U-PB GEOCHRONOLOGY IN THE SAINT-BARTHELEM Y MASSIF (PYRENEES, FRANCE)- DISCUSSION ABOUT THE AGE OF VARISCAN AND PRE-VARISCAN EVENTS

The Saint Barthelemy massif represents an example of the deepest Varis can crust in the Pyrenees, separated from the axial zone by the North Pyrenean Fault (NPF). It displays a quite complete pre-Variscan sedime ntary succession, from granulitic basal gneisses, of inferred Precambr ian age, to Carboniferous flysch sediments, crosscut by a 100 m-thick mylonitic shear zone (the so-called main mylonitic band or MMB). Altho ugh the present-day kinematics of this accident is that of a thrust to ward the North, structural and petro-structural data point out that th is shear-zone is in fact a major, crustal-scale ductile normal fault, formerly dipping south before being tilted either by late-Variscan eve nts or by Pyrenean thrusting. We carried out an U-Pb isotope study on two samples from this massif, a basal gneiss and a mylonite from the M MB. Zircons from the basal gneiss define a lower intercept age of 310 +/- 25 Ma, interpreted as the age of the granulitic metamorphism. Mona zite analyses yield ages of 295 +/-5 Ma for the basal gneiss, interpre ted as a cooling age after the peak of granulitic metamorphism, and 30 0 +/- 7 Ma for the mylonite. Petrostructural data, and P-T estimates f or the syn-tectonic mineral assemblages within the MMB suggests that t he temperature of 500 +/- 50-degrees-C measured in these rocks was hig h enough to reset the U-Pb isotopic system of these monazites. Therefo re, we interpret his 300 Ma age as the age of mylonitization during du ctile normal faulting in the Saint Barthelemy massif. We relate it to the similar extensional setting documented in other regions of the Var iscan belt, like the French Massif central, during Upper Carboniferous . In addition, single zircon Pb evaporation analyses carried out on bo th samples gave some indications about the age of the pre-Variscan bas ement, with apparent ages ranging from Cambrian to Archaean.