HIGH-SR VOLCANIC DOMES FROM THE LASSEN VOLCANIC REGION, SOUTHERNMOST CASCADE ARC, NORTHERN CALIFORNIA: IMPLICATIONS FOR ANDESITE AND DACITE MAGMA GENERATION

Abstract:

The Onion Butte (OB) and Barkley Mountain (BM) groups comprise two linear arrays of volcanic domes at a high angle to the Cascade arc axis (40 km SW of Lassen Volcanic Center). These groups have atypical petrographic and geochemical characteristics compared to other lithologies in the Lassen area. Petrographically, both groups are sparsely phyric containing hornblende and pyroxene, but lacking plagioclase; disequilibrium textures are not observed. The OB group is composed of dacite (64.0-65.2wt. % SiO₂) characterized by high Sr (> 1000ppm), with low ⁸⁷Sr/⁸⁶Sr (<0.7040), high (Sr/P)_N (2.8-3.5), no Eu anomaly, and moderate (La/Yb)_N (11-23). The BM group is composed of basaltic andesite to andesite (53.6-62.3wt. % SiO₂) characterized by lower Sr (354-611ppm), but higher ⁸⁷Sr/⁸⁶Sr (>0.7040), low (Sr/P)_N (1.1-2.1), no Eu anomaly, and moderate (La/Yb)_N (4.6-11.8). The distinctive characteristics of these two groups mirror those of the two end members of calc-alkaline primitive basalts recognized in the Lassen region (high Sr/P vs. low Sr/P). We invoke different sources and processes to explain the origin and evolution of these groups. The BM basaltic andesites are likely derived from fractionation of low Sr/P basalts and the BM andesites are successfully modeled by fractional crystallization of BM basaltic andesites with a fractionating assemblage that contains <10% plagioclase. In contrast, the OB dacites share geochemical characteristics with some adakites, suggesting an origin by partial melting of the subducting slab. However, partial melting models of the subducting Gorda plate at eclogite facies conditions failed to reproduce the observed trends of the OB dacites. Partial melting of an amphibolitic lower arc crust with a source composition similar to high Sr/P calc-alkaline basalts successfully reproduces observed trends. Geochemical characteristics and results from modeling suggest Sr concentrations may be a useful source indicator in the Lassen region.