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This paper presents the results of petrological observations and diffusion modelling on garnet from high‐pressure to ultrahigh‐pressure ((U)HP) metamorphic rocks of the Western Gneiss Region in the Nordfjord. Garnet from kyanite‐bearing micaschist preserves two generations of garnet growth that are related to the Pre‐Caledonian granulite facies and Caledonian eclogite facies metamorphic events. Mafic eclogite, forming lenses in the micaschist, contains only eclogite facies assemblages with partial recrystallization under amphibolite facies conditions. Caledonian garnet in both the micaschist and hosting eclogite indicates reaction overstepping and nucleation near or above 550°C/2.0 GPa. Maximum pressure and temperature, calculated using pseudosection modelling for the eclogite facies event, were ~2.6 GPa and 650°C. The interface between the Pre‐Caledonian and Caledonian garnet in the micaschist shows a strong compositional gradient or possibly a compositional jump. The preservation of such a gradient together with the hummocky‐shaped composition profiles in the Caledonian garnet from the eclogite indicates either no relaxation or a short‐time of relaxation of the rocks at their peak temperature conditions, as well as their exhumation by cooling. Possibly, heating or exhuming of the rocks by isothermal decompression could have easily modified such compositional irregularities along the garnet profiles. A cooling rate of ~187°C/Ma and exhumation rate in the vertical direction of ~2.5 cm/year for the HP rocks were obtained by considering that the temperature and transport distance changes from their maximum depth and peak temperature to the surface were proportional to the time (3.5 Ma) calculated by modelling for the garnet. [ABSTRACT FROM AUTHOR]

Exsolution lamellae of baryte, Fe sulfides, Cu sulfides and Fe oxides were observed in apatite enclosed in garnet and omphacite and their intergranular spaces in ultrahigh-pressure eclogite in the Moldanubian Zone, Czech Republic. Micro-textural relations and compositional mapping of the apatite indicates a close relationship between the density of the exsolution lamellae and compositional domains that are rich in sulfur and iron. No relation between compositional domains and fluorine or chlorine content or any evidence of apatite metasomatisation was observed. On the basis of cathodoluminescence images, the compositional domains reflect sector zoning in apatite crystals by preferential uptake of elements due to differences in surface charge and morphology on the growth plane. It is concluded that the lamellae are products of exsolution in a closed system resulting from temperature decrease during metamorphism. [ABSTRACT FROM AUTHOR]

High pressure granulite and granulite gneiss from the Rychleby Mountains in the East Sudetes form an approximately 7 km long and 0.8 km wide body, which is enclosed by amphibolite facies orthogneiss with a steep foliation. Well preserved felsic granulite is located in the central part of the body, where several small bodies of mafic granulite are also present. In comparison to other high pressure granulites in the Bohemian Massif, which show strong mineral and textural re-equilibration under granulite facies conditions, the mafic granulite samples preserve eclogite facies minerals (garnet, omphacite, kyanite, rutile and phengite) and their field and textural relations indicate that both mafic and felsic granulites shared common metamorphic history during prograde eclogite facies and subsequent granulite facies events. Garnet from both granulite varieties shows prograde compositional zoning and contains inclusions of phengite. Yttrium and REEs in garnet show typical bell-shaped distributions with no annular peaks near the grain rims. Investigation of major and trace elements zoning, including REEs distribution in garnet, was combined with thermodynamic modelling to constrain the early eclogite facies metamorphism and to estimate pressure-temperature conditions of the subsequent granulite facies overprint. The first (U)HP metamorphism occurred along a low geothermal gradient in a subduction-related environment from its initial stage at 0.8 GPa/460 °C and reached pressures up to 2.5 GPa at 550 °C. The subsequent granulite facies overprint (1.6–1.8 GPa/800–880 °C) affected the rocks only partially; by replacement of omphacite into diopside + plagioclase symplectite and by compositional modification of garnet rims. The mineral textures and the preservation of the eclogite facies prograde compositional zoning in garnet cores confirm that the granulite facies overprint was either too short or too faint to cause recrystallisation and homogenisation of the eclogite facies mineral assemblage. The results of this study are compared with other granulite massifs in the Moldanubian Zone. In addition, a possible scenario for the Variscan eclogite and subsequent granulite facies metamorphism in the Bohemian Massif is discussed. [ABSTRACT FROM AUTHOR]

