134 Microscopic Petrography – The Younger Dacite Flows – The Llao Dacite

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 Younger Dacite Flows

     The Llao Dacite

The great sheet of lava forming Llao Rock is mainly composed of black obsidian, which is locally spherulitic, and partly of pale-gray, more vesicular and lithoidal dacite. Commonly the dense obsidian is streaked with highly pumiceous layers. Basic inclusions are comparatively rare. Patton has already given a lengthy description of the Llao dacite, and since the principal varieties are almost identical with those of the dacite dikes beneath Llao Rock, of which an account may be found on page 143, a few words must suffice.

In thin section, the black obsidians show beautiful fluidal banding owing to the alignment of minute augite rods, acicular microliths of feldspar, and curved black trichites. Compared with most of the andesites, the Llao Rock and other dacites are relatively poor in phenocrysts. The main porphyritic mineral is plagioclase, marked by oscillatory zoning and ranging in composition from acid andesine to acid labradorite. Next in importance is augite, then hypersthene, and lastly a few small crystals of green and brown hornblende, apatite, and ore.

Locally, the black obsidian is traversed by red and chocolate-colored streaks. These are distinctly more porous than the enclosing lava, and apparently owe their origin to gas concentration and oxidation.

Here and there, particularly in the center of the flow, the Llao dacite has a dull-gray color and lithoidal appearance, where the base, instead of being clear glass, is composed of cryptofelsite charged with abundant streams of acicular feldspar.

Although the Llao lava is thus a pyroxene dacite poor in hornblende, all the basic inclusions examined from it are rich in hornblende. These inclusions vary from a few millimeters to a few inches in diameter, and are characterized by a peculiar “pseudo-lamprophyric” texture, like the basic inclusions in the dacites of the Lassen region.


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