128 Andesites of Mount Mazama and Scott – Basic Inclusions in the Andesites

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

     Basic Inclusions in the Andesites

In the Lassen region, basic inclusions (autoliths) are particularly abundant in the dacites and rare among the andesites. On Mount Mazama, the reverse holds true. These inclusions vary from a fraction of an inch to more than a foot in maximum dimension, the majority measuring 2 to 6 inches across. The larger ones are well defined and stand out distinctly as ovoid nodules; the smallest, seen only with a hand lens or microscope, have irregular outlines and merge indefinitely into the enclosing lava. The texture of these inclusions is diktytaxitic. Were it not that all contain an interstitial residue of glass, their texture would be described as lamprophyric. Their mineral content is surprisingly uniform. Essentially, they consist of a crisscross felt of slender plagioclase laths and equally slender prisms of hypersthene and augite, together with granular ore, between which lies a matrix composed partly of brown glass and partly of cristobalite or tridymite or both.

Some typical samples may be selected for description. In the Palisades andesite, the inclusions are abnormally abundant and have the following content: zoned laths of andesine-basic labradorite, 70 per cent; acicular prisms of hypersthene and augite, commonly in parallel growth and approximately equal amounts, 15 per cent; granular ore, 5 per cent; pale-brown glass, 2 per cent; cristobalite, 8 per cent. In the enclosing andesite, cristobalite is either rare or absent, though fan-shaped twins of tridymite are widespread.

In the adjacent andesite of Roundtop, the inclusions consist of the same minerals in slightly different proportions. Cristobalite is again plentiful, making up between 5 and 10 per cent of an average sample, though it is rare in the lava itself. Inclusions in other flows show a similar concentration of cristobalite. For instance, in the third flow from the lake edge on Redcloud Cliff, one inclusion has 25 per cent cristobalite though the mineral could not be detected in the surrounding andesite. Still more striking are the relations between the dacitic andesite on Vidae Ridge, 2 miles south of Garfield Peak, and the inclusions within it. In the lava, vesicles and cracks are so plentifully lined with tridymite that the mineral constitutes approximately 10 per cent of the rock. Yet the percentage composition of one of the inclusions is roughly as follows: labradorite, 65 per cent; hypersthene, 10 per cent; augite, 5 per cent; granular ore, 5 per cent; pale-brown biotite, green hornblende, and olivine, each less than 1 per cent; brown glass, 2 per cent; tridymite, 1 per cent; cristobalite, 10 per cent.