The Podolsko complex, Bohemian Massif, is a mid-crustal migmatite–granite dome exposed along a tectonic boundary separating the upper crust from the deeply eroded interior of the Variscan orogen, referred to as the Moldanubian Zone. This study examines metamorphic history of mafic and felsic granulites that occur in this complex as minor lenses or layers hosted in pervasively anatectic rocks. The mafic granulite contains garnet with preserved high-Ca cores, which based on pseudosection modelling indicates pressure conditions near the coesite stability field at temperatures of ca. 550–600 °C. The relicts of an earlier eclogite-facies stage have been overprinted by a later granulite-facies assemblage consisting of ternary feldspar, orthopyroxene, and spinel in the mafic granulite and sillimanite and spinel in the felsic granulite. Composition of younger garnet (in rims and as smaller grains) in both granulites suggests that a near isothermal decompression of these rocks was followed by heating that reached temperature of ca. 900 °C at pressure of ca. 0.5 GPa. It is thus concluded that the granulites underwent at least two temporally separate tectonometamorphic events: they were first subducted to mantle depths and exhumed rapidly at relatively low temperatures and then near isobarically heated at mid-crustal levels. The preservation of earlier eclogite-facies garnet in the mafic granulite indicates that the latter event was short-lived and was followed by near isobaric cooling. The geologically brief granulite-facies metamorphism was previously explained as a result of slab break-off and mantle upwelling after the main phase of microplate convergence in the Bohemian Massif. To put the Podolsko complex into a broader tectonic context, we synthesize the available petrologic and structural data from the correlative (U)HP assemblages of the Moldanubian Zone to suggest that they typically do not preserve structural record of the subduction stage, only rarely preserve an early exhumation phase to the middle crust, and most, if not all, have been extensively overprinted by late-stage processes at shallower crustal levels. This synthesis also reveals that kinematics and thus tectonic processes driving the final exhumation of the (U)HP rocks were diverse and may vary from one location to another. These local settings include exhumation in the footwall of major extensional detachments, due to folding and subsequent lateral extrusion, and along crustal-scale strike-slip shear zones. The extensive late-stage overprint masking the earlier (pre-340 Ma) subduction history is greatly exemplified in the Podolsko complex where the granulites are only small relics in pervasively anatectic rocks melted at mid-crustal conditions and then rapidly exhumed along extensional detachments to upper-crustal levels. [ABSTRACT FROM AUTHOR]

The largest body of metabasic rocks in the Bohemian Massif, the Mariánské Lázně Complex (MLC), represents an Early Paleozoic ophiolite complex, which experienced poly-metamorphic evolution with an early high-pressure amphibolite, followed by eclogite and subsequent amphibolite facies overprint. This study aims to investigate the fabric relationships among the rock bodies and to analyze the deformation-recrystallization history of the MLC using the anisotropy of magnetic susceptibility (AMS). Magnetic susceptibility of the MLC rocks varies according to the presence of para- and ferromagnetic minerals as well as according to the primary lithologies. In weakly magnetic rocks (Km < 1 x 10- 3 [SI]), the main carrier of susceptibility is paramagnetic mafic silicates, amphibole and biotite in amphibolites, clinopyroxene and garnet in eclogites, olivine and serpentine minerals in serpentinized ultramafite, and amphibole in gabbro. In highly magnetic rocks (Km ≫ 1 x 10- 3), ferromagnetic minerals, such as magnetite, titanomagnetite, general Fe-Ti oxides, and subordinately also pyrrhotite can play an important role. The results of AMS indicate that the orientation of magnetic minerals in most rocks is related to the last exhumation and retrogression event that occurred in amphibolite facies conditions. However, there are some well preserved eclogites possessing the magnetic fabrics that are at least partially related to prograde or peak PT conditions during metamorphism. In addition, massive metagabbros that suffered only weak deformation partially preserve the intrusive magnetic fabrics. [ABSTRACT FROM AUTHOR]

