U-PB ZIRCON ANALYSIS FROM THE WESTERN SIERRAS PAMPEANAS, NORTHWEST ARGENTINA: EVIDENCE FOR A COMPLEX PROTEROZOIC THROUGH SILURIAN TECTONIC HISTORY

Newly acquired U-Pb SHRIMP-zircon ages from the western Sierras Pampeanas of northwest Argentina refine existing age data on early Paleozoic deformation events and substantiate previous suggestions for the existence of a late Proterozoic basement east of the Cuyania terrane. U-Pb ages were obtained from the Sierra Pie de Palo, an area believed to constitute the basement of the allochthonous Cuyania terrane, and from the western Sierra de La Huerta, a region in the suture zone between the Cuyania and terranes to the east. The new ages extend the timing of early Paleozoic deformation and reveal a more complex tectonic history for the developement of the early Paleozoic Gondwana margin than formerly accepted.

Along the eastern margin of the Sierra Pie de Palo, a foliated sill sharing a pervasive fabric with the country rock yielded a crystallization age of 451.5±2.5 Ma. Within the upper units of the range, a potassium feldspar pegmatitic body contained within the nose of a syncline yielded an age of 431±7 Ma. Previous geochronology in the region has revealed a middle Ordovician deformation event at approximately 460 Ma. The age of the foliated sill extends this event further, at least into the late Ordovician. The early Silurian date either represents the latest stages of the Ordovician deformation or a later and separate episode of melt emplacement and deformation.

The next range to the east, the Sierra de La Huerta, contains a 30 km² orthogneiss along its western margin that yielded an age of 839±10 Ma. The orthogneiss occurs within a major N-S trending shear zone that separates high-P, lower-T metamorphic rocks on the western side from rocks to the east that have undergone low-P, high-T metamorphism. The date of 839±10 Ma is similar to K-Ar and Rb-Sr ages reported for amphibolites and gneisses respectively from the Valle Fertil, the northern continuation of the Sierra de La Huerta. These rocks with ages circa 850 Ma are unique in western Argentina. Regionally, they are bounded to the east by the Ordovician age Famatina arc and to the west by Grenville age metamorphic rocks and the Cambrian-Ordovician carbonate platform of the Cuyania terrane. The orthogneiss may constitute the basement of the Famatina arc or may be a separate allochthonous terrane.