Complex zoning and resorption of phenocrysts in mixed potassic mafic magmas of the Highwood Mountains, Montana

Disequilibrium phenocryst assemblages and complex compositional zoning in clinopy-roxene, mica, and olivine phenocrysts provide a detailed record of multiple mixing, fractional crystallization, and degassing events during the high-level evolution of potassic mafic magmas in the Eocene Highwood Mountains province. The contrasting phenocryst assemblages of minettes (diopside + phlogopite ± olivine) and mafic phonolites-shonkin-ites (salite + leucite ± olivine) permit unambiguous documentation of mixing between these two magma types and also between more primitive and more evolved members of each type.

The varied behavior of crystals mixed into disequilibrium liquids, deduced from detailed textural and microprobe analyses, is consistent with the results of experimental studies of plagioclase dissolution. Phlogopite and diopside xenocrysts within more evolved mafic phonolite liquids initially underwent peripheral resorption. Further dissolution of diopside produced a network of interior cavities that were subsequently plated by salite crystallized from the host magma. In contrast, salite xenocrysts within more primitive mafic phonolite liquids underwent partial dissolution at their margins, yet remained eu-hedral. In some cases dissolution was restricted to a single, compositionally distinct sector. Halos of Fe-Al-rich salite subsequently formed around the melt-filled cavities by diffusion.

Complexly zoned salite and biotite crystals containing one or more sharply bounded, epitaxial bands of diopside and phlogopite, respectively, are common in the mafic phono-lites and shonkinites. On the basis of features such as the different salite compositions on either side of diopside bands and their lack of leucite inclusions and oscillatory zoning, it is argued that each band in a crystal (rarely more than six) is a record of a mixing event with minette magma.

We infer that the contrasting phenocryst assemblages of the compositionally similar minettes and mafic phonolites largely reflect differences in H_zO activity during their crystallization, and we propose that degassing of ascending minette magmas was an important petrogenetic process. We conclude that the high-level evolution of the Highwood magmatic system involved repeated mixing in multiple reservoirs among batches of variably fractionated and degassed mafic phonolite magma and periodic influxes of more primitive, undegassed minette magma.