Date of Graduation

Spring 2019


Master of Science in Geospatial Sciences


Geography, Geology, and Planning

Committee Chair

Gary S. Michelfelder


The Mogollon-Datil volcanic field (MDVF), located in southern New Mexico, is the result of episodic volcanism and the transition between arc and rift magmatism. The MDVF has been the focus of several mapping, stratigraphic, petrologic, geochronological, and geochemical studies to understand the complete volcanic and tectonic history. However, the majority of previous studies lack geochemical analyses on intermediate composition volcanic rocks, giving more attention to the large-volume ignimbrites and rhyolite flows as well as the minor basalt flows that occurred between 36 and 20 Ma. I present new whole-rock major- and trace-element analyses and petrographic textures on one of the largest volume intermediate composition formations in the MDVF, known as the Bearwallow Mountain Andesite. Geochemical analyses compiled from literature and original sources are used to assess arc- versus rift-related trends and give insight into the temporal variation in crustal thickness overtime. Based on these data, I propose a two stage model of assimilation and fractional crystallization (AFC) of a basaltic composition magma during the arc-rift transition in the Mogollon-Datil volcanic field. Induced partial melting of subduction-modified lithospheric mantle incorporates geochemical signatures from deep-seated rutile-bearing pyroxenite cumulates and likely assimilates amphibole-rich pre-30 Ma material. Fractional distillation and filter pressing removed crystal cargo within the melts, resulting in the crystal-poor Bearwallow Mountain Andesite. Crustal extension provides conduits for rapid magma ascent via dike propagation and is controlled by further AFC processes to produce a spectrum of compositions that make up the Bearwallow Mountain Andesite, ranging from basalt to andesite.


Andesite, Arc Magmatism, MDVF, New Mexico, Crustal Evolution, Magma Evolution

Subject Categories

Geochemistry | Tectonics and Structure | Volcanology


© Conor Lee O'Dowd

Open Access