EVIDENCE FOR DOME COLLAPSE AND COEVAL MAFIC-FELSIC VOLCANISM IN THE CENTRAL MCCULLOUGH RANGE, NEVADA

The McCullough Range was not highly disrupted by faulting during Miocene extension and provides a unique view of a late-Tertiary volcanic field in the Northern Colorado River Extensional Corridor. Our work focused on the volcanic section lying beneath the tuff of Bridge Spring (15.2 Ma) and above Peach Springs Tuff (18.5 Ma) and identified three volcanic centers. A broad basalt-andesite cone, informally named the Cactus Hill volcano (CHV), nearly 2 km in diameter, contains a 200 m thick section of basalt and andesite flows and agglomerates. Basalt (46-52% SiO2) with olivine and clinopyroxene phenocrysts in a vitric matrix is interbedded with andesite (55-57% SiO2) containing hornblende, olivine and pyroxene. Two basalt dikes (each 2-3 m wide) cut the CHV. Intruding the CHV on its western flank are at least 8 dacite (61-64% SiO2) domes. Debris aprons associated with domes are interbedded with basalt and andesite. Dacite contains small plagioclase and occasional hornblende phenocrysts. Also present are xenocrysts and glomerocrysts of clinopyroxene and plagioclase. Located 2 km to the southwest is a dome complex (DC) composed of dacite (63-65% SiO2) with large (2-4 mm) plagioclase phenocrysts, hornblende, and glomerocrysts of plagioclase and clinopyroxene. Breccia representing dome growth and collapse crops out about the DC. The Eldorado Valley Volcano (EVV), a series of dacite domes and flows, 2.5 km west of the CHV, is the source of a 250 m thick breccia unit (Eldorado Valley breccia) that crops out between and is interbedded with rocks of the DC and CHV. Eldorado Valley breccia is a block and ash deposit (locally containing beds 1.5 m thick) containing bombs and clasts. Bombs are 10 cm to 6 m in size and are recognized by radial fractures and breadcrust surfaces. Clasts range in size from <1 cm to 3 m. Bombs and clasts are usually vitric aphanitic dacite (68% SiO2), but may include biotite and hornblende phenocrysts; all contain small clinopyroxene/plagioclase glomerocrysts. The thickness of the Eldorado Valley breccia represents both debris aprons from dome growth and deposits from partial collapse of domes. Coeval mafic-felsic volcanism in the CHV is demonstrated by the interbedding of EVV dacite breccia and CHV dacite dome debris with CHV andesite and basalt.