140 Microscopic Petrography – Domes and Flows East of Mount Scott

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

Microscopic Petrography


Domes and Flows East of Mount Scott

Except for the andesite forming the dome of Dry Butte, all the flows and domes beyond the eastern base of Mount Scott are composed of microvesicular pyroxene dacite. Since none of them were discussed by Patton, it seems advisable to summarize their principal features.

The andesite of the Dry Butte dome (analysis no. 17) is the only true hornblende andesite known among the surface extrusions of the Crater Lake region. Such lavas are in fact extremely rare in the southern part of the High Cascades. Hornblende andesite forms the summit dome of Rustler Peak and the Black Butte dome at the west base of Mount Shasta, but these are the only occurrences so far recognized throughout a distance of 80 miles. Thayer has likewise remarked on the paucity of hornblende-bearing andesites in the north-central High Cascades of Oregon. There, also, they were erupted as viscous flows or domes. Again, the plugs (domes) of Mount St. Helms are rarely devoid of hornblende. The mineral, therefore, seems to be typical of the viscous, acid andesite and dacite protrusions of the Cascade belt.

The andesite of Dry Butte is a dense, pale-gray to pink lava carrying abundant prisms of oxyhornblende up to 3 mm. in length. In some samples, the mineral constitutes as much as 10 per cent of the volume. Many of the crystals are bordered by magnetite, some are entirely replaced by ore, and a few are pseudo-morphed by a granular mosaic of augite and magnetite. Accompanying the oxyhornblende are small, well terminated prisms of strongly pleochroic hypersthene and anhedral grains of green augite. Together these accessories make up approximately 2 per cent of the whole. One-third of a typical sample is composed of large plagioclase phenocrysts, marked by intense oscillatory zoning, marginal embayments, and spongy inclusions of glass. In composition, these phenocrysts range from labradorite to acid andesine. The remainder of the rock consists of a dense pilotaxitic groundmass of microlithic oligoclase with intergranular pyroxene and ore, and abnormally abundant cristobalite and tridymite.

Occasional small crystals of oxyhornblende have been observed in some of the dacites east of Mount Scott, but even where they are present they are far subordinate to pyroxene. The typical dacite of both the domes and the flows is a pale-gray, crumbly, almost pumiceous, and highly porphyritic lava, characterized by large zoned crystals of andesine-labradorite (25-35 per cent), and smaller prisms of hypersthene and augite (5 per cent). Usually the two varieties of pyroxene are equally developed,-but where this is not the case, hypersthene predominates.

The main difference between the dacites is in the nature of the groundmass. Though none now contain glass, it is likely that in most the finely crystalline matrix is a product of devitrification, for perlitic cracks are widespread. In many samples, the matrix appears to be a cryptofelsite crowded with tiny microliths of oligoclase, irresolvable trichites, and specks of cristobalite. In others, the base consists of fibrous and brownish positive spherulites up to 5 mm. across, separated by clearer, micrographic areas that seem to be composed of feldspar and tridymite. Wherever open cavities exist, they are almost invariably lined in part by cristobalite.

Basic inclusions in the dacites are both rare and small, and resemble those already described from among the andesites of Mount Mazama.


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