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In continental crust, rapid melt flow through macroscopic conduits is usually envisaged as the most efficient form of melt transport. In contrast, there is growing evidence that in hot continental crust, grain‐scale to meso‐scale porous melt flow may operate over long distances and over millions of years. Here, we investigate the dynamics of such porous melt flow by means of two‐dimensional thermo‐mechanical numerical models using the code ASPECT. Our models are crustal‐scale and describe the network of pores through which the melt flows by permeability that depends on the spacing of the pores. Our results suggest that assuming realistic material properties, melt can slowly migrate in the hot and thick continental crust through pores with a characteristic spacing of 1 mm or larger. Despite its low velocity (millimeters to centimeters per year), over millions of years, such flow can create large partially molten zones in the middle‐lower crust and significantly affect its thermal state, deformation, and composition. We examined the role of the permeability, melt and solid viscosities, the slope of the melting curve and temperature conditions. We obtained contrasting styles of melt distribution, melt flow, and solid deformation, which can be categorized as melt‐enhanced convection, growth of partially molten diapirs and melt percolation in porosity waves. Our numerical experiments further indicate that grain‐scale porous flow is more likely in rocks where the melt productivity increases slowly with temperature, such as in metaigneous rocks. Plain Language Summary: Continental crust contains a significant proportion of rocks that can melt relatively easily. Geophysical methods that measure, for example, seismic velocities or electric conductivity revealed large partially molten regions in the continental crust. In addition, solidified melt pathways (veins, dykes) are often observed in rocks that were formerly in hot continental interiors. Usually, melt flow through macroscopic pathways is assumed to be the most efficient type of melt transport. However, several geological studies have shown that melt may flow through much finer (microscopic) pores, which form a network along boundaries of rock grains. The question arises, how efficient this microscopic melt flow is, and how it affects the properties of the continents. We address this issue using numerical models. According to our models, microscopic melt flow is very slow—melt moves millimeters to centimeters per year. Despite its low velocity, over millions of years, it forms large partially molten zones, significantly increases the temperature in the crust, changes its composition and softens it. The style of this process varies notably depending on the rock properties and temperature conditions. For example, in some models, kilometer‐sized melt batches move through nearly immobile rock, while in others, melt and rock both move together as a stirred mush. Key Points: Numerical thermo‐mechanical models simulate porous melt flow through hot felsic continental crustMelt can propagate through pores with the spacing of 1 mm or larger with a velocity of millimeters to centimeters per year and form large partially molten zonesStyle of melt flow and solid deformation vary between melt‐enhanced convection, diapirism, and porosity waves depending on model parameters [ABSTRACT FROM AUTHOR]

The Mongolian Altai Zone of the Central Asian Orogenic Belt has been traditionally interpreted as a mosaic of Paleozoic magmatic arcs, back‐arcs, and Precambrian continental terranes. In order to define its architecture and its tectonic evolution, three domains previously interpreted as terranes were investigated. The findings show that the Northern and Central domains are formed by a metamorphic sequence characterized by Barrovian S1 fabric transposed by recumbent folds and dominant sub‐horizontal amphibolite facies S2 schistosity. The latter is associated with the intrusions of late Devonian syntectonic granite sheets and anatexis in the south. The Southern domain is formed by early Permian migmatites and anatectic granites separated from the metamorphic envelope by amphibolite to green‐schist facies D3 shear zone cross‐cutting S2 fabrics. All domains have been reworked by E‐W upright folds associated with axial‐planar greenschist facies cleavage, reflecting the final mid‐Permian to Triassic D4 shortening. Lithological, geochemical, and U‐Pb zircon analyses of metasediments of all domains indicate that they are formed by Ordovician mature quartzite derived from Precambrian basement intruded by Cambrian‐Ordovician continental arc and Silurian immature graywacke which originated through erosion of an oceanic arc. Altogether, the whole sequence represents a fore‐arc basin in front of a migrating arc affected by thickening and late Devonian extension. The Southern domain is interpreted as an early Permian core complex amplified by mid‐Permian to Triassic compression. The apparent "terrane" architecture of the Mongol Altai Zone originated due to Devonian and Permian heterogeneous reworking of a giant Ordovician to Silurian fore‐arc basin. Key Points: The Mongolian Altai Zone is an Ordovician to Silurian fore‐arc basin affected by Barrovian prograde metamorphismLate Devonian extension resulted in intrusion of sub‐horizontal, syntectonic granite sheets and partial melting in deep crustal levelsEarly Permian extension resulted in the formation of metamorphic core complex later amplified by regional mid‐Permian to Triassic compression [ABSTRACT FROM AUTHOR]

