122 Microscopic Petrography – Lavas of the Union Peak Volcano

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

Microscopic Petrography


Lavas of the Union Peak Volcano

ALL who have worked among the lavas of the High Cascades have been faced with the troublesome problem of deciding upon a distinction between andesite and basalt. All agree that the crowning cones of the Cascade Range, Mounts Shasta, Mazama, Hood, Rainier, Baker, and Adams, to mention a few, are composed principally of hypersthene andesite. But these cones, huge as they are, form only a small fraction of the total volume of lava in the High Cascades. Certainly the dominant lavas of the range are more basic. Some, for example the intracanyon flows (the Santiam, Umpqua, Rogue River, and Butte Creek lavas), are unmistakable olivine basalts, but many more lie so close to the boundary between basalt and andesite that their proper position can only be established by chemical analysis. Among these problematic rocks must be included most of the pre-Mazama lavas within Crater Lake National Park, and most of those which form the High Cascades southward to Mount Shasta.

Thayer,1 working among the lavas of the north-central Cascades in Oregon, found it convenient to consider all the olivine-bearing lavas basalts. If olivine is regarded as a diagnostic mineral, then certainly all the lavas of Union Peak and virtually all the High Cascade lavas that stretch southward to Shasta are basalts. Many authors prefer to regard the nature of the plagioclase as diagnostic, and include among the basalts those lavas in which the average of the total feldspar is more basic than Ab1An1. The difficulty here, however, is that the porphyritic feldspar is strongly zoned in an oscillatory fashion and much of the microlithic feldspar is too minute for accurate determination. To distinguish between andesite and basalt on the basis of the plagioclase would therefore be an interminable task at best, and in the case of glass-rich flows it might be quite misleading. The only satisfactory solution of the matter is, of course, to prepare as many chemical analyses as possible. Of the four analyses made from the rocks of the Union Peak volcano, one has a silica percentage of 54.38; in the others the percentages are 55.63, 56.75, and 56.95 (see analyses 4, 5, 6, and 7; references here and below are to tables on pages 148-152). Bearing in mind the ratio of lime to alkalis, most geologists would hesitate to classify the three more siliceous rocks as basalt, even though they do contain a small amount of olivine. It is therefore suggested that they be called olivine-bearing basaltic andesites. Such rocks are probably the main lavas on Union Peak, Mount Thielsen, and Mount Bailey, and are more voluminous than true olivine basalts in the High Cascades to the south. In the opposite direction, however, the principal pre-Mazama lavas of the High Cascades are, as Thayer has demonstrated, olivine basalts with a silica percentage of less than 55.


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