The Calderas field of Northern Chile (Atacama region): eruptive mechanisms and structural control in the latest Cretaceous to Eocene Andean magmatic arc

Abstract

Thick volcanic and volcano-sedimentary deposits represent Late Cretaceous to Eocene arc magmatism in the Copiapó pre-cordillera (Chile at∼26° S), being the result of the activity of large caldera edifices. This volcanic caldera complex preserves typical features of the evolution of collapse-resurgent caldera systems and evidence for the interaction of local fractures related to collapse processes with large regional structures. This study presents a stratigraphical, structural and geochemical characterization of the Carrizalillo megacaldera, a huge, 60 km-long and 30 km-wide mega-structure, and its nested calderas, which represent the resurgence of magmatism after the collapse of the main plutonic-volcanic system. The formation and evolution of the Carrizalillo megacaldera and its nested calderas can be related to four well-defined stages of caldera evolution: pre-collapse, collapse, resurgence, and post-collapse. These stages are structurally arranged and influenced by segments of the La Ternera Fault system (in the eastern part) and sub-annular fractures. The pre-collapse stage is characterized by the development of sub-annular fractures in the basement rocks. Subsequently, the initial collapse stage is characterized by the deposition of pyroclastic density currents and fallout deposits, marked by continuous, massive, and/or graded lapilli-tuffs interbedded with thin tuff layers. This was followed by a vast effusive volcanism, which resulted in the deposition of significant volumes of material in the eastern part of the megastructure. Sedimentary deposits and erosional unconformities represent episodic volcanic quiescence. The main collapse stage of the Carrizalillo megacaldera is represented by a high volume extrusion of magma forming homogeneous rhyolitic ignimbrites and lavas. Their arrangement, slightly dipping inward, suggests the progressive subsidence of the roof of the magma chamber during the caldera collapse. Subsequently, the magmatic reservoir was reactivated after the extrusion of the latest ignimbrite deposits of the Carrizalillo megacaldera, resulting in smaller, 10 km-in-diameter nested calderas, aligned along the trace of the La Ternera fault system, to the east of the primary mega-structure. According to the arrangement of volcanic breccias and ignimbrites close to the highly dipping ring fractures of these nested calderas, a more energetic collapse occurred during the first stages of their evolution, followed by the deposition of ignimbrite plateaus. The post-collapse stage corresponds to a period of quiescence, which favored erosion and the accumulation of intra-caldera lacustrine deposits, volcanic edifices formed within the caldera with the emplacement of post-collapse lavas. Preliminary geochemical results point to low-pressure Pl and Hbl fractionation as the main differentiation mechanism in shallow magma reservoirs, suggested by the increasing Eu, Sr, and MREE anomalies in samples with higher silica contents. Besides, they exhibit a calc-alkaline signature, Nb-Ta anomalies, and enrichment in LREE and LILE, typical of an active continental margin setting. The Paleocene-Eocene calderas of Atacama pre-cordillera were strongly controlled by dextral transtension within the arc. According to the tectonic scenario proposed in southern regions during the Eocene, with the Farallon-Aluk mid-ocean ridge subducting obliquely beneath South America, we suggest that this transient plate configuration, previously established in northern Chile, may have favored the magma ascent to shallow crustal levels and roof collapse that represent the volcanic sequences presented in this work.