Phase equilibrium modelling of metagranitoids and metapelites in the MnO-[Na.sub.2]O-CaO-[K.sub.2]O-FeO-MgO-[Al.sub.2][O.sub.3]-Si[O.sub.2]- [H.sub.2]O system was used to characterize Variscan and Alpine metamorphism in the crystalline basement of the Vepor Unit, West Carpathians. The calculated P-T conditions range between 570 670[degrees]C and 6-8.5 kbar for the Variscan and 430-600[degrees]C and 5-11 kbar for the Alpine event. These two events show contrasting metamorphic field gradients and P T evolutions, indicating 22 27 [degrees]C [km.sup.-1] during the retrograde Variscan metamorphism and 15-18 [degrees]C [km.sup.-1] during the prograde Alpine metamorphism. The prograde Alpine metamorphism is associated with the Early Cretaceous overthrusting of the southern Gemer Unit, which resulted in apparently contemporaneous burial and horizontal ductile spreading of the underlying Vepor basement. The argon diffusion modelling was used to interpret the existing Variscan, Alpine and mixed [sup.40]Ar/[sup.39]Ar cooling ages, and to constrain the T-t evolution and Alpine thermal overprint. Constructed Alpine metamorphic isograds and isotherms show horizontal P-T gradients resulting from heterogeneous exhumation of the deeper parts of the Vepor basement along two narrow belts during later folding. Here the structurally deeper metapelites form large-scale anticlinal cusp-like structures that separate structurally higher metagranitoids.

New palaeomagnetic data from Lower Carboniferous granitoids of the orogenic root of the eastern Variscan belt (Moldanubian domain) show a polyphase palaeomagnetic record. Comparison of the data with existing palaeomagnetic measurements from Lower Palaeozoic sequences of the Bohemian Massif demonstrates a Carboniferous remagnetization of these latter units. All the data presented suggest that the Saxothuringian basement, the Moldanubian orogenic root system as well as the eastern Neoproterozoic Brunovistulian basement were already assembled during the Early Carboniferous. The whole Variscan belt subsequently rotated in a clockwise direction during Mid-Late Carboniferous times. Structural and geochronological data indicate that this rotation was accompanied by large-scale dextral wrenching along NW-SE-trending lithospheric faults. In a first stage, the blocks limited by wrench-faults rotated anticlockwise in bookshelf manner. The data presented rule out the existing model of oroclinal bending of the Rhenohercynian zone at the eastern termination of the Variscan belt. Keywords: palaeomagnetism, Carboniferous, Bohemian massif, rotation, wrenching.

