Development of an in-situ magmatic dome (Svecofennian accretionnary orogen, Finland)

Volume: 16This study focuses on the formation of the 100 km wide Vaasa magmatic dome (Svecofennian orogen, Finland). It is cored by diatexite migmatites and granitoids and gradually mantled by metatexite migmatites and mica schist. Geochemical data have demonstrated that the later are the sources of the melted core. This is an agreement with new petrological modeling showing an increase of temperature from 500C to up to 800C at constant pressure (5-6 kbar) from the border to the core of the dome (peak). Field work studies highlights the initial formation of a layered middle crust with a strong lateral increases of in-situ melt content towards the core of the dome. It is followed by a regional shortening and exhumation along sub-vertical shear zone during a persistent high-temperature thermal anomaly. This may be the result of distributed thickening competing with regional shortening and perturbed by lateral increase of in-situ melting within middle crust towards the dome core. Sub-crustal continuous underplating of magma in this accretionnary orogen might be responsible for the origin of the thermal anomaly and formation of this 1.87 Ga in-situ magmatic dome.Non peer reviewe

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

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

The Vaasa migmatitic complex : the birth, growth and death of a thermal dome

EGU GA. Volume: 17 Host publication title: EGU General Assembly 2015The Vaasa migmatitic complex, or Vaasa dome, is cored by diatexite migmatites and S-type granitoids and gradually mantled by metatexite migmatites and mica schist with thin metabasite-andesite intercalations. Previous geochemical studies have demonstrated that the metasediments are the sources of the melted core: it have been suggested that the complex have been formed by in-situ melting of a basin. Field work studies highlight the formation of a gently dipping metamorphic fabric with a lateral increase of the in-situ melt content towards the core of the dome (D1). This early layered and partially melted fabric is then affected by a regional N-S shortening forming km- to outcrop-scale E-W striking folds and new sub-vertical foliation (D2). Late sub-vertical shearing is visible along the dome border and within the diatexitic zone (D3). No late detachment structures have been observed. In the metamorphic belt, the grade increases from medium-T amphibolite facies to low-P granulite facies towards the core of the dome. Pseudosections in the MnNCKFMASHTO system have been performed in one mica schist (Grt+BtPl+Qz±Std+Sill+And) and one metatexite migmatite (Bt+Liq+Crd+Pl+Kfs+Grt+Qz±Sill+And). The metamorphic peaks are bracketed at 560°C at 5 kbar and 750-770°C at 4.5-5 kbar, respectively. The retrograde condition is situated at 540°C and <3 kbar for both lithologies. This implies an isobaric increase of the metamorphic grade towards the core of the dome. An isothermal decompression for the schist and a retrograde PT path for the migmatites are observed. Existing and new U/Pb monazite ages from mica schists, migmatites and clustered at 1860–1865 Ma whereas U/Pb ages from metamorphic and magmatic zircons are older and clustered at 1875 Ma. The latter might represent the peak of melting process and associated metamorphism whereas monazites ages might be related to the cooling of the orogenic middle crust. It has to be noticed that few monazites from metamorphic rocks of the dome mantle gave ages similar to those of upper crustal pegmatites, i.e about 1800 Ma. The formation of the Vaasa dome might be the result of thickening during a persistent high-temperature thermal anomaly. It forms a layered middle crust with a strong lateral increase of in-situ melt (D1), followed by a regional N–S horizontal shortening (D2). The culmination of the thickening and the associated metamorphism takes place at 1875 Ma. Exhumation and concomitant cooling of the dome along subvertical shear zone is bracketed at 1860–1865 Ma. This might be correlated with the geodynamic setting of the Svecofenian orogen. After its formation in a back-arc setting at around 1910 Ma, the basin is thickened within a fore arc system in front of a pre-existing arc. An orogenic scale N–S shortening, possibly associated with the formation of an orocline at 1870–1860 Ma might be responsible for the final configuration of the orogen. The origin of the thermal anomaly in the Vaasa dome is still disputed. Heat source inherited from the back-arc basin, radioactive decay, thickening, magma underplating in lower crust and/or delamination might have generated the thermal positive anomaly.Non peer reviewe

The Vaasa dome : its tectonic and metamorphic evolution during the Svecofennian orogen

Non peer reviewe

Développement in-situ d’un dôme migmatitique (Dôme de Vaasa, Svécofennides, Finlande)

