ALBERT

Description: The European Variscan orogeny can be compared to the Tibetan–Himalayan system for three main reasons: 1) The Variscan belt originated through progressive amalgamation of Gondwanan blocks that were subsequently squeezed between the Laurussia and Gondwana continents. Similarly, the Tibetan–Himalayan orogen results from amalgamated Gondwanan blocks squeezed between Asia and India. 2) The duration of the collisional period and the scale of the two orogens are comparable. 3) In both cases the collisional process resulted in formation of a thick crustal root and long lasting high-pressure granulite-facies metamorphism. Recent petrological data allow a more detailed comparison pointing to similarities also in the mid-crustal re-equilibration of the granulites and their association with specific (ultra-)potassic magmatic rocks. In both orogens, the origin of the granulites was attributed to relamination and thermal maturation of lower-crustal allochthon below upper-plate crust. Later evolution was explained by mid-crustal flow eventually leading to extrusion of the high-grade rocks. We propose that the lower and middle crustal processes in hot orogens are connected by gravity overturns. Such laterally-forced gravity-driven exchanges of material in the orogenic root were already documented in the Variscides, but the recent data from Tibet and Himalaya show that this process may have occurred also elsewhere. Using numerical models we show that the exchange of the lower and middle crust can be efficient even for a minor density inversion and various characteristics of the crustal layers. The modeled pressure–temperature paths are compatible with two-stage metamorphism documented in Tibet and Himalaya.

Description: New petrographic and microstructural observations, mineral equilibria modelling and U/Pb (monazite) geochronological studies were carried out to investigate relationship between deformation and metamorphism across the Rehamna massif (Moroccan Variscan belt). In this area, typical Barrovian (muscovite to staurolite) zones developed in Cambrian to Carboniferous metasedimentary rocks that are distributed around a dome-like structure. First assemblages are characterized by the presence of locally preserved andalusite, followed by prograde evolution culminating at 6 kbar and 620 °C in the structurally deepest staurolite zone rocks. This Barrovian sequence was subsequently uplifted to supracrustal levels, heterogeneously reworked at greenschist facies conditions, which was followed locally by static growth of andalusite, indicating heating to 2.5−4 kbar and 530−570 °C. The 206 Pb/ 238 U monazite age of 298.3 ± 4.1 Ma is interpreted as minimum age of peak metamorphic conditions whereas the ages of 275.8 ± 1.7 Ma and 277.0 ± 1.1 Ma date decompression and heating at low pressure, in agreement with previous dating of Permian granitoids intruding the Rehamna massif. The prograde metamorphism occurred during thickening and associated horizontal flow in the deeper crust (S1 horizontal schistosity). The horizontally disposed metamorphic zones were subsequently uplifted by a regional scale antiform during ongoing N-S compression. The re-heating of the massif follows late massive E-W shortening, refolding and retrograde shearing of all previous fabrics coevally with regionally important intrusions of Permian granitoids. We argue that metamorphic evolution of the Rehamna massif occurred several hundred kilometres from the convergent plate boundaries in the interior of continental Gondwanan plate. The tectonometamorphic history of the Rehamna massif is put into Palaeozoic plate tectonic perspective and Late Carboniferous reactivation of (Devonian)-Early Carboniferous basins formed during stretching of the north Gondwana margin and formation of the Palaeotethys Ocean. The inherited heat budget of these magma-rich basins plays a role in the preferential location of this intracontinental orogen. It is shown that rapid transition from lithospheric stretching to compression is characterized by specific HT type of Barrovian metamorphism, which markedly differs from similar Barrovian sequences along Palaeozoic plate boundaries reported from Variscan Europe. This article is protected by copyright. All rights reserved.