Highly deformed banded phengite–biotite metagranite from the Snieznik dome in the Bohemian Massif has been modified locally to have stromatic, schlieren or nebulitic textures typical of migmatites. This occurred mostly along subvertical deformation zones at eclogite-facies conditions, at a scale of several centimetres to several metres, mostly parallel to the foliation. The transition from banded to migmatite types of orthogneiss is marked by an increase in the amount of phases interstitial along grain boundaries in the dynamically recrystallized monomineralic feldspar and quartz aggregates, and by increasing consumption of recrystallized K-feldspar grains by fine-grained plagioclase and quartz, as well as myrmekite (intergrowth of Pl−Qz). The new minerals are in textural equilibrium with phengite. The myrmekite, quartz and feldspars can be coarse-grained (grain size ≤0·5 cm). These features are considered to be the result of grain-scale melt infiltration that caused dissolution–reprecipitation along grain boundaries in the presence of phengite. The infiltration was pervasive at the grain scale, but localized at hand-specimen to outcrop scales. All the rock types have the same mineral assemblage of Grt−Ph−Bt−Ttn−Kfs−Pl−Qz±Rt±Ilm; they have similar garnet, phengite and biotite compositions, and based on mineral equilibria modelling we infer equilibration at a pressure of 1·5–1·7 GPa and a temperature of 690–740°C. Because the rocks are inferred to be H2O-undersaturated and above the temperature conditions of the wet solidus, infiltration must have involved a hydrous melt, as opposed to an H2O fluid. Stability of melt-bearing mineral assemblages and mineral compositions are almost independent of the melt proportion in the system, thus explaining the identical assemblage and mineral compositions observed in all the migmatite types. This precludes the estimation of the amount of melt infiltrated. Migmatite textures, however, suggest that variable degrees of melt–rock interaction occurred, being low in the banded migmatite types and higher in the nebulitic and schlieren types. Retrograde equilibration was largely restricted to retrograde zoning in phengite, garnet and plagioclase, and crystallization of biotite around phengite and garnet, presumably in a continuous reaction consuming melt. This may have occurred down to ∼0·7–1·0 GPa. According to Sr–Nd isotope data, the infiltrating melt is probably derived from similar rocks, structurally beneath the observed ones. The infiltration may have facilitated exhumation of a 2 km wide structural domain from ∼1·7 to ∼0·7 GPa, within which are the subvertical deformation zones along which the infiltration occurred. [ABSTRACT FROM AUTHOR]

Kinematic significance and time scales of geodynamic processes forming the Altai Orogenic Belt are addressed through structural and petrological analysis combined with zircon and monazite geochronology. The study area is composed of orogenic lower crust represented by a Devonian migmatite‐magmatite complex and orogenic middle and upper crust formed by an amphibolite‐facies Ordovician sedimentary sequence and a weakly to unmetamorphosed Devonian volcano‐sedimentary cover, respectively. The orogenic lower and middle crust were first affected by moderate thickening, which formed subhorizontal Barrovian metamorphic schistosity. This fabric was reworked by deep crustal melting and intrusion of granite sheets during horizontal extension at 400–380 Ma. Soon after, this horizontal fabric was affected by NW‐SE shortening generating crustal‐scale upright folding associated with subvertical flow of still partially molten orogenic lower crust. During this event, the orogenic lower and middle crust were tightly juxtaposed with upper crustal sedimentary rocks. The last event was related with a NE‐SW oriented convergence resulting in large‐scale folding and megafold interference pattern in the Permian at 280–273 Ma. Combined with existing regional data, our results allow proposing a Devonian tectonic switching from compression to extension and back to compression, as a response to variations of subduction dynamics between slab advance and retreat in a Pacific‐type suprasubduction system. The Permian folding was associated with the progressive northward exhumation of thermally softened crust. This tectonic evolution is in response to the indentation of the rigid Junggar arc domain into the weak Altai wedge. Key Points: The Chinese Altai was affected by pre‐Devonian crustal thickening, Mid‐Devonian (400‐380 Ma) extension, and Late Devonian NW‐SE shorteningThe Devonian edifice was affected by NW‐SE zones of high‐temperature deformation and upright folding in the Permian at 280‐273 MaDevonian events reflect suprasubduction tectonic switching, while Permian folding indicates the Junggar‐Chinese Altai collision [ABSTRACT FROM AUTHOR]

