K-40 AR-40 DATING OF THE LATE MIOCENE TRANSITION FROM CALC-ALKALINE TO ALKALI BASALT SERIES, NORTHWESTERN ALGERIA

Neogene volcanic rocks from the northwestern coast of Algeria include calc-alkaline to shoshonitic andesites and dacites (Sahel of Oran and M'Sirda areas) and alkali basalts (Tafna valley). Seventeen new K-40-A r-40 ages indicate that these volcanics were emplaced during two disti nct periods, from 11.7 to 7.2 Ma and ca. 4 Ma, respectively. All the a ndesites and dacites were emplaced during the first period, and their trace element characteristics are typical of subduction- and/or collis ion-related magmas. They are enriched in large-ion lithophile elements and display negative Nb anomalies. The associated alkali basalts are chemically heterogeneous, and contain variable amounts of TiO2 and P2O S. Their multielement plots show either positive or slightly negative Nb anomalies. These features suggest that the Neogene volcanics studie d derive from two different types of mantle sources, i.e. an enriched subcontinental mantle and an upper mantle carrying a subduction-relate d geochemical imprint. The latter component may have inherited its spe cific signature from an earlier subduction event. This ''orogenic'' im print tends to disappear through time as the contribution of the subco ntinental mantle increases.