EARLY PRIMITIVE VOLCANISM AT CLEAR LAKE VOLCANIC FIELD, CA

Located ~135 km north of San Francisco, Clear Lake volcanic field (CLVF) is the northernmost and youngest (~10 ka to ~2 Ma) of the volcanic centers distributed along the San Andreas Transform in the California Coast Ranges. Extensive mapping, geochemistry, geochronology, and petrology have been done at the Geysers steam field at the SW end of the CLVF, and on the <1.3 Ma rocks within the central CLVF, but much less is known about the early volcanic rocks that erupted between ~1.3 and 2 Ma. The main axis of this volcanism, which extends ~35 km from the SE edge of Clear Lake to the NW tip of Lake Berryessa, forms a nearly continuous broad upland plateau capped with thick lava flows and eroded scoria cones, but there are also additional isolated volcanic outcrops on the periphery. The lavas and tephras along this axis are high-MgO (8-12 wt% MgO), med-K₂O, calc-alkaline basalts and basaltic andesites that contain olivine (Fo_90-92) phenocrysts with Cr-spinel (Cr# = 81) inclusions +/- subordinate clinopyroxene, orthopyroxene, and plagioclase phenocrysts. High Mg#s between 74 and 77 and high Ni and Cr abundances (140-316 and 424-756 ppm, respectively) indicate that these are primitive, primary melts, likely from a depleted harzburgitic source. Lavas sampled from peripheral outcrops are high-MgO andesites and dacites with adakitic trace element signatures, phenocrysts of orthopyroxene (En₉₀) with spinel inclusions (Cr# = 82), +/- subordinate clinopyroxene and plagioclase. These peripheral andesites and dacites have high Mg#s (75-82) and high Ni (108-203 ppm) and Cr (213-370 ppm), indicating they derive from or interacted with a depleted ultramafic source. Chondrite-normalized REE patterns for these early primitive CLVF compositions are characterized by a negative slope in the light to middle REE range and a flat trend in the middle to heavy REE elements, with significant depletion of the heavy REE (Yb = 0.6-1.2 ppm) in the adakitic andesites and dacites compared to the basalts and basaltic andesites (Yb = 1.5-2.1). Adakitic signatures of intermediate lavas and the primitive nature of the basaltic lavas likely reflect the shallow slab window origin of the melts. Ongoing analyses will elucidate source conditions for the early primitive CLVF lavas and allow comparison with other older Coast Range volcanic fields to the south.