data; H2O and CO2 contents of cordierites are accounted for with the solution model of Kurepin (1985); simple Landau theory is used to model lambda anomalies in heat capacity and the Al/Si order–disorder behaviour in some silicates, and Tschermak-substituted end-members have been derived for iron and magnesium end-members of chlorite, talc, muscovite, biotite, pyroxene and amphibole.
Article
Following and extending the early work of Velde (1965) the pressure-temperature dependence of the compositions of potassic white micas coexisting with K-feldspar, quartz, and phlogopite in the model system K2O-MgO-Al2O3-SiO2-H2O was investigated up to fluid pressures of 24 kbar by synthesis experiments. There is a strong, almost linear increase of the Si content per formula unit (p.f.u.) of phengite, ideally KAl2−xMgx[Al1−xSi3+xO10] (OH)2 with pressure, as well as a moderate decrease of Si (or x) with temperature. The most siliceous phengite with Si near 3.8 p.f.u. becomes stable near 20 kbar depending on temperature. However, contrary to Velde's assumption, these phengites coexisting with the limiting assemblage are invariably not of an ideal dioctahedral composition (as given by the above formula) but have total octahedral occupancies as high as about 2.1 p.f.u. The stability field of the critical assemblage phengite — K-feldspar — phlogopite — quartz ranges, in the presence of excess H2O, from at least 350° C to about 700° C but has an upper pressure limit in the range 16–22 kbar, when K-feldspar and phlogopite react to form phengite and a K, Mg-rich siliceous fluid. For the purpose of using these phase relationships as a new geobarometer for natural rocks, the influence of other components in the phengite (F, Fe, Na) is evaluated on the basis of literature data. Water activities below unity shift the Si isopleths of phengite towards higher pressures and lower temperatures, but the effects are relatively small. Tests of the new geobarometer with published analytical and PT data on natural phengite-bearing rocks are handicapped by the paucity of reliable values, but also by the obvious lack of equilibration of phengite compositions in many rocks that show zonation of their phengites or even more than one generation of potassic white micas with different compositions. From natural phengites that do not coexist with the limiting assemblage studied here but still with a Mg, Fe-silicate, at least minimum pressures can be derived with the use of the data presented.
Article
The mineral assemblages in the eclogites and meta-sediments of the Mnchberg gneiss complex suggest minimum pressures of about 15 kbar, and temperatures of 60030 C for the eclogite-facies metamorphism. Both rock types underwent a subsequent amphibolite-facies metamorphism at almost the same temperature range. In the Weissenstein borehole the eclogites and meta-sediments show an intimate interlayering on a centimetre scale. Contacts between the two rock types are often sharp and untectonized suggesting in-situ metamorphism of the eclogites. The following features indicate that the gneisses were subjected to eclogite-facies metamorphism:1. Phengite relics and phengite inclusions in garnet contain up to 3.45 Si per formula unit while amphibolite facies phengites have considerably lower Si-contents (3.0–3.25 p.f.u.). 2. Omphacite relics occur in the form of Na-augite (6–9 mol% jadeite)-oligoclase symplectites. 3. Garnets with up to 40 mol% of the pyrope component occur. The geological and mineral-chemical data indicate that large volumes of crustal material have been buried to depths possibly exceeding 70 km. The preservation of primary eclogitic textures as well as symplectitic textures in the presence of a fluid phase, are indicative of a rapid decompression. This would suggest a tectonic uplift (e.g. underthrusting) as is also indicated by the virtually constant temperature range during the uplift.
Article
Kyanite eclogites occur as part of the Mnchberger nappe pile in NE-Bavaria, West Germany. Eclogites are overprinted by subsequent amphibolite facies metamorphism. The preservation of primary eclogitic textures as well as symplectitic textures are indicative of rapid decompression. Eclogite formation is estimated to have occurred under conditions of high H2O-activities at pressures between 20 and 26 kbar and temperatures ranging between 590 and 660 C, as is shown by the coexistence of omphacite (Jd 50), kyanite, zoisite and quartz. Minimum pressure estimates, independent of the water activity, range between 9 and 16 kbar at the relevant temperatures. Detailed studies of fluid inclusion reveal two predominant groups of aqueous-brine inclusions: high salinity (14–17 wt% NaCl equiv.) and low salinity (0–8 wt% NaCl equiv.) inclusions. Fluid compositions of both groups of inclusions yield isochores passing close to the estimated amphibolite facies PT-field. The compositions of these fluids are in good agreement with fluid compositions considered from mineral equilibria. None of the fluid inclusions has densities appropriate for eclogite facies metamorphism, but probably reflect later amphibolite facies metamorphism.
