40 Ar/ 39 Ar geochronology of Alpine tectonism in the Betic Cordilleras (southern Spain)

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Abstract
The 40 Ar/ 39 Ar dating method has been applied to metamorphic rocks of the Alpujarride and Nevado-Filabride nappes (Alboran domain, SE Spain) in a first attempt to discriminate individual phases of deformation and metamorphism. The upper Nevado-Filabride nappes experienced an early eclogitic and blueschist metamorphism for which a barroisitic amphibole indicates a minimum age of 48 Ma. An Early Miocene age is attributed to the subsequent amphibolite facies metamorphism. Deformation associated with this metamorphic evolution is of unknown direction. The Alpujarride nappes record a plurifacial metamorphic evolution with the superimposition of low-pressure assemblages upon high-pressure ones with variable P – T ratios. Phengite from a carpholite-bearing high pressure-low temperature ( HP / LT ) assemblage gives an age of 25 Ma interpreted to reflect the end of the high-pressure evolution. Biotite and muscovite from high-grade metamorphic rocks overprinted under low-pressure conditions yield similar closure ages of 19 Ma dating cooling after the main episode of E – NE directed ductile deformation. This deformation was followed by W–SW directed extensional events producing brittle structures in the Alpujarride nappes and ductile-brittle shearing in the Nevado-Filabride nappes. Biotite and muscovite from ductile sheared rocks in the detachment zone between the two complexes have concordant ages of 16–17 Ma related to the end of the extensional ductile deformation. Therefore, a correlation of metamorphic and tectonic events between the two nappe complexes seems possible only since the Early Miocene and later.