Abstract

Lavas Las Pataguas unit is a unique expression of the Miocene forearc alkaline volcanism on Central Chile, with a restricted areal and volumetric distribution (<2 km2 and <1 km3, respectively), mound like structures (tumuli) and no evidence of a former volcanic landform at the vent. New 40Ar/39Ar Early Miocene ages (18,5 Ma) solve previous geochronology uncertainty and major and trace elements (SiO2 ~53,5%wt, 5,7%wt K2O+Na2O, La>30 ppm, TiO2 ~2%wt, P2O5 ~0,8%wt), show a clear difference with rocks from the coeval Miocene volcanic arc, located eastward. On the other hand, low Ni (<15 ppm) and Cr (<20 ppm) concentrations and Mg-number (<0,4), rule out the possibility of a primary magma, but at the same time the Sr, Pb and Nd isotopic values show weak crustal contribution. Furthermore, although Nb, Ti and Pb negative anomalies are present, Ba/La<20 and K/Ba ~20 ratios point to weak subduction components. A simple petrogenetic model shows fusion of a metasomatized mantle with partial melting degree <2%, which is consistent with the Nd isotope and Zr/Y ratios.