Late Variscan magmatism in the Nevado-Filábride Complex: U-Pb geochronologic evidence for the pre-Mesozoic nature of the deepest Betic complex (SE Spain)

Abstract

Orthogneisses crop out in different lithostratigraphic positions throughout the Nevado-Filábride metamorphic Complex (Betic Cordillera, southern Spain). Their abundance offers the opportunity to establish its origin and to date the ages of acid magmatism and of early deposition ages of metasedimentary host rocks of the orthogneisses, in this complex metamorphosed and deformed during the Alpine Orogeny. Petrography and bulk-rock chemistry indicate that the orthoderivates samples are peraluminous tourmaline leucogranites and strongly peraluminous biotite gneisses. Compositional similarity in different outcrops points to common genetic relationships. Although the studied gneisses are affected by metamorphic and deformational processes their compositional variations may be explained by differing degrees of crustal melting, source heterogeneity or minor feldspar fractionation. Variation in REE contents is attributed to accessory minerals fractionation. The geochemical data show that the protoliths originated by low degree of partial melting of biotite-rich, muscovite-plagioclase-bearing metasediments. U-Pb SHRIMP dating of magmatic zircons from six samples yielded ages between ~282±5 and 295±3Ma. Their ages and chemical composition indicate that these intrusive bodies strongly resemble the peraluminous, post-tectonic, leucogranites widespread in the Variscan Orogen, particularly those produced by crustal partial melting of the Central Iberian Zone. Zircons in host pelitic metasediments provide older ages ranging from ~336 to 2.050Ma. They testify to the recycling of Early Carboniferous and older granites, which are found in the Central Iberian zones of the Iberian Massif. The new geochronological data combined with detailed field observations allow drafting a new chronostratigraphy of the Nevado-Filábride Complex. Traditionally proposed Triassic ages for the metasedimentary upper series of the complex are not consistent with the ages reported here, which suggests that the entire complex consists of Paleozoic or older rocks. This conclusion has profound implications on the validity of the tectonic subdivisions proposed so far. Metamorphic zircon rims in two gneisses and a pelitic metasediment yield Miocene (17.3±0.4Ma) ages in line with what previously proposed for the high-pressure metamorphic peak in the Nevado-Filábride Complex. The new radiometric data moreover confirm independent metamorphic and tectonic evolutions for the Nevado-Filábride and the Alpujárride complexes of the Betic Cordillera.