Ar-40/Ar-39 dating of shear zones in the Variscan basement of Greater Kabylia (Algeria). Evidence of an Eo-Alpine event at 128 Ma (Hauterivian-Barremian boundary): geodynamic consequences

A ductile shearing event is recognized in gneisses and barite veins of the crystalline internal massifs of the Alpine Maghrebides in Algeria. Dating o f shearing movement is carried out by Ar-40/Ar-39 laser-probe experiments p erformed on synchronous neoformed micas. The proposed age at 128 Ma is cons trained by a two-component mixing model for the saddle-shaped age spectra o n phengites and by a plateau age at 128.3 +/- 0.3 Ma on muscovite. This Hau terivian-Barremian boundary age records a strong reworking of the Alboran p late (input of detritals in the flysch basin) and surrounding areas. It can be correlated with the J magnetic anomaly in the Atlantic and with the dis tensive events in the Pyrenees and external Alps. Therefore, isotopic ages (80-130 Ma) obtained on the Alpine-Mediterranean basement could reflect int erferences between this event at 128 Ma and late-Alpine stages. (C) 1999 El sevier Science B.V. All rights reserved.