132 Microscopic Petrography – The Southern Dacite Flows and Domes

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

Microscopic Petrography

 

The Southern Dacite Flows and Domes

The oldest dacites of Mount Mazama form the Vidae and Grayback ridges south of Tututni and Maklaks passes, respectively. Patton has discussed them under the title “Sun Creek flow,” noting that in general they are more lithoidal and have suffered more devitrification than the dacites erupted from the Northern Arc of Vents.

In general the dacites erupted as flows are much more strongly banded and less pumiceous than those which form the domes. All, however, have the following features in common: (1) abundant zoned phenocrysts of plagioclase, commonly corroded and rich in glass inclusions (10 to 20 per cent); (2) accessory and smaller phenocrysts of pyroxene, hypersthene usually predominating (1 to 5 per cent); (3) abundant tridymite. In many specimens, tridymite flakes are so plentiful that the rocks glisten like frost. Cristobalite, which is almost ubiquitous among the basalts and basaltic andesites of the Crater Lake region, has not been detected among these southern dacites.

Among the phenocrysts, quartz, mica, and orthoclase appear to be absent, and porphyritic oxyhornblende is extremely rare. Much of the porphyritic plagioclase shows intense oscillatory zoning, the general change being from labradorite in the center to oligoclase at the margins. To judge by the strong dispersion and small optic angle, some of the oligoclase may be rich in potash. The porphyritic hypersthene and augite not uncommonly show a peripheral separation of magnetite and hematite, and their normal greenish color gives way marginally to shades of reddish brown.

Glass may formerly have been plentiful in the groundmass of these rocks, for perlitic fractures are common. Now, however, it is generally absent or present only in insignificant amount. It may be that devitrification was accelerated by the action of hot siliceous vapors, since the coarsest felsitic intergrowths occur along cracks in which tridymite is most plentiful. In other samples, ovoid areas of clear microfelsite are scattered in a matrix of pale-brown cryptofelsite, and the margins of the ovoid spots may be accentuated by a concentration of “limonitic” pigment. In still other lavas, streams of microlithic oligoclase and acid andesine so far predominate over the interstitial microfelsite that the groundmass takes on a trachytic aspect. Spherulitic dacites have been recognized only among the flows; they seem to be lacking from the pumiceous, white dacites of the domes.

Only one specimen of the lavas in question has been analyzed (no. 24). This came from the cliffs on the east wall of Sun Creek approximately 1/2 mile south of Maklaks Pass, and is typical of the flows in that vicinity. Though the silica content is 71.32 per cent, the ratio of lime to alkalis and the presence of abundant tridymite suggest that the rock should be classified, not as rhyolite, but as tridymite-rich pyroxene rhyodacite. Its affinities with the quartz latites are obviously close. Possibly some of the finely banded lavas northwest of Lost Creek Ranger Station, which are shown on the map as andesites, are in reality dacites, though it would require chemical analysis to verify the suggestion.

 

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