GEOCHEMISTRY AND GEOCHRONOLOGY OF EARLY MESOZOIC THOLEIITES FROM CENTRAL MOROCCO

From Central Morocco (Central High Atlas, Middle Atlas, Haute Moulouya ) continental tholeiites were investigated geochemically and geochrono logically. These tholeiites are intercalated within continental redbed s of the Early Mesozoic (Triassic-Liassic). The major, trace and rare earth element contents classify these volcanic rocks as basaltic to an desitic-basaltic, quartz-normative tholeiites. Some trace element rati os (e.g. Zr/Nb, Zr/Y, Y/Nb, Ti/V) suffer a heterogeneous source with a composition similar to MORB (P- to N-type). The enriched LILE content s, the negative Nb anomaly and the initial Sr-87/Sr-86 ratios (0.7064- 0.7069) reveal the presence of a crustal component up to 13-17 wt%. Sr -87/Sr-86 ratios of carbonate mineral separates from different lava fl ows show different cycles of alteration; however, the major and trace element chemistry together with Sr isotope evidence, indicate that the alteration phases are not submarine in origin. Ar-40/Ar-39 age determ inations on translucent plagioclase phenocrysts yield extrusion ages w hich range between 210.4 +/- 2.1 Ma and 196.3 +/- 1.2 Ma. These ages c orrespond to a stratigraphic period between the Norian (Rhaetian?) and the Upper Sinemurian.