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27 p.International audience[1] 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-Snieznik 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-Snieznik mantled gneiss dome derives from polyphase recycling of Saxothuringian material

Eclogite, felsic orthogneiss and garnet-staurolite metapelite occur in a 5 km long profile in the area of Międzygo´rze in the Orlica-S´nie_ znik dome (Bohemian Massif). Petrographic observations and mineral equilibria modelling, in the context of detailed structural work, are used to document the close juxtaposition of high-pressure and medium-pressure rocks. The structural succession in all lithologies shows an early shallow-dipping fabric, S1, that is folded by upright folds and overprinted by a heterogeneously developed subvertical foliation, S2. Late recumbent folds associated with a weak shallowdipping axial-plane cleavage, S3, occur locally. The S1 fabric in the eclogite is defined by alternation of garnet-rich (grs = 22-29 mol.%) and omphacite-rich (jd = 33-36 mol.%) layers with oriented muscovite (Si = 3.26-3.31 p.f.u.) and accessory kyanite, zoisite, rutile and quartz, indicating conditions of 19-22 kbar and 700-750°C. The assemblage in the retrograde S2 fabric is formed by amphibole, plagioclase, biotite and relict rutile surrounded by ilmenite and sphene that is compatible with decompression and cooling from 9 kbar and 730°C to 5-6 kbar and 600-650°C. The S3 fabric contains in addition domains with albite, chlorite, K-feldspar and magnetite indicating cooling to greenschist facies conditions. The metapelites are composed of garnet, staurolite, muscovite, biotite, quartz, ilmenite and chlorite. Chemical zoning of garnet cores that contain straight ilmenite and staurolite inclusion trails oriented perpendicular to the external S2 fabric indicates prograde growth, from 5 kbar and 520°C to 7 kbar and 610°C, during the formation of the S1 fabric. Inclusion trails parallel with the S2 fabric at garnet and staurolite rims are interpreted to be a continuation of the prograde path to 7.5 and 630°C in the S2 fabric. Matrix chlorite parallel to the S2 foliation indicates that the subvertical fabric was still active below 550°C. The axial planar S2 fabrics developed during upright folding are associated with retrogression of the eclogite under amphibolite facies conditions, and with prograde evolution in the metapelites, associated with their juxtaposition. The shared part of the eclogite and metapelite P-T paths during the development of the subvertical fabric reflects their exhumation together.

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The Mollendo-Camana Block (MCB) is a 50 · 150 km Precambrian inlier of the Andean belt that outcrops along the Pacific coast of southern Peru. It consists of stromatic migmatites of Paleoproterozoic heritage intensely metamorphosed during the Grenville event (c. 1 Ga; U-Pb and U-Th-Pb ages on zircon and monazite). In the migmatites, aluminous mesosomes (FMAS) and quartzofeldspathic leucosomes (KFMASH), contain various amounts of K-feldspar (Kfs), orthopyroxene (X Mg Opx ¼ 0.86), plagioclase (Pl), sillimanite (Sil; exceptionally kyanite, Ky) ilmenite (Ilm), magnetite (Mag), quartz (Qtz), and minor amounts of garnet (X Mg Grt ¼ 0.60), sapphirine (X Mg Spr ¼ 0.87), cordierite (X Mg Crd ¼ 0.92) and biotite (X Mg Bt ¼ 0.83). The ubiquitous peak mineral assemblage is Opx-Sil-Kfs-Qtz-(± Grt) in most of the MCB, which, together with the high Al content of orthopyroxene (10% Al 2 O 3 ) and the local coexistence of sapphirine-quartz, attest to regional UHT metamorphism (> 900°C) at pressures in excess of 1.0 GPa. Fluid-absent melting of biotite is responsible for the massive production of orthopyroxene that proceeded until exhaustion of biotite (and most of the garnet) in the southern part of the MCB (Mollendo-Cocachacra areas). In this area, a first stage of decompression from 1.1-1.2 to 0.8-0.9 GPa at temperatures in excess of 950°C, is marked by the breakdown of Sil-Opx to Spr-Opx-Crd assemblages according to several bivariant FMAS reactions. High-T decompression is also shown by Mg-rich garnet being replaced by Crd-Spr-and Crd-Opxbearing symplectites, and reacting with quartz to produce low-Al-Opx-Sil symplectites in quartz-rich migmatites. Neither osumilite nor spinel-quartz assemblages being formed, isobaric cooling at about 0.9 GPa probably followed the initial decompression and proceeded with massive precipitation of melts towards the (Os) invariant point, as demonstrated by Bt-Qtz-(± pl) symplectites in quartz-rich migmatites (melt + Opx + Sil ¼ Bt + Grt + Kfs + Qtz). Finally, Opx rims around secondary biotite attest to late fluid-absent melting, compatible with a second stage of decompression below 900°C. The two stages of decompression are interpreted as due to rapid tectonic denudation whereas the regional extent of UHT metamorphism in the area, probably results from large-scale penetration of hot asthenospheric mantle at the base of an over-thickened crust.

A microstructural and metamorphic study of a naturally deformed medium-to high-pressure granitic orthogneiss (Orlica-S´nie_ znik 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.

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[1] The deformation study of midcrustal porphyritic granite reveals exceptionally high strain intensities of feldspar aggregates compared to stronger quartz. Three types of microstructures corresponding to evolutionary stages of deformed granite were recognized: (1) the metagranite marked by viscous flow of plagioclase around strong alkali feldspar and quartz, (2) quartz augen orthogneiss characterized by development of banded mylonitic structure of recrystallized plagioclase and K-feldspar surrounding augens of quartz, and (3) banded mylonite characterized by alternation of quartz ribbons and mixed aggregates of feldspars and quartz. The original weakening of alkali feldspar is achieved by decomposition into albite chains and K-feldspar resulting from a heterogeneous nucleation process. The subsequent collapse of alkaline feldspar and development of monomineralic layering is attributed to the onset of syn-deformational dehydration melting of Mu-Bi layers associated with production of $2% melt. The final deformation stage is marked by mixing of feldspars which is explained by higher melt production due to introduction of external water. An already small amount of melt is responsible for extreme weakening of the feldspar because of Melt Connectivity Threshold effect triggering grain boundary sliding deformation mechanisms. The grain boundary sliding controls diffusion creep at small melt fraction and evolves to particulate flow at high melt fractions. Strong quartz shows a dislocation creep deformation mechanism throughout the whole deformation history marked by variations in the activity of the slip systems, which are attributed to variations in stress and strain rate partitioning with regard to changing rheological properties of the deforming feldspars.Citation: Schulmann, K., J.-E. Martelat, S. Ulrich, O. Lexa, P. Š típská, and J. K. Becker (2008), Evolution of microstructure and melt topology in partially molten granitic mylonite: Implications for rheology of felsic middle crust,