Viscous collision in channel explains double domes in metamorphic core complexes. Comment

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TRACKING THE P–T PATH OF PRECAMBRIAN ECLOGITE USING PSEUDOSECTION, Ti-IN-QUARTZ AND Zr-IN-RUTILE THERMOBAROMETRY

International audienceSveconorwegian eclogite occur as a nappe within the high-grade metamorphic region in southernSweden, which constitutes a window into the deepest part of this Precambrian mountain belt. Distinctmicrostructural domains (i.e., garnet core, garnet rim, and matrix) in a Fe-Ti-rich eclogite variety containabundant quartz, rutile and zircon.A pseudosection approach was first applied and compared to results from a combination of Zr-in-rutileand Ti-in-quartz. The pressure input used for both thermometers was first deduced for each microstructuraldomain from the pseudosection. For the garnet core, Zr-in-rutile yields temperatures of 700-715°C and Ti-inquartz~ 635°C at 7 kbar. For the garnet rim, temperatures of 760-790°C (Zr-in-rutile) and 740-890°C (Ti-inquartz)at 12-18 kbar were calculated. Matrix rutile recorded temperatures of ~ 810°C, while quartz recordedtemperatures up to ~ 890°C. Additionally, direct combination of Ti content in quartz and Zr content in rutileisopleths (i.e., independent from the pseudosection) yield a prograde path in nearly perfect agreement withthe one deduced from the pseudosection.The pseudosection shows that rutile was produced by continuous breakdown of ilmenite during the earlystages of prograde metamorphism, a reaction that ran to completion at ~ 730°C. Most rutile grains in garnetrim and matrix are interpreted to subsequently form by recrystallization of smaller matrix grains. However,they generally do not record the peak-P temperatures and instead range mostly between 775 and 815°C,interpreted as a result of more efficient recrystallization during a dehydration reaction (progressivereplacement of hornblende by clinopyroxene).This study illustrates that both Zr-in-rutile and Ti-in-quartz thermometry cannot only robustly constrain aprograde evolution, but when combined with a pseudosection model can also yield information onrecrystallization processes. In fact, the combination of these three methods provides an unrivalled tool forpetrologic interpretation.The variation in Ti concentration in quartz is small regardless of crystal size. This P–T path reach veryhigh temperatures (up to 875°C) with a high dP/dT ratio, both during prograde and retrograde histories. Thesteep P–T path, together with preservation of garnet growth zoning, symplectitic textures and the lack ofsignificant Ti diffusion in quartz is consistent with a short residence time at high-temperature, implyingunusual fast burial and exhumation of the eclogite-bearing nappe

Repeated slip along a major decoupling horizon between crustal-scale nappesof the Central Western Carpathians documented in the Ochtinà tectonicmélange

International audienceThe Ochtiná Unit is situated in the ENE-WSW-trending contact zone between two crustal-scale nappes, the upper Gemer Unit and the lower Vepor Unit, in the Central Western Carpathians, Slovakia. The Ochtiná Unit consists mainly of Carboniferous phyllitic schists and sandstones enclosing lenses of diverse lithological nature and contrasting metamorphic history. Peak PT conditions obtained by means of phase equilibrium modelling from lenses of amphibolite and chloritoid schist in this unit indicate 500-600 °C and 4-6.5 kbar and 500-520 °C and 9-11 kbar, respectively. These PT conditions contrast not only with the greenschist-facies metamorphism of dominant phyllite but also with each other documenting two distinct metamorphic field gradients related to Variscan and Alpine metamorphic events. Geochemical data reveal an affinity of the amphibolite lenses to similar Variscan rocks in the basement of the upper Gemer Unit and of the chloritoid schist to similar Alpine rocks in the cover of the lower Vepor Unit. Such heterogeneous lithological and metamorphic record is consistent with a block-in-matrix rock arrangement and the Ochtiná Unit is interpreted as deep seated tectonic mélange. The mélange evolved via repeated slip along the rheologically weak sediments of the Ochtiná Unit during the building and collapse of the Eo-Alpine orogenic wedge of the Central Western Carpathians. Deformation record indicates that the mélange separates two distinct structural domains marked by a decoupled behaviour, i.e. the orogenic suprastructure represented by the Gemer Unit and the infrastructure represented by the Vepor Unit. With this respect, the Ochtiná Unit represents an unusual example of a suprastructure-infrastructure transition zone with its position being controlled by the mechanical weakness of this sedimentary horizon and not by the temperature-dependent rheological transition

Oligo-Miocene thinning of the Beni Bousera peridotites and their Variscan crustal host rocks, Internal Rif, Morocco†

