Seventh Annual V. M. Goldschmidt Conference 2422.pdf

MAGMA INTERACTIONS AT CHAOS CRAGS, LASSEN VOLCANIC NATIONAL PARK, CA. F. J. Tepley III1, J. P. Davidson1, and M. A. Clynne2. 1University of California, Los Angeles, Department of Earth and Space Sciences, Los Angeles, CA 90095, ([email protected], [email protected]); 2U.S. Geological Survey, 345 Middlefield Road MS910, Menlo Park, CA 94025

Chaos Crags comprises a group of silicic lava inclusions. More significantly though, the variability domes and associated tephra deposits that form part of 87Sr/86Sr in the interiors of the crystals, and the of the Lassen Volcanic Center, the southernmost core-to-rim decrease in 87Sr/86Sr ratios in both extension of current subduction-related magmatic resorbed and unresorbed dacite plagioclase crystals activity of the Cascades. The domes contain a suite suggest that mafic recharges occurred during earlier of mafic (basaltic andesite) magmatic inclusions phases of crystal growth. which increase in volumetric abundance with time throughout the emplacement sequence. The inclusions have distinctly lower 87Sr/86Sr ratios (~0.7037 to 0.7038) than the host dacite (~0.704 to 0.7041), which can be used to fingerprint the origin of mineral components in the mingled magmas. The difference in the isotope ratios between the magmatic inclusions and their host domes becomes smaller through the sequence of dome emplacement, which suggests that mixing between end-member sources is becoming more thorough before eruption. Large plagioclase crystals are found in both the inclusions and the host dacites. Electron probe traverses conducted on several plagioclase crystals enclosed within basaltic andesite inclusions show that the crystals are compositional (An40) similar to those in the host dacite. Many have spongy-texture overgrowths, interpreted as reflecting immersion of crystals originally crystallized in the dacite, into the hotter, higher Ca basaltic andesite magma. The 87Sr/86Sr ratios from the cores of the crystals in the inclusions are similar to those of the bulk host dacite, whereas the 87Sr/86Sr ratios of the rims are close to those of the basaltic andesite inclusions in which the crystal now resides, again suggesting derivation from the dacite. Micro-drilling of these plagioclase crystals has revealed a significant range in 87Sr/86Sr ratios across the crystals revealing a more complex history than simple incorporation of host phenocrysts into the inclusion magma. Nomarski imaging shows that the growth history of plagioclase crystals in both host and inclusions is defined by many fine oscillatory zones punctuated by multiple, major dissolution surfaces with irregular boundaries. Examination of phenocrysts in the dacite reveals that they too display core-rim variations in 87Sr/86Sr, but do not have the low 87Sr/86Sr spongy-textured overgrowths see in crystals from the inclusions. It appears then that recharge by basaltic andesite magmas occurred on several occasions, the most recent of which is represented by the mafic inclusions, preserved as evidence of incomplete hybridization by quenching on dome emplacement. The earlier recharge events are recorded by the mafic clots and resorbed olivine crystals in the dacite which are believed to represent thoroughly disaggregated