LITHOSPHERE | Volume 9 | Number 3 | www.gsapubs.org 441 RESEARCH Discerning Permian orogenic metamorphism from other tectonothermal events (Mesoproterozoic to Alpine, contact to orogenic or extensional) in the concealed basement of the Basque-Cantabrian Basin (northern Spain) Pablo Puelles 1 , Luis M. Agirrezabala 2 , Fernando Sarrionandia 1 , Benito Ábalos 1, *, Manuel Carracedo-Sánchez 3 , and José I. Gil-Ibarguchi 3 1 DEPARTMENT OF GEODYNAMICS, UNIVERSITY OF THE BASQUE COUNTRY (UNIVERSIDAD DEL PAÍS VASCO/EUSKAL HERRIKO UNIBERTSITATEA, UPV/EHU), P.O. BOX 644, E-48080 BILBAO, SPAIN 2 DEPARTMENT OF STRATIGRAPHY AND PALEONTOLOGY, UNIVERSITY OF THE BASQUE COUNTRY (UNIVERSIDAD DEL PAÍS VASCO/EUSKAL HERRIKO UNIBERTSITATEA, UPV/EHU), P.O. BOX 644, E-48080 BILBAO, SPAIN 3 DEPARTMENT OF MINERALOGY AND PETROLOGY, UNIVERSITY OF THE BASQUE COUNTRY (UNIVERSIDAD DEL PAÍS VASCO/EUSKAL HERRIKO UNIBERTSITATEA, UPV/EHU), P.O. BOX 644, E-48080 BILBAO, SPAIN ABSTRACT Recently discovered Albian submarine diatremes sample tectonites from the Basque-Cantabrian Basin concealed basement. Garnet-sillimanite gneiss xenoliths provide petrologic, petrofabric, and radiometric evidence on a so-far unidentifed early Permian (ca. 280–275 Ma) high- grade regional metamorphism. It is interpreted as a syntectonic event recorded at middle to lower crustal settings and related to the Variscan orogeny. This metamorphism is 25–30 m.y. younger than the Variscan high-grade regional metamorphism preserved in the axial zone of the Pyrenees and 15–30 m.y. younger than low-pressure plutonometamorphism documented in the core of the Ibero-Armorican arc. By contrast, it is 15–20 m.y. older than Variscan regional and contact metamorphism unraveled in basement outcrops around the Basque-Cantabrian Basin; it can be thoroughly discriminated from Proterozoic high-grade metamorphism preserved in lower crustal segments of the submerged north Iberian continental margin, and from low-pressure metamorphism related to mid-Cretaceous hyperextension predating the Alpine orogeny. The early Permian metamorphism reveals protracted and diachronic Variscan tectonomagmatic activity in the core of the Ibero- Armorican arc due to orocline buckling processes triggering lithosphere delamination, asthenosphere upwelling, and eventually heat and mass transfer through the continental crust. LITHOSPHERE; v. 9; no. 3; p. 441–452; GSA Data Repository Item 2017113 | Published online 20 March 2017 doi:10.1130/L619.1 INTRODUCTION The Pyrenees and their prolongation in the northern edge of the Iberian microplate (onshore and offshore) record a protracted geological history including various tectonometamorphic events. The most outstanding of them are those related to the Variscan (late Paleozoic) and Alpine (Paleo- gene) orogenies. These included various phases of regional deforma- tion, orogenic and contact metamorphism, and magmatism (Choukroune, 1992; Barnolas et al., 1996; Ábalos et al., 2002; Capote et al., 2002). The Alpine orogeny was preceded by a Cretaceous lithospheric hyperexten- sion process that also associated specifc deformation and metamorphism (Lagabrielle and Bodinier, 2008; Jammes et al., 2009). Nevertheless, the geological history of the northern Iberian continental crust can be traced back to the Archean, in order to include Neoarchean and Paleoprotero- zoic magmatism (Guerrot et al., 1989), Mesoproterozoic exhumation of metamorphic rocks (unraveled in lower crustal exposures from the sub- merged continental margin; Gardien et al., 2000), and Neoproterozoic peripheral orogen tectonics (recorded in the onshore basement outcrops; e.g., Ábalos et al., 2002; Díaz-García, 2006; Casas et al., 2015). Regional tectonic syntheses of the Variscan orogeny relate the basement of the Basque-Cantabrian Basin (BCB; Barnolas and Pujalte, 2004; Ábalos, 2016) to two major tectonic domains: the core of the Ibero-Armorican arc to the west (e.g., Weil et al., 2013; Murphy et al., 2016, and references therein) and the Pyrenean axial zone to the east (Barnolas et al., 1996; Car- reras and Druguet, 2014). The outcrops of these domains are separated by the Mesozoic–Cenozoic sedimentary infll of the BCB, which geologically constitutes the western prolongation of the Alpine Pyrenees. The nature of the BCB concealed basement has remained essentially unknown to date. In this article we report the occurrence of high-grade tectonites in the concealed basement of the BCB, sampled by submarine Albian dia- tremes. They bear fabrics (defned by a high-grade mineral assemblage) that resulted from orogenic metamorphism and contain neoformed metamorphic zircon grains and overgrowths. The relationships between © 2017 Geological Society of America | For permission to copy, contact editing@geosociety.org Benito Abalos http://orcid.org/0000-0003-4690-6468 *Corresponding author: benito.abalos@ehu.es Downloaded from https://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/9/3/441/999140/441.pdf by guest on 23 May 2020