Article
The petrography, mineral chemistry and petrogenesis of a sample from the Weissenstein eclogite, Bavaria, Germany, has been investigated. The total mineral assemblage comprises garnet, clinopyroxeneI+II, quartz, amphiboleI+II, rutile, phengite, epidote/allanite, plagioclase, biotite, apatite, pumpellyite, titanite (sphene), zircon, alkali feldspar and calcite. Textural observations combined with geothermobarometry (Fe/Mg distribution between clinopyroxene/garnet and phengite/garnet; jadeite-content of omphacite, Si-content of phengite, and An-content of plagioclase) provide indications of two different stages in the metamorphic evolution of the rock. The main phengitequartz-eclogite mineral equilibration occurred at minimum P=13–17kbar, minimum T=62050 C; the retrograde symplectite stage (clinopyroxeneII, amphiboleII, biotite, plagioclase) occurred at P total between 12 and 8.5 kbar.Reactions of the symplectite stage are:(1) phengite (core) + Na2Oaq + CaOaq=phengite (rim) + biotite + plagioclase + K2Oaq + H2O (2) phengite (core) + clinopyroxeneI + Na2Oaq=phengite (rim + biotite + plagioclase + amphiboleII + SiO2 + K2Oaq + CaOaq + H2O (3) clinopyroxeneI + SiO2 + K2Oaq + H2O=clinopyroxeneII + plagioclase+amphiboleII + Na2Oaq + CaOaq The phengite decomposition produces H2O, whereas the clinopyroxene decomposition consumes H2O.The estimated P-T-conditions for the Weissenstein eclogite are in the same order of magnitude as those for other eclogite bodies from the Alps and Caledonides believed to be related to subduction processes.
Article
Titanite and rutile are a common mineral pair in eclogites, and many equilibria involving these phases are potentially useful in estimating pressures of metamorphism. We have reversed one such reaction, {Mathematical expression} using a piston-cylinder apparatus. Titanite+kyanite is the high-pressure assemblage and our results locate the equilibrium between 15.5 15.9, 17.7-17.9, 18.8-19.0, and 20.0-20.2 kb at 900, 1000, 1050, and 1100°C, respectively. The experiments require a positive dP/dT of between 20.5 and 23.5 bars/°C for the reaction. We use the reversed equilibrium and two other reactions, {Mathematical expression} and {Mathematical expression} to calculate metamorphic conditions for three eclogite localities. Using these reactions in conjunction with garnet-clinopyroxene Fe2+-Mg exchange equilibria, conditions of metamorphism were 16 kb and 750°C for kyaniteeclogites from Glenelg, Scotland, 21 kb and 625°C for eclogite-facies mica schists from the Tauern Window, Austria, and 46 kb and 850°C for eclogite-facies biotite gneisses from the Kokchetav Massif, USSR. For the Scottish and Austrian eclogites, the pressures derived from the titanite-rutile reactions provide additional constraints on pressures for these localities, leading to precise estimates of metamorphic conditions. In the case of the Soviet Union eclogites, the results show that the silicate-oxide assemblage is consistent with the remarkable occurrence of diamond inclusions in the garnets. The results of this study suggest that titanite and rutile stably coexist in many eclogites and that titanite solid solutions are ideal or nearly so.
Article
Coesite is reported for the first time from the Caledonide orogen. It occurs as inclusion in clinopyroxene in the dolomite-eclogite at Grytting, Norway, and provides valuable new evidence supporting the hypothesis of extremely high pressure in certain Norwegian eclogites. This finding has implications for the geodynamics and kinetics of the concepts of microscale intracrystalline tectonic overpressure and of macroscale local or regional tectonic overpressures, and for hypotheses of subduction and obduction involving a more than 100 km vertical component of transport.
Article
A standardized, internationally applicable set of symbols for rock-forming mineral phases and mineral components is presented.-J.A.Z.
