83 Sun Creek Flow

The Geology and Petrography of Crater Lake National Park, 1902

 PART II.

HYPERSTHENE-DACITES.

DISTRIBUTION AND DESCRIPTION OF DACITE MASSES.

SUN CREEK FLOW.

This mass includes all the dacitic area to be seen on both sides of Sun Creek, and stretching to the east as far as Sand Creek. The specimens studied are Nos. 121, 122, 123, 124, 125, 126, 127. No. 144 comes from a secretion in dacite and will be described later. No. 123, which is distinctly different from the others, will be found described on page 139, under the head of secretions. No. 128 is a dacite tuff.

This is the only dacite mass to be seen on the south side of Mount Mazama. It differs from all the above-described dacites, in that, as Mr. Diller has shown, it belongs to an earlier period in the formation of the volcano and is overlain by andesite flows. Although the individual specimens above numerated vary greatly in their outward appearance, they are in general much more thoroughly crystalline than are the more recent dacites on the northern and eastern sides of the lake. A glassy groundmass is not wanting in portions of these rocks, but devitrification, either contemporaneous with the cooling of the rock or subsequent thereto, is well developed. These rocks, therefore, are better suited to a study of the lithoidal types and will receive more individual attention than has been given to most of the specimens thus far.

No. 121, taken from the west side of Sun Creek Canyon, is a light-gray, porous, and rough-fracturing rock, with inconspicuous phenocrysts, and with numerous rough cavities that are lined with minute white glassy crystals. Some of these crystals, scraped from the cavity with a knife, proved to be tridymite, together with some feldspar similar to that described later as visible in the thin section lining the cavities.

Under the microscope the groundmass appears to be in most places holocrystalline, or at least one in which a glass base plays a very subordinate role. Here and there, however, the thin section assumes a brownish cast, indicative of abundant glass. The colorless particles, which appear to make up the bulk of the groundmass, are neither elongated nor squarish in cross section, but have ill-defined, roundish, allotriomorphic form. Throughout this granular mass are scattered minute opaque dust-like ore particles, and also many minute yellowish to colorless dust-laden augite microlites. Irregular longish cavities abound, the larger of which are empty; the smaller ones are often filled with tridymite. Between this tridymite and the walls of the cavity is a narrow fringe of colorless grains, the same to all appearances as those that form a large part of the groundmass. Wherever these colorless grains come into contact with the Canada balsam they are seen to have an index of refraction lower, but only very slightly lower, than that of the balsam, so that the edge of the grain can hardly be distinguished. They can not, therefore, be quartz. They are structureless and without twinning bands. In polarized light the polarization colors are much higher than in the adjacent tridymite. Some of the largest of these grains (0.02 to 0.03 millimeter) give biaxial images in convergent polarized light. In one case a distinct cleavage was noticed with an extinction angle of 7-1/2° trace of the cleavage plane. Convergent polarized light gave a positive large-angled bisectrix in the center of the field, with the plane of the optical axes inclined 7-1/2° trace of the cleavage. These properties would certainly indicate orthoclase, in which case the grain just mentioned is cut nearly parallel to the clinopinacoid. These fringing grains are free from solid inclosures, but they appear to contain gas pores.

That, in spite of the above-described properties, these colorless grains are probably not orthoclase, may appear from the following two considerations: First, the index of refraction is too high for orthoclase, although, like orthoclase, it is lower than that of the Canada balsam, whose index of refraction is taken to be 1.540. Orthoclase, whose index of refraction is about 0.025 less than that of balsam, should present a sharply defined edge at contact with the balsam. Second, many or even most of these grains have an undulous extinction. This second consideration would suggest the presence of anorthoclase, which would also be corroborated by the preponderance of Na2O over K2O, as shown in the chemical analyses of the Crater Lake dacites. On the other hand, the index of refraction of anorthoclase as given by Professor Rosenbuscha is only 1.527, which is but 0.002 higher than that of orthoclase, Unless, therefore, anorthoclase should prove to possess a higher index of refraction than that here given it would seem that this feldspar must be placed in the albite-oligoclase series in spite of the fact that no twinning is to be seen.

aMikroskopische Physiographie, 3d edition, Vol. I, 1892, p. 157.