The formation and exhumation of Variscan high- to ultrahigh-pressure metamorphic rocks from the Moldanubian Zone in the Bohemian Massif are the subject of controversy regarding their unclear relationship to possible subduction zones. In this paper we present the results of a petrological study of eclogites that occur together with serpentinites within amphibolite facies gneisses in the Moldanubian Zone, east of the Teplá–Barrandian Block. More than 100 bodies of retrogressed eclogite and serpentinite follow an approximately 250km long SW–NE trending zone in the central part of the Bohemian Massif. Together with surrounding gneisses, the eclogites share a medium to low-pressure amphibolite facies metamorphism. P–T conditions estimated for the eclogite facies stage indicate a relatively low-temperature geothermal gradient similar to those observed in the Saxothuringian Zone, which occurs north and northwest of the Teplá–Barrandian Block. The presence and distribution of eclogites and comparison of their P–T and age data with HP–UHPM rocks in other units in the Bohemian Massif allow us to constrain the Variscan suture, which straddles the SE of the Teplá–Barrandian Block. The existence of this suture in relation to available geotectonic models and its possible continuation through other allochthonous units along the European Variscan Belt are discussed. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents monomineral and multiphase inclusions in garnet from eclogites and clinopyroxenites, which form layers and boudins in garnet peridotites from two areas in the Moldanubian zone of the Bohemian Massif. The garnet peridotites occur in felsic granulites and reached UHP conditions prior to their granulite facies overprint. In addition to complex compositional zoning, garnets from hosting eclogites and clinopyroxenites preserve inclusions of hydrous phases and alkali silicate minerals including: amphiboles, chlorites, micas and feldspars. Amphibole, biotite and apatite inclusions in garnet have a high concentration of halogens; CO2 and sulfur are involved in carbonates and sulfide inclusions, respectively. The inclusion patterns and compositional zoning in garnet in combination with textural relations among minerals, suggest that the ultramafic and mafic bodies are derived from lithospheric mantle above the subduction zone and were transformed into garnet pyroxenites and eclogites in the subduction zone. Based on compositional, mineral and textural relations, all of these rocks along with the surrounding crustal material were overprinted by granulite facies metamorphism during their exhumation. [Copyright &y& Elsevier]

Abstract: Felsic granulites from the Kutná Hora complex in the Moldanubian zone of central Europe preserve mineral assemblage that records transition from early eclogite to granulite facies conditions, and exhibits leucocratic banding, which is interpreted as an evidence for melt loss during the decompression path. The granulites are layered and consist of variable proportions of quartz, ternary feldspar, garnet, biotite, kyanite, and rutile. In the mesocratic layers, garnet grains show relatively high Ca contents corresponding to 28–41mol% grossular end member. They have remarkably flat compositional profiles in their cores but their rims exhibit an increase in pyrope and a decrease in grossular and almandine components. In contrast, garnets from the leucocratic layers have relatively low Ca contents (15–26mol% grossular) that further decrease towards the rims. In addition to modeling of pressure–temperature pseudosections, compositions of garnet core composition, garnet rim-ternary feldspar–kyanite–quartz equilibrium, ternary feldspar composition, and the garnet–biotite equilibrium provide five constraints that were used to reconstruct the pressure–temperature path from eclogite through the granulite and amphibolite facies. In both layers, garnet cores grew during omphacite breakdown and phengite dehydration melting at 940°C and 2.6GPa. Subsequent decompression heating to 1020°C and 2.1GPa produced Ca- and Fe-poor garnet rims due to the formation of Ca-bearing ternary feldspar and partial melt. In both the mesocratic and leucocratic layer, the maximum melt productivity was 26 and 18vol.%, respectively, at peak temperature constrained by the maximum whole-rock H2O budget, ~1.05–0.75wt.%, prior to the melting. The preservation of prograde garnet-rich assemblages required nearly complete melt loss (15–25vol.%), interpreted to have occurred at 1000–1020°C and 2.2–2.4GPa by garnet mode isopleths, followed by crystallization of small amounts of residual melt at 760°C and 1.0GPa. Phase formation and melt productivity were independently determined by experiments in the piston-cylinder apparatus at 850–1100°C and 1.7–2.1GPa. Both the thermodynamic calculations and phase equilibrium experiments suggest that the partial melt was produced by the dehydration melting: muscovite+quartz=melt+K-feldspar+kyanite. The presence of partial melt facilitated attainment of mineral equilibria at peak temperature thus eliminating any potential relics of early high-pressure phases such as phengite or omphacite. By contrast, adjacent mafic granulites and eclogites, which apparently share the same metamorphic path but have not undergone partial melting commonly preserve relics or inclusions of eclogite-facies mineral assemblages. [Copyright &y& Elsevier]

