496 My age of plagiogranites in the Chamrousse ophiolite complex (external crystalline massifs in the French Alps)" evidence of a Lower Paleozoic oceanization

Abstract Genuine ophiolitic series are extremely rare in the Variscan orogenic belt and the Chamrousse formation appears as an exception. The main lithological and geochemical characters of the ophiolite are summarized and a special attention is paid to the field relationships and the magmatic filiation of the leucocratic rocks. The time evolution of the mafic magmas is considered to express the transition of attenuated continental lithosphere to oceanic lithosphere. The silicic rocks represents the final products from a multisequence fractional crystallization of a tholeiitic LIL-enriched magma and their compositional diversity results from the local increase of the H 2 O partial pressure during syn-accretion tectonics. Dating the plagiogranites, using theUP/b method on zircons, also give the date of the oceanization. The resulting age of 496 ± 6Ma is closely comparable (1) to the accretion age of ophiolites from peri-Atlantic Paleozoic belts, and (2) to the protolith crystallization age of the “leptyno-amphibolitic groups” (LAGs) from Western Europe. But these relics (ophiolites and LAGs) of the Cambro-Ordovician distensional event later undergo very different tectonometamorphic evolution. The distinct post-magmatic evolutions may be related to their rather distinct original geodynamic significance and to their primitive location with respect to the subsequent zonation of the Hercynian belt.