123 Microscopic Petrography – Lavas of the Union Peak Volcano – Lavas of the Main Cone

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

     Lavas of the Main Cone

The typical flows of the Union Peak volcano are generally distinguishable from the lavas of Mount Mazama by the presence of small phenocrysts of olivine and by the paucity of porphyritic feldspar. Pyroxene is invariably present in large amount, the hypersthene normally in the form of euhedral or subhedral prisms of early growth, and the augite2 in the form of irregular grains in the groundmass. Texturally the flows vary widely, from intergranular to ophitic and hyalopilitic. There is no systematic variation in the relative proportions of olivine and hypersthene according to texture, for olivine may be just as plentiful in the coarse, ophitic flows as in the dense, fluidal types. Nevertheless, there is often a reciprocal relation. between the two minerals, irrespective of texture.

A common feature of the Union Peak lavas is the partial or complete replacement of olivine by granular magnetite or hematite or both. Usually this change is confined to the rims of the olivine. Alteration to serpentine is exceptional. In some instances, replacement of olivine by iron ores took place at an early stage, for shells of fresh augite surround the altered crystals. In other instances, the change may have been brought about at a late stage, for it affects olivine and augite alike in those lavas which have been most deeply reddened by gas action.

If porphyritic feldspar is present, it is invariably corroded and marked by intense zoning, and in general the composition ranges from basic labradorite inside to oligoclase or acid andesine at the rim. Occasionally the cores of the phenocrysts may be composed of acid bytownite. The microlithic feldspar, on the other hand, is predominantly medium to acid andesine. Two typical specimens were selected for analysis. The one from Red Blanket Canyon (analysis 5) has the texture described and figured by Patton as “fluidal interstitial.” A little black glass is present, but essentially the rock is composed of closely packed, subparallel microliths of acid labradorite, intergranular augite and ore, and a light sprinkling (3 per cent) of porphyritic olivine, partly altered to serpentine. The other specimen analyzed (no. 4) is an extremely fine-grained lava with the following content: microporphyritic olivine, 5 per cent, almost entirely replaced by magnetite; rare phenocrysts of hypersthene and augite, 2 per cent; stumpy, zoned phenocrysts of acid bytownite-acid labradorite, 25 per cent; and a dense, intergranular base of microlithic andesine, anhedral augite, and ore.

Finally, it should be noted that few of the Union Peak lavas are devoid of minute quantities of either tridymite or cristobalite, the former occurring usually in triangular interspaces in the groundmass, and the latter on the walls of vesicles.


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