International audienceDeciphering Variscan versus Alpine history in the internal Rif system is a key to constraining the tectonic evolution of the Alboran domain and hence the geodynamics of the western Mediterranean system during the Cenozoic. This study focuses on the evolution of the metamorphic envelope of the Beni Bousera massif and its relation to the underlying peridotites. Combining structural geology, metamorphic petrology, and LA-ICP-MS U-Th-Pb dating of monazite, this study contributes to the understanding of the tectonic history of the western internal Rif. The regional foliation (S2) is characterized by LP/HT mineral assemblages and obliterates a former foliation (S1) developed along a barrovian (MP/MT) metamorphic gradient. The dating of some metamorphic monazite grains from a micaschist and a migmatitic gneiss demonstrates that the crustal envelope of the peridotite recorded two distinct tectonometamorphic episodes. Data from monazite inclusions in S1 garnet suggest that the first event, D1, is older than 250-170 Ma and likely related to the Variscan collision, in agreement with the barrovian type of the metamorphic gradient. The second event, D2, is Alpine in age (at circa 21 Ma) and corresponds to a strong lithosphere thinning allowing subsequent subcontinental mantle exhumation. Such a tectonic context provides an explanation for the LP/HT metamorphic gradient that is recorded in the regional foliation of the western Betic-Rif system. This extension is probably related to a subduction slab roll-back in the western end of the Mediterranean realm during the Oligo-Miocene times. No evidences for a Tertiary HP/LT metamorphism have been identified in the studied are

Two-stage partial melting during the Variscan extensional tectonics (Montagne Noire, France)

International audienceOne of the striking features that characterise the late stages of the Variscan orogeny is the development of gneiss and migmatite domes, as well as extensional Late Carboniferous and Permian sedimentary basins. It remains a matter of debate whether the formation of domes was related to the well documented late orogenic extension or to the contractional tectonics that preceded.Migmatization and magmatism are expected to predate extension if the domes are compression-related regional anticlines, but they must both precede and be contemporaneous with extension if they are extensional core complexes. In the Montagne Noire area (southern French Massif Central), where migmatization, magmatismand the deformation frameworkare well documented, the age of the extensional event was unequivocally constrained to 300-0Ma.Therefore,dating migmatization in this area is a key point for discriminating between the two hypotheses and understanding the Late Palaeozoic evolution of this part of the Variscan belt. For this purpose, a migmatite and an associated anatectic granite from the Montagne Noire dome were dated by LA-ICP-MS (U-Th/Pb onzircon and monazite) andlaser probe40Ar-39Ar (K-Ar on muscovite). Although zircon did not record any Variscan ageunequivocally related to compression (380-330Ma),two age groups were identified from the monazite crystals. A first event, at ca. 319 Ma (U-Th/Pb on monazite),is interpreted as a first stage of migmatization and as the emplacement age of the granite, respectively. A second event at ca. 298-295 Ma, recorded by monazite (U-Th/Pb) and by the muscovite 40Ar-39Ar system in the migmatite and in the granite, could be interpreted as a fluid-induced event, probably related to a second melting event identified through the syn-extensional emplacement of the nearby Montalet leucogranite ca. 295 Ma ago. The agesof these two events post-date the Variscan compression and agreewith an overall extensional context for the development of the Montagne Noire dome-shaped massif.Comparison of these results with published chemical (EPMA) dating of monazite from the samerocks demonstrates that the type of statistical treatment applied to EPMA data is crucial in order to resolve different monazite age populations

Syn-deformational melt percolation through a high-pressure orthogneiss and the exhumation of a subducted continental wedge (Orlica-Śnieżnik Dome, NE Bohemian Massif)

High-pressure granitic orthogneiss of the south-eastern Orlica-Śnieżnik Dome (NE Bohemian Massif) shows relics of a shallow-dipping S1 foliation, reworked by upright F2 folds and a mostly pervasive N-S trending subvertical axial planar S2 foliation. Based on macroscopic observations, a gradual transition perpendicular to the subvertical S2 foliation from banded to schlieren and nebulitic orthogneiss was distinguished. All rock types comprise plagioclase, K-feldspar, quartz, white mica, biotite and garnet. The transition is characterized by increasing presence of interstitial phases along like-like grain boundaries and by progressive replacement of recrystallized K-feldspar grains by fine-grained myrmekite. These textural changes are characteristic for syn-deformational grain-scale melt percolation, which is in line with the observed enrichment of the rocks in incompatible elements such as REEs, Ba, Sr, and K, suggesting open-system behaviour with melt passing through the rocks. The P-T path deduced from the thermodynamic modelling indicates decompression from ~15−16 kbar and ~650-740 ºC to ~6 kbar and ~640 ºC. Melt was already present at the P-T peak conditions as indicated by the albitic composition of plagioclase in films, interstitial grains and in myrmekite. The variably re-equilibrated garnet suggests that melt content may have varied along the decompression path, involving successively both melt gain and loss. The 6-8 km wide zone of vertical foliation and migmatite textural gradients is interpreted as vertical crustal-scale channel where the grain-scale melt percolation was associated with horizontal shortening and vertical flow of partially molten crustal wedge en masse

