The Ollo de Sapo gneisspetrology, geochemistry, partial melting

Resumen

The Ollo de Sapo gneiss has been the object of several studies due to its petrological interest and for the implications on the tectonic evolution of Gondwana, and this contribution is a brief overview. The Ollo de Sapo Formation, which contains not only the gneiss but also tuffs, ignimbrites and metarrhyolites, has a calcalkaline affinity and a Upper Cambrian – Lower Ordovician age and formed from magmas originated from metagreywackes subducted in the active margin of Gondwana, which later ascended through diapirism to a back-arc basin undergoing a rifting process. The Formation crops out along 600 km from NW Spain to the easternmost region of the Spanish Central System and the main lithology metamorphized to an orthogneiss during the Variscan orogeny. The gneiss has a silicic and peraluminous composition, with a high REE content and a negative Eu anomaly. Melts generated from the gneiss are silicic, peraluminous and felsic, and under low-T water-rich conditions, they are depleted in REE and have a positive Eu anomaly. In the Sanabria region the gneiss underwent partial melting under water-present conditions and generated large volumes of melt, which produced migmatites and leucogranites.