CENOZOIC EXTENSION AND VOLCANISM IN THE SOUTHERN LAKE MOUNTAINS, CENTRAL UTAH

The Late Cenozoic transition from subduction-related to extension-related volcanism is recorded in the southern Lake Mountains of western Utah. An early suite (35 to 33 Ma) of dacite tuff, low-silica rhyolite tuff, and basaltic-andesite to andesite lavas has all of the characteristics of magma formed at a continental subduction zone. They are very high K, magnesian rocks with large negative Nb anomalies and positive spikes for Sr and Pb. These alkali-calcic to alkalic rocks have minerals that crystallized at high oxygen and water fugacities. Magma mixing is evidenced by high concentrations of compatible elements in the intermediate composition rocks, plagioclase compositions and textures, and disequilibrium mineral assemblages. There is no evidence of extension during the eruption of the Oligocene suite. Nearly horizontal lake and hot-spring deposits of the Sage Valley Limestone are interlayered with and younger than the volcanic rocks.

Following a volcanic lull of about 15 million years, a mafic lava 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 (Fe#25). Trace element patterns lack negative Nb and Ti anomalies and Pb and Sr spikes. 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#, Ni, Cr, and olivine compositions show that it is not primary.

This transitional sequence is probably the result of the progressive foundering of a shallowly dipping subducting slab that began during the Oligocene below this part of the Great Basin. Foundering or slab breakoff produced widespread dehydration of the subducted lithosphere and generated voluminous arc-like magma of Oligocene age which intruded, hydbridized, 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 asthenospheric mantle by 17 Ma. The young basaltic rocks have not been tilted but the Lake Mountains horst is bounded on east and west by normal faults.