Journal of Petrology, Volume 40, Issue 5: May 1999.

Magmatic Interactions as Recorded in Plagioclase Phenocrysts of Chaos Crags, Lassen Volcanic Center, California

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, USA
²UNITED STATES GEOLOGICAL SURVEY, MENLO PARK, CA 94025, USA

The silicic lava domes of Chaos Crags in Lassen Volcanic National Park contain a suite of variably quenched, hybrid basaltic andesite magmatic inclusions. The inclusions represent thorough mixing between rhyodacite and basalt recharge liquids accompanied by some mechanical disaggregation of the inclusions resulting in crystals mixing into the rhyodacite host preserved by quenching on dome emplacement. ⁸⁷Sr/⁸⁶Sr ratios (~0·7037-0·7038) of the inclusions are distinctly lower than those of the host rhyodacite (~0·704-0·7041), which are used to fingerprint the origin of mineral components and to monitor the mixing and mingling process. Chemical, isotopic, 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, plagioclase, 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, partially resorbed plagioclase crystals enclosed within basaltic andesite inclusions, and partially resorbed plagioclase crystals in the rhyodacitic host are all similar. Rim ⁸⁷Sr/⁸⁶Sr ratios of the partially resorbed plagioclase 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 recycled back into the host rhyodacite. Textural evidence, in the form of sieve zones and major dissolution boundaries of the resorbed plagioclase crystals, indicates immersion of crystals into a hotter, more calcic magma. The occurrence of partially resorbed plagioclase together with plagioclase microlites and olivine crystals reflects disaggregation of inclusions and mingling of this material into the silicic host. These processes are commonplace in some orogenic magma systems and may be elucidated by isotopic microsampling and analysis of the plagioclases crystallizing from them.

Keywords: Chaos Crags;inclusion disaggregation;magma mixing; magmatic inclusions; microsampling

Pages 787-806