Modular Secretions Llao Rock Flow

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The Geology and Petrography of Crater Lake National Park, 1902

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PART II.

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HYPERSTHENE-DACITES.

DISTRIBUTION AND DESCRIPTION OF DACITE MASSES.

LLAO ROCK FLOW.

NODULAR SECRETIONS.

In the description of the vitrophyric dacite (102) from the south end of the Llao Rock flow, reference is made to inclosed angular fragments of compact, grayish-brown, dull-lustered material. In the thin section of this rock there occur three of these inclusions, the largest of which is about 6 millimeters in diameter. They are composed of a loosely felted mass of slender hornblende prisms and also of almost equally slender plagioclase laths, with a few octahedrons of magnetite. In the interstices of this felt is to be seen a brown glass that composes at most one-quarter of the mass. The hornblende and plagioclase are present in about equal amounts. The hornblende is very uniformly about 0.1 millimeter in width and from 0.5 to 1 millimeter in length; the plagioclase is about the same in width, but not quite so long. In one of these inclusions the color and pleochroism of the hornblende are almost identically the same as that of the reddish-brown hornblende that is mentioned above as occurring in many of the dacites. It is to be distinguished only by the very slender form and by its great abundance. The extinction angles in the prism zone are very small, rarely over 3°.

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In another of these inclusions the hornblende has the color and pleochroism of the greenish-brown hornblende of the dacites, while in the third one the color is intermediate between the two. This change of color, corresponding as it does to the variations of color of the hornblende in the different dacites or in the same dacite, forms one of the arguments in favor of this being an older secretion. The form of this hornblende does not vary with the change of color. Usually only the unit prism is to be noted. The terminations are not often sharp. The prisms either taper out at the end, or end roughly, as though broken off.

The plagioclase is usually simply twinned, with only two or three bands visible. The largest extinction in a symmetrical section observed was 30° very slender laths contain long brown glass inclosures that often show the form of the host. The prisms of plagioclase and hornblende do not often interfere, but when they do it does not appear that one of these ingredients is older than the other. In addition to these lath-shaped plagioclase, this mineral also occurs in two or three comparatively large and decidedly spongiform, stout crystals that contain much colorless glass, and bits of hornblende of the same color as in the section outside of the feldspar. It is rather remarkable that this mineral should be thus inclosed in the plagioclase, inasmuch his elsewhere in these dacites hornblende belongs to the youngest of the phenocrysts. It is to be noted, however, that, as it occurs thus inclosed in plagioclase, it does not have the same sharp form as is otherwise to be seen. In fact, it presents exactly the appearance of having been formed as a secondary mineral in the feldspar.

Both hypersthene and augite seem to be missing.

There does not appear to the writer to be much doubt that these inclosures are fragments of older secretions from the dacitic magma, although the absence of the pyroxenes is hard to explain on this supposition. But the tendency for the ingredients to assume long slender forms, quite distinct from those in the rock in which they occur, appears to be very characteristic not only of this but of other secretions that will be described later as occurring in the dacites, and also of secretions in the andesites.

Fig. A of Pl. XVIII (p. 132) is a photomicrograph from No. 102, and shows both the secretion and the inclosing vitrophyric dacite.

Reference is made elsewhere to the resemblance between some of the vitrophyric dacites of Crater Lake and the dacite from Lassen Peak, California, of which Mr. Diller has given a brief description.^a In this description Mr. Diller mentions and gives a photograph of angular nodules inclosed in the dacite. His description of the microscopic appearance of these inclusions, corroborated by study of the thin sections kindly loaned the writer for the purpose, discloses a very close resemblance to the inclusions from the Llao Rock dacite. In color, form, and structural relationships of the hornblende and in the occurrence of plagioclase and a small amount of brown glass the resemblance is very close. The main points of difference are these, that the Lassen Peak inclusion is coarser grained, the hornblende prisms are not so slender, and the hornblende not infrequently incloses plagioclase laths; also biotite amid pyroxene and a little olivine and tridymite are present. The appearance of biotite in the Lassen Peak secretion is to be expected, as the same mineral occurs in the dacite itself.