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

摘要

Major and trace element concentrations and strontium, neodymium, and osmium isotopic compositions were determined for a series of samples from a thick, symmetrically zoned pyroxenite layer from the Beni Bousera massif. The two main rock types included in the layer, garnet pyroxenites and websterites, have distinct, well-defined elemental and isotopic compositions. The websterites are, in most respects, more refractory 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 magma conduit at medium to high pressures, given that the source and composition of the magma varies with time. Re-Os model ages for three samples of different lithology converge to about 1.3 b.y., which is interpreted as the age of formation of the layer. In contrast, Sm-Nd model ages from the layer are mutually conflicting or indeterminate, largely due to the similarity between the measured neodymium isotopic ratios and that of the depleted mantle. On the other hand, the Sm-Nd model age obtained from an extremely depleted peridotite (Nd-143/Nd-144 = 0.51391) 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 closely related Ronda Ultramafic Complex of southern Spain. This may suggest that the mantle lithosphere currently exposed along the southern margin of Spain and the northern margin of Morocco has been linked for over a billion years. [References: 63]