PRE-VARISCAN, VARISCAN AND EARLY ALPINE THERMO-TECTONIC HISTORY OF THE NORTH-EASTERN BOHEMIAN MASSIF - AN AR-40 AR-39 STUDY/

The Orlica-Snieznik and Jesenik Mountains correspond to three main dom es from west to east: the Snieznik, Keprnik and Desna domes. They are composed of a basement of autochthonous gneisses, a thick series of bl astomylonites and a supposed para-autochthonous or allochthonous metam orphic pre-Devonian to Devonian cover. Their broad direction is NNE-SS W. Ar-40-Ar-39 radiometric measurements allow three main groups of age s to be defined. (1) 300-310 Ma, represented in the Keprnik and Desna domes. This age is interpretated following the constraints on the age of the metamorphism, which is linked with the extensional process occu rring during the Westphalian. (2) 320-340 Ma, represented mainly in th e Snieznik Dome, but not in the Keprnik Massif. The nappe structure of Orlik-Vysoka hole, in the northern area of the Desna Dome, also exhib its this age, which is interpretated as reflecting the period of the m ajor Variscan Barrowian metamorphism, which accompanied the compressio nal process. It is only represented in the zones where the extensional process was not strong enough to result in a complete overprinting. ( 3) 340-440 Ma, corresponding to a very strictly defined area in the ea stern rim of the Desna Dome occupied by ultramylonites and mylonites. These ages, obtained an muscovites, result from an incomplete resettin g of the minerals developed during the cooling of a granitic protolith and mylonitized during the extensional process. A laser probe analysi s confirms the extreme inhomogeneity of the ages of the muscovites and their different resetting from one grain to another. The Late Alpine overprinting is more discrete, but can be deciphered through the low e xtraction temperatures with ages between 80 and 120 Ma. These ages can be compared with Alpine ages in the close Western Carpathians.