Award Date

12-1997

Degree Type

Thesis

Degree Name

Master of Science in Geoscience

Department

Geoscience

First Committee Member

Eugene l. Smith, Chair.

Second Committee Member

Rodney V. Metcalf

Third Committee Member

Terry L. Spell

Graduate Faculty Representative

Steven Lepp

Number of Pages

149

Abstract

Alkali basalts on Citadel Mountain form the southern margin of the Lunar Crater Volcanic Field (LCVF) in the central Great Basin, Nevada. Citadel Mountain comprised of a faulted, north tilted section of Tertiary andesite and ash-flow tuff is capped by Pliocene and Quaternary alkali basalt flows that erupted from six major cinder cones. The basalt flows on Citadel Mountain can be divided into two groups (older and younger) based on age and isotopic signatures. The older basalt group is characterized by higher 87 Sr/86Sr and lower ENd and the younger group has lower 87Sr/86Sr and higher ENd. Geochemical evidence suggests that the older basalt group is contaminated by a lithospheric mantle melt. A model is presented that shows contamination of the older alkali basalts by magma commingling/mixing of rising asthenospheric melts with lithospheric mantle veinlets in the mantle lithosphere. Fractional crystallization of olivine and clinopyroxene can explain the chemical variation in the older basalt group, and although small amounts of assimilation of upper crustal material are permissible, assimilation is not required in the models. The younger basalt group evolved solely by fractional crystallization of olivine and clinopyroxene and their isotopic signatures may reflect their asthenospheric mantle source.

Keywords

Basalt—Composition; Geochemistry; Nevada; Radioactive dating; Strontium—Isotopes; United States--Great Basin

Disciplines

Geochemistry | Geology | Volcanology

Language

English

Comments

Best copy available.

Refer to PDF file for exact formulas.

Signatures have been redacted for privacy and security measures.


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