It is more than likely that quartz forms a large part of the crystalline groundmass, but owing to the small size of the particles this could not be demonstrated.

The tridymite of this rock does not possess the customary shingled aspect. It does probably occur in more or less overlapping scales, but the edges are irregular and the overlapping not very conspicuous. The occurrence, however, is exactly like that of the tridymite in the lithoidal rhyolite from Obsidian Cliff in the Yellowstone National Park, as examined by the writer in sections from that rock in the possession of the mineralogical laboratory of Harvard University. In describing the occurrence of tridymite in the rhyolite from Obsidian Cliff, Iddings says:b “The spaces * * * are in most instances occupied by tridymite in comparatively large crystals, often twinned and carrying numerous gas cavities.” The tridymite in this Sun Creek dacite may be recognized by the following properties. It occurs in minute crystals or grains that measure 0.05 to 0.15 millimeter, usually filling the cavities. Twinning is very common. Sometimes the twins resemble those of orthoclase in the form of Carlsbad twins, the twinned halves having widely differing extinction angles; in other eases there appear to be twinning planes in more than one direction. Undulous extinction is very common, but where the twinning is well marked this is not much in evidence. Exactly similar undulous extinction was also noted in the sections from Obsidian Cliff above referred to. Interference colors are very low, sometimes being almost invisible. The refraction is much lower than that of the adjacent balsam. This great difference in refractive powers is strongly brought out by the extreme roughness of the tridymite, as well as by the sharpness of the edges as compared with the great smoothness of the surface of the feldspar grains fringing the cavities. Except for the presence of gas cavities. the tridymite is free from inclosures.

bSpherulitic crystallization: Bull. Philos. Soc. Washington. vol. XI, 1891, pp. 445-464.

Among the phenocrysts of this rock are fairly abundant plagioclase, rather sparingly developed hypersthene, and only an occasional minute augite crystal. Hornblende was not to be seen.

Specimen No. 122, collected lower down on the west side of Sun Creek Canyon, not far from No. 121, is a slightly porous light-gray rock of lithoidal character, mixed to a certain extent locally with dark grayish-green glassy portions. Both the lithoidal and glassy portions are crowded with small spherulites, 1 to 3 millimeters in diameter. Most of these spherulites are solid externally, and of a dark-gray color within on the freshly fractured face. Some are hollow and show a slight tendency toward the formation of concentric shells or lithophysae.

Under the microscope the greater part of the rock is seen to be composed of spherulites of distinctly recognizable fibers and shreds that radiate in branching, feathery forms from common centers. These apparently feldspathic fibers have both positive and negative, but more commonly positive, extension, and extinguish both parallel and oblique. Whether they are actually intergrown with quartz can not positively be asserted. The outer portion of the spherulite usually contains deep brownish-red scales, presumable of hematite, that polarize light strongly. They radiate from the center and branch at low angles. To these brownish-red scales is due the reddish color of the outer portion of the spherulites as seen in the hand specimen. The two thin sections prepared from this rock were both cut from the lithoidal part of the hand specimen, so that they do not show any glass under the microscope. The groundmass outside of the spherulites seems to be holocrystalline, but extremely fine grained. The individual grains are too fine to be determined, but they appear to have no particular elongation. Through this minutely granulated groundmass, as well as through the spherulites, run streams of slender, rod-like augite microlites and of minute but not very abundant black ore particles, to which should be added an occasional lath-shaped plagioclase microlite. Phenocrysts occur the same as in No. 121.

No. 124 is an aggregate of very light-gray, porous, and distinctly crystalline, as well as of very dark-gray, dense parts. In the thin section the granular parts have mostly disappeared in the grinding, but from what is left they seem to be composed of an aggregate of colorless, allotriomorphic grains, probably of feldspar and quartz, with occasional tridymite scales. The dense, darker-colored parts probably contain a glass base completely obscured by the devitrification products, which consist of augite microlites granulated with adhering and inclosed opaque ore particles and of extremely minute, colorless allotriomorphic shreds and grains. As in No. 122, this rock also contains a very few lath-shaped plagioclase microlites.

The phenocrysts consist of the customary plagioclase, with also a little hypersthene and two or three fragments of brown hornblende with black resorption rims.

