Cenozoic tholeiitic volcanism in the Colbún area, Linares Precordillera, central Chile (35º35'-36ºS)

Abstract

Upper Eocene to Middle Miocene volcanic rocks are found in the Colbún area, Linares Precordillera, central Chile (35º35'-36ºS). Based on stratigraphic characteristics, K-Ar and Ar-Ar dating, two different units are recognized: 1-a Lower Unit (Upper Eocene to Upper Oligocene), in which silicic volcanic rocks predominate in the lower level and basic volcanic rocks in the upper, and 2- an Upper Unit (Lower to Middle Miocene), consisting of aphanitic basalts and rhyolitic welded tuffs. Geochemical characteristics of the Colbún volcanic rocks, such as their low K, O contents, high MgO/FeO ratios, flat REE patterns, MORB normalized Nb to Sc close to unity, relatively low initial Sr isotopic ratios (0.703575-0.704028) and high initial Nd isotopic ratios (0.512919-0.513003) are all consistent with a tholeiitic affinity and a low degree of crustal involvement in the evolution of the parental magmas. Pb isotopic relationships and the enrichment of some incompatible elements such as K, Rb, Sr and Ba suggest recycling of subducting Nazca plate material. This was accompanied by crustal thinning and development of caldera type structures. The Colbún rocks are the isotopically most primitive rocks of the Chilean Andes. Geochemical relationships suggest that these rocks were emplaced in a back-arc, intracontinental, postorogenic extensional geotectonic environment. The Colbún magmas evolved mainly by closed crystal fractionation from primary magmas generated in the mantle. The chemical and isotopic similarities exhibited by rocks ranging in age from the Late Eocene to Middle Miocene suggest that the sources and processes, involved in the generation and evolution of these magmas, were more or less constant during this time span of 20 m. y.