Abstract Assessment of sediment redistribution by end-Ordovician ice sheets is crucial for the reconstruction of Lower Paleozoic source-to-sink patterns. Focusing on the ice-distal, deepwater Tazekka depocenter (Moroccan Meseta), we performed a provenance study that combined whole-rock geochemistry, petrography and insights from high-resolution detrital zircon ages. The results show that the glacigenic sediments are compositionally — mineralogically and geochemically — more mature than preglacial strata. This observation points to a preferential cannibalization of the “great Lower Paleozoic quartz-rich sandstone sheet”, with a limited input of first-cycle, far-travelled clastic sediments. Differentiation of glacial units is not straightforward, yet the glaciation acme is typified by a highly mature sedimentary source and an age spectrum lacking Mesoproterozoic zircon grains, both features strongly indicating derivation from the Cambrian–Lower Ordovician cover of the Tuareg Shield. More regional sources are expressed during the earlier glaciation stages, during which lowstand remobilizations unrelated to subglacial erosion are also suspected. Subordinate but notable late Tonian (∼0.8 Ga) and latest Stenian to early Tonian (∼1 Ga) zircon populations are also evidenced in Morocco, which may have implications for future paleogeographic reconstructions. Graphical abstract Unlabelled Image Highlights • Glacial recycling of the great Lower Paleozoic quartz-rich sandstone sheet • Zircon record: Changing glacier flows vs. lowstand cannibalizations? • Stenian and Tonian zircon populations off the West African craton [ABSTRACT FROM AUTHOR]

During continental collision, a part of the lower‐plate material can be subducted, emplaced at the base of the upper plate, and eventually incorporated into its crust. This mechanism of continental‐crust transformation is called relamination, and it has been invoked to explain occurrences of high‐pressure felsic rocks in different structural positions of several orogenic systems. In the present study we reproduced relamination during continental collision in a thermomechanical numerical model. We performed a parametric study and distinguished three main types of evolution regarding the fate of the subducted continental crust: (i) return along the plate interface in a subduction channel or wedge, (ii) flow at the bottom of the upper‐plate lithosphere and subsequent translithospheric exhumation near the arc or in the back‐arc region (“sublithospheric relamination”), and (iii) nearly horizontal flow directly into the upper‐plate crust (“intracrustal relamination”). Sublithospheric relamination is preferred for relatively quick convergence of thin continental plates. An important factor for the development of sublithospheric relamination is melting of the subducted material, which weakens the lithosphere and opens a path for the exhumation of the relaminant. In contrast, a thick and strong overriding plate typically leads to exhumation near the plate interface. If the overriding plate is too thin or weak, intracrustal relamination occurs. We show that each of these evolution types has its counterpart in nature: (i) the Alps and the Caledonides, (ii) the Himalayan‐Tibetan system and the European Variscides, and (iii) pre‐Cambrian ultrahot orogens. [ABSTRACT FROM AUTHOR]

Neoproterozoic to early Paleozoic accretionary processes of the Central Asian Orogenic Belt have been evaluated so far mainly using the geology of ophiolites and/or magmatic arcs. Thus, the knowledge of the nature and evolution of associated sedimentary prisms remains fragmentary. We carried out an integrated geological, geochemical, and zircon U-Pb geochronological study on a giant Ordovician metasedimentary succession of the Mongolian Altai Mountains. This succession is characterized by dominant terrigenous components mixed with volcanogenic material. It is chemically immature, compositionally analogous to graywacke, and marked by significant input of felsic to intermediate arc components, pointing to an active continental margin depositional setting. Detrital zircon U-Pb ages suggest a source dominated by products of early Paleozoic magmatism prevailing during the Cambrian-Ordovician and culminating at circa 500 Ma. We propose that the Ordovician succession forms an 'Altai sedimentary wedge,' the evolution of which can be linked to the geodynamics of the margins of the Mongolian Precambrian Zavhan-Baydrag blocks. This involved subduction reversal from southward subduction of a passive continental margin (Early Cambrian) to the development of the 'Ikh-Mongol Magmatic Arc System' and the giant Altai sedimentary wedge above a north dipping subduction zone (Late Cambrian-Ordovician). Such a dynamic process resembles the tectonic evolution of the peri-Pacific accretionary Terra Australis Orogen. A new model reconciling the Baikalian metamorphic belt along the southern Siberian Craton with peri-Pacific Altai accretionary systems fringing the Mongolian microcontinents is proposed to explain the Cambro-Ordovician geodynamic evolution of the Mongolian collage system. [ABSTRACT FROM AUTHOR]

