Early Jurassic petrogenesis of the retro-arc volcanism of northwestern Patagonia, Argentina: Evidence from Lu-Hf isotopes and whole-rock geochemistry

Abstract

Located in the north-western Patagonian Massif, the Loncón Complex offers insight into the Upper Sinemurian syn-rift phase of this Patagonian sector. New whole-rock geochemistry and zircon Lu–Hf isotopic data, combined with adjacent volcanic depocentres such as the Lonco Trapial Formation, the Cañadón Chileno Complex, the Cerro Piche Graben Formation, the Comallo Volcanic Sedimentary Complex as well as the Colomichicó Formation from the Neuquén syn-rift phase, allow us to constrain the tectonic characteristics and evolution of the Jurassic retro-arc system of Patagonia. The effusive and explosive volcanism registered by andesitic lava flows and dykes and rhyolitic pyroclastic deposits defines subalkaline and calc-alkaline series. Additionally, the eruptive rocks show significant enrichment in light rare-earth elements and large-ion lithophile elements, and lower La/Ta, Ba/Ta and higher Th/Ta than the effusive rocks. Both units show depletion in high field strength elements (e.g., Nb, Ta, Zr, and Hf), and relatively high Nb/U, Ta/U, moderate La/Yb and low Th/Ta and Sr/Y ratios. Negative and positive εHf zircon values (−4.8 to +4.1) reveal the rocks were derived from the partial melting of a metasomatised mantle, which underwent moderate crustal contamination during its emplacement. These I-type hybrid magmas are generated on an extensional tectonic setting, probably caused by the retreating of the orogen, during Sinemurian times. This intraplate extension is considered slightly before the Early Jurassic Chon Aike (V1) episode.