TECTONOSTRATIGRAPHIC RELATIONSHIPS AND STRUCTURAL EVOLUTION OF THE WESTERN MARGIN OF THE SIERRA DE PIE DE PALO, CUYANIA (PRECORDILLERA) TERRANE, ARGENTINA

The Sierra de Pie de Palo comprises metaigneous rocks of the oceanic Pie de Palo Complex (PdPC) and metasedimentary rocks of the Caucete Group. The two units were structurally amalgamated during a collision between the Laurentian Cuyania and Gondwanan Pampia terranes. A pegmatitic gabbro near the contact between the serpentinites and gabbro within de PdPC yield a U-Pb (TIMS) age of c. 1.2 Ga, and crosscutting trondhjemite and tonalite dikes and sills have yielded ages between 1174 and 1150 Ma (U-Pb TIMS & Shrimp). These ages combined with intrusive relationships and geochemistry suggest that these rocks represent a Mesoproterozoic fragment of suprasubduction zone oceanic crust that became the foundation to a more evolved magmatic arc. The Caucete Group comprises 3 major units: 1) El Quemado Formation (Fm), containing reddish yellow and green feldspathic arenite and coarse, dark greywacke, with igneous (volcanic?) detritus; 2) Puntilla Blanca Fm, composed of red to white dolomite and a poorly layered, talc-bearing red marl; and 3) the Angaco Fm, composed of white to yellowish marble and marl. The latter two formations can be correlated with basal Cambrian units of the eastern Precordillera stratigraphic succession (e.g. Cerro Totora Fm). The origin of the El Quemado Fm is enigmatic and correlation with the unmetamorphosed Precordillera stratigraphy is difficult. First, its relationships with both the structurally overlying PdPC and underlying calcareous rocks are tectonic (contacts marked by flaggy mylonites) and second, ages of detrital zircons comprise two major populations that fall between c.1-1.2 Ga and c.0.67 Ga, which are compatible with either a Gondwanan or Laurentian provenance. Structural relationships suggest that the El Quemado Fm was first imbricated with the PdPC and subsequently both were emplaced westwards on top of the calcareous Caucete Group rocks. Early ductile thrusting was followed by large scale, west-verging recumbent sheath folds, out-of sequence thrusting and open E-W trending cross-folds. Metamorphic zircon rims dated at c. 455±10Ma (Shrimp) and significant discordance in the U-Pb system with lower intercepts between c. 460 and 490 Ma indicate a penetrative Ordovician tectonometamorphic event (thrusting and recumbent folding), which probably is due to collision.