Magmatic interactions as recorded in plagioclase phenocrysts of Chaos Crags, Lassen Volcanic Center, California

The silicic lava domes of Chaos Crags in Lassen Volcanic National Park cont ain a suite of variably quenched, hybrid basaltic andesite magmatic inclusi ons. The inclusions represent thorough mixing between rhyodacite and basalt recharge liquids accompanied by some mechanical disaggregation of the incl usions resulting in crystals mixing into the rhyodacite host preserved by q uenching on dome emplacement. Sr-87/Sr-86 ratios (similar to 0.7037-0.7038) of the inclusions are distinctly lower than those of the host rhyodacite ( similar to 0.704-0.7041), which are used to fingerprint the origin of miner al components and to monitor the mixing and mingling process. Chemical, iso topic, and textural characteristics indicate that the inclusions are hybrid magmas formed from the mixing and undercooling of recharge basaltic magma with rhyodacitic magma. All the host magma phenocrysts (biotite, plagioclas e, hornblende and quartz crystals) also occur in the inclusions, where they are rimmed by reaction products. Compositional and strontium isotopic data from cores of unresorbed plagioclase crystals in the host rhyodacite, part ially resorbed plagioclase crystals enclosed within basaltic andesite inclu sions, and partially resorbed plagioclase crystals in the rhyodacitic host are all similar Rim Sr-87/Sr-86 ratios of the partially resorbed plagioclas e crystals in both inclusions and host are lower and close to those of the whole-rock hybrid basaltic andesite values. This observation indicates that some crystals originally crystallized in the silicic host, were partially resorbed and subsequently overgrown in the hybrid basaltic andesite magma, and then some of these partially resorbed plagioclase crystals were recycle d back into the host rhyodacite. Textural evidence, in the form of sieve zo nes and major dissolution boundaries of the resorbed plagioclase crystals, indicates immersion of crystals into a hotter, more calcic magma. The occur rence of partially resorbed plagioclase together with plagioclase microlite s and olivine crystals reflects disaggregation of inclusions and mingling o f this material into the silicic host. These processes are commonplace in s ome orogenic magma systems and may be elucidated by isotopic microsampling and analyses of the plagioclases crystallizing from them.