DOME ERUPTION, COLLAPSE, AND PYROCLASTIC FLOWS OF A MID-MIOCENE VOLCANIC SECTION IN THE CENTRAL MCCULLOUGH RANGE, SOUTHERN NEVADA

The central McCullough Range contains a record of over four million years of mid-Miocene volcanism. Work has focused on characterizing the section lying between Precambrian basement and the 15.2 Ma Tuff of Bridge Spring (TBS). Three main centers are associated with the section: the Cactus Hill (CH) volcano, a basalt-andesite composite cone and cinder cones; the Eldorado Valley volcano (EVV), composed of glassy andesite flows and a 200 m thick dacite block-and-ash deposit; and the McCullough Wash dome complex (MWDC), a series of dacite domes, flows, and a 100 m thick block-and-ash deposit. New findings include: 1) Variability within the Eldorado Valley breccia (EVB). The EVV breccia is composed of largely aphanitic (<5% phenocrysts) dacite blocks and bombs (10 cm-6 m). EVV breccia near the MWDC includes 1-2 m diameter hornblende and plagioclase bearing bombs, and blocks (5 m) of flow banded dacite. Crude bedding in EVV breccia reflects several 4-5 m thick block-and-ash flows, suggesting numerous explosive and/or dome collapse events. 2) Thin tuffs were discovered near both the EVV and MWDC, further supporting the interpretation of explosive volcanism during eruption of the two volcanoes. A 10 m thick tuff underlain by a surge deposit occurs at the top of the MW section and just below TBS. Two thin (4 m) tuffs underlain by surge deposits crop out above an EVV andesite flow and below TBS. The lower tuff is a moderately welded pyroclastic flow with aligned fiamme and inclined, partially collapsed pumice fragments. The upper tuff is also welded and similar in mineralogy and outcrop appearance to the 18.5 Ma Peach Springs Tuff. Although these tuffs are typically found stratigraphically close to the base of TBS, they do not resemble TBS texturally or mineralogically. Welded tuffs associated with dome eruption are rare; therefore these thin tuffs may have regional significance. 3) Detailed mapping of the MWDC indicates that it is composed of coalescing domes concentrated in a 1 km² area. Flows related to domes are short and plagioclase-rich. Toward the top of the MWDC section, debris apron breccia containing cold clasts gives way to bombs and a thick block-and-ash deposit. The MWDC is the oldest volcano. Evidence includes: CH dikes cutting the MWDC, a CH flow overlying a MWDC dome, and EVB pinching out against the southern margin of the MWDC