GEOCHEMISTRY OF THE TRIASSIC-JURASSIC ALPINE CONTINENTAL DEPOSITS - ORIGIN AND GEODYNAMIC IMPLICATIONS

Mid-Triassic to mid-Jurassic Alpine continental deposits are known all along the former Brianconnais peninsula. They constitute small karsti c pockets on the thick Triassic calcareous series and their chemistry evolves between bauxites s.s. and aluminous argillites. Most of them w ere deeply buried during the Alpine orogenesis as recorded by HP-LT me tamorphism. Only the deposits of the Prealps were submitted to lower P T conditions (diagenesis-anchizone boundary) during their incorporatio n in the thrust wedge of the ''Prealpes Medianes''. These formations a re known for containing traces of light elements (Li, F) and heavy ele ments (Zn, REE...). In order to understand the possible origin of thes e elements, we studied the geochemistry (major and trace elements) of two representative deposits, one in Vanoise which underwent a HP-LT me tamorphism, the other one in the Prealps, which was only submitted to diagenesis. Trace elements patterns allow us to preclude an autochthon ous origin for these formations as well as the intervention of metasom atism, and demonstrate a granitic origin. Moreover, discrimination dia grams for granites indicate an obvious alkaline granitic origin for th ese deposits. In the framework of the Alpine palaeogeography, we then discuss the possible granitic sources. Two main sources can be invoked : either a Brianconnais s.s. formation (crystallines or sediments), wh ich supposes a more intense erosion as classically admitted, or more d istant sources such as the Corso-Sardinian alkaline acid-rocks, which supposes a complex palaeohydrography. This confirms the sedimentary or igin of the light elements in these rocks and precludes the interventi on of light elements-rich hydrothermal fluids migrating through Alpine metamorphic units.