150 Microscopic Petrography – Cinder Cones and Associated Flows – Wizard Island

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

Microscopic Petrography


Cinder Cones and Associated Flows

     Wizard Island

The lavas of Wizard Island have a decided basaltic appearance in the field, and in thin section the abundance of ferromagnesian crystals and the dark color of the interstitial glass distinguish them clearly from the typical andesites of Mount Mazama. Yet analysis (no. 13) shows that the Wizard Island lavas are also andesites.

Almost all of them are hyalopilitic hypersthene-augite andesites with accessory olivine. Exceptionally, they may be holocrystalline. But the chief interest of the lavas is the presence in them of sporadic inclusions. There are, as in most of the flows related to cinder cones, xenoliths of older andesite and basalt, but these, being quite unaltered, merit no discussion. Here and there one may find white or pale-gray, sugary, and vesicular inclusions from a fraction of an inch to 4 feet in diameter. Some of these are so cellular as to resemble pumice. Under the microscope, many are seen to be composed almost entirely of colorless glass (R.I., 1.492-1.497) in which lie partially vitrified phenocrysts of labradorite up to 2 mm. long. Accompanying the vitrified feldspar are occasional prisms of hypersthene and augite and granules of ore around which the glass is commonly stained brown, though the minerals themselves appear to be unaffected. Occasionally the vesicular glass is cut by veins of quartz mosaic. The inference is that these cellular, glassy inclusions represent partially dissolved feldspathic (dioritic to gabbroid) clots picked up and metamorphosed by the enclosing lava. Rarely one finds, in addition to the well defined inclusions, irregular strings of similar material, as if, locally, the glass produced by solution of the feldspars was rendered sufficiently fluid to be streaked parallel to the banding of the surrounding lava.

With these vitrified inclusions, there are occasional pale-gray inclusions of dioritic or gabbroid appearance. These are composed chiefly of large, finely twinned phenocrysts of acid labradorite, separated by slender laths of andesine, granular ore, biotite, a little hypersthene, and less augite. The dark constituents form but a small part of the whole. Presumably these coarsely crystalline feldspathic inclusions represent fragments torn from the margins of the underlying magma chamber, and the vitrified, pumiceous inclusions are the result of their partial solution. Patton has described somewhat similar rocks among the ejecta of the culminating eruptions of Mount Mazama, under the title “light-colored granophyric secretions.” These are the only rocks of the Crater Lake region which are known to contain biotite.


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