The Sloan Sag: A Mid-Miocene Volcanotectonic Depression, North-Central McCullough Mountains, Southern Nevada

In the Hidden Valley area of the north-central McCullough Mountains, southern Nevada, mid-Miocene andesite and dacite domes, flows and pyroclastic units (the Sloan volcanics) partially fill a sag in the underlying Hidden Valley volcanics. The 13.5 km diameter sag formed during and/or after the eruption of the Sloan volcanics. Sagging was accommodated by a combination of movement on the McCullough Wash fault system, and subsidence into evacuated chambers. Major, trace and rare-earth element geochemistry suggests that the rocks of the Sloan volcanics belong to four groups, each of which were produced by partial melting of chemically distinct sources. With the exception of the Center Mountain dome complex, magmas rose rapidly without significant crystal fractionation or crustal contamination. The Mount Hanna andesite member of the Sloan volcanics erupted as a hot, dry aphyric lava by a mechanism of fire-fountaining from a depth of up to 25 km, precluding a Plinian-style ash-flow event. Eruptions of felsic-to-intermediate lavas by a lava-fountaining event have been described in other areas, but the Mount Hanna andesite represents the first documentation of such an eruption in the southern Basin-and-Range