152 Microscopic Petrography – Andesite Dikes

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

Microscopic Petrography


     Andesite Dikes

It would serve no good purpose to describe in detail the petrographic characters of the andesite dikes, since they closely resemble the andesite flows already discussed. Certain features, however, are worthy of mention.

Three dikes of andesite are exposed on the walls of Steel Bay. The easternmost of these has margins of black, glassy lava. The core, on the other hand, consists of almost holocrystalline, pale-gray andesite. Viewed in thin section, the paler core is seen to be made up largely of a dense pilotaxitic felt of oligoclase microliths and specks of augite with a small amount of interstitial glass (55 per cent). In this matrix lie phenocrysts of bytownite-andesine (30 per cent), granular ore (5 per cent), hypersthene (6 per cent), and augite (2 per cent). The pale core is somewhat more vesicular than the glassy selvage, and many of the vesicles are coated with crystals of cristobalite (2 per cent). The selvage, on the contrary, contains no cristobalite.

West of this dike is a much larger intrusion which connects in its upper part with a long flow of lava. This intrusion is distinctive in having no glassy selvage and in carrying unusually abundant and large basic inclusions. Locally these inclusions are as voluminous as the enclosing andesite. The latter is a pilotaxitic hypersthene-augite andesite, and except in one particular does not differ from the normal andesite flows. This exceptional feature is the presence of abundant wedge-shaped twins of tridymite which partly fill cavities. In one section, the mineral constitutes as much as 3 per cent of the whole. Cristobalite, on the other hand, is absent.

The inclusions in this dike are very porous. Their texture is best described as diktytaxitic and pseudolamprophyric. Essentially, they are composed of a crisscross felt of labradorite laths (65 per cent), slender hypersthene prisms partly rimmed by secondary ore and occasionally enclosed in jackets of augite (15 per cent), stumpy prisms of augite (5 per cent), and granular magnetite (5 per cent). Between these is a matrix partly made up of devitrified glass (2 per cent), but mainly of tridymite (6 per cent) and cristobalite (2 per cent). The occurrence of both forms of silica in the inclusions is thus in contrast with the development of tridymite alone in the enclosing andesite.

The third dike on the walls of Steel Bay also served as a feeder to a surface flow. This differs from the preceding chiefly in having a glassy selvage and pale, vesicular, holocrystalline core, and in being almost wholly devoid of basic inclusions.