Award Date

8-1996

Degree Type

Thesis

Degree Name

Master of Science (MS)

Department

Geoscience

Advisor 1

Eugene I. Smith, Chair

First Committee Member

Rodney V. Metcalf

Second Committee Member

Wanda J. Taylor

Graduate Faculty Representative

Peter L. Starkweather

Number of Pages

135

Abstract

Andesite dominated volcanic systems produced during regional crustal extension remain a relatively unexplored aspect of andesite petrogenesis. In the southern Basin and Range province, thick sections of andesite and basaltic andesite have been documented in areas including the Piute Range, California, the Eldorado Range, Nevada, and the McCullough Range, Nevada. The northern McCullough Range, located 20 miles south-southeast of Las Vegas, is an ideal place to study the petrogenesis of an andesite dominated system. This area contains a mid-Miocene stratocone composed mainly of a 1200 m thick section of andesite flows that formed synchronous with regional extension.

The northern McCullough Range andesites are calc-alkaline and contain phenocrysts of olivine, clinopyroxene, and plagioclase. Although the andesites are similar in major element chemistry, they exhibit wide variations in trace element chemistry. Resulting trace element Harker diagrams show considerable scatter and no discernible petrogenetic trends. on 87Sr/86Sr and Nd isotope plots, however, andesites fall into 3 distinct groups: group A (87Sr/86Sr=0.70758 to 0.7077; Nd=-6.2 to -6.9), group B (87Sr/86Sr=0.7081 to 0.7083; Nd=-6.7 to -7.4), and group C (87Sr/86Sr=0.70897 to 0.70904; Nd=-8.8 to -9.8). Chemical variations and petrogenesis were assessed using fractional crystallization, assimilation and fractional crystallization, magma recharge, and magma mixing models. Geochemical variations, isotopic data and petrogenetic modeling suggest that the andesites in the northern McCullough Range formed primarily from mixing together several different andesitic sources (represented by isotope groups A, B, and C) that are similar in chemistry but isotopically distinct. This proposed model involving mixing of different andesitic end members explains the andesite dominated section observed in the McCullough Range and provides and alternative explanation for andesite petrogenesis in extensional terranes. A preliminary study of Xitle volcano in central Mexico shows similarities to the McCullough Range results. It is possible, therefore, that the petrogenetic model developed in the northern McCullough Range may be applied to extensional environments in other areas of the world.

Keywords

Andesite; Magmas; Mexico -- Xitle; Nevada -- McCullough Range; Petrology; Volcanoes

Disciplines

Geochemistry | Geology | Volcanology

Language

English

Comments

Signatures have been redacted for privacy and security measures.

(Refer to PDF file for exact formulas.)


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