Abstract

Kyanite-bearing basic granulites occur in the kinzigite envelope of the Beni Bousera peridotite massif (main Sebtide nappe, Rif, Morocco). Their major and trace elements chemistry support an origin from an Al-rich tholeiitic melt. The primary mineral assemblage consists of pyrope-rich garnet (Pyr = 37-60%), jadeite-rich pyroxene (Cpx I with Jd = 20-32%), kyanite, rutile, plagioclase, quartz. Corona structures developed during retrogression, mainly with sapphirine-plagioclase symplectites after kyanite, and Cpx II-Opx-plagioclase-amphibole symplectites after garnet and Cpx I. For the high-pressure stage of metamorphism. PT conditions were around 16 kbar-20 kbar, 760-820 degrees C, and early unloading was nearly isothermal. The corresponding geotherm contrasts with the Alpine HP-LT geotherm of the Sebtide-Alpujarride nappes. By reference to the West European setting, we suggest that the Beni Bousera HP-granulites originated in a Variscan overthickened crust of the Himalayan-Tibetan type. This is supported by recent isotopic datings of armoured monazite crystals from the kinzigitic country-rocks [Montel et al., 1995].

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