Analysis of triassic volcanic rocks in two pendants, eastern Sierra Nevada, California with implications for continental arc paleogeography and volcanology

Brennan, Kelsey Elizabeth (2022) Analysis of triassic volcanic rocks in two pendants, eastern Sierra Nevada, California with implications for continental arc paleogeography and volcanology. Masters thesis, Northern Arizona University.

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Abstract

The continental passive margin along the west coast of Laurentia transitioned from transpressional faulting to a subduction zone and continental magmatic arc in Pennsylvanian to Permian time, but the exact timing of initial arc magmatism, eruptive styles and sequences, and the depositional setting remain poorly understood. Several pendants in the Sierra Nevada in California preserve the transition from passive margin to active magmatic arc in early Triassic time. Mapping and facies analysis, whole-rock and zircon trace element geochemical data, and three new zircon U-Pb ages from pyroclastic rocks in the Ritter Range and Mount Morrison pendants provide insight into the timing and style of the initial stages of Triassic arc development at ~37°N. A series of tuffs in the Agnew Meadows area in the Ritter Range pendant are poorly sorted deposits of juvenile and accidental lithic clasts and broken quartz and feldspar phenocrysts in a fine-grained groundmass and were emplaced by pyroclastic density currents. A pumice conglomerate unit within this sequence is composed of elongate pumice clasts in a volcanic ash matrix and is the result of the effusive emplacement of a subaqueous silicic dome in the Agnew Meadows area. Compositionally similar rocks crop out along strike in the Mount Morrison pendant to the southeast; lithic clast and phenocryst sizes increase southward and the tuff of Skelton Lake becomes more chaotic in the farthest southeast corner of the pendant. The Red and White Spur area in the southwest corner of the Mount Morrison pendant exposes a thick unit containing caldera-wall megabreccia in fine-grained tuff of Skelton Lake. The tuff of San Joaquin Mountain in the Ritter Range pendant, the tuff of Skelton Lake at Skelton Lake, and the lithic-rich facies in the Red and White Spur area are compositionally similar, suggestive of cogenesis. Previous studies of zircon in pyroclastic rocks in the Mount Morrison, Saddlebag Lake, and Ritter Range pendants yielded crystallization ages of 232 to 217  2 Ma, which marks the earliest arc volcanism in this area. To determine eruptive sequences and constrain the relation between similar pyroclastic units across these pendants, new zircon U-Pb samples were collected and analyzed from two pyroclastic units in the Ritter Range and one in the Mount Morrison pendant. All three samples revealed identical ages within error centered at ca 220  2 Ma, suggesting these units are coeval. Textural, stratigraphic, and whole-rock and zircon geochemical data collectively suggest these units are also cogenetic. Zircon geochemical signatures indicate contamination by continental crust in a convergent-margin setting. Primary volcanic textures suggest subaqueous silicic dome carapace pumice and pyroclastic deposits in the Ritter Range pendant, and thick (>1 km) intra-caldera and caldera-wall breccia in the Mount Morrison pendant, indicating a caldera with a diameter of at least 13 km centered in the southeastern portion of this pendant. The earliest volcanic activity related to the Late Triassic arc involved large silicic systems of calderas and domes, which likely erupted subaqueously, producing vesicular pumiceous clasts that floated in the water column before settling into pyroclastic material from coeval eruptions. Studies of these volcanic sections provide insight into juvenile volcanic arc evolution, paleogeography, and volcanism in an induced-subduction initiation setting.

Item Type:	Thesis (Masters)
Publisher’s Statement:	© Copyright is held by the author. Digital access to this material is made possible by the Cline Library, Northern Arizona University. Further transmission, reproduction or presentation of protected items is prohibited except with permission of the author.
Keywords:	California; Sierra Nevada; Triassic; Volcanism; Zircon; Lithic clasts; Megabreccia; Pyroclastic rocks
Subjects:	Q Science > QE Geology
NAU Depositing Author Academic Status:	Student
Department/Unit:	Graduate College > Theses and Dissertations College of the Environment, Forestry, and Natural Sciences > School of Earth Sciences and Environmental Sustainability
Date Deposited:	08 Jun 2023 16:06
Last Modified:	04 Jan 2024 08:30
URI:	https://openknowledge.nau.edu/id/eprint/5988

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