Abstract

The late Cenozoic transition from subduction-related to extension-related volcanism is recorded in the Soldiers Pass volcanic field of the southern Lake Mountains, north-central Utah. The Soldiers Pass Formation (new formal name) is a Paleogene (35–33 Ma) suite of intermediate to silicic volcanic rocks interstratified with and overlain by lake and hot-spring deposits. In ascending order, its volcanic units include the trachydacite tuff member, Chimney Rock Pass Tuff Member, breccia member, and andesite member. Nearly horizontal lake and hot-spring deposits (White Knoll Member) are interlayered with and cap the volcanic strata. The volcanic rocks are very high-K, magnesian rocks with large negative Nb anomalies on normalized trace-element diagrams. Mineral compositions show they crystallized at high oxygen and water fugacities. Magma mixing is evidenced by high concentrations of compatible elements in the intermediate composition rocks, plagioclase and sanidine compositions and textures, and disequilibrium mineral assemblages. In short, the Paleogene suite has the characteristics of magmas formed at continental subduction zones. There is no structural evidence of extension during the eruption of the Paleogene suite.

Following a lull in volcanic activity of about 14 million years, the 19 Ma Mosida Basalt (new formal name) erupted as one of the oldest basaltic magmas in the eastern Great Basin. This mildly alkaline, potassic trachybasalt has phenocrysts of olivine (Fo₆₀), plagioclase (An₆₅), and clinopyroxene. Trace-element patterns lack large negative Nb and Ti anomalies, consistent with a nonsubduction origin. Tectonic discrimination diagrams also imply a within-plate, alkalic character. We conclude that it is one of the oldest asthenosphere-derived magmas in the Great Basin, but low Mg/(Mg+Fe), Ni and Cr concentrations, and relatively Fe-rich olivine compositions show that it is not primary.

This transitional magmatic sequence is probably the result of the progressive foundering of a shallowly dipping subducting slab that began during the Eocene below this part of the Great Basin. Foundering produced widespread dehydration of the subducted lithosphere and generated voluminous arc like magma that intruded, hybridized, and differentiated in the crust. Compensating inflow of asthenospheric mantle beneath the Great Basin along with the development of a transform boundary and lithospheric extension, eventually resulted in decompression melting of the mantle by 19 Ma. The Mosida Basalt has not been tilted, but the Lake Mountains horst is bounded on the east and west by normal faults.