Géologie et métamorphisme du complexe ophiolitique de la Haute-Ubaye (Basses-Alpes, France)

Abstract

The area studied is located in the Haute-Ubaye Valley (Basses-Alpes, France). A characteristic ophiolitic sequence with associated sedimentary rocks (Chabriere series) is described. From bottom to top we find: 1. metadolerites; 2. a complex formation consisting mainly of brecciated pillow lavas partially reworked by younger lava (Facies de passage); 3. a thick mass of pillow lava grading towards the top into pillow breccias; 4. radiolarian cherts; 5. marmorean limestones; 6. Formation de la Replatte. The last formation is made up of an alternation of thin siliceous limestone and clayey slaty beds. The radiolarian cherts and the marmorean limestones are not always present and may be proxied for by volcano-sedimentary breccias. Serpentinites occur within the sequence in a position which cannot be clearly defined. Two distinctly different types of ophicalcites are associated with these ultramafics. Indirect evidence indicates an Upper Jurassic to Lower Cretaceous age for the ophiolitic activity and the deposition of the associated sediments. Similar sequences are known to exist in a number of other ophiolitic bodies in the Piemont zone. It is not possible to specify whether there is a tectonic or stratigraphic relation between the sequence just described and the calcareous schists (Schistes lustres) of the Piemont zone. The rocks of the ophiolite suite are typical of the Steinmann trilogy. Metadolerites and metabasalts are by far the most abundant rocks. They form dikes, sills, pillow lavas and several types of volcanic breccias. Coarse metagabbros occur only as blocks in a volcano-sedimentary breccia. Serpentinites are mainly derived from harzburgites, more rarely from dunites or lherzolites. Some antigorite develops during the principal phase of alpine metamorphism. The contact between serpentinites and wall-rock is characterized by reaction rims of rodingitic affinity with development of calc-silicates (diopside, tremolite, pumpellyite, lawsonite) and of chlorite. Metaalbitites, metadolerites and sedimentary rocks are found as inclusions in serpentinites. The margins of these inclusions were transformed during three successive stages. During a first stage calc-silicates, principally lawsonite and pumellyite, crystallized. During the second and third stages, a chloritic zone of varying composition developed towards the edge. It is suggested that the ophiolites with the associated sediments belong to a different paleographic area from that of the Schistes lustres. The Chabriere series may have deposited in a more or less extended oceanic basin which was formed during a sea- floor-spreading phase of Jurassic and lower Cretaceous age. The alpine metamorphism had a plurifacial character in the region under study. The following age sequence can be postulated: 1. Blueschist facies. In some instances two different assemblages can be observed: a) jadeitic pyroxene (chloromelanite of jadeite) - lawsonite. b) glaucophane — lawsonite and/or pumpellyite. 2. Greenschist facies as a result of retrograde metamorphism. Consideration of variations of fluids pressure and especially of fluids composition is essential to the understanding of the irregular distribution of the different blueschist assemblages. Total pressures between 5,5 and 9,5 kb are necessary to form the assemblages 1a and lb. The ingenious model of subduction may be a probable explanation of the extreme P-T conditions.