Award Date

May 2019

Degree Type

Thesis

Degree Name

Master of Science (MS)

Department

Geoscience

First Committee Member

Terry Spell

Second Committee Member

Simon Jowitt

Third Committee Member

Arya Udry

Fourth Committee Member

Dennis Bazylinski

Number of Pages

159

Abstract

The El Rechuelos Rhyolite represents volcanic activity preceding the multiple caldera collapse events of the Jemez Mountains volcanic field (JMVF) in northern New Mexico. This study focuses on seven rhyolitic units clustered north of the Valles caldera. These units have been studied in order to interpret their relationships to one another and the eruptive history of the JMVF. Additionally, this study adds to the research on the relationships between effusive and explosive rhyolitic volcanism, and understanding how caldera-forming eruptions develop from silicic systems.

Limited previous research includes K/Ar dates and a few inconsistent 40Ar/39Ar dates, in addition to whole rock geochemistry, petrography, and radiogenic isotope analyses. This previous work has been expanded using comprehensive 40Ar/39Ar geochronology, and more extensive whole rock geochemistry, detailed petrography, and electron microprobe analyses. Five distinct eruptive episodes are characterized by distinctive petrography and geochemistry, which when combined with their ages, suggests that these units are the products of separate, independent magma batches. The Early Rhyolite (7.10 + 0.04 Ma), is comprised of phenocrysts of plagioclase, quartz, biotite, sanidine, and accessory oxides in a devitrified groundmass. The Intermediate Rhyolite (7.05 + 0.24 Ma) consists of plagioclase, sanidine, biotite, and minor accessory oxides in a devitrified, altered groundmass. The Pumice Ring Rhyolite (5.61 + 0.48 Ma) is composed of phenocrysts of plagioclase, biotite, quartz, amphibole, and accessory oxides in a finer crystalline groundmass. The three units comprising the El Rechuelos Rhyolite (2.23 + 0.15 Ma) appear almost identical to each other petrographically; these units are sparsely porphyritic, comprised primarily of rhyolitic glass and sparse, small crystals of plagioclase, sanidine, biotite, quartz, and accessory oxides. The Young Rhyolite (1.19 + 0.01 Ma) consists of sanidine, quartz, plagioclase, sparse fayalite, and accessory oxides and zircon in an altered glassy groundmass. All seven of these units plot as rhyolite on the Le Bas Classification Diagram, however there are five distinct suites based on major element comparisons, such as SiO2 vs. K2O, Al2O3, and CaO. Additionally, trace element geochemical analyses, including Nb vs. Sr, Rb, and Th, indicate five separate magma batches, with only the three 2.23 + 0.15 Ma units being geochemically related to one another.

Previous studies have disputed the relationships of these units to one another, and which units should be referred to as the El Rechuelos Rhyolite. This study concludes that these seven units represent five separate eruptive episodes, and that only the three 2.23 + 0.15 Ma units should retain the name of El Rechuelos Rhyolite. Additionally, this study suggests that the Young Rhyolite is a phase of Bandelier Tuff. Geochemical modeling and analysis suggests that the four pre-caldera units are likely products of varying degrees of partial melting and crustal assimilation, while the syn-caldera unit may be a product of fractional crystallization, however more research is needed to fully constrain the petrogenesis of these rhyolites.

Keywords

Geochemistry; Geochronology; Geoscience; Igneous Petrology; Valles Caldera; Volcanology

Disciplines

Geology

Language

English


Included in

Geology Commons

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