Sprit_Mt

Geology’s graduate classes in Volcanology and Igneous Petrology teamed up this semester (fall 2008) for a weekend field trip to the Spirit Mountain Batholith in southern Nevada. The pluton is a large (250 km2), Miocene-age, composite silicic intrusion. Tilting, uplift and erosion have exposed a complete cross section, from the roof to deep levels of the intrusion. U/Pb data on zircon reveal 2 million years of igneous activity, and exposed field relations suggest a complex history of injection, mingling, and mixing of a variety of magma types.
	Mallory and Matt examine an outcrop at Hiko Canyon. The rock contains three magma types: diorite, fine-grained granite, and coarse-grained granite. This and other exposures in the canyon dramatically display magma mingling and varying degrees of mixing.
	Students at a mafic sill with Spirit Mountain as back drop. The mafic sill is near the contact of the Spirit Mountain batholith and the Proterozoic granite-gneiss. The main granitic body of the batholith is in the close background, and the ridge in the far background is the high-silica leucogranite roof of the batholith.
	Sean and Joanne study dikes in Hiko canyon, where the deeper levels of the batholith are exposed.

Northern Arizona University