COMPLEX CRUSTAL PROCESSES RECORDED IN PLAGIOCLASE PHENOCRYSTS IN A SINGLE FLOW IN THE POISON LAKE CHAIN, LASSEN REGION, SOUTHERN CASCADES

The Poison Lake Chain (PLC) is a series of 39 contemporaneous (~100ka) basaltic cones and flows, located east of the Lassen Volcanic Center in the southernmost Cascades. The basalts have been subdivided into 9 groups based on major-element geochemistry and mineralogy and are dominated by primitive basaltic compositions (MgO >6% and high Ni and Cr). However, one group, the basalts of Bogard Buttes (BoBB), represents more evolved compositions with MgO<6% and can provide insight into crustal processes experienced by basalts on their traverse through the crust in continental arcs. Previous work on trace element compositions in the BoBB suggests that mapped flows are homogenous and likely represent single, monogenetic eruptions. We focus on a series of 10 channel overflows associated with a single flow of BoBB (unit 5) and present new petrographic and chemical data for plagioclase phenocrysts that allow us to determine a variety of crustal processes occurring before and during eruption of a single batch of basalt. Plagioclase core compositions in BoBB unit 5 are variable but are dominated by An₈₄-An₈₇; rim compositions are consistent with An₇₂-An₇₇. However, BSE images reveal complex textures indicating that a range of crustal processes occurred. Plagioclase phenocrysts in BoBB unit 5 show a variety of complex textures that include simple zoning, oscillatory zoning, sieved texture, and glomerocrysts, as well as complex combinations of two or more textures. These textures are evidence of complex crustal processes that may include remobilization of earlier crystallized material, mixing with similar composition magmas, circulation of crystals within a crustal magma chamber, simple fractionation or combinations of several of the aforementioned processes. Consistent core and rim compositions confirm the idea that these units represent a single batch of magma; however, phenocryst populations reflect more complex processes within the crust. The complex textures found in unit 5 of BoBB provide insight into the compositions of basaltic magmas generated in arcs and the complex crustal processes experienced on their journey from mantle to surface.