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Paper No. 62-4
Presentation Time: 8:30 AM-4:30 PM
GEOCHEMICAL CORRELATION OF BASALTS IN THE SYLVANIA MOUNTAINS, CALIFORNIA AND NEVADA
Isolated Neogene basalt flow outcrops in the Sylvania Mountains of eastern California and western Nevada overlie Paleozoic sedimentary rock, Jurassic granitoids and Neogene rhyolite flows. The lack of local feeder dikes suggests a distal eruptive source, possibly in the White Mountains 40 km to the west. Studies of basalt flows in the western Basin and Range demonstrate a compositional change at the latitude of the Sylvania Mountains at about 5 Ma that is related to magma passing through the subducting Farallon plate and the Mendocino triple junction. In this study, we combine whole-rock x-ray fluorescence (XRF) analysis on basalt sample along with 40Ar/39Ar dating in an attempt to correlate these isolated basalt flows. A basalt flow on the north side of Sylvania Canyon has an age of 11.5-11.7 Ma and a geochemical composition (Zr/Ba vs. Ce/Y) similar to other 11.5-11.7 Ma basalt flow to the west, including flows in the White Mountains. Basalt flows to the north along Palmetto Wash have a geochemical composition similar to the other 11.5-11.7 Ma basalts as well. A basalt flow that overlies the 4.5 Ma (whole-rock K-Ar on rhyolite) Pidgeon Spring tuff also has the same trace-element composition as the 11.5-11.7 Ma flows. We interpret the 11.5-11.7 Ma, geochemically similar, basalt flows as having erupted from the White Mountains and flowed 40 km east to the Sylvania Mountains, which must have been a topographic low at that time. There are three interpretations of the geochemical similarity of the Pidgeon Spring basalt to older basalt flows. First, the 50-year-old 4.5 Ma K-Ar date is incorrect and the Pidgeon Spring basalt is actually an older basalt flow. Second, the 4.5 Ma K-Ar date is correct and the timing of the change in basalt composition is not precisely 5 Ma. Third, the 4.5 Ma K-Ar date is correct and trace element composition is insufficient to differentiate basalt flows as previously thought. We are processing additional geochemical and geochronology samples to test these hypotheses.