The alkaline intraplate Pliocene-Quaternary lavas from the Middle Atlas Volcanic Field (Morocco): Petrology, mineralogy and geochemistry

Abstract

The Middle Atlas Volcanic Field (MAVF) covers an area of 1500 km2, with a total erupted volume of solid products (DRE) estimated at more than 80 km3. In this study we present and discuss new petrological, mineralogical, and geochemical data of mafic rocks from the Timahdite and Lechmine N'kettane maars and of the Hebri, Habri (H1) and Boutagarouine scoria cones. For these volcanic areas we report the exclusive presence of alkaline and sodic undersaturated basaltic rocks (nephelinites, alkaline basalts and basanites). The studied lavas exhibit compositions typical of mantle-derived melts that underwent weak magmatic evolution processes (high Mg# = 56–68; Ni = 140–420 ppm; Cr = 150–500 ppm). Olivine and diopside are the main fractionation phases. Elemental data and Sr and Nd isotopes indicate a relatively homogeneous source region for the three lithotypes, which were generated in the presence of garnet as residual phase ([Tb/Yb]N = 2.4 to 3.0) through variable degrees of partial melting, decreasing from alkaline basalts to basanites and nephelinites. Magma genesis in equilibrium with garnet suggest that the sources were placed at depths in excess of 82 km, i.e., at asthenospheric depths in a region where the lithosphere has an estimated depth of about 77 km. The present-day Sr and Nd isotope signatures (87Sr/86Sr = 0.7031–0.7036; εNd = +2.5 to +5.5) of the lavas indicate a similarity with the Canary Islands lavas and, identically, the contribution of the HIMU and EM1 to the mantle sources. The sub-chondritic Ti/Eu ratios (down to 4471) and their negative correlation with Ca/Sr, Sr/Sm, Ga/Nb suggest carbonatitic metasomatism fingerprints, which is here considered the responsible for the introduction of amphibole in the lithosphere base. We propose that this carbonatite metasomatism was originated from the earliest low-degree melts, issued from the adiabatic decompression of the ascending mantle upwelling. This created low-solidus lithospheric domains which later interacted with the ascending silicate magmas. The fact that this mantle upwelling is rooted, like the Canary plume, in the same mantle anomaly at the top of the lower mantle explains the similar elemental and isotope characteristics between the studied MAFV lavas and those observed in Canary Islands.