GRANITOIDS FROM THE ZONE HOUILLERE BRIANC ONNAISE (SAVOIE AND VALLE-DAOSTA, WESTERN ALPS) - GEOLOGY AND U-PB GEOCHRONOLOGY

The ''Zone Houillere Brianconnaise'' (ZHB) is the westernmost continuo us unit of the Pennine domain that overlies the Pennine Front. The ZHB comprises Namurian and Westphalian formations locally overlain by Per mian and Triassic cover rocks. As with other basements of the Pennine domain (Brianconnais id France, Grand Saint Bernard in Italy and Switz erland), the ZHB comprises gneissic units, whose age, origin and tecto nic significance have been discussed for a long time. This study deals with the Sapey gneisses, defined near Modane (Savoie), and the Costa Citrin metagranites (Valle d'Aosta, Italy). The alpine tectonic evolut ion of the ZHB involves an early thrusting event that produced a non-p enetrative foliation in the Carboniferous formations. This foliation o ccurs only in the vicinity of tectonic contacts between individual sli ces. The thrusting event is overprinted by a regional large-scale, eas tward-verging, deformation event and then by a partly extensional even t. Pre-alpine relict amphibolite-facies mineral assemblages found in t he Sapey gneisses confirm that they belong to a poly-metamorphic basem ent. Ages from U-Pb geochronology on zircons (three samples of differe nt lithology and from different regions) confirm this interpretation. Concordia diagrams evidence that zircons have a Proterozoic inheritanc e and that minimum ages are older than 360 Ma (lower intercepts with t he concordia). By contrast, two samples of the Costa Citrin metagranit e yielded well-defined Visean-Namurian ages at 324 +/- 17 MA and 323 /- 8 Ma. This old basement age (early Hercynian or older) associated w ith evidences for a Visean-Namurian magmatic event in the ZHB are cons istent with the recent reappraisal of the age of the sedimentary forma tions of the ZHB (now dated of Namurian to Westphalian). These data su pport the possible ''exotism'' of a ''Brianconnais'' terrane with resp ect to external domains of the Alps. The ages of the basement (pre-Car boniferous), of the sediments (Visean-Namurian) and of the recorded ma gmatic event (Visean-Namurian) of the ZHB cannot be related to the lat e orogenic basin formation and magmatic evolution of the Eastern Massi f Central or of the External Basement Massifs during Stephanian times. The significance of the Visean-Namurian plutonism of the Costa Citrin is discussed with respect to other parts of the european hercynian be lt. It is interpreted either to be a consequence of a late Hercynian ' 'eastern'' subduction or rather, to relate to an early extensional sta ge during the orogeny.