40AR/39AR DATING AND GEOCHEMISTRY OF THOLEIITIC MAGMATISM RELATED TO THE EARLY OPENING OF THE CENTRAL ATLANTIC RIFT

Tholeiitic effusive and intrusive magmatism from Iberia, Morocco, Algeria and Mali, related to the early opening of the Central Atlantic rift, was dated by the 40Ar/39Ar step-heating method. Four plateau ages, ranging from 203.7 +/- 2.7 to 197.1 +/- 1.8 Ma, were obtained on plagioclase from dykes from the Taoudenni area (Mali) and two lava flows from Morocco. The Messejana dyke (Iberia), which previously yielded discrepant conventional K-Ar dates, did not furnish any 40Ar/39Ar plateau dates. However, there is a clear inverse relationship between apparent age and K/Ca ratio for gas fractions from a plagioclase separate (proportional to the alteration degree) which, combined with dates obtained on amphibole, biotite and pyroxene, allows us to determine an age of around 200 Ma for this body. These data, and those obtained on the Foum Zguid (Morocco) and the Ksi Ksou (Algeria) dykes, give evidence of a brief magmatic event (between 206 and 195 Ma ago) which affected a large area ca. 2500 km long. Trace-element modelling shows that most of these formations originated from a homogeneous, enriched, source material. Such a brief magmatic episode related to the opening of a continental rift is in agreement with findings in other magmatic provinces (e.g. the Deccan traps and the Red Sea rift, precisely dated by the 40Ar/39Ar method as well).