151 Microscopic Petrography – Dacite Dikes

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

Microscopic Petrography



     Dacite Dikes

Most of the dikes on the walls of Crater Lake are composed of pyroxene andesite; only two are of dacite. These two dikes cut across the cliffs immediately beneath Llao Rock, and Diller maintained that the more westerly was one of the feeders of the Llao lava. Elsewhere, however, evidence has been offered to indicate that this dike is not a feeder to the Llao lava, but an offshoot from a conduit common to both.

The dike in question varies between 6 and 7 feet in width. For a distance of approximately 6 inches from the edge, it consists of well banded black obsidian, composed of alternating layers of light- and dark-brown glass crowded with microliths of oligoclase and augite and with varicolored globulites. Scattered throughout are corroded phenocrysts of basic plagioclase, together with a few crystals of hypersthene and augite and occasional prisms of green hornblende.

The interior of the dike is strikingly different, being pale gray and holocrystalline. By far the bulk consists of dense cryptofelsite heavily charged with clouds of trichites and globulites. Zoned phenocrysts of feldspar (labradorite-andesine) up to 2 mm. in length (12 per cent), slender prisms of hypersthene (2 per cent), green hornblende (1 per cent), granular ores, and scanty apatite are distributed throughout. But perhaps the most interesting feature of the center of the dike is the fact that it is much more vesicular than the glassy margins and that the vesicles carry abundant tiny spheroids of cristobalite. In some samples, indeed, cristobalite constitutes almost 5 per cent of the total volume.

The second dacite dike beneath Llao Rock shows an inward passage from black obsidian through pale-gray, almost pumiceous lava, to a core of darker-gray and much more strongly banded dacite. The obsidian selvage, seen under the microscope, appears as a pale-buff glass charged with trichites of plagioclase and ?augite, and carrying phenocrysts of zoned andesine-labradorite (12 per cent), olive-green hornblende, and hypersthene (together, 3 per cent). No cristobalite was detected.

Within an inch or two, this obsidian merges into microvesicular dacite. Locally the two are interlaminated. In proportion, size, and nature, the phenocrysts are identical with those in the obsidian, but the groundmass contains either no glass at all or at most an insignificant amount, being composed of an irresolvable, cloudy cryptofelsite. Most of the vesicles are empty, but a few carry grains of cristobalite.

The dark-gray core of the dike is particularly distinguished by two features: first, a strong fluxion structure, caused by the parallel orientation of swarms of oligoclase microliths in the cryptofelsitic base; second, the abundance of cristobalite lining vesicles.

Briefly, then, this eastern dacite dike shows an inward transition from a glassy skin to increasingly coarse, holocrystalline lava, and a corresponding increase both in vesicularity and in the content of cristobalite. Throughout the dikes are scattered inclusions, some of them true xenoliths of hyalopilitic hypersthene andesite, and some pseudo-lamprophyric segregations like those common in the adjacent flows.


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