Tectonic Auto-contamination as a Mechanism of Compositional Variations in the Beni Bousera Massif (Internal Rif, Morocco)

In Beni Bousera ultramafic massif, clinopyroxene and spinel composition show a heterogeneous variation in major elements, which could not reveal such a simple melt extraction but was controlled by secondary recrystallization. Since most of the studied clinopyroxene in spinel lherzolite appears enriched in Na-Ti-Al refers to fertile lherzolite which did not experience significant melt extraction. In contrast, they are depleted in light rare earth elements, suggesting that those rocks have suffered a high degree of partial melting. Furthermore, an enrichment in Al spinel could not be recorded as a melt extraction. These conflicting observations and the present-day composition of clinopyroxene and spinel may result from tectonic auto-contamination of the oceanic lithosphere. This is based on the following considerations: 1) The petrogenesis of garnet pyroxenite as high-pressure recrystallization of plagioclase gabbro dykes. 2) The tectonic dispersion of garnet pyroxenite within the surrounding spinel peridotites involving the recrystallization of garnet dunites with Fe-rich olivine. This has resulted in low temperatures (800 ºC, stress over 200 MPa) subsequently expanding the garnet lherzolite stability field. This tectonic dispersion should be a discrete process at high temperatures, yielding an important homogenization and re-enrichment.