Subduction and exhumation dynamics can be investigated through analysis of metamorphic and deformational evolution of associated high-grade rocks. The Erzgebirge anticline, which forms at the boundary between the Saxothuringian and Teplá-Barrandian domains of the Bohemian Massif, provides a useful study area for these processes owing to the occurrence of numerous meta-basites preserving eclogite facies assemblages, and coesite and diamond bearing quartzofeldspathic lithologies indicating subduction to deep mantle depths. The prograde and retrograde evolution of meta-basite from the Czech portion of the Erzgebirge anticline has been constrained through a combination of thermodynamic modelling and conventional thermobarometry. Garnet growth zoning indicates that the rocks underwent burial and heating to peak conditions of 2.6 GPa and at least 615 °C. Initial exhumation occurred with concurrent cooling and decompression resulting in the growth of amphibole and zoisite poikiloblasts overgrowing and including the eclogite facies assemblage. The development of clinopyroxene-plagioclase-amphibole symplectites after omphacite and Al-rich rims on matrix amphibole indicate later heating at the base of the lower crust. Omphacite microstructures, in particular grain size analysis and lattice-preferred orientations, indicate that the prograde evolution was characterized by a constrictional strain geometry transitioning into plane strain and oblate fabrics during exhumation. The initial constrictional strain pattern is interpreted as being controlled by competing slab pull and crustal buoyancy forces leading to necking of the subducting slab. The transition to plane strain and flattening geometries represents transfer of material from the subducting lithosphere into a subduction channel, break-off of the dense slab and rebound of the buoyant crustal material. [ABSTRACT FROM AUTHOR]

Rift-related regional metamorphism of passive margins is usually difficult to observe on the surface, mainly due to its strong metamorphic overprint during the subsequent orogenic processes that cause its exposure. However, recognition of such a pre-orogenic evolution is achievable by careful characterization of the polyphase tectono-metamorphic record of the orogenic upper plate. A multidisciplinary approach, involving metamorphic petrology, P-T modelling, structural geology and in situ U-Pb monazite geochronology using laser-ablation split-stream inductively coupled plasma mass spectrometry, was applied to unravel the polyphase tectono-metamorphic record of metapelites at the western margin of the Teplá-Barrandian domain in the Bohemian Massif. The study resulted in discovery of three tectono-metamorphic events. The oldest event M1 is L P- H T regional metamorphism with a geothermal gradient between 30 and 50 °C km−1, peak temperatures up to 650 °C and of Cambro-Ordovician age ( c. 485 Ma). The M1 event was followed by M2-D2, which is characterized by a Barrovian sequence of minerals from biotite to kyanite and a geothermal gradient of 20-25 °C km−1. D2-M2 is associated with a vertical fabric S2 and was dated as Devonian ( c. 375 Ma). Finally, the vertical fabric S2 was overprinted by a D3-M3 event that formed sillimanite to chlorite bearing gently inclined fabric S3 also of Devonian age. The high geothermal gradient of the M1 event can be explained as the result of an extensional, rift-related tectonic setting. In addition, restoration of the deep architecture and polarity of the extended domain before the Devonian history - together with the supracrustal sedimentary and magmatic record - lead us to propose a model for formation of an Ordovician passive continental margin. The subsequent Devonian evolution is interpreted as horizontal shortening of the passive margin at the beginning of Variscan convergence, followed by detachment-accommodated exhumation of lower-crustal rocks. Both Devonian shortening and detachment occurred in the upper plate of a Devonian subduction zone. The tectonic evolution presented in this article modifies previous models of the tectonic history of the western margin of the Teplá-Barrandian domain, and also put constraints on the evolution of the southern margin of the Rheic ocean from the passive margin formation to the early phases of Variscan orogeny. [ABSTRACT FROM AUTHOR]

