Abstract

Recent volcanism in the Black Rock Desert and Markagunt Plateau volcanic fields in south-central Utah is mainly subalkaline, which is uncommon elsewhere in the Basin and Range Province. The Black Rock Desert volcanic field (BRD) is composed of five subfields and spans about 170 km along the eastern margin of the Basin and Range Province in Utah. The subfields are: (1) Cove Fort, (2) Twin Peaks, (3) Beaver Ridge, (4) Ice Springs, and (5) Fumarole Butte. Although volcanism in BRD began in the late Miocene, about 6 Ma, most activity has occurred in the past 2.5 million years. Black Rock Desert is composed of a tholeiitic suite ranging in composition from basalt to dacite, and calc-alkaline andesite, dacite, and rhyolite. During the 6 million year life of the field, alkali basalts erupted only rarely and comprise no more than three flows. The Markagunt Plateau volcanic field (MP) is located east of Cedar City at the south end of the High Plateaus physiographic province (a structural and stratigraphic transition zone between the Basin and Range Province to the west and the Colorado Plateau to the east) and has been active since the earliest Pliocene, with the most recent eruptions occurring in latest Pleistocene or possibly Holocene time. It contains over 25 cinder cones and associated flows. Flows vary in composition from calc-alkaline basalt, basaltic andesite, and andesite, to olivine tholeiite and alkali basalt.

One of the most puzzling attributes of MP and BRD, and of volcanic fields in western Utah in general, is the presence of abundant subalkaline volcanic units. In most other volcanic fields in an intraplate tectonic setting, and in particular in the Basin and Range Province, alkali basalt is the dominant eruptive product with little, if any, calc-alkaline intermediate or silicic rocks. The chemical characteristics of both BRD and MP, while unlike typical intra-plate volcanism, are similar to those of continental rift zones like the Rio Grande Rift on the east side of the Colorado Plateau. We suggest that recent subalkaline volcanism at the western margin of the Basin and Range Province may be signaling the initiation of a rift along the western margin of the Colorado Plateau, similar to the Rio Grande Rift. Alternatively, these chemical signatures could simply be a characteristic of volcanism at, or near, the margins of the Colorado Plateau.