Textural analysis of mafic enclaves as an insight into magma mixing processes at Chaos Crags, Lassen Volcanic Center, California

Thesis (M.S., Geology) -- California State University, Sacramento, 2011.The Chaos Crags are a series of volcanic domes located in Lassen Volcanic Center, southernmost Cascades Range. The six domes erupted approximately 1100 years ago. The host rock is dacite, which is compositional similar in all domes at (66-69 wt. % SiO2), and differs from the mafic enclaves that range in composition from (53-61 wt. % SiO2). The enclaves result from two distinct and thermally different magmas mixing and can provide an insight into the processes of magma mixing. Five texturally different enclaves types were identified. To determine abundance of the enclaves in each dome, 113 point count stations were completed in the dome complex talus slopes. Previously collected and new samples were photomircrographed and plagioclase crystals were hand traced to be processed in Crystal Size Distribution (CSD), which determines nucleation and growth time of crystals. Previously completed geochemical data was used to determine if the enclaves and host were similar or different in composition. The results of observation at Chaos Crags were 1) The total abundance of enclaves increases with eruption of domes. 2) There are distinctive abrupt increases in the total abundance of enclaves between eruption of domes B and C, domes C and D. \ud 3) There are more modest increases in the total abundance of enclaves between eruption of domes A and B, domes E and F. 4) Although it seems likely that all enclave types are present in each dome, changes in distribution of enclave type seem to correlate with the increase in total abundance. 5) Host dacites show a narrow range in composition while enclaves show a mixing trend from a more mafic source toward the host dacite. 6) There is a clear link between enclave type and geochemistry. The most mixed enclave types are type 1. The least mixed are types 3 and 4. The magma mixing model proposed is one of repeated injections of small batches of mafic magma, either injected as a fountain or ponded at the base of the magma chamber. Also the mafic magma injections are the suggested cause of eruption. Enclaves present are directly related to the type of recharge event. Disaggregation of the enclaves occurred in the conduit during the eruption. This thesis was just an initial step using CSD and geochemical data leading to some surprising results and further research should be conducted.Geolog