Decoding subcontinental lithosphere processes: The key role of fractional crystallization in Central Andes monogenetic volcanism - Insight from El Negrillar volcanic field, Chile

Abstract

Subduction zones are complex geodynamic settings in which volcanic arcs form due to the interaction between subducting slabs, water, and continental materials. Investigating this extensive Earth recycling system offers valuable insights into the exchange of materials between the crust and mantle. While previous research has primarily focused on analyzing polygenetic volcanoes to explore this interaction, monogenetic volcanic fields (MVF) present an opportunity to delve into the mechanisms operating within the upper and lower mantle. Monogenetic volcanoes are a class of volcanoes that are typically smaller in size but showcase a range of eruptive styles and compositions. These volcanoes can be found in various locations along the arc, including the main Central Volcanic Zone (CVZ) (arc-front volcanoes), and even far from the tectonic plate boundaries in the back-arc region (intraplate volcanism).We carried out a detailed geochemical study of El Negrillar (EN) MVF, located in the Central Andes main-arc, on the Altiplano-Puna Plateau's southwestern border. The volcanic field has 35 eruptive centers and a total of 86 eruptive phases originating from its three clusters: Northern, Central, and Southern. EN magmas range in composition from basaltic andesite to dacite and have produced over 6.8 km3 (DRE) of lava and pyroclastic material, which makes it the most voluminous volcanic field in the Central Andes, and therefore, an excellent example to study MVF in arc-systems. We have conducted an extensive study encompassing major and trace elements, as well as isotopic analysis using whole rock Sr-Nd-Pb. We also present an analysis of our results with available geochemical data from the back-arc and main-arc volcanoes, including other monogenetic volcanoes near EN (<80 km, EN's neighbors). The results revealed that EN magmas and their neighbors are strongly related and exhibit a clear similarity in major and trace element compositions and isotopic ratios, indicating a common source and origin of their melts. Our comparison across the arc surprisingly reveals similarities between EN and back-arc monogenetic volcanism. Both regions indicate geochemical signatures that do not support melting via slab metasomatism, typical of subduction zones. Instead, they show low Th/Ce (<0.1 ppm) and Ba/La (<30 ppm) and high Ce/Pb (>6 ppm), Ba/Th (>100 ppm), high 143Nd/144Nd > 0.51240, and La/Yb ratios at a given La/Sm compared to the main-arc polygenetic volcanoes. Interestingly, EN also exhibits adakite-like signatures with increasing SiO2 content (high Sr > 600 ppm, Sr/Y > 40, and high La/Yb > 20), a term used for igneous rock suites with chemical characteristics that are identical to those of adakites but produced through petrogenetic processes that do not include a slab component. We explore how fractional crystallization and crustal assimilation participate in the development of monogenetic volcanoes, and our findings underscore the significant influence of fractional crystallization on the monogenetic volcanic processes in the Salar de Atacama region. The geochemical changes observed in this area can be attributed to a combination of varying degrees of fractional crystallization within magmas generated through the partial melting of a garnet-enriched environment, which could be primary lower crust or partial melting of a peridotite mantle that has later equilibrated with garnet-bearing crustal melts.