125 Microscopic Petrography – Lavas of the Union Peak Volcano – The Micronorite Plug 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

     The Micronorite Plug of the Union Peak Volcano

The summit tuff cone of the Union Peak volcano is intruded by a large, vertical-sided plug, similar to the Minto plugs described by Thayer4 and to the plugs of the Mount Thielsen and Howlock Mountain volcanoes.5 The summit pinnacles of several other Cascade cones are of the same kind.

The dominant rock of the Union Peak plug is a pale-gray, minutely porous micronorite cut by broadly spaced joints. The grain size is fairly uniform and the texture is pseudo-lamprophyric to ophitic. Slender laths of feldspar are crossed haphazard by thin prisms of hypersthene, and are partly enclosed or separated by augite. The feldspar laths are zoned from labradorite in the center to albite along the rims. Discrete crystals of albite may also be found in the fine base and lining cavities. Olivine is present only in minor amount, but euhedral grains of magnetite are abundant. No feature of these rocks is more distinctive, however, than the unusual development of silica minerals. In parts of the plug, the joint faces are “frosted” with myriads of tiny plates of tridymite, and the dense matrix is composed largely of fan-shaped twins of this mineral. Cristobalite is also widespread. Some of it occurs in the groundmass, where it may have developed from glass under the influence of mineralizers, as suggested by Larsen;6 but by far the bulk occurs in the form of spheroids, up to 1 mm. in diameter, on the walls of cavities. Dutton,7 who first detected them, observed that sanidine and euhedral augite are associated with them on the cavity walls. Equally characteristic is the association of both tridymite and cristobalite with abundant, well formed crystals of hematite. There can be no doubt that both the silica minerals and the hematite are a product of fumarolic gases. That they are far more plentiful in the plug than in any of the extruded lavas is only to be expected, for hot gases must have continued to rise up the central conduit for a long time.

Associated with the pale-gray rocks just described are darker, denser, less vesicular, and more closely jointed rocks which seem to have been intruded later. In these, the feldspars show a larger range in size. Moreover, the augite is no longer related ophitically to the feldspar, as in the coarse, pale facies, but is entirely intergranular. The hypersthene-augite ratio is extremely variable, though the former always predominates and is generally three to five times as plentiful as the latter. Commonly augite forms jackets enclosing the hypersthene prisms. In some specimens, brown glass may form as much as 10 per cent of the volume.

In the coarse parts of the Minto plugs, Thayer found olivine to be rare, and he suggested that resorption during slow cooling might account for the fact. In the Union Peak plug, on the contrary, there is no systematic relation between the coarseness of the rocks and the amount of olivine. Indeed, the mineral is least common and most altered in the fine-grained rocks. The content of cristobalite, tridymite, and hematite does, however, vary almost directly with the coarseness. The open-textured, coarser rocks afforded easier passage to the fumarolic vapors responsible for those minerals.


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