126 Microscopic Petrography – Pre-Mazama Lavas in the Northwest Corner of the Park

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

Microscopic Petrography


Pre-Mazama Lavas in the Northwest Corner of the Park

The lavas just described do not differ essentially from those which outcrop beyond the northwest base of Mount Mazama, and those related to Bald Crater and Red and Desert cones. All these flows are characterized by the presence of porphyritic olivine. In many, tridymite is developed in the dense groundmass and granules of cristobalite may be found lining vesicles. They vary principally in the character of the groundmass, as Patton has set forth in detail. Only two analyses are available, one of a lava from the base of Red Cone and the other from the ridge north of Desert Cone (nos. 9 and 8). The former is the most basic rock in the park and is an unmistakable basalt; the other contains 58.65 per cent of silica and, like the dominant lavas of Union Peak, is best referred to as an olivine-bearing basaltic andesite. Between these two types there are all gradations.

Examples of hypersthene-free, olivine-rich basalt have been recognized southeast of Bald Crater. They may also be found on and around the bases of Desert and Red cones. Essentially, the hypersthene-free flows consist of porphyritic crystals of olivine (5 to 8 per cent), many of which are partly or wholly replaced by magnetite or hematite or both, accompanied by occasional phenocrysts of basic labradorite, in a fluidal groundmass composed of subparallel microliths of medium labradorite, intergranular augite, and ore. In some specimens, the vesicles are lined with opal, and a little tridymite may be detected in the groundmass. Patton noted that some cavities in the lavas at the east base of Red Cone are also lined with apatite and pseudobrookite.

Associated with the flows just enumerated are others in which the content of porphyritic feldspar is so much higher that they resemble some of the dark andesites of Mount Mazama. Patton referred to these as “andesitic basalts.” Examples are common on Desert and Crescent ridges. Some are intergranular, some hyalopilitic. The feldspar phenocrysts are invariably embayed, charged with glass inclusions, and strongly zoned. The cores of some are at least as basic as acid bytownite, though the rims may be andesine. Commonly they include grains of augite and ore. Olivine, partly altered to iddingsite, is also present.

An unusual type of lava comes from the crest of 6789 Ridge, a spur running eastward from Desert Ridge. This is a dense, pale-gray lava with a pseudolamprophyric habit, consisting of a crisscross felt of zoned labradorite laths and prismatic hypersthene, porphyritic olivine, accessory augite and ore, and abundant interstitial tridymite (8 per cent). Hypersthene makes up no less than 15 per cent of the volume, and is approximately ten times as common as augite. The rock resembles much of the coarser facies of the Union Peak plug, and it may not be far removed from the original conduit of the Desert Ridge volcano.

Taken as a whole, the lavas of the Desert Ridge volcano carry more hypersthene than any of the other pre-Mazama rocks; on the other hand, the flows adjacent to Bald Crater and Red and Desert cones tend to be richest in olivine.

Finally, it may be pointed out that whereas basic inclusions are plentiful among the lavas of Mount Mazama, they are scarce among the earlier flows. Sporadic clots of coarse olivine-hypersthene gabbro occur in the lavas of Desert Ridge, but elsewhere inclusions are extremely rare.


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