ANCESTRAL CASCADES VOLCANIC CENTERS OF THE FEATHER AND YUBA RIVER DRAINAGES, NORTHEASTERN CALIFORNIA

New geochemistry and Ar/Ar geochronology show extinct volcanic centers in the northern Sierra Nevada are consistent with the ancestral Cascades arc in this region. Westward migration of the arc is thought to result from post Laramide steepening of the Farallon slab, with volcanism active in the northern Sierra during the middle Miocene and Pliocene. Samples were collected at these centers to observe potential changes in the arc as it migrated westward. Volcanic activity in this region ceased as the Mendocino Triple Junction (MTJ) migrated north of the area.

Two of the basalts erupted through the Middle Jurassic Haypress Creek pluton. A basalt sampled at Haskell Peak (4.7 ± 0.04 Ma) and a Miocene basalt (9.9 ± 0.9 Ma) about 9 km southeast erupted through granodiorite, though the latter may have cut gabbroic-monzonite rocks at depth. There are significant differences in some of the trace elements, with the younger Haskell Peak basalt having lower concentrations of large ion lithophile elements (LILE) and light rare earth elements (LREE).

About 26 km west of Haskell Peak, a basalt from Democrat Peak (4.5 ± 0.13 Ma) erupted through the Feather River ultramafic belt. Geochemistry of this basalt shows slightly lower LILE and LREE compared to the basalt at Haskell Peak. Approximately 32 km northwest of Democrat Peak, a basalt geochemically alike to Democrat Peak shows further decreases for some of the LILE, and had the least amount of silica of the samples. Previous researchers found modern Cascades basalts to be lower in LILE and LREE compared to the ancestral arc. These new data suggest as the arc moved west, it became more geochemically similar to the modern Cascades.

Andesites are < 60% silica, except for a platy andesite along Eureka Ridge (4.1 ± 0.24 Ma). This andesite erupted through metasediments of the Shoo Fly Complex. Other andesites erupted through a variety of metamorphic rocks. LILE and LREE do not show the same patterns as with the basalts but reveal some covariance with the amount of silica.

These geochronological and geochemical data suggest the ancestral Cascades arc likely reached its current configuration before migration of the MTJ shut down volcanic activity. Composition of basalts may reflect changes in subducting slab geometry, while geochemistry of andesites may be more influenced by the variable Sierran crustal rocks.