Dans les Svécofennides, le dôme migmatitique géant de Vaasa (>150 km de diamètre) est né de l’anatexie partielle et progressive d’un bassin sédimentaire paléoprotérozoïque. Son étude permet de contraindre les processus orogéniques de transferts de matière et de chaleur dans les orogènes chauds précambriens. Le dôme ne présente pas une architecture typique avec foliations et linéations concentriques. En effet, l’étude structurale mets en évidence un épaississent du bassin formant une foliation plate suivi par la formation des plis droits et de cisaillements verticaux localisés en bordure du dôme et au sein des diatexites délimitant son cœur. L’ensemble de ces trois phases est associé sur le terrain à la production de liquide silicaté. Aucun mouvement tardif associé à des structures plates et concentriques n’a pu être observé. Ni l’étalement latéral ni l’extension post-orogénique n’expliquent son architecture. La structure en dôme se matérialise par un fort gradient du pic métamorphique du faciès amphibolite jusqu’à l’anatexie, de la bordure vers le coeur du dôme. La modélisation pétrologique (Perple_X) suggère une augmentation de 500 à plus de 800 °C à une pression constante de 4,5−5 kbar, donc sans exhumation différentielle. De nouvelles datations U/Pb associées à la compilation des données géochronologiques existantes suggère un pic de déformation et de fusion partielle à 1875 Ma, et un refroidissement global du dôme à environ 1860–1865 Ma dans un environnement métamorphique estimé à 450 °C et 3 Kbar. Le dôme de Vaasa reflète la fusion partielle d’un bassin sédimentaire lors de son accrétion au-dessus d’un front thermique dont l’origine (sous-placage de magma dans la croûte inférieure ? délamination de la lithosphère ? maturation thermique ? …) reste pour le moment discutée. N’ayant pas les caractéristiques classiques des dômes structuraux, le dôme de Vaasa pourrait être un parfait exemple de dôme migmatitique formé in-situ par anatexie.Non peer reviewe

Le Massif des Rehamna (Meseta marocaine) : témoin d’un changement majeur de la dynamique des plaques au Carbonifère supérieur - Permien inférieur au sein de l’orogène varisque– alléghénien

L’absence d’étude moderne dans le varisque marocain associant étude structurale, métamorphique et géochronologique freine toute avancée significative sur son évolution tectonique et sur les corrélations géodynamiques avec les autres branches adjacentes de l’orogène varisque-alléghénien en Europe et en Amérique du Nord au Paléozoïque supérieur. Dans le massif des Rehamna (Meseta marocaine), trois épisodes tectoniques ont été mis en évidence. (1) Charriage vers le SSO de formations ordoviciennes sur le socle néoprotérozoïque à cambrien et ses bassins intracontinentaux dévono-carbonifères. Cet événement entraîne un cisaillement ductile horizontal et un métamorphisme prograde Barrovien au sein des roches enfouies. (2) Il résulte de ce raccourcissement la formation d’un dôme syn-convergent d’allongement ~E−O permettant l’extrusion des unités inférieures et le détachement des unités supérieures métamorphiques. Ces épisodes sont contraints à 310−295 Ma par les âges 40Ar/39Ar de refroidissement et de cristallisation métamorphique d’amphiboles et de micas. (3) Une convergence de direction ONO, orthogonal au précédent, permet l’accrétion finale de toutes les unités sur le socle continental plus à l’ouest. Les âges 40Ar/39Ar de refroidissement d’amphiboles d’un leucogranite syn-tectonique et de son encaissant, ainsi que les âges de cristallisation de muscovites d’une mylonite, démontrent que cet épisode prend place entre 295 et 280 Ma. La fin de l’orogène varisque dans cette partie de la Meseta marocaine est contrainte par l’âge de refroidissement d’un batholithe granitique post-tectonique à 275 Ma. Ces événements, confrontés d’abord à l’évolution de la Meseta marocaine, sont enfin mis en relation avec la géodynamique globale des continents Laurentia et Gondwana à la fin de l’orogène varisque–alléghénien, montrant le changement des contraintes aux limites dans la chaîne au Carbonifère supérieur−Permien inférieur au Maroc, en Europe et en Amérique du Nord (Appalaches).Non peer reviewe

Tectonometamorphic evolution of the Rehamna dome (Morocco)

Volume: 42 Host publication title: The 2014 CETeG Conference "Lądek" Host publication sub-title: The Orlica-Śnieżnik Dome and the Upper Nysa Kłodzka Graben, the Sudetes 23-26 April 2014, Lądek Zdrój, Poland : Proceedings and Excursion GuideNon peer reviewe

Three new PAX6 mutations including one causing an unusual ophthalmic phenotype associated with neurodevelopmental abnormalities

The PAX6 gene was first described as a candidate for human aniridia. However, PAX6 expression is not restricted to the eye and it appears to be crucial for brain development. We studied PAX6 mutations in a large spectrum of patients who presented with aniridia phenotypes, Peters' anomaly, and anterior segment malformations associated or not with neurological anomalies.Journal ArticleResearch Support, Non-U.S. Gov'tSCOPUS: ar.jinfo:eu-repo/semantics/publishe