THE DIVERSITY OF DUNITES

Abstract

The Wadi Tayin massif in the Oman ophiolite which is representative of a fast spreading situation offers the thickest mantle section (up to 15km, exposed on 1250 km2). It has been the site of the most extended petrostructural investments. In this massif, the size, structure and distribution of dunites is far from being uniform. The basal section (~3km thick) affected by the pre-detachment solid state flow contains large dunites (1 to 40%, up to 50m thick), which are flat-lying and concordant with respect to the peridotite low-T foliation; they constitute the ‘banded unit’. The main section (up to 12km thick) contains large dunites, few tens of meters thick (like the Batin dunite, Fig.1) which are also flat-lying but discordant with respect to the peridotite high-T foliation. There are also thin dunites (1 to 50cm thick), which are either concordant and parallel to the banding, or discordant and often expressed as reaction zones along gabbro or pyroxenite dykes or chromite schlierens (maximum 1m thick). The thick Moho transition zone (~500 m in the western part of the massif) is formed by large concordant flat dunites, interlayered dominantly with depleted harzburgites downsection and with gabbro sills upsection. In the main section, as shown in the Batin area by detailed mapping, the thick dunites are not transposed parallel to the high-T foliation of the enclosing harzburgites. They have retained their flat attitude and initial thickness. These large flat dunites are interpreted, based on trace elements geochemistry, as resulting from melt-rock reaction at high melt/rock ratios at the asthenosphere/lithosphere thermal boundary. The same interpretation is proposed for the dunites set at the base of the mantle section. Associated or not with reactive mafic dikes, only thin discordant dunite veins and concordant bands could represent traces of late and early melt extraction channeling, respectively. These structural constraints are discussed in reference to previous melt extraction models. In the Lanzo massif of the Piemont Alps, dunite lenses as well as gabbro dikes, both reactive and intrusive, are concentrated in a western domain represented by the southwestern body and the western margin of the central body. They lie presumably close below the overlying oceanic crust represented by the metamorphic serpentinites and mafic rocks of the Viu zone. One group of dunite lenses are subconcordant and associated with olivine gabbro dykes, thus coeval of high-T mantle flow; the second group located in the Monte Arpone area are discordant and associated with parallel ferrogabbro dikes. Whatever the melt extraction model leaving residual dunite lenses, this process is controlled by a thermal boundary, and dunites do not image a steady state process of melt extraction from the mantle.