A structural and geochronological Ar/Ar study was performed in kilometre-scale middle and lower crustal lens-shaped domains dominated by a preserved subvertical foliation, surrounded by horizontally foliated migmatites. These domains occur within the Moldanubian nappe overlying the Brunia microcontinent at the eastern margin of the European Variscides. Three main deformation phases were recognized: subvertical S2 fabric trending NW-SE in lower crustal rocks and NE-SW in mid-crustal rocks. It is reworked by HT/MT horizontal fabric S3 along margins of crustal domains and in surrounding migmatites. S3 bears a prolate NE lineation parallel to the S2-S3 intersection in the lower crustal domain. In the middle crustal units, L3 is weak, connected to oblate strain and trends NE-SW parallel to the S2-S3 intersection. D4 non-coaxial shear deformation is mainly localized at the boundary between the Moldanubian nappe and Brunia and bears strong top to the NNE shear criteria. In order to constrain kinematics of the D3 deformation, strain modelling was performed to show that the Moldanubian hot nappe was frontally thrust over the Brunia indentor. The renewed D4 tangential movement only heterogeneously reactivates the horizontal S3. This evolution is recorded in Ar/Ar amphibole cooling ages, which show two statistically significant Carboniferous peaks at ~342 and 332 Ma, which are also reflected by published detrital muscovite Ar/Ar ages in the adjacent foreland basin. This geochronological record is correlated with progressive erosion of the topographically elevated upper crustal part of the Moldanubian nappe during D3 frontal thrusting, followed by greenschist facies D4 transpressive reactivation and subsequent erosion of high-grade parts of the nappe. [ABSTRACT FROM AUTHOR]

New petrographic and microstructural observations, mineral equilibria modelling and U/Pb (monazite) geochronological studies were carried out to investigate the relationships 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 206Pb/238U 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. [ABSTRACT FROM AUTHOR]

Granitoid magmatism and its role in differentiation and stabilization of the Paleozoic accretionary wedge in the Chinese Altai are evaluated in this study. Voluminous Silurian-Devonian granitoids intruded a greywacke-dominated Ordovician sedimentary succession (the Habahe Group) of the accretionary wedge. The close temporal and spatial relationship between the regional anatexis and the formation of granitoids, as well as their geochemical similarities including rather unevolved Nd isotopic signatures and the strong enrichment of large-ion lithophile elements relative to many of the high field strength elements, may indicate that the granitoids are product of partial melting of the accretionary wedge rocks. Whole-rock geochemistry and pseudosection modeling show that regional anatexis of fertile sediments could have produced a large amount of melts compositionally similar to the granitoids. Such process could have left a high-density garnet- and/or garnet-pyroxene granulite residue in the deep crust, which can be the major reason for the gravity high over the Chinese Altai. Our results show that melting and crustal differentiation can transform accretionary wedge sediments into vertically stratified and stable continental crust. This may be a key mechanism contributing to the peripheral continental growth worldwide. [ABSTRACT FROM AUTHOR]

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 midcrustal 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. Subsequent evolution was explained by midcrustal 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 demonstrate 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. [ABSTRACT FROM AUTHOR]

ABSTRACT 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 subhorizontal 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 post-tectonic andalusite indicates further decompression to 2-3 kbar and 600-650 °C. Widespread crystallization of post-tectonic muscovite is explained by the release of H2O 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 to 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 analysed by LASS yielded elevated HREE concentrations. The grain with the best-developed oscillatory zoning is 356 ± 1.0 [±7] Ma (207Pb-corrected 238U/206Pb), considered to date the crystallization from melt in the cordierite stability ~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. [ABSTRACT FROM AUTHOR]

