El Cortijo Formation: an OPS (ocean plate stratigraphy) type sequence in the Palaeoproterozoic basement of the Tandilia System. El Cortijo Formation is exposed near the Tandil city, within an isoclinal fold and vertical fault system, with an E-W general orientation. It is constituted by an intercalation of low metamorphic grade metabasites, metacherts and metawackes. Structural, petrological and geochemical characteristics of the aforementioned rocks could be interpreted as an OPS (ocean plate stratigraphy) type sequence. This new interpretation of El Cortijo Formation genesis provides reliable evidences to the collisional theory of Tandilia System basement evolution
