The Montgenevre ophiolite (Hautes Alpes, France): Meta—morphism and trace-element geochemistry of the volcanic sequence

Abstract

The Montgenevre ophiolite near Briançon in the French Alps consists of pillow lavas, pegmatitic and flaser gabbros and serpentinite in tectonic contact. These rocks have been metamorphosed under prehnite—pumpellyite to greenschist facies conditions during two stages, an early sub-sea-floor metamorphism prior to emplacement followed by the Alpine metamorphism. The pillow lavas can be divided into two groups, the Chenaillet Sequence showing moderate differentiation, and the Grand Charvia Sequence with a more restricted composition. Forty-seven samples of the volcanic rocks have been analysed for Ti, Zr and Y by X-ray fluorescence, and eleven for rare-earth elements (REE) and Th, Ta, Hf and Sc by instrumental neutron activation (INAA). Although the original stratigraphic relationship of the lavas has been obscured by emplacement tectonics, geochemical modelling suggests that they can all be related by up to 60% fractionation of olivine, plagioclase and clinopyroxene. Their chemical composition and phenocryst assemblages are similar to modern ocean-floor basalts.