UNDERSTANDING AND EXTENDING THE WESTERN BOUNDARY OF THE CENOZOIC MARYSVALE VOLCANIC FIELD, SOUTHWEST UTAH

The Marysvale Volcanic Field (MVF), located in southwest Utah, underwent a period of calc-alkaline igneous activity (~34 to 21 Ma) as part of the regional Cenozoic Reno-San Juan magmatic zone, produced by a series of arc-derived sub-volcanic magma chambers formed through the roll-back of the Farallon Plate after the shallow-plate induced Laramide Orogeny. Slab roll-back produced “ignimbrite” flare-up events that are surficial reflections of batholith formation at depth. The MVF lies at the eastern end of the northeast-trending Pioche-Marysvale igneous belt where extensive, mostly Oligocene and Miocene, volcanism occurred above a major batholith complex.

Gravitational collapse of the southern MVF produced catastrophic mega-scale gravity slides in the area, including the recently discovered Black Mountains Gravity Slide (BMGS). The western MVF boundary is traditionally, though arbitrarily, placed along the west side of the Mineral Mountains (MM) batholith. This traditional western boundary does not extend far enough to include a western BMGS source area, suggesting that its placement may be due for change. An exposed granite in the northern Star Range was sampled to the west of the current boundary, which yielded a new U-Pb zircon age of 23.01± 0.17 Ma, similar to previously published K-Ar ages of other nearby intrusions and similar calc-alkaline subduction related geochemical signatures to those of the MVF. Based on this combination of old and new data, we extend the western boundary of the MVF at least 26 km west of the MM. This new placement would include the aforementioned intrusions and a western volcanic source area for the BMGS, albeit partially buried in the Milford Valley Graben.