Petrology and Geochemistry of the Calingasta Caldera Complex, San Juan Province, Argentina

Jasmine Gomez

Jasmine Peach
BS Candidate

Advisor: Dr. Dave Kimbrough
Monday, May 10th, 2021
11:30 am – via zoom

Abstract
This study seeks to test the possible link between a Permian-aged globally significant spike in explosive silicic volcanism and climate change and mass extinction events. The Permian-Triassic Choiyoi Magmatic Province (ChMP) is distributed across a ~500,000 km2 area of southern South America, mostly in Argentina. The ChMP is comprised of thick sequences of predominately, rhyolite-dacite pyroclastic rock that record a non-steady-state eruptive history with peaks of activity occurring at ~267 and ~247 Ma (Kimbrough et al. 2016). Because of its large areal extent and silicic composition, the ChMP constitutes a Silicic LIP as described by Bryan and Ferrari (2013).  Temporal data of the volcanic history of the ChMP suggests a possible link to the Kamura Cooling event, a 3–4-million-year cooling anomaly that occurred during the Capitanian stage of the Permian.
This study focuses on the Calingasta Caldera Complex (CCC), located in the Calingasta River Valley in the San Juan Province, hosting some of the best exposures of the ChMP. Mapping by Rocher et al. (2015) has shed ling on features of the physical volcanology, eruptive history, and petrogenetic evolution of the area. The CCC likely evolved as part of an extensional magmatic arc resulting from slab rollback in the final phases of the Gondwanan Orogeny (Rocher et al. 2015). The ChMP here is divided into three formations; the Vega de Los Machos Formation, the El Palque Formation, and the Horcajo Formation (Rocher et al. 2015). Geochemical analysis indicates a trend of increasing in volume and rhyolitic volcanism from the lower unit to the upper units (Rocher et al. 2015). The youngest upper unit of the Horcajo Formation U-Pb zircon age has been determined to be 265 Ma (Rocher et al, 2015). Previous K-Ar ages for the Vega de los Machos (288+/-5 to 294+/-9), El Palque (233+/-10 to 267+/-5), and Horcajo Formations (254+/-22 to 269+/-40) were gathered but had wide error ranges (Rocher et al. 2015).
In this study, detrital zircon U-Pb dating from the Calingasta drainage area documents peak activity for the CCC occurring at 272-261 Ma. New zircon U-Pb ages are also reported for the the Alumbera Pluton (263.6­+/- 2.2 and 269.89 +/- 0.96 Ma), the Cordon del Diablo ignimbrite (269.26+/- 0.88 Ma), the Cordon del Carrizalito ignimbrite (268.11+/-0.68 Ma), and the Vega de Los Machos Fm (270.9 +/- 1.4). Thin section analysis of 7 samples from the caldera complex show 1.) Calingasta units represent silicic pyroclastic deposits of recrystallized pumice lapilli, lithic fragments, and remnant glass shards 2.) phenocryst minerology is consistent with rhyolitic bulk composition, and 3.) volcanic units have undergone static greenschist facies recrystallization. These petrological features such as rhyolitic bulk composition and remnant glass shards suggest that the volcanism of the CCC could have the capacity for extensive ash fall that could lead to iron fertilization of oceans associated with the Gondwana supercontinent.  These results reinforce the hypothesized link between Choiyoi magmatism and the Kamura Cooling event.