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Laboratoire de Pétrologie Physique, IPGP-Univ. P7, 2 Place Jussieu F-75251, Paris cedex 05, France
1 University of Nantes, 2 Rue de la Houssiniére, F-44072, Nantes cedex, France
From studies of ophiolites, it is thought that plagioclase-peridotites represent a non-negligible constituent of the oceanic lithosphere that reflects magmatism at oceanic ridges. To date, very few of them have been described from the abundant peridotites recovered from oceanic domains at present day oceanic ridges, except from transform faults. However, some plagioclase-lherzolites, interpreted as cumulate rocks, were drilled at Site 334 of DSDP Leg 37 on the Mid-Atlantic Ridge.
These plagioclase-lherzolites display poikilitic textures with oikocrysts of clinopyroxene and interstitial plagioclase that include xenocrysts of strained and partly annealed olivine crystals and subidiomorphic spinels. Petrofabric data for olivine point to a plastic deformation stage for these rocks as they display weak lattice fabrics which may have resulted from high-temperature plastic flow. Mineral compositions bear a strong resemblance to ophiolitic and oceanic rocks interpreted as residues from partial melting, except for clinopyroxenes which show a slight titanium enrichment. It is considered that these plagioclase-lherzolites represent residual harzburgites which have been pervasively impregnated by a melt from which clinopyroxene and plagioclase crystallized. Due to this impregnation episode, the primary structure and chemical composition of olivines were partly modified. It is suggested that these peridotites were locally so sufficiently lubricated by interstitial melt that they behaved as a crystal mush that had intruded the high-level crustal gabbros of the Mid-Atlantic Ridge. Their formation reflects pervasive magma percolation in the upper-most residual upper mantle at the Mid-Atlantic Ridge axis.
