Geochronology and petrochemistry of Late Cretaceous-(?)Paleogene volcanic sequences from the eastern central Patagonian Cordillera (45°-45°40'S)

Abstract

In the eastern part of the central Patagonian Cordillera (45°15-45°40S), northeast of the town of Coihaique, the chronology of volcanic sequences are documented by detailed field work and twelve new K-Ar dates that have established their Late Cretaceous-(?)Paleogene age. The oldest sequence corresponds to the rhyolitic and rhyodacitic domes of the Casa de Piedra Volcanic Complex, of Campanian age, which were emplaced on volcanic rocks of the Aptian-(?) Albian Divisadero Formation. The plagioclase and two pyroxenes andesitic to dacitic lavas of the El Toro Formation and the basalts from Morro Negro have a similar Maastritchian age. However, dacitic lavas with chemical signature similar to that of El Toro Formation crop out below the Morro Negro sequence showing that the basalts were the latest erupted lavas. Petrologic and geochemical data are used to characterize the three volcanic episodes. The Casa de Piedra volcanic rocks and the andesitic to dacitic lavas of the El Toro Formation have the mineralogy and geochemical signature of subduction-related magmas: enrichment in light rare earth elements (LREE), highly incompatible elements (Rb, Th, U and K), and strong depletion in Nb-Ta. Basalts from Morro Negro show distinct REE and multi-element patterns. Their geochemical signature is similar to that of the Murta Quaternary basalts, inferred to be related to slab window opening, but distinct from that of the Eocene Balmaceda basalts, more akin to within-plate volcanism.