85 Dacite Dikes

The Geology and Petrography of Crater Lake National Park, 1902




Among the dikes that cut the andesitic rocks of the crater wall are two underneath Llao Rock that rise from the water’s edge and consist of dacite. The more westerly of these two dikes, immediately under the summit of Llao Rock, is represented by two specimens, Nos. 130 and 131. The first of these is a very light gray, porous, lithoidal dacite and comes from the central part of the dike. The second is a grayish-black glass and was collected at the margin of the dike. The two together may be taken as the equivalents of Nos. 103 and 102, respectively, which represent the lithoidal and the vitrophyric dacites of the Llao Rock mass. Under the microscope the resemblance is still more striking. Nos. 130 and 131 both contain the same phenocrysts already described in the equivalent rocks above mentioned. There are comparatively abundant plagioclase with large extinction angles and zonal structure, scanty hypersthene and augite, and occasional greenish-brown hornblende, together with accessory magnetite and apatite. The lithoidal rock from the main part of the dike does not differ sufficiently from the lithoidal dacite of the summit to justify a special description. The glassy margin, however, presents minor features that distinguish it from No. 102. Fluidal structure is very perfectly developed and is accentuated both by the straight augite microlites and by streams of roundish, nearly colorless globulites. The color of the glass streams alternates between whitish and brownish. Here and there a little clear brown glass, free from microlites and globulites, is to be seen in close proximity to a phenocryst. The thin section of this specimen also contains several small fragments of a fine-grained holocrystalline, porphyritic basalt, in the groundmass of which are to be seen slender prisms of augite and hypersthene, also grains of magnetite and plagioclase laths.

This dike is undoubtedly one of the feeders of the Llao Rock dacite.

The more easterly of the two dacite dikes is represented by only one specimen, No. 132. It is a light-gray, coarse, rough-fracturing rock. Under the microscope it presents features somewhat suggestive of the Grouse Hill dacite, but the similarity is not marked enough to justify the conclusion that this dike is a feeder for this dacite mass. The rock is not altogether fresh. It contains, in addition to the usual phenocrysts, a very little reddish-brown hornblende in small crystals and fragments with black borders. The groundmass contains the usual short rectangular feldspar and a good deal of feldspathic matter that has no well-defined shape. Brownish and reddish ferritic matter, together with some black magnetite, renders the thin section clouded and dirty looking. Abundant groups of tridymite are to be seen filling what appear to be cavities.

This dike contains an inclusion, No. 133, the nature of which is not clear. It has a slightly greenish-gray color, is distinctly but finely granular, and is decidedly porous. In thin section it is seen to be composed essentially of plagioclase laths and of hypersthene and augite in mostly long, slender prisms. Grains of black magnetite, as well as reddish and brownish ferritic staining matter, are abundant. Porphyritic crystals are entirely absent. The shape and arrangement of the main ingredients, but more especially of the plagioclase laths, are closely analogous to those of interstitial basalts, although it does not agree with any of the basalts from Crater Lake. But there are other respects in which this inclusion distinguishes itself from the basalts of this region. It contains considerable reddish-brown hornblende, and in some of the interstices between the plagioclase laths are colorless aggregates that appear to be tridymite, similar to that in the dike rock. It is probable that there is, or was, considerable glass in the form of a mesostasis. But if so it is largely devitrified and colored a dullish green. The thin section is too thick to allow a satisfactory examination, but the writer is inclined to place this specimen among the secretions of dacite. It calls to mind a not very dissimilar specimen described among the dacite secretions, No. 123, from Sun Creek. (See page 139.)

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