Publication Date

8-2016

Date of Final Oral Examination (Defense)

4-20-2016

Type of Culminating Activity

Thesis

Degree Title

Master of Science in Geology

Department

Geosciences

Major Advisor

Mark Schmitz, Ph.D.

Advisor

Brittany D. Brand, Ph.D.

Advisor

V. Dorsey Wanless, Ph.D.

Advisor

James Crowley, Ph.D.

Abstract

Intracontinental magmas are stored at shallow crustal depths and commonly produce compositionally variable deposits when erupted. A core objective of igneous petrology is to understand the processes and pathways by which these magmas are generated and differentiated. Despite decades of research, debate persists regarding the comparative effects of processes like assimilation, fractional crystallization, and magma mixing in the production of compositional zonation. There are two end member hypotheses of how compositional variability develops in pre-eruptive magma bodies; one proposes long-term storage and crystal fractionation of a mafic or intermediate parent magma, while the other proposes a more transient system resulting from crustal melting, high magma flux rates, and magma mixing. This study examines the relative influences of fractional crystallization and magma mixing in the development of compositional heterogeneities in crustal intracontinental magmas and the timescales over which these processes occur.

The Picture Gorge Ignimbrite (PGI) and Rattlesnake Tuff (RST) in east central Oregon are contrasting examples of compositional variability in rhyolitic ashflow tuffs that provide case studies for both gradational and abrupt styles of compositional variability, respectively. Disequilibrium textures in zircon crystals from both tuffs offer evidence of open system recharge and magma mixing in the pre-eruptive magma chambers. Trace element chemistry and Ti-in-zircon crystallization temperatures of zircon crystals from the PGI record a reheating event coupled with a change in magma chemistry that coincides with an episode of zircon resorption followed by recrystallization. The zircon crystal record of the RST magma system is markedly more complicated as illustrated by complex crystallization textures and intracrystal thermochemical zoning profiles. Complexity of the RST system is further revealed by bimodal feldspar compositions and widely variable glass compositions that form fractional crystallization and mixing arrays between and within compositional groups.

Tandem LA-ICPMS and CA-TIMS U-Pb geochronology on crystals demonstrate multigenerational antecrystic growth in both the PGI and RST systems occurring over 104 year timescales. Petrochronologic analysis of these two rhyolites preclude protracted closed system differentiation as a mechanism for compositional evolution and development of a compositionally zoned pre-eruptive magma body. Rather, thermochemical gradients in both systems were likely imparted primarily by magma rejuvenation and hybridization. Results suggest that, in the PGI magma body, a single recharge episode allowed for homogenization of the hybrid magma prior to eruption, while greater magma flux rates in the RST magma resulted in incomplete pre-eruptive mixing and preservation of distinct magma compositions that were mingled upon eruption.

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