title: High-Sr Volcanic Domes from the Lassen Volcanic Region, Southernmost Cascade Arc, Northern California: Implications for Andesite and Dacite Magma Generation
Creators: name: Stout, Christina M.
Publisher: name: Western Washington University
Year: 2015

description: "<p>The Onion Butte (OB) and Barkley Mountain (BM) groups are two lineaments of volcanic domes and lava flows located with the Lassen segment of the southernmost Cascades, northern California. The OB group (<sub>~</sub>2.5 Ma) consists of 13 domes that are dominantly dacitic, but span the range from andesite to dacite. The BM group (<sub>~</sub>1.5 Ma) comprises 21 domes that range from basaltic andesite to andesite, but are mostly andesitic.</p>
<p>The lavas of both groups are petrographically similar, but differ geochemically. The lavas are fine-grained, sparsely phyric containing needle-like hornblende phenocrysts, but lack the large plagioclase phenocrysts so characteristic of typical andesitic to dacitic suites. While the groups share some geochemical characteristics, their differences are significant. The lavas of the OB group have high Sr concentrations (> 1,000 ppm), low abundances of Rb and Pb, high (Sr/P)<sub>N</sub>, and have isotopic compositions that are similar to the high (Sr/P)<sub>N</sub> primitive magmas in the Lassen region. This group has the highest Sr concentrations and lowest <sup>87</sup>Sr/<sup>86</sup>Sr compositions of any silicic lavas in the Lassen segment. The BM group is defined by higher Pb abundances, lower Th concentrations, low (Sr/P)<sub>N</sub>, and isotopic compositions similar to the low (Sr/P)<sub>N</sub> primitive magmas of the Lassen segment. The andesites of the BM group are subdivided into two subgroups based on HREE concentrations: subgroup 1 has no variation in HREE concentrations and subgroup 2 has variable HREE concentrations.</p>
<p>Regional calc-alkaline mafic volcanism in the Lassen segment forms a compositional continuum that has been generally divided into two geochemical groups: high (Sr/P)<sub>N</sub> (> 3.3) with MORB-like isotopic compositions and low (Sr/P)<sub>N</sub> (< 3.3) with OIB-like compositions. The basaltic andesites from the Barkley Mountain group are geochemically and isotopically similar to the low (Sr/P)<sub>N</sub> mafic magmas. It is likely the basaltic andesites were generated by fractionation from those parental mafic magmas. The OB group are geochemically similar to the high (Sr/P)<sub>N </sub>magmas which are likely sources for the dacites of the Onion Butte group.</p>
<p>The Barkley Mountain andesites can be produced by simple fractional crystallization from Barkley Mountain basaltic andesites. Fractional crystallization models require moderate amounts of fractionation (20-40%) with hornblende ± garnet in the fractionating assemblage to recreate observed REE patterns and HREE concentrations.</p>
<p>In contrast, the origin of the single sample of the Onion Butte andesite proved more difficult to constrain. While major element data suggests simple mixing between the OB basaltic andesite and dacites to produce intermediate andesites, the Ni, Sr, and Nb concentrations of the andesites belies such a simplistic explanation. Addition or subtraction models of mafic material (e.g. olivine) to the dacites to reproduce andesite compositions were unsuccessful. The relationship between the Onion Butte andesite and the dacites is unclear, but both share the same geochemical and physical characteristics of the group.</p>
<p>Partial melting of the lower continental crust with a source composition similar to the high (Sr/P)<sub>N</sub> mafic magmas results in geochemical characteristics of the Onion Butte dacites. Models were successful at reproducing observed OB dacite concentrations after 20% melting with hornblende ± garnet in the melting assemblage. Partial melting is consistent with published thermal calculations that demonstrate emplaced basaltic magmas in the lower continental crust as the heat source to induce melting.</p>
<p>Although some geochemical characteristics of the Onion Butte dacites (high Sr, high Sr/Y, and low <sup>87</sup>Sr/<sup>86</sup>Sr isotopic compositions) are shared with proposed slab derived melts, partial melting modeling using an eclogite mineralogy fails to reproduce observed Sr/Y concentrations. Low La/Yb ratios in the OB dacites also preclude this slab melt origin.</p>
<p>A petrogenetic model is offered that demonstrates that: 1) BM basaltic andesites are related to regional low (Sr/P)<sub>N</sub> magmas, 2) BM andesites can be generated through fractional crystallization of BM basaltic andesites; 3) OB dacites are thought to be generated by partial melting of amphibolitic lower continental crust similar in composition to the high (Sr/P)<sub>N</sub> primitive magmas; 4) the distinctive geochemical characteristics (high Sr, high Sr/Y, low <sup>87</sup>Sr/<sup>86</sup>Sr isotopic compositions) of the Onion Butte group likely reflects the rarity of the high (Sr/P)<sub>N</sub> primitive magmas in the Lassen segment; and 5) Sr concentrations can be used as a discriminator of magma sources in the Lassen region; and 6) both groups equilibrate with a mineral assemblage that requires hornblende ± garnet and minor plagioclase.</p>"