Document Type

Open Access Thesis

Embargo Period

8-31-2018

Degree Program

Geosciences

Degree Type

Master of Science (M.S.)

Year Degree Awarded

2018

Month Degree Awarded

September

Abstract

The Atascosa Lookout lava flow is a mid-Tertiary trachyandesite flow that caps the rhyolitic to dacitic volcanic sequence exposed in the Atascosa Mountains of southern Arizona. The flow erupted near the beginning of extension in the southern Basin and Range following the floundering of the Farallon plate and during the development of the San Andreas fault. The flow hosts a variety of disequilibrium crystals and textures including resorbed and overgrown feldspar phenocrysts with inclusion-rich zones, quartz-bearing enclaves, and clusters of plagioclase +/- chromium diopside, magnesian augite, quartz, hornblende, and orthopyroxene crystals and envelopes of contrasting composition with both the groundmass and the enclaves.

Current evidence suggests that magma generation and differentiation commonly take place mainly in the lower crust and batches of magma are emplaced and equilibrate across a range of crustal levels. Crystallization depths and temperatures of various phases in the flow were obtained with new and revised geothermometers and geobarometers to examine the petrogenesis of the lava flow. Major elements of parental melts for most of the mineral phases were estimated using thermobarometry equilibrium tests and rare earth and trace element concentrations of parental melts of hornblendes and clinopyroxenes were calculated using known partition coefficients elements.

Thermobarometry shows distinct ranges of temperatures and pressures for each component of the flow and calculated parental melts of various phases are distinct from one another. Orthopyroxenes crystallized at depths greater than 25 km, at the highest temperatures from the most mafic parent, estimated to be a picro-basalt. Clinopyroxenes crystallized at 11.5 – 30 km, lower temperatures and a more evolved parent of basalt or trachybasalt composition. Plagioclase crystallized throughout the crust from a range of intermediate melts and hornblendes crystallized 12 – 13 km from a parental melt similar in composition to the groundmass. This study demonstrates the lava flow hosts minerals that crystallized from different parent melts at various crustal levels. Extension and previous magmatism provided a rapid path for magma to ascend, subduing crustal assimilation and enhancing the probability of a diverse crystal cargo that retains the record of the plumbing system beneath a volcanic complex.

First Advisor

Sheila Seaman

Second Advisor

Michael Williams

Third Advisor

Michael Jercinovic

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