Description: The Chandman massif, a typical structure of the Mongolian Altai, consists of a migmatite-magmatite core rimmed by a lower-grade metamorphic envelope of andalusite and cordierite-bearing schists. The oldest structure in the migmatite-magmatite core is a sub-horizontal migmatitic foliation S1 parallel to rare granitoid sills. This fabric is folded by upright folds F2 and transposed into a vertical migmatitic foliation S2 that is syn-tectonic, with up to several tens of metres thick granitoid sills. Sillimanite-ilmenite-magnetite S1 inclusion trails in garnet constrain the depth of equilibration during the S1 fabric to 6–7 kbar at 710–780°C. Reorientation of sillimanite into the S2 fabric indicates that the S1-S2 fabric transition occurred in the sillimanite stability field. The presence of cordierite, and garnet rim chemistry point to decompression to 3–4 kbar and 680–750°C during development of the S2 steep fabric, and postectonic andalusite indicates further decompression to 2–3 kbar and 600–650°C. Widespread crystallization of post-tectonic muscovite is explained by the release of H 2 O from crystallizing partial melt. In the metamorphic envelope the subhorizontal metamorphic schistosity S1 is heterogeneously affected by upright F2 folds and axial planar subvertical cleavage S2. In the north, the inclusion trails in garnet are parallel to the S1 foliation, and the garnet zoning indicates nearly isobaric heating from 2.5–3 kbar and 500-530°C. Cordierite contains crenulated S1 inclusion trails and has pressure shadows related to the formation of the S2 fabric. The switch from the S1 to the S2 foliation occurred near 2.5–3 kbar and 530–570°C; replacement of cordierite by fine-grained muscovite and chlorite indicates further retrogression and cooling. In the south, andalusite containing crenulated inclusion trails of ilmenite and magnetite indicates heating during the D2 deformation at 3–4 kbar and 540–620°C. Monazite from a migmatite analyzed by LASS yielded elevated HREE concentrations. The grain with the best-developed oscillatory zoning is 356±1.0 [±7] Ma ( 207 Pb-corrected 238 U/ 206 Pb), considered to date the crystallization from melt in the cordierite stability around 680°C and 3.5 kbar, whereas the patchy BSE-dark domains give a date of 347±4.2 [±7] Ma interpreted as recrystallization at subsolidus conditions. The earliest sub-horizontal fabric is associated with the onset of magmatism and peak of P–T conditions in the deep crust, indicating important heat input associated with lower crustal horizontal flow. The paroxysmal metamorphic conditions are connected with collapse of the metamorphic structure, an extrusion of the hot lower crustal rocks associated with vertical magma transfer and a juxtaposition of the hot magmatite-migmatite core with supracrustal rocks. This study provides information about tectono-thermal history and time scales of horizontal flow and vertical mass and heat transfer in the Altai orogen. It is shown that, similar to collisional orogens, doming of partially molten rocks assisted by syn-orogenic magmatism can be responsible for the exhumation of orogenic lower crust in accretionary orogenic systems. This article is protected by copyright. All rights reserved.

Description: Continental core complexes are generally interpreted to result from extensional doming due to gravity-driven upflow of lower crust. In contrast, the Vepor Dome is characterized by the lack of inverted density profile and relatively cold metamorphic field gradient which precludes an activation of Rayleigh-Taylor instability. Instead, the crustal structure of the Vepor Unit is marked by dense and weak metapelitic lower crust and light and strong granitoid upper crust inherited from Variscan nappe stacking. It is shown that the Cretaceous Eo-Alpine tectonic evolution of the Vepor Dome is controlled by the dynamics of two neighboring mechanically strong continental blocks, i.e., the overthrusting of the suprastructural Gemer Unit from the south and the underthrusting of the Fatric basement from the north. Structural, metamorphic and geochronological data from the Vepor Unit imply two main phases of the convergent process: (1) Lower Cretaceous crustal thickening due to overthrusting and internal deformation of the Gemer Unit together with upper crustal folding in the Vepor Unit led to the progressive development of the orogenic front parallel pressure gradient. The instantaneous response of the lower crustal and low-viscosity metapelites led to an along-strike lower crustal flow accompanied by prograde Barrovian-type metamorphism. (2) As the south vergent underthrusting of the Fatric basement propagated to greater depths during the Upper Cretaceous, the convergent process switched from top driven to bottom driven, and the exhumation of the lower crust occurred via polyharmonic folding. Overall doming of the Vepor Unit induced upper crustal detachment faulting and eastward unroofing of the dome.

