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

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.

Timing of tectonic events in the Alpujárride Complex, Betic Cordillera, southern Spain

“…The difference in age between the two samples is small, and individual grain ages are somewhat scattered. These ages are likely to represent the results of cooling and deformation adjacent to the major detachment fault separating the Alpujárride and Nevado-Filábride Complexes, which Ar-Ar and FT data from elsewhere in the Betic Cordillera suggest was active in mid-to late Miocene time (Monié et al 1991;Johnson & Harbury 1997). The scatter in the ages may Table 3.…”

“…Isotopic dates on most Alpujárride rocks are dominated by the effects of rapid cooling in the early Miocene, which largely obscures the earlier history (Monié et al 1991;Zeck et al 1992;Monié et al 1994;Platt et al 1998Platt et al , 2003b. Little geochronological work has been published on the lowest-grade rocks of the Alpujárride Complex, yet these are the ones most likely to retain evidence for the early compressional events.…”

“…Little geochronological work has been published on the lowest-grade rocks of the Alpujárride Complex, yet these are the ones most likely to retain evidence for the early compressional events. Monié et al (1991) published an age of 25 Ma from Alpujárride phyllites, and suggested that this was the time of the high-P metamorphism. The presence of kyanite in these rocks (Monié et al 1991) suggests that they may have experienced temperatures higher than the closure temperature of the mica, however.…”

“…Monié et al (1991) published an age of 25 Ma from Alpujárride phyllites, and suggested that this was the time of the high-P metamorphism. The presence of kyanite in these rocks (Monié et al 1991) suggests that they may have experienced temperatures higher than the closure temperature of the mica, however. Here we present Ar-Ar dates from the lowest-grade Alpujárride units and relate them to their structural history, to disentangle early and late structural events.…”

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“…ago) by Bourgois et al (1972). On the other hand, Monié et al (1991) and Jabaloy et al (1993) propose on age of 25 m.a. for the end of the HP/LT event affecting the Alpujarride nappes, that is, at the end of the Oligocene or beginning of the Aquitanian, depending on the chronostratigraphic scale used.…”