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Complex Magmatic Processes in El Negrillar Monogenetic Field: A Case Study of the Central Andean Volcanic Zone

Patricia Larrea 1, Daniela Parra-Encalada2, Camila Loaiza1, Deborah Cáceres-Baez1, Rubén Cartagena1, Sergio Salinas3, Teresa Ubide2, Benigno Godoy4

  • Affiliations: 1Departamento de Geología, Facultad de Ciencias Físicas y Matemáticas, Universidad de Chile, Chile; 2School of the Environment, University of Queensland, Australia; 3Colegio de Geografía, División SUAyED, Universidad Nacional Autónoma de México, México; 4Facultad de Ingeniería, Arquitectura y Diseño, Universidad San Sebastián, Chile

  • Presentation type: Poster

  • Presentation time: Monday 16:30 - 18:30, Room Poster Hall

  • Poster Board Number: 48

  • Programme No: 1.7.35

  • Theme 1 > Session 7

Abstract

Monogenetic volcanic fields, traditionally viewed as simple magmatic systems with direct source-to-surface pathways, are increasingly recognized as complex magmatic systems providing crucial insights into arc magmatism and crustal processes. We present a study of El Negrillar, the largest monogenetic field in the Central Volcanic Zone of the Andes, comprising 51 eruptive centers and 98 lava flows across three clusters (Northern, Central,  and Southern). New 40Ar/39Ar dating reveals volcanic activity between 0.982 ± 0.008 and 0.141 ± 0.072 Ma, with synchronous eruptions (within error) throughout the field. The system produced 6.8 km³ (DRE) of magma, ranging from basaltic andesite to dacite, with systematic compositional variations from SW to NE. The Southern cluster displays primitive olivine-rich characteristics, while the Northern cluster exhibits evolved amphibole-rich compositions. Detailed petrological analysis reveals a complex pre-eruptive history involving fractional crystallization within a main stagnation zone (9-19 km depth) and multiple magmatic processes, including recharge, mixing, and crustal assimilation. Whole-rock major and trace elements and Sr-Nd-Pb isotopic analysis show stronger adakite-like signatures in low-silica members, suggesting magma generation through extremely low degrees of melting of a garnet-rich lower crust source. These signatures resemble those observed in regional back-arc monogenetic volcanism, implying shared aspects of magmatic history. Our findings reveal complex magmatic processes and demonstrate significant structural control on mafic volcanism in the Altiplano-Puna Volcanic Complex, challenging traditional views of monogenetic volcanism in arc settings.