Migmatite formation in a crustal-scale shear zone during continental subduction: an example from a high-pressure granitic orthogneiss from the Orlica-Śnieżnik Dome (NE Bohemian Massif)

Petrological study and pseudosection modelling have been carried out in high-grade orthogneisses of the southern domain of the Orlica- Snieznik Dome (NE Bohemian Massif). The studied samples are from an outcrop dominated by two deformation fabrics, a sub-horizontal S1 foliation defined by bands of recrystallized K-feldspar, quartz and plagioclase folded by centimetre- to several metre-scale close to isoclinal folds associated with development of a new subvertical N-S trending foliation S2. Based on field features and textural observations, a gradual transition from banded mylonitic orthogneiss (Type I) to stromatitic (Type II), schlieren (type III) and nebulitic (type IV) textures typical of migmatities can be distinguished. The banded orthogneiss is composed of almost monomineral recrystallized K-feldspar layers (2 to 10 mm thick) alternating with layers of plagioclase and quartz (1 to 4mm thick), parallel to the S1 limb and the axial planar S2 foliation. The stromatitic migmatite shows 1 to 4 mm thick layers with macroscopically diffuse boundaries between plagioclase, quartz and K-feldspar rich domains. Boundaries between quartz and feldspar layers are poorly defined and interlobed with adjacent minerals. The schlieren migmatite is almost isotropic preserving small K-feldspar-rich domains within a matrix characterized by random distribution of phases, whereas in the nebulitic migmatite the microstructure is completely isotropic characterized by random distribution of phases. The transition from the Type I to IV is characterized by increasing nucleation of interstitial phases along like-like grain boundaries, by a decrease of grain size of all phases and by progressive disintegration of recrystallized K-feldspar grains by embayments of fine-grained myrmekite. The mineral assemblage of all types consists of biotite, white micas, garnet, quartz, K-feldspar and plagioclase, and accessory apatite, ilmenite, zircon and monazite. In the mineral equilibria modelling, the core of garnet (alm0.58, py0.02-0.03, grs0.34, sps0.05) and phengite (Si = 3.38-3.20 p.f.u) is consistent with a P-T peak at 10-13 kbar and 720-750 C in the dominant grt-bt-ph-rt-qtz-pl-kfs mineral assemblage. The garnet rim (alm0.68, py0.02-0.03, grs0.11, sps0.21), white mica rim (Si = 3.10 p.f.u) together with unzoned biotite (XFe = 0.76-0.78) match the modelled isopleths in the middle-P part of the grt-bt-ph-ilm-qtz-pl-kfs field to reach the solidus at 78 kbar and 630650 C. In addition, the absence of prograde garnet zoning in the Type I to III suggests that the garnet was completely re-equilibrated during the retrograde history, whereas in the Type IV the HP garnet chemistry was preserved. This is discussed in frame of melt presence in different migmatite types along their P-T path. Based on mineral equilibria modelling it is argued for fluid/melt-fluxed melting at HP conditions and on exhumation. The migmatite textural types are a result of grain-scale melt migration process and not of a localized melt transport in dykes as known from metasediments

Histoire Géologique du massif Armoricain : Actualité de la recherche

National audienceUne part essentielle de l'histoire géologique de la France (et même d'Europe occidentale, avec des roches ayant environ 2000 Ma) est déchiffrable dans le Massif armoricain. Si celui-ci est réputé pour ses excellentes qualités d'affleurement sur le littoral (Armor, ou pays de la mer), certains objets ou structures ne peuvent être observés que dans le bocage (Argoat, ou pays des arbres), où leur lisibilité est souvent problématique. En Armor comme en Argoat, de nombreux sites constituent un réel patrimoine géologique (l'intérêt de certaines localités sera mis en exergue dans le texte), dont la valeur ne peut être jaugée qu'au regard de son intérêt scientifique. Ainsi ce travail - une mise en perspective de nos connaissances scientifiques sur l'évolution géologique du Massif armoricain - est-il basé sur plusieurs synthèses antérieures (par ex. Le Corre et al., 1991 ; Ballèvre et al., 2009), qu'il complète en intégrant les nouvelles données disponibles. Plus qu'un exposé complet des faits, nous visons à clarifier certains débats, et montrer en quoi les recherches en cours changent notre image globale du Massif armoricain. Nous restreindrons notre analyse à la période qui couvre la fin du Protérozoïque (Ediacarien : 635-540 Ma), le Paléozoïque (540-250 Ma) et le début du Mésozoïque (Trias : 250-200 Ma)

Crustal influx, indentation, ductile thinning and gravity redistribution in a continental wedge: Building a Moldanubian mantled gneiss dome with underthrust Saxothuringian material (European Variscan belt)

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