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Volcanic facies and geochemical analysis of the Triassic Mammoth Rock breccia as a test of early-arc paleogeography and subduction inception, eastern Sierra Nevada, California

Berglin, Samantha Kristine (2021) Volcanic facies and geochemical analysis of the Triassic Mammoth Rock breccia as a test of early-arc paleogeography and subduction inception, eastern Sierra Nevada, California. Masters thesis, Northern Arizona University.

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Abstract

The breccia of Mammoth Rock crops out in the Mount Morrison pendant in the eastern Sierra Nevada. The Triassic volcaniclastic facies of Mammoth Rock are a product of early arc magmatism that initiated after convergence began along a pre-existing transform fault. Early-arc paleogeography and subduction initiation that built the Sierra Nevada arc are poorly understood, despite decades of study. Early-arc paleogeography and induced subduction inception are recorded in the arc by tectonics including uplift, thrust faulting, and early-arc magmatism. The deposition, composition, magmatic source, and timing of magmatism for the breccia of Mammoth Rock provide an understanding of the arc paleogeography.The breccia facies dominantly contain angular to subrounded clasts with variable amounts of cuspate, jigsaw-fit, and fluidal clast shapes and flow-banded clast textures dispersed throughout. Interbedded sandstone facies are composed of alternating fine- to coarse-grained sandstone with bedding characteristics including planar laminated, lenticular, low-angle tabular cross-bedded, and massive. The volcaniclastic and siliciclastic facies of Mammoth Rock suggest deposition in a shallow-marine environment likely between shoreface and a shallow-continental shelf. Collapse of solidified lava flows resulting in submarine mass wasting is suggested by the clasts in the polymictic and monomictic facies. Whole-rock geochemical analysis of clasts in the breccia of Mammoth Rock reveals a trachyandesitic composition. Whole-rock geochemical analyses also show a high K2O and Al2O3 weight %, low Mg#, LILE and LREE enrichment, and HREE depletion that suggest dominantly an upper crustal magma source that fractionated from a primitive melt to an evolved andesitic melt and reveal geochemical signatures consistent with a thick continental crust. The breccia and sandstone zircon trace element geochemistry reveal medium to high U/Yb concentrations that aligns with whole-rock geochemical data, supporting the inference of a thick continental crust. Geochronologic analysis of zircon confine the timing of magmatism for the breccia of Mammoth Rock to ca. 225 - 216 Ma. Maximum depositional ages from detrital zircon in the sandstone facies suggest deposition as a result of mass wasting of breccia of Mammoth Rock occurred between ca. 222 Ma and 216 Ma.

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: Volcanic facies, Triassic, Mammoth Rock breccia
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: 01 Feb 2022 18:19
Last Modified: 01 Feb 2022 18:19
URI: https://openknowledge.nau.edu/id/eprint/5621

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