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

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: similar to 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 K2O (0.7-5.3 wt.%), Nb (8-19 ppm) contents and higher Sr-87/Sr-86 (0.70773-0.71016) than the shoshonitic series (K2O: 2.4-7.2 wt.%, Nb: 21-38 ppm, Sr-87/Sr-86: 0.70404-0.70778). Pliocene alkaline basalts have a sodic tendency (Na2O/K2O: 1.7-3.5), high Nb content (up to 52 ppm), and low 87Sr/86Sr ratio (0.70360-0.70413). The variations through time of K2O, Nb and Sr isotopic ratio reflect different mantle sources: (i) calc-alkaline, potassic calc-all;aline 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 87Sr/86Sr 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. (C) 1998 Elsevier Science B.V. All rights reserved.