CENOZOIC MAGMATIC EVOLUTION IN THE WESTERN VIRGINIA RANGE, WESTERN NEVADA: TRANSITION FROM SUBDUCTION- TO EXTENSION-RELATED MAGMATISM IN THE WESTERN GREAT BASIN

Magmatism in the Basin and Range has been largely driven by subduction and/or extension. The volcanic rocks of the Virginia Range, particularly middle Miocene intermediate volcanics, are thought to be associated with an ancestral Cascade arc generated from the subducting Juan De Fuca plate beneath western North America. The tectonic origin of younger (less than 12 Ma) basalt and rhyolite in the western Virginia Range (WVR) is not clear, however, because extension and subduction were both ongoing during this time. Several new 40Ar/39Ar dates and major and trace element geochemical analyses (18 samples) elucidate the petrogenesis and tectonic setting of Miocene and Quaternary volcanic rocks in the WVR.

Geochemistry of the middle to late Miocene volcanic rocks in the WVR shows a strong calc-alkaline signature and suggests a link with subduction-generated magmatism and the ancestral Cascade arc. Middle Miocene intermediate composition volcanism (Kate Peak volcanics) gave way to bimodal volcanism within the WVR area about 10 Ma, which may reflect the onset of regional extension at about 12.6 Ma. However, early bimodal volcanic rocks (~10 Ma Washington Hill rhyolite, 9.6 Ma Clark Mountain basalt, and 7.5 Ma Lousetown basalt) continue to show a subduction-related signature with low Niobium (Nb) and high Barium (Ba) concentrations. A gap in volcanism occurred from about 7.5 Ma until deposition of the 1.4 Ma McClellan Peak basalt. The McClellan Peak basalt shows a close affinity to extension-related magmatism, as evidenced by high Nb concentrations. Therefore, volcanic rock compositions suggest a change from subduction- to extension-related magmatism in the WVR between 7.5 and 1.4 Ma. This transition coincides with the passing of the Mendocino Triple Junction near the latitude of the WVR at about 4 to 6 Ma.