A large database of structural, geochronological and petrological data combined with a Bouguer anomaly map is used to develop a two-stage exhumation model of deep-seated rocks in the eastern sector of the Variscan belt. An early sub-vertical fabric developed in the orogenic lower and middle crust during intracrustal folding followed by the vertical extrusion of the lower crustal rocks. These events were responsible for exhumation of the orogenic lower crust from depths equivalent to 18)20 kbar to depths equivalent to 8)10 kbar, and for coeval burial of upper crustal rocks to depths equivalent to 8-9 kbar. Following the folding and vertical extrusion event, sub-horizontal fabrics developed at medium to low pressure in the orogenic lower and middle crust during vertical shortening. Fabrics that record the early vertical extrusion originated between 350 and 340 Ma, during building of an orogenic root in response to SE-directed Saxothuringian continental subduction. Fabrics that record the later subhorizontal exhumation event relate to an eastern promontory of the Brunia continent indenting into the rheologically weaker rocks of the orogenic root. Indentation initiated thrusting or flow of the orogenic crust over the Brunia continent in a north-directed sub-horizontal channel. This sub-horizontal flow operated between 330 and 325 Ma, and was responsible for a heterogeneous mixing of blocks and boudins of lower and middle crustal rocks and for their progressive thermal re-equilibration. The erosion depth as well as the degree of reworking decreases from south to north, pointing to an outflow of lower crustal material to the surface, which was subsequently eroded and deposited in a foreland basin. Indentation by the Brunia continental promontory was highly noncoaxial with respect to the SEoriented Saxothuringian continental subduction in the Early Visean, suggesting a major switch of plate configuration during the Middle to Late Visean.

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 of 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 reequilibrated to different degrees under high-temperature/low-pressure (HT/LP) conditions. The D1 metamorphic history is attributed to the progressive burial related to Early-Middle Palaeozoic crustal thickening and the Accepted ArticleThis article is protected by copyright. All rights reserved. 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 HT/LP 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.

The structure of the Moldanubian domain is marked by felsic granulites of Ordovician protolith age forming the cores of domes that are separated from mid-crustal Neoproterozoic and Palaeozoic metasedimentary rocks that occur in synclines by a late Ordovician to Silurian metabasic unit. Reflection and refraction seismic sections combined with gravity inversion modelling suggest the presence of a low density layer at the bottom of the crust (interpreted as felsic granulite) overlain by a denser layer (interpreted as amphibolite) with layers of intermediate density at the top (interpreted as metasedimentary rocks). It is proposed that the granulite domes surrounded by middle crustal rocks reflect transposed horizontal layering originally similar to that preserved in the deep crust and imaged by the geophysical surveys. This geological and geophysical structure is considered to be a result of Vise´an gravity redistribution initiated by radioactive heating of felsic crust tectonically emplaced at the bottom of a Palaeozoic orogenic root. The radioactive layer with heat production of 4 lW m )3 corresponds geochemically and isotopically to Ordovician felsic metaigneous rocks of the Saxothuringian domain that have been emplaced at Moho depth under thickened crust during late Devonian-early Carboniferous continental subduction. Part of the continental crust continued to be subducted and produced fluids ⁄ low-volume melts which directly contaminated and enriched the local lithospheric mantle by lithophile elements, most notably Cs, Rb, Li, Pb, U, Th and K. Thermal incubation of 10-15 Myr was sufficient to heat and convert the underplated felsic layer into granulites via dehydration melting and melt segregation. The process of melt loss was responsible for the removal of radioactive elements and for switching off the heat at the beginning of the exhumation process. At the same time, the metasomatized underlying mantle was heated producing characteristic ultrapotassic magmas. Gravitational instability was then induced by the density contrast between the light granulites and the overlaying denser mafic lower crustal layer and a viscosity drop related to thermal weakening and partial melting of the latter.

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The Chandman massif, a typical structure of the Mongolian Altai, consists of a migmatitemagmatite core rimmed by a lower-grade metamorphic envelope of andalusite and cordieritebearing 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 Accepted ArticleThis article is protected by copyright. All rights reserved. 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. 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 subhorizontal 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, ...

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A section of anatectic felsic rocks from a high-pressure (>13 kbar) continental crust (Variscan Bohemian Massif) preserves unique evidence for coupled melt flow and heterogeneous deformation during continental subduction. The section reveals layers of migmatitic granofels interlayered with anatectic banded orthogneiss and other rock types within a single deformation fabric related to the prograde metamorphism. Granofels layers represent high strain zones and have traces of localized porous melt flow that infiltrated the host banded orthogneiss and crystallized granitic melt in the grain interstices. This process is inferred from: (1) gradational contacts between orthogneiss and granofels layers; (2) grain size decrease and crystallographic preferred orientation of major phases, compatible with oriented growth of crystals from interstitial melt during granular flow, accommodated by melt-assisted grain boundary diffusion creep mechanisms; and (3) pressuretemperature equilibria modeling showing that the melts were not generated in situ. We further argue that this porous melt flow, focused along the deformation layering, significantly decreases the strength of the crustal section of the subducting continental lithosphere. As a result, detachment folds develop that decouple the shallower parts of the layered anatectic sequence from the underlying and continuously subducting continental plate, which triggers exhumation of this anatectic sequence.

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