Thesis

Enclave compositions indicate multiple felsic components at Chaos Crags, Lassen Volcanic National Park, California

The Chaos Crags, a series of 6 silicic lava domes located in the Lassen Volcanic Center of northern California, erupted approximately 1100 years ago and exhibit excellent outcrop exposure and abundant evidence of magma mixing, making them an ideal target for study of magmatic processes. Host material at Chaos Crags is rhyodacitic in composition whereas magmatic enclaves are basaltic to andesitic in composition. Enclaves from the study were classified into two suites, named “porphyritic” or “aphyric” based on the abundance of host-derived crystals. “Porphyritic” enclaves have a grey, fine-grained groundmass with abundant host-derived phenocrysts, while “aphyric” enclaves have a coarser groundmass with fewer host-derived phenocrysts. Both textural suites of enclaves range from fine-grained to coarse-grained. Field point counting was conducted on the talus slope of Dome B and in Chaos Jumbles of Dome C. Results from point counting show that “porphyritic” enclaves are more abundant than “aphyric” enclaves in both domes. Crystal Size Distribution (CSD) analysis revealed a weak link between enclave texture and CSD curve shape. A stronger link was noticed between enclave texture and plagioclase phenocryst shape. “Porphyritic” enclaves contain plagioclase phenocrysts that are abundant, acicular and needle-like, whereas “aphyric” enclaves contain plagioclase phenocrysts that are less abundant and blockier. Geochemical analysis of major oxides, trace elements and rare earth elements (REE) for Chaos Crags revealed two distinct chemical trends: a more dominant trend of primarily “porphyritic” samples that mixed towards the host rhyodacite, and a less dominant trend of only “aphyric” samples that mixed away from the host. It was determined that these two trends originate from the same mafic endmember, the composition of which was calculated using linear regression. It was determined that the small group of samples forming the secondary trend were contaminated by an unknown component, likely felsic in origin. The composition of this cryptic felsic component (CFC) was calculated at 70% SiO2 using linear regression. The geochemistry of regional crustal rocks from the LVC was compared to the composition of the CFC but none of them were a match. The calculated composition of the CFC bears some resemblance to minimum melts of granitic material. Several studies of minimum melts were examined in order to find compositions of rocks and emplacement scenarios similar to what was seen at Chaos Crags. It was concluded that crustal rocks seen in the LVC could provide the starting composition needed to generate a minimum melt of similar composition to the CFC.

Relationships

Items