Diverse, discrete, mantle-derived batches of basalt erupted along a short normal fault zone: The Poison Lake chain, southernmost Cascades

Abstract

The Poison Lake chain consists of small, monogenetic, calc-alkaline basaltic volcanoes located east of the Cascade arc axis, 30 km ENE of Lassen Peak in northeastern California. This chain consists of 39 distinguishable units in a 14-km-long and 2-km-wide zone trending NNW, parallel to nearby Quaternary normal faults. The 39 units fall into nine coherent groups based on stratigraphy, field characteristics, petrography, and major-element compositions. Petrographic differences among groups are expressed by different amounts and proportions of phenocrysts. MgO-SiO 2 , K 2 O-SiO 2 , and TiO 2 -SiO 2 variation diagrams illustrate clear differences in compatible and incompatible elements among the groups. Variation of K 2 O/TiO 2 and K 2 O/P 2 O 5 with MgO indicates that most of the basalts of the Poison Lake chain cannot be related by crystal fractionation at different pressures and that compositions have not been affected significantly by incorporation of low-degree silicic crustal melt or interaction with sialic crust. Limited trace-element and whole-rock isotopic data also suggest little if any incorporation of upper-crustal material, and that compositional variation among groups primarily reflects source compositional differences. Precise 40 Ar/ 39 Ar determinations show that the lavas were erupted between 100 and 110 ka. The migration of paleomagnetic remanent directions over 30° suggests that the entire Poison Lake chain could represent three short-lived episodes of volcanism within a period as brief as 500 yr. The diverse geologic, petrographic, chemical, paleomagnetic, and age data indicate that each of the nine groups represents a small, discrete magma batch generated in the mantle and stored briefly in the lower crust. A NNW normal fault zone provided episodic conduits that allowed rapid ascent of these batches to the surface, where they erupted as distinct volcanic groups, each aligned along a segment of the Poison Lake chain. Compositional diversity of these primitive magmas argues against widespread, long-lived ponding of uniform basalt magma at the base of the crust in this region and against interaction with a zone of melting, assimilation, storage, and homogenization (MASH) in the lower crust.