GEOLOGY, GEOCHEMISTRY AND K-40 AR-40 GEOCHRONOMETRY OF MIOCENE MAGMATISM IN ALGIERS AREA, NORTHERN ALGERIA

Miocene magmatic rocks outcrop within a narrow coastal strip east of A lgiers. They include basaltic and andesitic lava flows and intrusions (Dellys, Cap Djinet), the Thenia granodioritic plug and the dacitic to rhyolitic lavas and pyroclastic flows from Zemmouri El Bahri and El K erma. Despite the effects of hydrothermal alteration, K-40-Ar-40 ages coupled with micropalaeontological data lead to recognition of two emp lacement events at 16-15 and 14-12 Ma, respectively. All the studied c alc-alkaline to potassic calc-alkaline rocks are enriched in highly in compatible elements and display negative Nb anomalies. Acid magmas hav e a pronounced crustal imprint (Sr-87/Sr-86(i) = 0.7082 to 0.7155; del ta(18)O = +9 to +13 parts per thousand) which together with La/Nb rati os argues for the occurrence of upper crustal contamination processes. However, the Nb-depletion of the associated basalts suggest that the studied magmas derive from a mantle source which underwent subduction- related metasomatic enrichments prior to their Miocene emplacement.