Physical and chemical evolution of the largest monogenetic lava field in the Central Andes: El Negrillar Volcanic Field, Chile

Abstract

El Negrillar volcanic field is one of the most voluminous monogenetic volcanic fields so far identified in the Central Volcanic Zone (CVZ) of the Andes: its lava field comprises 84 eruptive phases from 35 small eruptive centers. The study of this volcanic field offers a unique opportunity to assess the volumetric, morphometric, rheological, and compositional evolution of the effusive activity in an entire monogenetic cluster; the great extent and exposure of these small eruptive centers is difficult to observe in other monogenetic volcanoes from the Central Volcanic Zone, as they normally appear as isolated vents or in small clusters (< 3 eruptive centers) with less associated eruptive phases. Our methodology utilizes GIS mapping tools to outline the different eruptive phases of El Negrillar to create a detailed lava flow map, allowing us to estimate the volume emitted by each eruptive center. This analysis was also possible through the paleoreconstruction of the buried lava flows, allowing the determination of the variation in the magma supply. This yielded a total bulk volume of ~7.6 km3 (6.8 km3 DRE, considering the low vesicle abundance of the eruptive products), which exceeds by more than one order of magnitude the volume emitted by other monogenetic centers from the Salar de Atacama region (e.g., El País lava flow, Tilocálar Norte, Tilocálar Sur, La Albondiga and Cerro Overo). We also determined the composition of the entire effusive activity of El Negrillar; andesitic magmas represent more than 46 vol% of the lava field. We identify three main clusters (i.e., Northern El Negrillar (NEN), Central El Negrillar (CEN), and Southern El Negrillar (SEN)), that revealed a compositional variation together with a change in the calculated morphometrical and rheological parameters, implying the existence of a local trend within the monogenetic lava field. The NEN cluster represents the most differentiated magmatism in the field, characterized by amphibole-rich lavas with almost absent olivine, and the highest crystallinity and vesicle content, which led to an effusive activity sustained for more than 3 years, with lava accumulation predominating over its transport, leading to the shortest and thickest flows of the field. The presence of “cauliflower shape bombs” in an early stage of one of its eruptive centers indicates that the NEN cluster may have been influenced by environmental factors leading to an early explosive episode during its activity. On the contrary, the SEN cluster represents the less differentiated magmatic activity of the field, characterized by olivine-rich lavas with almost absent amphibole, low crystallinity, and poor vesicles content, which along with a large magma supply led to effusive activity sustained for approximately two years where the lava flows achieved the longest distances in the field. The CEN cluster is compositionally closer to the SEN, but morphometrical and rheologically resembling to the NEN, suggesting that it represents an intermediate behavior between the other two clusters.