Article
Mid-Proterozoic (∼ 1000 Ma) granulite facies calc-silicates from the Rauer Group, East Antarctica, contain grossular-wollastonite-scapolite-dinopyroxene ( + quartz or calcite) assemblages which preserve symplectite and corona textures typically involving the growth of secondary wollastonite. The textures include (1) wollastonite rims between quartz and calcite; (2) wollastonite-plagioclase rims and intergrowths between quartz and scapolite; (3) wollastonite-scapolite-clinopyroxene inter-growths replacing grossular; and (4) wollastonite-plagioclase symplectites replacing grossular or earlier symplectites (3). Reactions between grossular, scapolite, wollastonite, calcite, quartz, anorthite, and vapour, have been modelled in the CaO-Al 2 O 3 SiO 2 -H 2 O-CO 2 and more complex systems using the internally consistent data-set of Holland & Powell (1990). Reactions producing scapolite and wollastonite consume vapour as temperature increases (i. e., carbonation), in agreement with the results of Moecher & Essene (1990α). These calc-silicates can therefore behave as fluid sinks under high-grade conditions. Conversely, they may be important fluid sources on cooling and contribute to the formation of post-metamorphic CO 2 rich fluid inclusions in isobarically cooled granulites. P-T-α CO2 diagrams calculated for typical phase compositions (e. g., garnet, scapolite) demonstrate that the observed textures are a record of near-isothermal decompression at 800–850 � C, consistent with P—rpath determinations based on other rock types from the Rauer Group. For example, texture (2) results from crossing the reaction Scapolite + Quartz = Wollastonite + Plagioclase + V on decompression, at ∼ 6. 5–7 kb, 820 � C, and a CO2 of 0–4–0–5. Furthermore, correlations between modes of product phases (e. g., wollastonitexlinopyroxene) and reactant garnet composition preclude open-system behaviour in the formation of these textures, consistent with post-peak vapour-absent reactions such as Grossular + Calcite + Quartz = Wollastonite + Scapolite occurring on decomposition at high temperatures (>800�C). Reaction textures developed in calc-silicates from other granulite terranes often involve the formation of grossular ( + quartz � calcite) as rims on wollastonite-scapolite, or replacement of wollastonite by calcite-quartz. These textures have developed principally in response to cooling below 780–810 � C and may be signatures of near-isobaric cooling. Infiltration of hydrous fluid is not a necessary condition for the production of garnet coronas in wollastonite-scapolite granulites. *Present address: Department of Earth Sciences, University of Melbourne, Parkville, Victoria 3052, Australia
Article
The polymetamorphic Moldanubian (MO) of the northeasthern margin of the Bohemian Massif has been thrust to the north onto the mainly Paleozoic sedimentary Saxothuringian of the Fichtelgebirge (FG). These two units have undergone polyphase deformation and the last regional event to affect both units was a low-pressure metamorphism in which temperatures decreased towards the north.In contrast, the nappe units of the Erbendorf-Vohenstrauss Zone (ZEV) and the Erbendorf Greenschist Zone (EGZ), which partly cover the border of the Moldanubian and the Saxothuringian, and the Münchberg nappe pile (MM), which lies on the Saxothuringian, were in parts subjected to a late medium-pressure metamorphic event.The ZEV, the EGZ, the MO and the FG are intruded by Late Carboniferous granites.Conventional K-Ar analyses, mainly of hornblendes and muscovites from the autochthonous FG and MO, the units beneath the nappes, have yielded exclusively Carboniferous dates. The oldest dates point to a regional cooling of the rocks which outcrop at the present-day surface at about 330-320 Ma, i.e., at the Early-Late Carboniferous boundary. The Late Carboniferous cooling history was largely governed by the thermal influence of the post-kinematic granites (320-295 Ma), especially in the FG and the northern MO.The high-grade metamorphic rocks in the western part of the ZEV and in the upper three nappes of the MM mostly yield dates around 380 Ma i.e., Early Devonian. The results show a relatively wide scatter. Moreover, biotites frequently appear to be older than the coexisting muscovites. Both observations indicate that the rocks underwent a later thermal influence. Whether some groups of older dates (e.g., 400 Ma) are due to excess argon or to inherited argon is still open to discussion.Slightly scattered muscovite dates around 366 Ma were obtained for the prasinite-phyllite series, one of the lower nappes of the MM. A single hornblende from the EGZ gave the same age. These two nappes have, therefore, probably been affected by a Late Devonian thermal and/or tectonic event.The muscovite dates obtained from the Paleozoic Bavarian lithofacies, the lowermost nappe of the MM∗, and the hornblende dates from the eastern part of the ZEV are indistinguishable from those of the autochthonous units FG and MO.
