PRIMITIVE VOLCANISM AT CLEAR LAKE VOLCANIC FIELD, CA INDICATES A PERIOD OF DEEP MAGMA ACCESS FACILITATED BY TRANS-TENSIONAL STRESS FIELD

Clear Lake Volcanic Field (CLVF) is the youngest (~2.8 Ma to 8 ka) and northernmost of several volcanic centers distributed along the San Andreas transform fault system in the California Coast Ranges. Sonoma Volcanic Field (SVF) is generally older than the CLVF with ages ranging from ~8 Ma to 2.5 Ma, however deposits from the two volcanic fields spatially overlap at the southern end of the CLVF and temporally overlap by ~300 ka. The geochemistry and petrology of these volcanic rocks provide insight into tectonic and magmatic processes associated with melt generation, storage, and eruption, improving our understanding of volcanic hazards in the region.

The earliest eruptive interval (~2.8 Ma to 1.3 Ma) in the CLVF produced dominantly primitive (Mg#: 65-80), basaltic to andesitic volcanism widely distributed over an area of about 50 x 60 km². The main axis of this volcanism is centered on a lineament of vents that generally young to the north which produced coalesced lava flows that presently cap a NW-SE trending plateau with peripheral isolated flow remnants outcropping to the east. Major and trace element whole-rock compositions demonstrate equilibration of these primitive magmas with mantle harzburgite, and thermobarometers using whole-rock compositions and olivine-spinel pairs indicate temperatures (1208-1325°C) and pressures (0.7 to 1.4 GPa) consistent with their derivation in the upper mantle. These characteristics, and a lack of significant fractionation or crustal assimilation indicate that there was no major crustal magma reservoir during this period. Instead, magmas were derived directly from the mantle with widespread eruptive vents concentrated along a northwest-trending zone of access to deep magma likely controlled by the regional right-lateral stress system.

In stark contrast are the preceding and contemporaneous late SVF eruptions and the following two intervals of CLVF eruptions, all of which are highly evolved dacites and rhyolites that were generated in long-lived crustal magma systems. The development of long-lived crustal magma reservoirs in this slab-window setting may be facilitated by increased flux into the system, changes in the crustal stress field, or both. The transition between the waning volcanism of SVF and the start of the CLVF provides an ideal setting investigate these processes.