RAPID AND DIACHRONOUS COOLING OF THE RUBY MOUNTAINS METAMORPHIC CORE COMPLEX THAT PREDATED THE YELLOWSTONE HOTSPOT

Plutonic rocks in the Basin and Range province are known to have experienced widespread extension and uplift at ca. 16.5 Ma, coincident with timing of earliest volcanism of the Yellowstone hotspot. However, the overall magnitude and history of pre-early Miocene cooling and exhumation of mid- to deep-level crust is unclear, as most published studies in central and eastern Nevada focus on volcanic rocks and low temperature (<200°C) thermochronometers. The Ruby Mountains East Humboldt (RMEH) metamorphic core complex exposes deep crustal sections and has been the subject of extensive previous study. The published data for the RMEH bearing on the higher temperature exhumation history, prior to the mid-Miocene, are insufficient to provide an unambiguous evaluation of the early exhumation history of the Ruby Mountains. It is critical to understand the pre-early Miocene exhumation history of this region to properly evaluate hypotheses for the origin of regional extension in the Ruby Mountains. Samples were collected from the RMEH for single-crystal ⁴⁰Ar/³⁹Ar dating, with emphasis on muscovite, to provide new constraints to the ca. 450-300°C thermal history of deep crustal sections presently exposed. Samples were prepared in the ANIMAL facility and include relatively undeformed, euhedral muscovite phenocrysts up to 2 mm in diameter from post-kinematic granitic intrusives. Using single crystal fusion, muscovite and biotite crystals from the Ruby Mountains were determined to have ⁴⁰Ar/³⁹Ar ages of approximately 36-23 Ma in the Harrison Pass Pluton in the southern RMEH and 31-23 Ma in Secret Pass in the north. Samples collected from a transect across the RMEH yielded ages of approximately 31-28 Ma east of Lamoille Canyon, 30-25 Ma in the central RMEH, and 22-20 Ma in the west. The data suggests unroofing of the RMEH metamorphic and igneous basement lithologies and retention of 40Ar* in micas began by ca. 35 Ma to the east, and progressed to final closure of 40Ar* in micas by ca. 18 Ma in the west. Cooling through the 40Ar* closure interval seemed to have occurred at rapid rates (ca. 10 °C/m.y. or higher) in any particular location. The new data suggest substantial ongoing extension and rapid uplift of the basement footwall beneath a west-migrating Ruby Mountains detachment fault occurred during the Oligocene and into the early Miocene.