PETROLOGIC ANALYSIS OF RAILROAD VALLEY RHYOLITE, GRANT RANGE, NYE COUNTY, NV

In the northern Grant Range, deposition of the Railroad Valley Rhyolite (33.0 to 33.4 Ma) marked the initiation of volcanism in the east-central Basin and Range province. Previously, the Railroad Valley Rhyolite was considered to be a lava flow with an autobrecciated top. However, this is unusual behavior for rhyolite, and field observations of the rhyolite did not indicate any solid flow-interior textures. A suite of samples were collected across the extent of this unit’s exposures. Slabs and thin sections were made from these samples, and examined under magnification for textures that would indicate the eruptive style of the rhyolite. All samples showed clastic textures. None of the samples showed intact flow textures, making it unlikely that this unit had flowed cohesively at the time of eruption. In addition, the samples all lacked pumice fragments. Matrix and clastic fragments all appeared to be devitrified, so the original presence, or not, of volcanic ash could not be verified, but the absence of pumice makes an ignimbrite origin of the rhyolite unlikely, as well. High-silica eruptions frequently take the form of domes, which commonly undergo a series of building and collapse events over the course of an eruptive cycle. Therefore, given the assembly of textures, the best interpretation is that the Railroad Valley Rhyolite is the result of pyroclastic flows produced by dome collapse.