Structure and Petrology of Tertiary Volcanic Rocks in Parts of Toms Cabin Spring and Lucin NW Quadrangles (Box Elder Co.), Utah

A series of late Tertiary rhyolitic and dacitic flows, domes, and minor pyroclastic rocks form an elongate volanic mass along the northwestern Utah-northeastern Nevada border . The structure of the flow banding and the linear arrangement of vents indicate that the mass represents a multi-sourced extrusive complex which erupted through many fissure-type conduits. A 39 km2 area at the southern end of the mass was studied in detail in order to gain a better understanding of the eruptive nature and history of these Tertiary volcanic rocks. Age dating reveals that volcanism in the study area was episodic, and covered a period of at least 4 to 5 million years. The silicic volcanic rocks in the study area are similar chemically and mineralogically to other eruptive units within the Rhyolite Mts., which range from dacite (Si0 2 69%) to high-silica rhyolite (SiOz 75-77%). They also exhibit chemical characteristics similar to other silicic volcanic rocks of bimodal association in the western United States. Two-feldspar high-K rhyolite is the dominant volcanic rock in the study area, commonly found overlying rhyolitic vitric tuffs and agglomerates. Rhyolite from the southern portion of the study area is dated at 7.6 to 8.6 m.y.b.p. Dacitic samples contain phenocrysts of plagioclase, quartz, biotite, hornblende, and orthopyroxene. Dacitic volcanism is dated at 12.4 m.y.b.p. By analogy with other bimodal volcanic fields in the western U.S., it is assumed that these si 1 icic magmas are products of partial melting of crustal rocks. Evidence from a least squares differentiation model, along with the overall geochemical characteristics, indicates crystal fractionation as the dominant mechanism for the transition from dacite to rhyolite, with plagioclase as the dominant fractionating phase