Master's Theses



Degree Name

Master of Science (MS)


The Owyhee Basalt is a multi-flow unit of calc-alkaline lavas ranging in composition from basaltic andesite to andesite. The basaltic andesite are relatively primitive (Mg # > 50) and show a general trend of increasing SiO2 with decreasing age. Interruptions occurring in this trend may suggest periods where more primitive magma is injected into the chamber and mingled with a more evolved melt. Selected major oxide and trace element analysis [Al2O3 (> 13 wt. %), TiO2 (average > 1.2 wt. %), and Ba/La (>13)] from the Owyhee flows are typical of lavas within volcanic arcs. These values suggest a possible slab signature that potentially reflects subduction and migration of the Farallon plate prior to its destruction around 20 Ma. Eruption of the Owyhee Basalt occurred during a short period around 14.3 and 14.5 Ma. Computer modeling of fractional crystallization and fractional crystallization with assimilation (AFC) suggests it is possible to derive the more evolved upper flows from liquid compositions of the more primitive lower flow units. The small deviation of trace elements form one flow to another indicates that any assimilation or mingling would have occurred with a material that possessed similar values. Sr-Pb-Nd isotope ratios do not adequately constrain potential sources, but the nature of these values suggest an enriched source that was altered by metasomatic fluids prior to melt generation in the Neogene. Owyhee magmas were most likely derived from the asthenospheric mantle and then stored in a low to mid-crustal chamber where they fractionated prior to eruption. Contamination occurred en route and/or during residence time in the chamber. The characteristics of these lavas suggest renewed magmatism within a post-subduction setting and place additional constraints on the interpretation of intra-plate calc-alkaline volcanic.


Gary L. Millhollen

Date of Award

Spring 1999

Document Type

Thesis - campus only access


© 1999 Terry J. Crotwell


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