Structural, petrological and textural studies are combined with phase equilibria modelling of metapelites from different structural levels of the Roc de Frausa Massif in the Eastern Pyrenees. The pre-Variscan lithological succession is divided into the Upper, Intermediate and Lower series by two orthogneiss sheets and intruded by Variscan igneous rocks. Structural analysis reveals two phases of Variscan deformation. D1 is marked by tight to isoclinal small-scale folds and an associated flat-lying foliation (S1) that affects the whole crustal section. D2 structures are characterized by tight upright folds facing to the NW with steep NE- SW axial planes. D2 heterogeneously reworks the D1 fabrics, leading to an almost complete transposition into a sub-vertical foliation (S2) in the high-grade metamorphic domain. All structures are affected by late open to tight, steeply inclined south-verging NW- SE folds (F3) compatible with steep greenschist facies dextral shear zones of probable Alpine age. In the micaschists of the Upper series, andalusite and sillimanite grew during the formation of the S1 foliation indicating heating from 580 to 640 °C associated with an increase in pressure. Subsequent static growth of cordierite points to post-D1 decompression. In the Intermediate series, a sillimanite-biotite-muscovite-bearing assemblage that is parallel to the S1 fabric is statically overgrown by cordierite and K-feldspar. This sequence points to ~1 kbar of post-D1 decompression at 630-650 °C. The Intermediate series is intruded by a gabbro-diorite stock that has an aureole marked by widespread migmatization. In the aureole, the migmatitic S1 foliation is defined by the assemblage biotite-sillimanite-K-feldspar-garnet. The microstructural relationships and garnet zoning are compatible with the D1 pressure peak at ~7.5 kbar and ~750 °C. Late- to post-S2 cordierite growth implies that F2 folds and the associated S2 axial planar leucosomes developed during nearly isothermal decompression to <5 kbar. The Lower series migmatites form a composite S1-S2 fabric; the garnet-bearing assemblage suggests peak P-T conditions of >5 kbar at suprasolidus conditions. Almost complete consumption of garnet and late cordierite growth points to post-D2 equilibration at <4 kbar and <750 °C. The early metamorphic history associated with the S1 fabric is interpreted as a result of horizontal middle crustal flow associated with progressive heating and possible burial. The upright F2 folding and S2 foliation are associated with a pressure decrease coeval with the intrusion of mafic magma at mid-crustal levels. The D2 tectono-metamorphic evolution may be explained by a crustal-scale doming associated with emplacement of mafic magmas into the core of the dome. [ABSTRACT FROM AUTHOR]

Ordovician metasedimentary rocks are the oldest and most extensive sedimentary sequence in the Chinese Altai. They experienced two major episodes of deformation (D1 and D2) resulting in the formation of juxtaposed Barrovian-type and migmatite domains. D1 is characterized by a penetrative sub-horizontal fabric (S1), and D2 is marked by upright folds (F2) with NW- SE-trending axial planes in shallow crustal levels and by sub-vertical transposition foliations (S2) in the high-grade cores of large-scale F2 antiforms. In the Barrovian-type domain, successive growth of biotite, garnet and staurolite is observed in the S1 fabric. Kyanite included in garnet and plagioclase in the migmatite domain is interpreted to have formed also in S1. In the biotite and garnet zones, the spaced S2 cleavage is marked by biotite and muscovite, and in the staurolite and kyanite zones, the penetrative S2 fabric is characterized by sillimanite, locally with late cordierite. Phase equilibria modelling indicates that the S1 fabric was associated with an increase in pressure and temperature under Barrovian-type conditions in both domains. The S2 fabric was related to decompression, in which rocks in the biotite and garnet zones well preserve the peak assemblage, and the higher grade rocks in the staurolite and kyanite zones re-equilibrated to different degrees under high-temperature/low-pressure ( H T/ L P) conditions. The D1 metamorphic history is attributed to the progressive burial related to Early-Middle Palaeozoic crustal thickening and the metamorphism associated with D2 is interpreted to result from exhumation by vertical extrusion. The extrusion of hot rocks was contemporaneous with the formation of gneiss domes accompanied by the intrusion of juvenile magmas at middle crustal levels during the Middle Palaeozoic. Consequently, there is a genetic link between the Barrovian-type and migmatite domains related to continuous transition of the Barrovian-type fabric into the H T/ L P one during development of domal structures in the southern Altai orogenic belt. This study has a broad impact on the understanding of the thermo-mechanical behaviour of accretionary orogenic systems worldwide. The lower crustal flow and doming of hot crust, so far reported only in continental collisional settings, seems to be also an integral mechanism responsible for both horizontal and vertical redistribution of accreted material prior to continental collision. [ABSTRACT FROM AUTHOR]

