2008 Alum

Katherine Fornash

Katherine Fornash

Years participated in RESESS:
2007 →
2009 →
2010 →

An Overview

Reassessing spatial, temporal, and compositional trends in western North American magmatism using NAVDAT

Academic Affiliation: Junior, University of Arizona, Geosciences
Science Research Mentor: G. Lang Farmer – CU
Writing and Communication Mentor:Susan Eriksson – UNAVCO


Plate tectonic setting and magmatic activity are intimately related, as evidenced by the fact that 95% of all magma generation (by volume) occurs along active continental margins. However, in Cenozoic western North America, magmatism occurred more than1,000 km from the active continental margin, even after corrections for Basin and Range extension. Previous research suggests that these magmas may be the product of changing plate configurations between the Farallon and North American plate and subductionrelated processes.

These hypotheses were re-examined using MATLAB-generated animations and visualizations of data in the North American Volcanic and Intrusive Rock Database (NAVDAT, navdat.kgs.ku.edu). Continental basaltic rocks (ages: 80 m.y.-present, SiO2 wt.%: 40-52%, 7,987 samples) and volcanic rocks from four mid-Tertiary volcanic fields (ages: 50 m.y.-20 m.y., SiO2 wt.%: 0-100%, 3,639 samples) were examined for the presence of a subduction-related geochemical signature.

Animations of the temporal and spatial changes in the Na/K ratio of basaltic rocks showed an increase in the prevalence of sodic magmatism through time, as well as a general concentration of more potassic rocks toward the interior of the continent. Also notable was the observation that potassic volcanism in the Sierra Nevadas (~3.5 m.y.) thought to be related to the lithospheric delamination, was the only location of potassic volcanism in the entire western United States province during that time period.

Harker variation diagrams of various major and trace element abundances in the mid-Tertiary volcanic fields revealed that the Trans-Pecos volcanic field had a consistently distinct chemical composition, characterized by low Ba/Nb, Th/Nb, and Sr/Ta ratios, low wt.% CaO and wt.% MgO, and high wt.% TiO2. Differences between the Challis/Absaroka, Mogollon-Datil, and San Juan volcanic fields were subtle but still evident. Taken together, these observations suggest that (1) spatial variations in the intensity of metasomatism of the mantle might have existed at the time, and that (2) magmas in the Trans-Pecos volcanic field may have been generated in a back-arc basin environment.