Role of the Ollo de Sapo massive felsic volcanism of NW Iberia in the Early Ordovician dynamics of northern Gondwana

The Ollo de Sapo Fm. in the Sanabria region of NW Iberia consists of subvolcanic, volcanic and volcano-sedimentary rocks underlying and partly intercalated with Early Ordovician, platform-facies sediments. Granitic plutons of the same age, presently transformed into orthogneisses, are genetically related. The metavolcanic rocks are dacites and rhyolites with potassic and peraluminous character, and volcanic arc signature, possibly inherited from the melting of calc-alkaline crustal material. In the study area, the Ollo de Sapo Fm. is made up of two large superposed volcanic domes separated by a thin screen of contemporaneous metapelites and sandstones. The domes underwent endogenous growth with subvolcanic, phenocryst-rich magma intruding and cooling under a volcanic and volcaniclastic shield. The western dome tapers to the east and underlies the younger eastern dome, suggesting that the whole formation consists of several volcanic edifices replacing each other along the 570 km outcrop belt of the Ollo de Sapo Fm.Two new U–Pb protolith ages of 488 ± 6 Ma and 472 ± 12 Ma match, within error, the known age span for the Ollo de Sapo Fm. in Iberia (495–483 Ma). The younger age dates an augen gneiss at the core of the eastern volcanic dome, whereas the older date comes from the overlying volcanic sequence, suggesting intrusion of the augen gneiss protolith into the overlying volcanic rocks.An incipient rift involving necking of crustal-scale boudinage a few hundred km landward of the edge, but still within the wide passive margin of northern Gondwana may explain the linear distribution of the Ollo de Sapo Fm. and the limited time span of massive volcanism. The heat for crustal melting was probably supplied by mafic magmas intruding or underplatting the lower crust. The Early Ordovician felsic magmatism of NW and Central Iberia seems to form part of a large siliceous province linked to break-up and terrane dispersion of the northern Gondwana realm.