INVESTIGATING MAGMATIC LULLS ALONG THE CORDILLERAN MARGIN, FROM THE NORTHERN SIERRA NEVADA TO THE KLAMATH MOUNTAINS PROVINCE

Arc magmatism fluctuates between periods of intense magmatic activity (flare-ups) separated by intervals where magma addition rates are lower (lulls). While flare-ups leave clear geological evidence, lulls are less well exposed, complicating our understanding of the causes of arc episodicity. By examining multiple arc segments, we may better define magmatism during lulls. This study investigates whether the Early Cretaceous Sierra Nevada lull period exhibits compositional uniformity or variability along strike by comparing plutons from the northern Sierra Nevada to the Klamath Mountains province.

Plutonic samples from the western region of the northern Sierra Nevada, emplaced between 140-130 Ma (Langenheim et al. 2021) were analyzed to determine their mineralogy and macroscopic to microscopic textures. Samples from the Bucks Lake (BL) and Granite Basin (GB) plutons range from diorite and quartz diorite to tonalite and granodiorite with small but measurable variation in mineralogy between samples. The BL and GB plutons share a similar abundance of plagioclase (35%-65%) micas (5%-25%), and pyroxene (15%-25%). However, amphibole is only present in GB, and quartz is absent in some BL samples. Accessory minerals include titanite (GB only), ilmenite, and magnetite. Texturally, these rocks are hypidiomorphic and phaneritic. GB plagioclase is oscillatory zoned, in contrast to the BL samples, where sieve textures were predominantly observed.

These samples were compared with literature data on contemporaneous plutons in the Klamath Mountains province where tonalite-trondhjemite magmas were emplaced, from 144 Ma to 136 Ma (Barnes et al. 1996). These plutons share similar textural features of plagioclase zoning, presence of hornblende, and accessory apatite, zircon, magnetite, and ilmenite (Barnes et al. 1996). Preliminary compilation of the Bucks Lake, Granite Basin, and Klamath Mountains plutons indicates that the studied plutons formed in the lull are relatively similar in composition, and all of the plutons from north- to-south are plagioclase-rich and alkali feldspar-poor. The similarity of coeval plutons along strike of the arc, suggests that the magma source and/or processes may be consistent across regions during the lull. Geochemistry of the samples will aim to test the spatial and compositional relationships further.