GLOMEROCRYSTS, ANTECRYSTS, AND PHENOCRYSTS: EVIDENCE FOR COMPLEX MAGMATIC EVOLUTION OF THE BASALTS OF STEVENS CAMPGROUND, POISON LAKE CHAIN, SOUTHERNMOST CASCADES

We present new electron microprobe (EMP) mineral data focused on one lithologic group in the basalts of the Poison Lake Chain (PLC). The PLC is located east of Lassen Peak in the southern Cascades and encompasses 39 units that have been subdivided into 9 groups based on chemistry and mineralogy (Muffler et al., 2011). One group of 5 basaltic units, the basalts of the Stephens Campground (bs group), has major and trace elements that are highly varied. Our previous work in the bs group revealed two spatially distributed sub-groups with distinct whole rock trace element compositions: units 1-3 with similar trace element compositions over a range of major elements and units 4-5 with more varied trace element compositions with respect to major elements (Salchert et al., 2018). Our previous study also found that mineral textures were consistent with these two sub-groups: glomerocrysts are more prevalent in units 4-5 compared to units 1-3. The current study focuses on the relationship of crystal cargo to the whole rock. Crystal cargo in all units of the bs group includes glomerocrysts, antecrysts and phenocrysts of olivine, plagioclase, and clinopyroxene in a groundmass dominated by plagioclase and clinopyroxene. EMP analyses of the crystal cargo reinforce the subdivisions determined by whole rock analyses. The core compositions of all crystal cargo in units 4-5 are close to equilibrium with the whole rock and rims show normal zoning, consistent with fractionation. Units 1-3 have evidence of a more complex magmatic history. Glomerocryst and antecryst core compositions are out of equilibrium with the whole rock and rims are reversely zoned. Phenocrysts in units 1-3 are in equilibrium with the whole rock and rims and groundmass are normally zoned reflecting simple fractionation. Examination of crystal cargo compositions present in bs group basalts distinguishes their magmatic history and characterizes crustal processes that modified the bs group. This study has bearing on our understanding of ways that basalt is modified as it moves through the crust in the southern Cascades.