CHARACTERIZATION OF THE YANA VOLCANIC CENTER

The remnants of the Yana Volcanic Center are located south of Lassen Volcanic National Park and represent the southernmost extent of Cascade volcanism at 3 Ma bp. Volcanic activity in the Yana Volcanic Center (YVC) occurred between 2,695 +/- 79 and 3,030 +/- 38 ka based on radiometric K-Ar and Ar-Ar dating of samples collected from the YVC. This coincides with the 2.6 - 3.4 Ma ages established for two tuff units found within the Tuscan formation (Evernden et al., 1964 & Blake Jr. et al., 1999). The exposed remnants of the central region of the YVC are hydrothermally altered, K-depleted mafic to intermediate lavas and breccias. This central region is surrounded by block and ash flow deposits interspersed with lavas. In many places these flows were remobilized, formed lahar flows and lithified. Previous work using lithologic, geographic, and geochemical data indicates that the YVC is a source for the Tuscan Formation lahar flows, which form the western foothills of the Sierra Nevada in Northern California. Preliminary fieldwork within the Tuscan Formation has identified a diversity of lahar flow units based on variations of clast lithologies, which range from mafic to intermediate composition. SiO2 concentrations range from 52-57% and MgO concentrations range from 4-12%. Petrographic observations and XRF geochemical analyses are used in this study to describe the lavas of the YVC and to correlate them with Tuscan Formation lahar flow units. The Lassen Volcanic Center is also used as a better exposed analog to better describe the range of volcanic products of the YVC. Early results of this work indicate the YVC included a variety of mafic and intermediate vents and flow units, similar to the range of compositions seen in monogenetic volcanoes that construct the platform of the modern Cascades.