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

The Orlica-Snieznik and Jesenik Mountains correspond to three main domes from west to east: the Snieznik, Keprnik and Desna domes. They are composed of a basement of autochthonous gneisses, a thick series of blastomylonites and a supposed para-autochthonous or allochthonous metamorphic pre-Devonian to Devonian cover. Their broad direction is NNE-SSW. 40Ar-39Ar radiometric measurements allow three main groups of ages 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 occurring during the Westphalian. (2) 320-340 Ma, represented mainly in the Snieznik Dome, but not in the Keprnik Massif. The nappe structure of Orlik-Vysoka hole, in the northern area of the Desna Dome, also exhibits this age, which is interpretated as reflecting the period of the major Variscan Barrowian metamorphism, which accompanied the compressional 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 eastern rim of the Desna Dome occupied by ultramylonites and mylonites. These ages, obtained an muscovites, result from an incomplete resetting of the minerals developed during the cooling of a granitic protolith and mylonitized during the extensional process. A laser probe analysis 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 extraction temperatures with ages between 80 and 120 Ma. These ages can be compared with Alpine ages in the close Western Carpathians.