No. 125, from near the head of the West Fork of Sand Creek, is somewhat similar to the last-described specimen, but the groundmass is much coarser grained. The shredded appearance of the colorless ingredients is very marked, hut their exact mineralogical nature can not be made out. These shreds do not reach dimensions greater than 0.05 millimeter, and are too small to clearly disclose twinning striation, although the larger ones leave the impression that polysynthetic twinning is present. In white light the granulated augite microlites show up very clearly and abundantly, even with moderate magnifying powers. Plagioclase and hypersthene are the only phenocrysts noted. This rock hears a close resemblance to No. 113 from the Cleetwood Cove mass.

No. 126, which was collected close to No. 125, has a very dense structure and dark-gray color. It looks decidedly like an andesite. In thin section it does not appear altogether even grained, but discloses coarser-grained patches. It is, however, as a rule, very fine grained, or, at least, appears so in white light, in which it appears to be composed of a uniform white substance very thickly sprinkled with opaque black dust particles. When strongly magnified these dust particles appear as black ore grains, and also as transparent or translucent globulitic matter. The granulated augite microlites so common in other specimens from this lava mass are almost absent. In polarized light the continuous white substance appears to be made of very irregular, interweaving allotriomorphic patches that remind one forcibly of the allotriomorphic feldspar patches of many of the holocrystalline andesites of this region. They are to be distinguished from such by the absence of inclosed plagioclase laths. Plagioclase and hypersthene occur among the phenocrysts, as usual. One augite crystal was noticed; also a grain of magnetite surrounded with a rim of leucoxene.

No. 127, from near Sand Creek, furnishes a beautiful illustration of fluidal structure, both in the hand specimen and in the thin section. In the former it appears in very thin alternating parallel bands of light reddish-gray and of dark-gray and very compact materials. Phenocrysts are small, but quite abundant, and are made more conspicuous by the evident flowing of these thin bands around them.

In thin section the fluidal structure is beautifully brought out, both by the alternation of coarser, white bands with finer-grained and brownish-colored bands and by the presence, especially in the finer-grained portions, of streaks of black dust. When examined with strong powers this apparent ore dust is seen to be composed in part of inclusions of air; in part also of really opaque particles that are usually to be seen inclosed in augite microlites. The white bands appear to be entirely crystalline. They contain comparatively little microlitic material, either in the shape of augite or ore particles. In polarized light the colorless material breaks up into very distinct, longish shreds quite similar to those described above in other specimens from this dacite mass. They are, perhaps, somewhat more distinct here than elsewhere. Occasionally some of the larger and better-defined shreds (0.1 millimeter or more in length) show distinct twinning bands, but this is exceptional. More frequently the extinction is more or less undulous and sometimes markedly so. In general it is nearly parallel to the extension and is always negative.

These shred-like strips show a strong tendency toward grouping themselves around common centers. Three, four, or five or more of these may be seen diverging from a common point, each of them narrowing down wedge-like at the center. As in the case of the more isolated shreds these are also negative. They thus produce what Rosenbusch calls “pseudospherulites” with negative crosses. In the midst of these “pseudospherulites” may also be seen the ordinary “true spherulites” of Rosenbusch, also with negative crosses. These latter do not appear to be composed of any recognizable mineral species. They show the customary black cross with arms parallel to the principal planes of the nicol prisms and remain stationary upon revolving the stage. They do not succeed in developing externally spherical forms, as their growth appears to be interfered with by contact with the coarser-grained portions of the groundmass. Every possible gradation appears to present itself between these so-called “true spherulites” and the radial groups of crystal particles that appear to be undoubtedly feldspathic.

The finer-grained bands with brownish color are composed, apparently, of glass with inclosed dust particles. But upon closer inspection the apparent glass seems to be composed of ill-defined particles that, owing to their parallel arrangement, very faintly polarize light, so that the whole band extinguishes parallel, with negative extensions. This is undoubtedly the substance so frequently referred to in describing such rocks as microfelsite. On each side of these uniformly polarizing bands of microfelsite occurs a very narrow, continuous spherulitic streak, the individual spherulitic parts of which are quite similar to the above-mentioned “true” sphemaulites.

The phenocrysts in this rock are similar to those in the rest of these dacites, namely, plagioclase and hypersthene. Hornblende could not be seen.

 << previousnext >>