Volcanic rocks from Amensif-Tnirt district in the Western High Atlas (Morocco): Geochemistry, magma features and new age dating

The High Atlas Mountains include mafic, intermediate and felsic volcanic-subvolcanic intrusive rocks, representing a result of magmatism related to “Cambrian rift” during the Lower Cambrian. The investigated magmatic rocks in the Amensif-Tnirt area are divided into three main groups as the mafic (SiO₂ 48.31 to 53.06 wt%), intermediate (SiO₂ 59.16 to 67.38 wt%), and felsic (SiO₂ 76.39 to 79.65 wt%) rocks. The mafic rocks correspond to microgabbros with high TiO₂ contents (1.98–2.12 wt%) and FeO_t/MgO ratios (1.65–1.90), La/Yb (9.09–11.60) and Nb/Zr (0.03–0.04) ratios, providing good evidence of extensive intracontinental tholeiite. The Nb/Yb and Zr/Y ratios ranging between 0.36 to 0.52 and 12.76 to 13.69, respectively suggest their derivation from an enriched mantle source during the opening of the Rheic ocean along the NW edge of Gondwana. The mafic rocks may be interpreted as the mixtures of melts derived from two contrasting mantle sources: (1) a garnet-bearing clinopyroxenitic ultramafic source and (2) an ultramafic source composed of spinel and peridotite. The intermediate and felsic rocks correspond to andesite-dacite and rhyolite with TiO₂ contents and FeO_t/MgO ratios ranging from 0.09 to 1.36 wt% and 1.32 to 4.69, respectively, suggesting their calc-alkaline affinities setting up during a compressive context. The youngest ²⁰⁶Pb/²³⁸U ages for intermediate rocks are 290.29 ± 4.68 and 291.27 ± 4.31 Ma, while the youngest felsic rocks yield a concordia age at 272.83 ± 0.73 Ma (MSWD = 0.25). Petrographic and geochemical data indicate that mafic magmatism in the Amensif-Tnirt region may be the equivalent of the first Cambrian volcanic episode in the High Atlas. Intermediate rocks are chronologically correlated to the Permian volcanic activities recorded in Central Morocco (Tiddas, Khenifra, Chougrane …) whereas felsic activity is correlated to the Permian Azegour granite from the High Atlas.