127 Microscopic Petrography – Andesites of Mount Mazama and Scott

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

The bulk of Mount Mazama and the whole of Mount Scott are composed of hypersthene andesites. Since they do not differ materially from the andesites already described from the Lassen region, from Mount Shasta, and from Mount Rainier, and since Patton has presented a detailed account of their principal variations, extended notice is unnecessary. As might be expected, they vary widely in texture, from holo-crystalline and subophitic to extremely glassy, the dominant types being pilotaxitic. Few are devoid of large phenocrysts of feldspar, and by this criterion alone they are usually distinguishable from the pre-Mazama flows. Besides, olivine, which is almost ubiquitous among the older flows, is an unusual accessory among the hypersthene andesites of Mazama itself. Among the more acid andesites, rare crystals of hornblende make their appearance. Biotite andesites are unknown.

As compared with the pre-Mazama flows and those erupted from parasitic cinder cones, the Mazama andesites are rarely vesicular. They are further typified by many basic inclusions. Indeed, locally these may be so plentiful as to constitute a quarter of the total volume.

Whether the lavas are holocrystalline or hyalopilitic, they have the following characteristics in common: Phenocrysts of plagioclase usually make up between a quarter and half the bulk. Almost without exception, they show pronounced oscillatory zoning, the normal range in composition being from basic labradorite to basic andesine. Occasionally cores of bytownite are present. Ferromagnesian phenocrysts are subordinate both in size and in abundance; in some flows their total content may reach 10 per cent, but generally the percentage falls below 5. Exceptionally, porphyritic augite exceeds hypersthene in amount. Typically, however, hypersthene is twice or three times as plentiful, and in a few flows it is the only ferromagnesian phenocryst. Olivine is absent from most of the lavas and even where present rarely exceeds 1 per cent by volume. Though usually fresh, it may be altered to green serpentine, iddingsite, or a mixture of granular ore and hematite. Where hornblende is developed, it has the strong pleochroism from pale yellow to deep brown and the dark rims of ore characteristic of hornblendes found among andesites in general.

Though the porphyritic constituents of the Mazama andesites differ relatively little in variety and proportion, the groundmass exhibits a wide range of textures. On this basis alone, Patton has already presented a detailed classification. In the pilotaxitic andesites, the matrix consists of slender microliths of oligoclase or acid andesine or both, and irregular grains of augite and ore, set in a mesostasis of crypto- to microgranular material, of uncertain composition but presumably composed mainly of acid feldspar and a little quartz. Hypersthene is rarely present in the groundmass; hornblende and olivine, never. Tridymite, on the contrary, is almost a constant accessory and may constitute more than 5 per cent of the total volume. Cristobalite may occasionally be seen in the groundmass and on the walls of vesicles, though it is far less widespread than in the pre-Mazama andesites.