Petrology and K-Ar chronology of the Neogene-Quaternary Middle Atlas basaltic province, Morocco

Abstract

AbstractThe Middle Atlas basaltic province is the largest and youngest volcanic field in Morocco. A hundred well-preserved strombolian cones and maars emitted numerous mafic lava flows, which cover a surface of ca. 960 km2. The study of 103 samples from these volcanic units allows us to distinguish four lava types, to map their distribution, to provide new K-Ar ages and to discuss their petrogenesis on the basis of their petrologic and geochemical (major and trace element) features. Nephelinites represent only 1.2% of the total surface of volcanic units; they are scattered all over it as small monogenic volcanoes built during the Middle and Late Miocene (16.25-5.87 Ma) and the Plio-Quaternary (3.92-0.67 Ma). The three other types are exclusively Plio-Quaternary (3.77-0.60 Ma). Basanites cover 22.5% of the volcanic field area, and generally overlie the more widespread alkali basaltic flows (68.5% of the plateau surface). Finally, subalkaline basalts form the El Koudiate cone and associated flows (7.8% of the surface of the volcanic units) and their origin is ascribed to contamination of alkali basaltic magmas by the upper continental crust. We show that nephelinites, basanites and alkali basalts derive from temporally increasing but small degrees of partial melting of variably enriched spinel– and garnet-bearing peridotites from the base of the North-African lithospheric mantle, at depths of ca. 70 km. Partial melting was caused by an asthenospheric uprise, which induced dehydration melting of parga-site-bearing enriched peridotites, metasomatised during an earlier Cenozoic plume-related magmatic event.