EVOLUTIONARY MODEL OF THE MELIATA BLUESCHISTS

The Meliata blueschists are subdivided into four groups which are characterized by different litholo-gies and metamorphic histories: (1) Marbles, intercalated with metabasalts and glaucophane-bearing phyllites. Phengite from this group metabasalts and phyllites records the Middle Jurassic age which is interpreted to date the high-pressure metamorphism; (2) Metabasites and micaschists, devoid of marbles contain relic muscovite and newly formed high-Si phengite. The relic muscovite gives Palaeozoic age; (3) Quartz phyllites indicate a polyphase evolution characterized by greenschist facies metamorphism followed by blueschist-facies over-print; the high-Si micas provide evidence of an early Middle Jurassic age of the high-pressure metamorphism; (4) Amphibolite-facies slices and blocks, overprinted by blueschist-facies metamorphism, represent old base-ment rocks. Lithological and metamorphic characteristics together with radiogenic ages of high-pressure micas indicate a subduction of a continental wedge prior to that of oceanic crust can be assumed

Exhumation of the Meliata high-pressure rocks (Western Carpathians): Petrological and structural records in blueschists

The Meliata unit, situated in the SE part of the Western Carpathians, represents an accretionary complex assembled during the closure of the Triassic-Jurassic Meliata oceanic basin. Blueschists, ophiolites and very low-grade metamorphosed sedimentary rocks are imbricated in a tectonic zone between the Gemericum and the Silica nappe. Petrological and microstructural analyses indicate a single progressive deformation coincident with prograde metamorphism at blueschist facies conditions. The foliation is defined by preferred orientation of mica, blue amphibole and rarely also by Na-pyroxene. The exhumation path is documented by ductite deformation, formed at blueschist-greenschist facies boundary and at greenschist facies conditions. The E-W directed thrust faults which are parallel to the foliation seem to be responsible for exhumation of the blueschists. Later stages of deformation in phyllites are documented by shear bands that crosscut the blueschist facies foliation. Low-temperature Cretaceous nappe tectonics resulted in brittle deformation and used mostly the older tectonic systems that formed at blueschist-greenschist facies condition

Exhumation of the Meliata high-pressure rocks (Western Carpathians):

Petrological and structural records in blueschist

Les marquers microstructuraux, pétrologiques et géochimiques du transport pénétratif de liquide de fusion dans la croûte

Cette thèse de doctorat présente les résultats d'études détaillées de terrain, microstructures, pétrologie et géochimie, effectuées sur deux large terrane migmatitique d'âge Varisque: l'unité Gföhl de la zone Moldanubienne, du massif de Bohême et Vosges (France). Les données ont été utilisé pour contraindre l'origine des roches migmatitiques, ainsi que permettre une meilleure compréhension du transport de produits fondus dans la croûte. Un nouveau modèle est proposé pour l'infiltration des produits fondus, à partir d'une source externe, pour expliquer (1) les variations dans l'apparence des migmatites et (2) le modèle reactive porous melt flow par le mouvement de produits fondus dans la croûte. Ce conception implique une infiltration de matériel à partir d'une source externe pendant l exhumation, ou ce matériel se déplace le long des limites de grains, à travers l'ensemble du volume des roches, changeant ainsi l'apparence macroscopique et microscopique des roches.This thesis represents an outcome of detailed field, microstructural, petrological and geochemical studies on two large Variscan migmatitic terranes, Gföhl Unit of the Moldanubian Zone in Bohemian Massif and Vosges Mts. in France. Obtained data are used to constrain the origin of the migmatitic rocks and for better understanding of the melt transport in the crust. We introduce a new possible model of melt infiltration from external source to explain the variations in the migmatite appearance and reactive porous melt flow for the melt movement on a crustal scale. In this concept melt passes pervasively along grain boundaries through the whole rock volume and changes macroscopic and microscopic appearance of the rock. We suggest that the individual migmatite types represent different degrees of equilibration between the host rock and migrating melt. It should be emphasized, that all these processes occur along a retrograde path during exhumation of the Gföhl Unit.STRASBOURG-Sc. et Techniques (674822102) / SudocSudocFranceCzech RepublicFRC

Ultrapotassic Dykes in the Moldanubian Zone and Their Significance for Understanding of the Post-Collisional Mantle Dynamics During Variscan Orogeny in the Bohemian Massif

We report mineral textures, geochemistry and age relations of two ultrapotassic dykes from a dyke swarm in the Western part of the Moldanubian Zone at contact to the Teplá-Barrandian Block. The dykes have orientation almost perpendicular to the NNE–SSW trending Central Bohemian plutonic complex and cross cut metamorphic foliation in basement gneisses and migmatites. Based on mineral compositions and geochemistry, the dykes show close relations to Mg-K syenite plutons in the Moldanubian Zone. The two dykes are vaugnerite and syenite in compositions and contain talc pseudomorphs after olivine within a fine-grained matrix that consists of K-feldspar, phlogopite with small amounts of clinopyroxene and accessory quartz, apatite, titanite and sulphides of Fe, Cu, Ni. The syenite porphyry dyke cross cuts the vaugnerite. It contains quartz phenocrysts and comparing to vaugnerite has lower modal content of talc pseudomorphs. The vaugnerite and syenite porphyry have high K2O (6 to 7 wt.%) and mg-number (0.66–0.74), but low CaO and Na2O contents. The vaugnerite is markedly rich in P2O5 (\u3e2 wt.%) and comparing to syenite porphyry has higher amount of mantle-incompatible elements (e.g. Rb, Cs, Ba, Pb, Th, U), V and Cr. ID-TIMS analyses on titanite in vaugnerite and on zircon in syenite porphyry yielded 338.59 ± 0.68 and 337.87 ± 0.21 Ma, respectively. Mineral and bulk rock chemistry of the dykes indicates that the source magma was formed by a low degree of partial melting of a highly anomalous domain in the upper mantle which subsequently fractionated and was contaminated with crustal material during its ascent. Formation of ultrapotassic magma is explained by transcurrent shear zones in the mantle lithosphere that occurred due to block rotation and post-collisional mantle dynamics initiated by slab break-off and asthenosphere upwelling into the Moldanubian accretionary complex during the Variscan Orogen