Abstract: We investigated several mineral phases and their replacement products which occur as inclusions in garnets from felsic and mafic granulites of the Gföhl Unit in the Moldanubian Zone. The most important mineral inclusions, Ti-rich muscovite and omphacite, were used for the reconstruction of the metamorphic history of granulites. Some inclusions were transformed during high-temperature granulite facies metamorphism, partial melting and decompression to other phases, and so the original mineral can only be deduced from the inclusion morphology and reaction products. These inclusions have columnar shapes and consist of K-feldspar+kaolinite, albite+Fe-oxide, plagioclase+Fe-oxide, or albite+K-feldspar, respectively. The pseudomorphs with albite/plagioclase occur in a Ca-rich garnet that shows prograde zoning. Pressure–temperature (PT) evolution, derived from mineral assemblages in granulite and based on the inclusions, suggests a prograde metamorphism from amphibolite through eclogite to granulite facies conditions with subsequent amphibolite facies overprint during exhumation. The estimated PT trajectory for the studied granulites, which also host lenses or boudins of eclogites and garnet peridotites, allows reconstruction of the complete clockwise metamorphic path that is consistent with subduction geotherm prior to the tectonic amalgamation within the continental collisional root. [Copyright &y& Elsevier]

Abstract: Two tectonic lenses of garnet peridotites and one of eclogite, all occurring in high-pressure granulites from the Kutná Hora Complex, were investigated. These rocks are thought to be correlative with the high-grade metamorphic Gföhl unit of the Moldanubian zone. The garnet peridotites contain small bodies of eclogites and thin layers of garnet clinopyroxenite. Also present are two varieties of eclogites (kyanite-bearing and kyanite-rfree) and one variety of garnetite, which form blocks and boudins in the garnet peridotites. The eclogite occurring in granulite without garnet peridotite belongs to the kyanite-bearing variety. Textural relations and mineral compositional zoning of garnet in peridotites and in eclogites were used to constrain PT evolution of these rocks and to determine whether they passed a PT path similar to that in the host peridotite or represent fragments with different metamorphic histories. The results of thermobarometric calculations show that the peridotites were cooled during an increase of pressure at 4.5/950 °C, and the pressure and temperature obtained for garnet clinopyroxenite are also close to these PT values. The eclogites seem to be derived from different rock compositions and to have passed different thermal evolutions prior to their emplacement into garnet peridotites. Temperatures calculated for the eclogites hosted in peridotites range between 850 and 1000 °C, but pressure obtained for kyanite-bearing variety is about 3.4 GPa at 960 °C. The eclogite present in granulite contains prograde zoned garnet and gives temperature of 900 °C at 3.4 GPa. All these rocks seem to represent tectonic fragments that were subducted to different depths and tectonically assembled during their exhumation. [Copyright &y& Elsevier]