The magmatism of Atienza (NW Iberian Chain, Spain): age, origin and architecture of the magmatic plumbing system

Abstract

A sill covering an area of more than 15 km2, several dykes up to 5 m thick and a volcanoclastic deposit crop out with similar petrology in the sector of Atienza (NW Iberian Chain, Spain). They consist of calc-alkaline porphyritic andesites with phenocrysts of plagioclase, amphibole, biotite, garnet and orthopyroxene. Based on U–Pb zircon analysis, an age of 290 ± 3 Ma (Sakmarian-Artinskian in the Cisuralian) has been calculated for this magmatism. The chemistry and geothermobarometry on amphibole crystals revealed crystallisation at different depths between 31 and 16 km, involving several events of magma recharge and fractional crystallisation. Magma ascent led to destabilisation of the amphibole crystals, their replacement by biotite, and the formation of thick microcrystalline coronas. Whole-rock trace element and isotopic compositions support a strong crustal influence in the origin of the magma. Crustal melting was produced by heating generated after lithospheric thinning, delamination, and asthenospheric rise produced after the uplift of the Variscan Orogen and the oroclinal folding of the Iberian Massif.