A MAJOR AND TRACE-ELEMENT AND STRONTIUM, NEODYMIUM, AND OSMIUM ISOTOPIC STUDY OF A THICK PYROXENITE LAYER FROM THE BENI-BOUSERA ULTRAMAFIC COMPLEX OF NORTHERN MOROCCO

Major and trace element concentrations and strontium, neodymium, and o smium isotopic compositions were determined for a series of samples fr om a thick, symmetrically zoned pyroxenite layer from the Beni Bousera massif. The two main rock types included in the layer, garnet pyroxen ites and websterites, have distinct, well-defined elemental and isotop ic compositions. The websterites are, in most respects, more refractor y than the garnet pyroxenites, but, surprisingly, are more enriched in the highly incompatible elements. The observed characteristics can be explained by formation of the layer by crystal deposition along a mag ma conduit at medium to high pressures, given that the source and comp osition of the magma varies with time. Re-Os model ages for three samp les of different lithology converge to about 1.3 b.y., which is interp reted as the age of formation of the layer. In contrast, Sm-Nd model a ges from the layer are mutually conflicting or indeterminate, largely due to the similarity between the measured neodymium isotopic ratios a nd that of the depleted mantle. On the other hand, the Sm-Nd model age obtained from an extremely depleted peridotite (Nd-143/Nd-144 = 0.513 91) located about 30 m from the layer falls within error of the Re-Os ages, providing support for an extensive magmatic event at about that time. This same age was obtained by several techniques from the closel y related Ronda Ultramafic Complex of southern Spain. This may suggest that the mantle lithosphere currently exposed along the southern marg in of Spain and the northern margin of Morocco has been linked for ove r a billion years.