REMNANTS OF ADAKITIC VOLCANISM ALONG THE EASTERN PERIPHERY OF THE CLEAR LAKE VOLCANIC FIELD TRACK THE EDGE OF THE RETREATING JUAN DE FUCA SLAB

Clear Lake volcanic field (CLVF) is the northernmost and youngest (~2.8 Ma to 8 ka) of the volcanic centers distributed along the San Andreas transform system in the California Coast Ranges. These volcanic centers are attributed to the northward migration of a slab window as the plate margin evolved from subduction to transform beginning ~30 Ma. The compositions and petrologic features of the volcanic rocks within these centers provide insight into tectonic and magmatic processes associated with melt generation and eruption, improving our understanding of volcanic hazards in the region.

Along the eastern periphery of the CLVF are eroded remnants of adakitic volcanism preserved as capping flows that form inverted valleys near Zim Zim Creek (ZZ) at the southern end of the CLVF, Grizzly Peak (GP) near the former McLaughlin gold mine, and Chalk Mountain (CM) at the northernmost end of the CLVF. Lava compositions vary from weakly adakitic (Sr/Y ~60) high-MgO (7.8 wt%) andesite at the ZZ flow to strongly adakitic (Sr/Y ~70-120) high-MgO (~5 wt%) dacite at the GP and CM flows. The high MgO (~5-7.8 wt%), Ni (110-210 ppm), and Cr (200-350 ppm) concentrations of the adakites indicate that silicic (60-68 wt% SiO₂) slab melts reacted with the mantle as they ascended.

The petrologic features of the adakitic lavas are variable with phenocrysts of olivine (Fo_88-90), enstatite, and augite in the ZZ lavas, enstatite and plagioclase in the GP lavas, and enstatite, plagioclase, and trace amphibole in the CM lavas. Enstatite phenocrysts (En_84-94) in the GP lavas contain small (~20 μm) partially resorbed cores of olivine (Fo_87-93) with the highest Ni concentrations (6500-9500 ppm) yet reported in natural olivine, consistent with an elevated olivine-melt partition coefficient for olivine growing from high-Ni silicic melt. The high-Ni olivine cores were preserved by enstatite growth as the magma ascended and aggregated in the crust. The ages of the adakites young to the north tracking the opening of the slab window which allowed hot sub-slab asthenosphere to ascend into the gap and melt the edge of the retreating Juan de Fuca slab. The compositions and petrologic features of the adakites record the conditions of slab-edge melting and reaction of the melts with the mantle prior to aggregation and eruption.