CONTRASTING STYLES OF VOLCANISM ALONG THE EAST SIDE OF NAPA VALLEY, CA

Along the east side of Napa Valley, three volcanic centers in the Sonoma Volcanic field differ in eruptive style, age, chemistry and extent of hydrothermal alteration. Recognition of eruptive units associated with each of these volcanic centers may provide a basis for unraveling the amount of offset in the western, more disrupted parts of the Sonoma volcanic field.

Volcanic rocks at the southern end of Napa Valley include a folded sequence of widespread 5.4-4.65 Ma ash-flow tuffs and tephra overlain by a field of coalescing 4.8-4.6 Ma flow-domes. A circular gravity low east of the city of Napa may represent a caldera that was the source of the older rocks; small volume flow-domes may represent subsequent eruptions along the caldera margin. Unconformably overlying these rocks is a gently dipping rhyolitic sequence that includes both flows and ash-flow tuffs. Hydrothermal alteration is absent throughout southern Napa Valley.

A thick accumulation of basaltic-andesite lava flows and lahars cut by north-striking rhyolite dikes crop out near Stags Leap in central Napa Valley. Fine-grained 6.0 Ma granitic rocks near the center of the basaltic-andesite exposures may represent a cogenetic pluton in the core of the andesitic stratovolcano. Hydrothermal alteration in the pluton consists of vein-related argillic alteration with elevated concentrations of Ag, As, Hg, and Zn. Volcanic rocks are unaltered.

In the northern part of Napa Valley, regionally extensive young ash-flow tuffs are associated with collapse calderas on the eastern side of the valley. Mt. St. Helena is a resurgent caldera that preserves >1200 m of intracaldera tuff and breccia beds, with a series of ring domes along the caldera margin. Based on similar 2.83 +/-0.08 Ma age, composition, and phenocryst assemblage, the Franz Valley tuff is an outflow unit from this caldera. Megabreccia beds exposed south of Mt. St. Helena suggest the presence of additional caldera collapse structures that may correlate with other ash-flow tuffs in northern Napa Valley. Widespread hydrothermal alteration occurred shortly after development of the Mt. St. Helena caldera and may be responsible for low magnetic values in the northern part of Napa Valley. Correlation of outflow units with their sources may provide piercing points for the West Napa and Maacama Faults.