Metamorphic processes in the Variscan crust of the central segment In: Proceedings of the Third Workshop in the European Geotraverse Project: the Central Segment
  • P Bliimel
Bliimel, P., 1986. Metamorphic processes in the Variscan crust of the central segment. In: Proceedings of the Third Workshop in the European Geotraverse Project: the Central Segment (eds Freeman, R., Mueller, St. & Giese, P.), pp. 149-155. European Science Foundation, Strassbourg
Phengite geobarometry based on the limiting assemblage with K-feldspar, phlogopite, and quartz Die Eklogit-vorkommen des kristallinen Grundgebirges in NE-Bayern. VII Calculation and application of clinopyroxene-garnet-plagioclas-quartz geobarometers
  • H J Massonne
  • W Schreyer
  • S Matthes
  • P Richter
  • K Schmidt
  • Abhandlungen
  • D P Moecher
  • E J Essene
  • L M Anovitz
Massonne, H. J. & Schreyer, W., 1987. Phengite geobarometry based on the limiting assemblage with K-feldspar, phlogopite, and quartz. Contributions to Mineralogy and Petrology, 96, Matthes, S., Richter, P. & Schmidt, K., 1974. Die Eklogit-vorkommen des kristallinen Grundgebirges in NE-Bayern. VII. Ergebnisse aus einer Kernbohrung durch den Eklogitkorper des Weiaensteins. Neues Jahrbuch fur Mineralogie, Abhandlungen, Moecher, D. P., Essene, E. J. & Anovitz, L. M., 1988. Calculation and application of clinopyroxene-garnet-plagioclas-quartz geobarometers. Contributions to Mineralogy and Petrology, 100, 92-106.
The reaction 113-125. 221 -229. titanite + kyanite = anorthite + rutile and titanite-rutile baro-metry in eclogites High-pressure, low-temperature meta-morphism of pelitic and other protoliths based on experiments in the system
  • C E Manning
  • S R K Bohlen
Manning, C. E. & Bohlen, S. R., 1991. The reaction 113-125. 221 -229. titanite + kyanite = anorthite + rutile and titanite-rutile baro-metry in eclogites. Contributions to Mineralogy and Petrology, Massonne, H. J., 1991. High-pressure, low-temperature meta-morphism of pelitic and other protoliths based on experiments in the system K,O-MgO-AI,O,-SO,-H,O. Unpubl. Habilita-tion thesis, Ruhr Universitat Bochum.
Proceedings of the Third Workshop in the European Geotraverse Project: the Central Segment
  • P. Blümel
Equilibrium Thermodynamics in Petrology. An Introduction
  • R. Powell
Diskrete früh-devonische Ar/Ar Alter der Hangendserie (Münchberger Masse)
  • Kreuzer
Die Eklogit vorkommen des kristallinen Grundgebirges in NE-Bayern. VII. Ergebnisse aus einer Kernbohrung durch den Eklogitkörper des Weiβensteins
  • S. Matthes
  • P. Richter
  • K. Schmidt
Metamorphic processes in the Variscan crust of the central segment
  • P Bliimel
  • R Freeman
  • Mueller
  • St
  • Giese
Bliimel, P., 1986. Metamorphic processes in the Variscan crust of the central segment. In: Proceedings of the Third Workshop in the European Geotraverse Project: the Central Segment (eds Freeman, R., Mueller, St. & Giese, P.), pp. 149-155.
An enlarged and updated internally consistent thermodynamic dataset with uncertainties and correlations: 3. Applications to geobarometry, worked examples and a computer program
  • Powell
Powell, R. & Holland, T. J. B., 1988. An enlarged and updated internally consistent thermodynamic dataset with uncertainties and correlations: 3. Applications to geobarometry, worked examples and a computer program. Journal of Metamorphic Geology, 8, 89-124.
PT-evolution and fluid inclusion characteristics of eclogites and country rocks in the Münchberg gneiss complex, Germany. 29th International Geological Congress, Abstract volume
  • R Klemd
  • S Matthes
  • M Okrusch
Klemd, R., Matthes, S. & Okrusch, M., 1992a. PT-evolution and fluid inclusion characteristics of eclogites and country rocks in the Münchberg gneiss complex, Germany. 29th International Geological Congress, Abstract volume, Kyoto, 2, 599.
The influence of post-peak metamorphic overprint on 40Ar/39Ar white micaages-an example from eclogites, Münchberg Massif, Northern Bavaria
  • Hammerschmidt
Die Eklogit vorkommen des kristallinen Grundgebirges in NE-Bayern. VII. Ergebnisse aus einer Kernbohrung durch den Eklogitkörper des Weiβensteins
  • Matthes