Description: Garnet (10 vol.%; pyrope contents 34–44 mol.%) hosted in quartzofeldspathic rocks within a large vertical shear zone of south Madagascar shows a strong grain-size reduction (from a few cm to ∼300  μ m). Electron back-scattered diffraction, transmission electron microscopy and scanning electron microscope imaging coupled with quantitative analysis of digitized images (PolyLX software) have been used in order to understand the deformation mechanisms associated with this grain-size evolution. The garnet grain-size reduction trend has been summarized in a typological evolution (from Type I to Type IV). Type I, the original porphyroblasts, form cm-sized elongated grains that crystallized upon multiple nucleation and coalescence following biotite breakdown: biotite + sillimanite + quartz = garnet + alkali feldspar + rutile + melt. These large garnet grains contain quartz ribbons and sillimanite inclusions. Type I garnet is sheared along preferential planes (sillimanite layers, quartz ribbons and/or suitably oriented garnet crystallographic planes) producing highly elongated Type II garnet grains marked by a single crystallographic orientation. Further deformation leads to the development of a crystallographic misorientation, subgrains and new grains resulting in Type III garnet. Associated grain-size reduction occurs via subgrain rotation recrystallization accompanied by fast diffusion-assisted dislocation glide. This plastic deformation of garnet is associated with efficient recovery as shown by the very low dislocation densities (10 10  m −3 or lower). The rounded Type III garnet experiences rigid body rotation in fine-grained matrix. In the highly deformed samples, the deformation mechanisms in garnet are grain-size- and shape-dependent: dislocation creep is dominant for the few large grains left (〉1 mm; Type II garnet), rigid body rotation is typical for the smaller rounded grains (300  μ m or less; Type III garnet) whereas diffusion creep may affect more elliptic garnet (Type IV garnet). The P–T conditions of garnet plasticity in the continental crust (≥950 °C; 11 kbar) have been identified using two-feldspar thermometry and GASP conventional barometry. The garnet microstructural and deformation mechanisms evolution, coupled with grain-size decrease in a fine-grained steady-state microstructure of quartz, alkali feldspar and plagioclase, suggests a separate mechanical evolution of garnet with respect to felsic minerals within the shear zone.

Description: A microstructural and metamorphic study of a naturally deformed medium- to high-pressure granitic orthogneiss (Orlica–Śnieżnik dome, Bohemian Massif) provides evidence of behaviour of the felsic crust during progressive burial along a subduction-type apparent thermal gradient (∼10 °C km −1 ). The granitic orthogneisses develops three distinct microstructural types, as follows: type I – augen orthogneiss, type II – banded orthogneiss and type III – mylonitic orthogneiss, each representing an evolutionary stage of a progressively deformed granite. Type I orthogneiss is composed of partially recrystallized K-feldspar porphyroclasts surrounded by wide fronts of myrmekite, fully recrystallized quartz aggregates and interconnected monomineralic layers of recrystallized plagioclase. Compositional layering in the type II orthogneiss is defined by plagioclase- and K-feldspar-rich layers, both of which show an increasing proportion of interstitial minerals, as well as the deformation of recrystallized myrmekite fronts. Type III orthogneiss shows relicts of quartz and K-feldspar ribbons preserved in a fine-grained polymineralic matrix. All three types have the same assemblage (quartz + plagioclase + K-feldspar + muscovite + biotite + garnet + sphene ± ilmenite), but show systematic variations in the composition of muscovite and garnet from types I to III. This is consistent with the equilibration of the three types at different positions along a prograde P−T path ranging from 〈15 kbar and 〈700 °C (type I orthogneiss) to 19–20 kbar and 〉700 °C (types II and III orthogneisses). The deformation types thus do not represent evolutionary stages of a highly partitioned deformation at constant P−T conditions, but reflect progressive formation during the burial of the continental crust. The microstructures of the type I and type II orthogneisses result from the dislocation creep of quartz and K-feldspar whereas a grain boundary sliding-dominated diffusion creep regime is the characteristic of the type III orthogneiss. Strain weakening related to the transition from type I to type II microstructures was enhanced by the recrystallization of wide myrmekite fronts, and plagioclase and quartz, and further weakening and strain localization in type III orthogneiss occurred via grain boundary sliding-enhanced diffusion creep. The potential role of incipient melting in strain localization is discussed.

