130 Andesites of Mount Mazama and Scott – Andesites of the Phantom Vent and Cone

The Geology of Crater Lake National Park, Oregon With a reconnaissance of the Cascade Range southward to Mount Shasta by Howell Williams

Microscopic Petrography

Andesites of Mount Mazama and Scott

     Andesites of the Phantom Vent and Cone

The material composing the plug of the Phantom vent is mainly a gray-green, vesicular pyroxene andesite. Darker, less vesicular andesites cut the paler lava in almost vertical bands. The microscope shows that the difference between these two types is merely one of texture. Whereas in the paler lava the groundmass is pilotaxitic, in the darker lava it is hyalopilitic. In both types, zoned and corroded phenocrysts of andesine-labradorite are plentiful. Porphyritic augite is invariably subordinate to hypersthene, and olivine is only a minor accessory. Most of the olivine and hypersthene has been altered to greenish serpentine and talc; the augite, on the other hand, is only occasionally altered. In some sections, relic grains of quartz, possibly xenocrysts, are present. Tridymite occurs sparingly. Finally, in the more vesicular lavas there is an abundance of waxy, olive-green chlorophaeite.

Flows of similar, slightly altered pyroxene andesite form the greater part of the Phantom cone. Interbedded with them are thick deposits of tuff breccia. These are well exposed on Phantom Ship and the adjacent headland. Characteristically, the pyroclastic rocks consist of greenish, angular fragments up to 2 feet in diameter, set in a paler grayish-green amygdaloidal matrix. Obviously, these rocks have been subjected to widespread hydrothermal action. None of the original ferromagnesian minerals remain. The irresolvable brownish-green substance replacing them is presumably serpentine, possibly accompanied by chlorite. Despite these alterations, both the porphyritic and the microlithic feldspars are unchanged.

The alterations noted above are more pronounced in the angular fragments than in the enclosing tuff, and account for their darker-green color. Presumably, therefore, the fragments were derived by explosive  shattering of lavas that had already suffered propylitization. Hydrothermal activity continued after the eruptions and was perhaps responsible for the cavernous weathering of the tuffs. It was at this time that the well formed crystals of quartz and calcite were deposited in the geodes and veins.

Many dikes cut the tuff breccias on Phantom Ship. The interior of most of these intrusions consists of relatively fresh, pilotaxitic hypersthene andesite, but the margins have been deeply bleached by late solutions. The pyroxenes are entirely replaced by serpentine (?bowlingite) and quartz, though the feldspar is fresh. Blotches of “limonite” and veins of hematite cut the rocks haphazard. In the interior of the dikes, on the contrary, the hypersthene and augite are either wholly unaltered or at most but slightly changed to serpentine.


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