The Cambro-Ordovician Ollo de Sapo magmatism in the Iberian Massif and its Variscan evolution: A review

Abstract

We present here a comprehensive and updated review on the gneissic Ollo de Sapo Formation (OSF) in the Iberian Massif and discuss multiple aspects (lithostratigraphy, petrography, geochemistry, geochronology, and partial melting relationships) of its evolution through time. These aspects arise from the superposition of a multi-magmatic Cambro-Ordovician assemblage and a complex Variscan metamorphic overprint. The OSF comprises plutonic, subvolcanic, volcanic, and volcaniclastic units of fel and peraluminous compositions and calcalkaline affinity of the Upper Cambrian to the Lower Ordovician age (495–470 Ma), which crops out at the core of a 600-km-long antiform in the Ollo de Sapo Domain of the Iberian Massif and are overlain by the Armorican Quartzite. The most striking petrographic features of the OSF are the large phenocrysts of K-feldspar, plagioclase, and blue quartz, giving the name “Ollo de Sapo” (toad's eyes), and the large fraction of inherited zircons of Ediacaran or older ages. Geochemical and petrographic indicators point to a medium-P, high-T melting event of Neoproterozoic metasedimentary rocks, most probably the Schist and Greywacke Complex, and a subsequent rapid ascent of melts. Augen gneisses of mostly plutonic origin, of similar age, petrography, and geochemistry and emplaced in the same sedimentary sequences in the Iberian Massif but in lower stratigraphic levels than the classical Ollo de Sapo Domain, also exist. These plutonic augen gneisses are considered to be genetically related to the OSF. The magmatic event that produced the OSF and the equivalent augen gneisses in the Iberian Massif, as well as other augen gneisses present in the European Variscan Belts, has been supposedly related to the opening of the Rheic Ocean in the northern margin of Gondwana. The Variscan metamorphism affected the OSF and the equivalent augen gneisses to variable degrees, from low-grade to anatectic conditions, in a clockwise P-T-t path, that produced its characteristic gneissic foliation. No model for the generation of the OSF has been unanimously accepted yet because of several, and often contradictory, geodynamic scenarios suggested by the petrographic and geochemical characteristics, further complicated by current debates on the paleogeography of northern Gondwana terranes. The stepwise investigation of this complex formation allows at present a general review of its evolution through time, a review that is presented here.