VOLCANISM ON THE FISH LAKE PLATEAU, CENTRAL UTAH

Volcanic rocks exposed on the Fish Lake Plateau (FLP) in south-central Utah lie at the northeastern margin of the extensive Marysvale Volcanic Field (MVF). Volcanic rocks on the FLP form a 250 to 700 m thick sequence with at least five distinctive and mappable units. In the southernmost FLP, the lowermost exposed unit consists of interlayered volcanic breccia, trachyte and minor andesitic ash. The basal volcanic unit across the northern and western part of the FLP is the Johnson Valley porphyritic trachyandesite, a densely welded unit composed of numerous individual flows that, in aggregate, are 100 to 300 m thick. The trachyandesite is composed of 56-58% SiO₂, 8-9% FeO, and 6-7% Na₂O + K₂O (enriched in the matrix). Multiple flows of the Lake Creek trachyte overlie the Johnson Valley trachyandesite and the unit ranges from ~70 m to >300 m thick. This phenocryst-poor trachyte is composed of 65-67% SiO₂, 4-5% FeO, and 10% Na₂O + K₂O. The regionally extensive, 30 to 120 m thick, Osiris trachyte caps the older volcanic sequence and contains abundant plagioclase and biotite phenocrysts with minor sanidine and pyroxenes in a microlitic feldspar and glassy matrix. It is composed of 62-65% SiO₂, 3-4% FeO, 2-3%, and 9-10% Na₂O + K₂O. A whole rock ⁴⁰Ar/³⁹Ar age dates the Lake Creek trachyte to 25.15 ± 0.14 Ma, and several ⁴⁰Ar/³⁹Ar total fusion ages for sanidines in the Osiris trachyte average 23.03 ± 0.08 Ma. Pliocene olivine-bearing trachybasalt unconformably overlies the older volcanic rocks in the southern FLP, Awapa Plateau, and in a small isolated flow at Forsyth Reservoir near the northeastern edge of the FLP.

In contrast to previous interpretations, we interpret the older volcanic units on the FLP as a series of potassium-rich ash flow sheets rather than lava flows. The areally extensive Johnson Valley, Lake Creek and Osiris units all contain widespread fragmented phenocrysts. In lava flows, broken phenocrysts are rarely present and are usually concentrated near flow margins, which is not the case with the FLP units. These observations coupled with mechanical considerations of subsolidus trachytic magmas are consistent with an explosive, rather than effusive, origin for these welded lava-like tuffs. This origin makes it likely that the older volcanic units on the FLP were derived from eruptive centers in the central MVF.