Stratigraphic, lithologic, and major element geochemical constraints on magmatic evolution at Lassen volcanic center, California
- Geological Survey, Menlo Park, CA (USA)
The evolution of the Lassen volcanic center is described in three stages. Stages 1 and 2 comprise the Brokeoff volcano, and 80 km{sup 3} andesitic stratocone, active from 600 to 400 ka. Brokeoff volcano is compositionally equivalent to the regional basaltic andesite to andesite volcanism in the Lassen region and is the result of structurally controlled focusing of the diffuse regional magic magmatism. Stage 3 comprises a silicic dome field and adjacent area of hybrid andesites and has a total volume of about 100 km{sup 3}. Volcanism during stage III was episodic and is subdivided into four sequences of lithologically and temporarily distinct lavas. Stage 3 began at 400 ka with a rhyolitic, caldera-forming pyroclastic eruption and chemically related lavas. Additional sequences of dacite erupted between 250-200 ka and 100-0 ka. Hybrid andesites erupted adjacent to the silicic dome field between 300 and 0 ka. Porphyritic andesite and dacite with high Al{sub 2}O{sub 3}, low TiO{sub 2}, medium K{sub 2}O and FeO/MgO ratios of 1.5-2.0 are the most abundant rock types in the Lassen volcanic center. However, the single most voluminous unit is sparsely phyric rhyolite pumice. Although major element variation can be modeled by fractional crystallization, petrographic and stratigraphic evidence indicates that magma mixing is an important but subtle process in Brokeoff lavas and suggests that lavas evolved in small independent batches. Disequilibrium mineral assemblages in the stage 3 lavas indicate that they are not directly derived from Brokeoff andesite by fractional crystallization. Mixing of silicic magma with regional mafic magma and disaggregation of andesite quenched magmatic inclusions play dominant roles in the compositional diversity of stage 3 lavas.
- OSTI ID:
- 5195620
- Journal Information:
- Journal of Geophysical Research; (United States), Vol. 95:B12; ISSN 0148-0227
- Country of Publication:
- United States
- Language:
- English
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