Geochemical Investigations of Ancient Silicic Magmatism in Western Arizona and Southern Brazil: Timing, Longevity, and Evolution

Abstract

Silicic magmas pose threats to society in the form of explosive volcanism. Understanding silicic magmatism is thus crucial for evaluating eruption-related risks, but numerous questions are unresolved: How are silicic magmas generated and how do they evolve? Over what timescales? What are the connections between shallow silicic magma reservoirs, their associated volcanic centers, and tectonic setting? This study uses petrography, whole rock and zircon elemental and isotopic geochemistry, and U-Pb zircon geochronology to investigate these questions in ancient silicic volcanic centers of the western U.S. and southern Brazil. The Silver Creek caldera in the Miocene southern Black Mountains volcanic center (SBMVC) of western Arizona produced the 18.8 Ma, ~1000 km3 Peach Spring Tuff (PST) supereruption. The PST was bracketed by 2 million years (19 – 17 Ma) of compositionally diverse intrusive and volcanic magmatic activity. Analysis of pre- and post-PST SBMVC units reveal that 1) the SBMVC experienced ~200 k.y. (19 – 18.8 Ma) of localized high-temperature magmatism (≥900 °C), coincident with the PST eruption; 2) SBMVC magmas were derived from Precambrian Mojave crust and enriched lithospheric mantle; 3) pre- and post-PST magmas are isotopically more diverse and have stronger mantle fingerprints than the PST, suggesting they were less homogeneous and less crustal than the PST magma body; and 4) mantle input into the SBMVC increased following the PST eruption. Late Proterozoic to early Cambrian silicic intrusions, lavas, and pyroclastic flows are sporadically exposed for ~1500 km throughout northern Uruguay and southern Brazil and are associated with a series of 600 – 530 Ma rift-generated basins. Zircon geochemistry and U-Pb geochronology of silicic units associated with the two largest basins, the Camaquã and Itajaí basins, indicate that 1) Neoproterozoic-Cambrian silicic magmatism in the region was episodic, occurring at ~520 Ma in the Itajaí basin and ~540 Ma and ~580 Ma in the Camaquã basin; 2) ~580 Ma magmatism was widespread, extending from the Graciosa region at the northern extent of the magmatic system to the Camaquã basin in the south; and 3) ~580 Ma magmatism is coincident with the proposed widespread rifting event that was responsible for basin development.