Enclave Compositions Indicate Multiple Felsic Components at Chaos Crags, Lassen Volcanic National Park, California

Abstract:

Chaos Crags, located in Lassen Volcanic National Park, is a series of 6 rhyodacite domes that exhibit dramatic evidence of magma mixing. Mafic enclaves from the Chaos Crags form two distinct textural groups: a finer-grained group containing abundant plagioclase crystals from the host rhyodacite, and a coarser-grained group generally lacking host phenocrysts. Enclave samples were collected from Dome B of the Chaos Crags with the intent of obtaining complete suites of the two textural groups. Geochemical data for the enclaves shows a weak correlation between texture and geochemistry. Notably, the geochemical data also shows two distinct chemical trends that appear to represent mixing with two different felsic components. The dominant trend shows mixing between the host rhyodacite and the mafic end member. The second trend is interpreted to result from mixing between the mafic end member and a cryptic felsic component that is distinct from the host rhyodacite. Modeling of major oxides, selected trace elements and rare earth elements suggests that, although the two clusters of enclaves mix towards different felsic end members, they appear start from the same mafic end member, which has a composition of approximately: SiO₂ 51.84%, CaO 10.45%, K₂O 0.69%, P₂O₅ 0.10%, MgO 5.35%, Na₂O 2.8%, FeO 8.25%, Fe₂O₃ 1.84%, Al₂O₃ 19.47% and TiO₂ 0.76%, a composition common in the Lassen region. Preliminary modeling results indicate that, at 75% SiO₂, the cryptic felsic component has a composition of: K₂O 5.25%, P₂O₅ 1.2%, Na₂O 8.25%, Al₂O₃ 14.65%, Fe₂O₃ 0.18%, FeO 0.88% and TiO₂ 1.46%, with negligible amounts of CaO and MgO. At 70% SiO₂ this composition is approximately: K₂O 4.3%, P₂O₅ 0.95%, Na₂O 7.05%, Al₂O₃ 15.66%, TiO₂ 1.31%, FeO 2.51% and Fe₂O₃ 0.55% with negligible amounts of CaO and MgO. It is likely that this previously unrecognized mixing trend represents crustal contamination of the mafic end member. A suite of samples with compositions intermediate between these two trends is also present.