Revised isotopic ( 40Ar/ 39Ar) age for the lamproite volcano of Cabezos Negros, Fortuna Basin (Eastern Betics, SE Spain)

Abstract

A new isotopic age of 7.71 ± 0.11 Ma (1 σ) has been obtained with the 40Ar/ 39Ar incremental heating technique on phlogopites from the Miocene volcanic rocks (lamproites) of the Fortuna Basin in southeast Spain. This age is significantly older than earlier reported K/Ar ages, but confirms recent magnetostratigraphic ages on the sedimentary sequences of the eastern Betics. Because the volcanic rocks are intercalated in the stratigraphic sequence of the Fortuna Basin directly above the transition from evaporitic to continental sediments, it implies that the Fortuna evaporites are older than 7.71 Ma, and thus of Tortonian age. In addition, it shows that the volcanism in the Fortuna Basin must be attributed to a late Tortonian deformation phase and not to Messinian tectonics. Consequently, these results form an important step towards a consistent chronological framework for the sedimentary, tectonic and paleogeographic evolution of the Fortuna Basin during the late Miocene.