Volcanological evolution of the maar of Lechmine n’Aït el Haj (Middle Atlas, Morroco)

Abstract:

Monogenetic volcanoes are the most common type of subaerial volcanoes. The phreatomagmatic type results from the interaction between the ascending magma and superficial water leading to the formation of a maar. The Lechmine n’Aït el Haj maar (LNH) located in the Middle Atlas, Morocco is an excellent example of this type of volcanos.

This work aims to investigate the volcanic mechanism of the LNH based on tephrostratigraphy, in order to improve the knowledge about the evolution of tephra ring around LNH. The geochemical, mineralogical, and petrographic features constrain the origin and type of magma.

The LNH environment is characterized by an outcrop of lacustrine deposits at the base of the pyroclastic deposits. Based on stratification criteria and deposition, transport, and fragmentation mechanisms, five main units were distinguished. The first three units (U1, U2, U3) consist of beds of lapilli tuff, composed of juvenile clasts and accidental lithics and a small proportion of xenoliths. The distinction between the three units is based on the observation of the distribution, the size, the shape, and the proportion of accidental lithics and juvenile clasts. This variation depends on the intensity of the explosion, the water/magma ratio, and the location of the eruptive vent. The fourth Unit (U4) consists of massive breccia tuffs. It contains scoria, lava blocks, and volcanic bombs. The upper and last unit (U5) is represented by a thin level of lapilli tuff, located mostly in the SE flank.

The tephrostratigraphic study allowed to reconstructing the evolution of the maar, which were phreatomagmatic mostly, this is attested by the deposition of U1, U2, and U3. the water depletion causes a changing in the eruptive style to a strombolian one attested by the fourth Unit. A second transition occurs at the end of the LNH activity when the ascending magma meets karstic water again and provoked the final phreatomagmatic explosion.

The mafic lavas of LNH are basanites. They derived from a primary magma that is near the primary. The phenocrysts of LNH lavas consist of olivine and diopside. The enrichment of incompatible elements (Ni, Cr) suggests a low degree of partial melting (2%) of a mantle source located at 65 km deep (2 GPa). This depth corresponds to the lithosphere-asthenosphere boundary under the Middle Atlas (60-80 km).