Description: The contribution of lateral forces, vertical load, gravity redistribution and erosion to the origin of mantled gneiss domes in internal zones of orogens remains debated. In the Orlica−Śnieżnik dome (Moldanubian zone, European Variscan belt), the polyphase tectono-metamorphic history is initially characterized by the development of subhorizontal fabrics associated with medium- to high-grade metamorphic conditions in different levels of the crust. It reflects the eastward influx of a Saxothuringian-type passive margin sequence below a Teplá-Barrandian upper plate. The ongoing influx of continental crust creates a thick felsic orogenic root with HP rocks and migmatitic orthogneiss. The orogenic wedge is subsequently indented by the eastern Brunia microcontinent producing a multiscale folding of the orogenic infrastructure. The resulting kilometre-scale folding is associated with the variable burial of the middle crust in synforms and the exhumation of the lower crust in antiforms. These localized vertical exchanges of material and heat are coeval with a larger crustal-scale folding of the whole infrastructure generating a general uplift of the dome. It is exemplified by increasing metamorphic conditions and younging of 40Ar/39Ar cooling ages toward the extruded migmatitic subdomes cored by HP rocks. The vertical growth of the dome induces exhumation by pure shear-dominated ductile thinning laterally evolving to non-coaxial detachment faulting, while erosion feeds the surrounding sedimentary basins. Modeling of the Bouguer anomaly grid is compatible with crustal-scale mass transfers between a dense superstructure and a lighter infrastructure. The model implies that the Moldanubian Orlica−Śnieżnik mantled gneiss dome derives from polyphase recycling of Saxothuringian material.

Description: We combine structural observations, petrological data and 40 Ar– 39 Ar ages for a stack of amphibolite-facies metasedimentary units that rims high-pressure (HP) granulite-facies felsic bodies exposed in the southern Bohemian Massif. The partly migmatitic Varied and Monotonous units, and the underlying Kaplice unit show a continuity of structures that are also observed in the adjacent Blanský les HP granulite body. They all exhibit an earlier NE−SW striking and steeply NW-dipping foliation (S3) which is transposed into a moderately NW-dipping foliation (S4). In both the Varied and Monotonous units, the S3 and S4 foliations are characterized by a Sil–Bt–Pl–Kfs–Qtz–Ilm±Grt assemblage, with occurrences of post-D4 andalusite, cordierite and muscovite. In the Monotonous unit, minute inclusions of garnet, kyanite, sillimanite and biotite are additionally found in plagioclase from a probable leucosome parallel to S3. The Kaplice unit shows rare staurolite and kyanite relicts, a Sil–Ms–Bt–Pl–Qtz±Grt assemblage associated with S3, retrogressed garnet−staurolite aggregates during the development of S4, and post-D4 andalusite, cordierite and secondary muscovite. Mineral equilibria modelling for representative samples indicates that the Varied unit records conditions higher than ~7 kbar at 725 °C during the transition from S3 to S4, followed by a P−T decrease from ~5.5 kbar/750 °C to ~4.5 kbar/700 °C. The Monotonous unit shows evidence of partial melting in the S3 fabric at P−T above ~8 kbar at 740–830 °C and a subsequent P−T decrease to 4.5–5 kbar/700°C. The Kaplice unit preserves an initial medium-pressure prograde path associated with the development of S3 reaching peak P−T of ~6.5 kbar/640 °C. The subsequent retrograde path records 4.5 kbar/660 °C during the development of S4. 40 Ar– 39 Ar geochronology shows that amphibole and biotite ages cluster at c . 340 Ma close to the HP granulite, whereas adjacent metasedimentary rocks preserve c . 340 Ma amphibole ages, but biotite and muscovite ages range between c . 318 and c . 300 Ma. The P − T conditions associated with S3 imply an overturned section of the orogenic middle crust. The shared structural evolution indicates that all mid-crustal units are involved in the large-scale folding cored by HP granulites. The retrograde P – T paths associated with S4 are interpreted as a result of a ductile thinning of the orogenic crust at a mid-crustal level. The 40 Ar– 39 Ar ages overlap with U–Pb zircon ages in and around the HP granulite bodies, suggesting a short duration for the ductile thinning event. The post-ductile thinning late-orogenic emplacement of the South Bohemian plutonic complex is responsible for a re-heating of the stacked units, reopening of argon system in mica and a tilting of the S4 foliation to its present-day orientation. This article is protected by copyright. All rights reserved.