156 Petrology

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

Microscopic Petrography



The rocks of the Crater Lake region, like those of all the High Cascade volcanoes studied thus far, belong to the calcic igneous series as defined by Peacock.8 The alkali-lime index, that is, the silica percentage at which the content of lime equals that of the alkalis combined, is approximately 62 (see figure 32). For the lavas of the Lassen region, the index is 63.9; for those of Mount Shasta, 63.7; and for those of Mount St. Helens, 63.2.

According to the scheme proposed by Niggli,9 the magma types of the Crater Lake region are mainly quartz dioritic (peleitic), dioritic, and trondhjemitic. Burri,10 who was struck by the resemblance of the Crater Lake rocks to the Tertiary lavas of the Sierra Nevada, grouped them together as the Sierra Nevada (effusive) type of the Pacific Province. However, the Crater Lake lavas are more similar in many respects to those of the Lassen region than they are to the effusive rocks of the Sierras. Thus, the values foral in the Crater Lake series are consistently closer to those of the Lassen lavas. So are the values for alk and al-alk among the basic lavas (si<200). The values for c in the Crater Lake rocks, on the other hand, are closer to those of the Sierran lavas.

The principal differences between the Lassen and Crater Lake series have already been shown in an earlier paper.11 Briefly, they are as follows: lime is consistently higher among the Lassen rocks; potash is also higher except among lavas with less than 56 per cent SiO2; soda is distinctly higher among the rocks of Crater Lake; so is iron, except in the most basic flows; and finally, magnesia is higher among the basic lavas of Crater Lake, but lower among lavas with more than 55 per cent SiO2. Noteworthy is the wider range in silica content of the Lassen series.

Among the acid rocks of the Lassen region (si>300) the k value, that is, the molecular ratio of K2O to total alkalis, may reach as high as 0.5, though in the Crater Lake series the k value never exceeds 0.28 (see figure 33). This low potash content of the Crater Lake lavas is reflected in the absence of orthoclase and the extreme rarity of biotite in even the most siliceous types. The acid dacites of the Lassen country, on the contrary, contain biotite in abundance. The acid effusive rocks and the Mesozoic intrusives of the Sierra Nevada also show high k values, the magma types tending toward the granodioritic. If assimilation has played any role in the differentiation of the Lassen magmas, their high k values may imply contamination with rocks of the granodiorite basement. Presumably the upper parts of the magma chambers beneath the Crater Lake region lay far above such a basement and among the thick series of calc-alkaline lavas which outcrop in the Western Cascades. Causes other than assimilation, however, may well account for the observed differences.

In their low k values, the Crater Lake lavas resemble those of Mounts Shasta and St. Helens. They are not, however, so poor in potash as the lavas of Montserrat.12Nevertheless there are strong similarities between the Crater Lake rocks and those of the Lesser Antilles in general.

In brief, the Mazama and pre-Mazama flows of the Crater Lake region belong to the same calcic magma type as those of Lassen Peak and the Lesser Antilles, though exhibiting many similarities to the Tertiary lavas of the Sierra Nevada. That they also bear a notable resemblance to the Mesozoic batholithic rocks of the Sierra Nevada suggests the persistence, over a very long period and with little modification, of similar magmas beneath the Sierra Nevada-Cascade orogenic belt.