METASOMATIC INTERACTIONS IN THE LITHOSPHERIC MANTLE - PETROLOGIC EVIDENCE FROM THE LHERZ MASSIF, FRENCH PYRENEES

To better understand the processes of metasomatism in the lithospheric mantle, we have undertaken a detailed petrologic study of wallrock ad jacent to late stage mafic dikes at the Lherz massif. Traverse I conta ins a garnet-pyroxenite dike, along with homblendite dikelets which cu t harzburgite. Traverses II and III contain an anhydrous garnet-pyroxe nite dike and a hornblendite dike in Iherzolite, respectively, There i s evidence for three successive metasomatic events in these samples: ( 1) crystallisation of Al-rich grain-boundary spinels +/- associated ph ases from a silicate melt, (2) formation of apatite related to carbona tite metasomatism, (3) Fe-Ti metasomatism related to the intrusion of garnet-pyroxenite dikes and hornblendite dikes. The first two events w ere diffuse in character, and although they penetrated a relatively la rge volume of peridotite, interaction with the peridotite was not perv asive, The result was the development of localised chemical gradients at the mm scale, The carbonatite metasomatism caused a net oxidation a nd chemical enrichment of the peridotite, Metasomatic effects related to the intrusion of the garnet-pyroxenite and hornblendite dikes are s patially restricted. Primary and secondary spinel and clinopyroxene co mpositions indicate that melt migrating from the hornblendite dikes wa s mostly responsible for pervasive metasomatism at the scale of < 5 cm . Further distant, up to similar to 15 cm, pervasive overprinting give s way to a zone where variable degrees of interaction have occurred. L ittle to no metasomatic interaction is observed adjacent to a thick an hydrous garnet-pyroxenite dike. Successive overprinting of one metasom atic event by another has led to the production of enriched peridotite s with complex geochemical signatures. The variable extent of melt-per idotite interaction can lead to geochemical heterogeneities at a varie ty of length scales. Textural relations reveal little, if any, differe nce in the mechanism of infiltration of a silicate or carbonatite melt through harzburgite or Iherzolite.