Post-collision neogene volcanism of the Eastern Rif (Morocco): magmatic evolution through time

Abstract

Neogene volcanism in the Eastern Rif (Morocco) comprises a series of calc-alkaline, potassic calc-alkaline, shoshonitic and alkaline volcanic rocks. According to new stratigraphical, along with new and previous chronological and geochemical data, the orogenic volcanism was successively (1) calc-alkaline (basaltic andesites and andesites: 13.1 to 12.5 Ma, rhyolites: 9.8 Ma), (2) K-calc-alkaline (basaltic andesitic to rhyolitic lavas and granodiorites: 9.0 to 6.6 Ma), and (3) shoshonitic (absarokites, shoshonites, latites, trachytes: ∼7.0 to 5.4 Ma). The later Pliocene volcanism was basaltic and alkaline (5.6 to 1.5 Ma). The calc-alkaline and K-calc-alkaline series exhibit lower K 2O (0.7–5.3 wt.%), Nb (8–19 ppm) contents and higher 87 Sr/ 86 Sr (0.70773–0.71016) than the shoshonitic series (K 2O: 2.4–7.2 wt.%, Nb: 21–38 ppm, 87 Sr/ 86 Sr : 0.70404–0.70778). Pliocene alkaline basalts have a sodic tendency (Na 2O/K 2O: 1.7–3.5), high Nb content (up to 52 ppm), and low 87 Sr/ 86 Sr ratio (0.70360–0.70413). The variations through time of K 2O, Nb and Sr isotopic ratio reflect different mantle sources: (i) calc-alkaline, potassic calc-alkaline and shoshonitic series are derived from a mantle source modified by older subduction, (ii) alkaline basalts are derived mainly from an enriched mantle source. Through time, incompatible elements such as Nb increased while 87 Sr/ 86 Sr decreased, suggesting a decreasing influence of metasomatized mantle (inherited subduction). Such evolution is related to the post-collision regimes operating in this area, and could be linked to the succession of extensional, compressional and strike-slip fault tectonics.