EVOLUTION OF MASS-TRANSFER DURING PROGRESSIVE OBLIQUE UNDER-THRUSTINGOF THE VARISCAN FORELAND - EASTERN BOHEMIAN MASSIF

Four ductile shear zones were sampled in the autochthonous Thaya basem ent and the Upper Bites nappe (Moravian unit) at the Eastern margin of the Bohemian massif. In both studied units, the tectono-metamorphic e volution and the chemical mass transfer are different. Two deformation al events are recognised: the first deformation stage under amphibolit e facies conditions is overprinted by a second event under greenschist facies conditions. The first deformation affected the western margin of the Thaya basement and the whole Bites nappe: microstructures are c haracterised by dynamic recrystallisation of feldspars and quartz, and occurrence of myrmekites and grain-boundary migration of quartz. None or weak chemical mass transfer is related to this medium to high temp erature deformation. This deformation corresponds to the thrusting of Moldanubdian units on the Brunovistulian units (Moravian nappes and au tochthonous Thaya basement). The second deformation generated shear zo nes in the until then preserved Thaya basement and reactivated both sh ear zones of the western margin of the Thaya basement and those of the Bites nappe. This deformation is retrograde and mainly associated wit h chemical mass transfer: a decrease of CaO, FeO, FeO/Fe2O3 and an inc rease of MgO, K2O and H2O. These chemical changes are related to green schist metamorphic reactions leading to the destabilisation of feldspa rs and the crystallisation of white micas and Ca-silicates. The large chemical mass transfer is associated with the circulation of a large v olume of fluids. A model of progressive fluid circulation correlated w ith Variscan prograde and retrograde metamorphism during the collision of Moldanubian and Brunovistulian units is proposed.