The temporal evolution of monogenetic volcanism in the Central Andes : 40Ar/39Ar geochronology of El Negrillar volcanic field, Chile

Abstract

El Negrillar volcanic field has the largest extent and erupted volume (~ 6.8 km3 DRE) of all the monogenetic centers of the Andean Central Volcanic Zone (CVZ). The volcanic field comprises 51 eruptive centers and 98 lava flows distributed in three clusters: Northern El Negrillar (NEN), Central El Negrillar (CEN), and Southern El Negrillar (SEN). Here, we present a geological map of El Negrillar, with detail of effusive and explosive volcanic deposits not previously mapped in the southern sector of the CEN and SEN clusters. Ten samples of El Negrillar’s deposits associated with effusive and phreatomagmatic activity were dated using 40Ar/39Ar geochronology, establishing, along with previously published dates, a geochronological characterization of the development of El Negrillar’s monogenetic field. The collected age data yields a range of 0.982 ± 0.008 to 0.141 ± 0.072 Ma, compared to previously published K–Ar ages for the same deposits range from < 1.5 Ma to 0.6 ± 0.4 Ma. The new ages presented here indicate that the effusive activity at El Negrillar (NEN, CEN, and SEN), and the phreatomagmatic activity in the CEN (dated for first time) occurred quasi-simultaneously (within error). The end of the volcanic activity within the monogenetic field occurred in the eastern sector of the CEN at 0.141 ± 0.072 Ma, which represents the youngest eruption ages of El Negrillar. If these new ages are revisited within the regional context of the SW sector of the Altiplano-Puna Volcanic Complex (APVC), the monogenetic volcanoes appear to be the result of a migration of mafic vents along a southwest-northeast trend, as shown by their age variation from the oldest to the youngest volcanic center: Morro Punta Negra, La Negrillar, El Negrillar, Tilocálar Sur, Tilocálar Norte, Cerro Tujle, El País, Puntas Negras, La Albóndiga Grande, and Cerro Overo. These results highlight the structural control on the emplacement of monogenetic mafic volcanism in the APVC.