The Early Aptian volcanic episode of Gutiolo (N Spain): Expression of the Bilbao Rift Fault Zone

The Gutiolo volcanic unit belongs to the Aptian Bilbao Formation and crops out 6 km SE of Bilbao (N Spain) within marine marlstones. It is 300 m thick and up to 1 km long. The volcanics consist of lava in the lower part and volcaniclastic sediments above, with metatrachytes and metatrachybasalts, similar to others of the alkali Cretaceous sequence in the Basque-Cantabrian Basin. The palaeontological age is top early Aptian (Dufrenoyia furcata Zone) near the Bedoulian-Gargasian substage transition, and the isotopic age (K-Ar) is 123 +/- 3 Ma but most probably, 122 Ma. E-W and N-S trending faults bounding the Gutiolo volcanics controlled magma eruption. Other NW- and SE-oriented major faults in the Bilbao area favoured several Valanginian to Santonian eruptions including Gutiolo. The eruptions moved progressively in space and time from SW to NE. These faults make up a 15-km-wide tectonic belt, the magma-leaky Bilbao Fault Zone. A similar magmatic fault zone NE of Bilbao, Gernika-Elgoibar, poured magma onto the sea floor during the late Albian-Cenomanian up to the Santonian, continuing the north-easterly younging trend of volcanic inceptions recognized in Bilbao. All this Cretaceous volcanism is related to the Bay of Biscay opening processes, and the Gutiolo event is correlated with the first appearance of oceanic crust in the Bay of Biscay. From stratigraphic, volcanic, geochemical, and mineralization evidence, we attribute Gutiolo to an early phase of mafic intrusion emplacement in the Bilbao area, underplated by magmatism during the Valanginian to Santonian (Cretaceous). Volcanic hydrothermal fluids associated to the Gutiolo event are considered responsible for metallic ore emplacement in the Basque-Cantabrian Basin during the early Aptian.