40Ar/39Ar Diffusion and Age Constraints in Muscovite from the Ruby Mountains, Northeastern Nevada

Abstract

Plutonic rocks exposed in the Basin and Range province are known to have experienced widespread extension and uplift at ca. 16.5 Ma. This extension is broadly coincident with timing of earliest volcanism of the Yellowstone hotspot (notably in the Northern Nevada rift and Oregon-Idaho graben and manifest by widespread basalts of the early Columbia River basalt province). Although the coincidence in timing leads to suggestions of a causal relationship, the overall magnitude and history of pre-early Miocene cooling and exhumation of mid- to deep-level crust is unclear. One reason for this existing uncertainty is that most of the geochronologic data available in central and eastern Nevada are for volcanic rocks and low temperature (<200°C) thermochronometers. The Ruby Mountains East Humboldt (RMEH) metamorphic core complex exposes deep crustal sections in the eastern Basin and Range province that have been the subject of extensive previous study. Previous published studies focused on low-temperature history (e.g. Colgan et al. [2010]) and emphasize a mid- to late-Miocene episode of exhumation that might coincide with the early Yellowstone hotspot [Camp et al., 2015]. The published thermochronologic 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 in order to properly evaluate competing hypotheses for the origin of regional extension in the Ruby Mountains, such as I) decompression of over-thickened crust following the Sevier orogeny, or II) uplift caused by the Miocene Yellowstone hotspot. Samples were collected from traverse and longitudinal sections of the Ruby Mountains metamorphic core complex for single-crystal 40Ar/39Ar dating, with emphasis on muscovite, in order to provide new constraints to the ca. 450-300°C thermal history of deep crustal sections presently exposed in the RMEH. Samples were prepared for analysis in the ANIMAL facility, and include relatively undeformed, euhedral muscovite phenocrysts up to 2 mm in diameter that were collected from syn- to 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 35-30 Ma in locations in the eastern Ruby Mountains, 26-25 Ma in central locations, and 22-20 Ma in northern and western locations in the footwall near the detachment fault. 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. 20 Ma in the west. In contrast to the regional variation of up to ca. 17 million years in cooling ages, cooling through the 40Ar* closure interval seemed to have occurred at rapid rates (ca. 30°C/m.y. or higher) in any particular sample location. The new data are best reconciled with ages published previously for crystallization of zircon and monzanite in felsic intrusives and low-temperature thermochronometers by cooling during ongoing extension with rapid uplift in the Oligocene and early Miocene. The present study derives cooling rates from the 40Ar/39Ar ages of single mica crystals, with closure temperatures calculated on the basis of the crystal size. The ages and calculated closure temperatures were used to further constrain cooling histories of the Ruby Mountains and formulate a more complete time-temperature model. Most of the extension that formed the metamorphic complex occurred before regional extension that could have arisen from the upwelling of the Yellowstone hotspot, and thus was not driven by Yellowstone volcanism.