A section of the orogenic middle crust (Orlica-Śnieżnik Dome, Polish/Czech Central Sudetes) was examined to constrain the duration and significance of deformation (D) and intertectonic (I) phases. In the studied metasedimentary synform, three deformation events produced an initial subhorizontal foliation S1 (D1), a subsequent subvertical foliation S2 (D2) and a late subhorizontal axial planar cleavage S3 (D3). The synform was intruded by pre-, syn- and post-D2 granitoid sheets. Crystallization-deformation relationships in mica schist samples document I1-2 garnet-staurolite growth, syn-D2 staurolite breakdown to garnet-biotite-sillimanite/andalusite, I2-3 cordierite blastesis and late-D3 chlorite growth. Garnet porphyroblasts show a linear Mn-Ca decrease from the core to the inner rim, a zone of alternating Ca-Y- and P-rich annuli in the inner rim, and a Ca-poor outer rim. The Ca-Y-rich annuli probably reflect the occurrence of the allanite-to-monazite transition at conditions of the staurolite isograd, whereas the Ca-poor outer rim is ascribed to staurolite demise. The reconstructed P- T path, obtained by modelling the stability of parageneses and garnet zoning, documents near-isobaric heating from ~4 kbar/485 °C to ~4.75 kbar/575 °C during I1-2. This was followed by a progression to 4-5 kbar/580-625 °C and a subsequent pressure decrease to 3-4 kbar during D2. Pressure decrease below 3 kbar is ascribed to I2-3, whereas cooling below ~500 °C occurred during D3. In the dated mica schist sample, garnet rims show strong Lu enrichment, oscillatory Lu zoning and a slight Ca increase. These features are also related to allanite breakdown coeval with staurolite appearance. As Lu-rich garnet rims dominate the Lu-Hf budget, the 344 ± 3 Ma isochron age is ascribed to garnet crystallization at staurolite grade, near the end of I1-2. For the dated sample of amphibole-biotite granitoid sheet, a Pb-Pb single zircon evaporation age of 353 ± 1 Ma is related to the onset of plutonic activity. The results suggest a possible Devonian age for D1, and a Carboniferous burial-exhumation cycle in mid-crustal rocks that is broadly coeval with the exhumation of neighbouring HP rocks during D2. In the light of published ages, a succession of telescoping stages with time spans decreasing from c. 10 to 2-3 Ma is proposed. The initially long period of tectonic quiescence (I1-2 phase, c. 10 Ma) inferred in the middle crust contrasts with contemporaneous deformation at deeper levels and points to decoupled P- T- D histories within the orogenic wedge. An elevated gradient of ~30 °C km−1 and assumed high heating rates of c. 20 °C Ma−1 are explained by the protracted intrusion of granitoid sheets, with or without deformation, whereas fast vertical movements (2-3 Ma, D2 phase) in the crust require the activity of deformation phases. [ABSTRACT FROM AUTHOR]