Geological Survey Professional Paper 3 The Geology and Petrography of Crater Lake National Park

BASALTS.
(continued)

ANDESITIC BASALTS.

In nearly all of the basalts thus far described in these pages the structures have been distinctly different from what has been described as characteristic of the andesites. To a certain extent this may also be said of the composition. But in the rocks here designated as andesitic basalts these differences in structure and also in composition to a large extent disappear, so much so that were it not for close association with more distinctly basaltic types, some of those here included would certainty be classed with the andesites. In the absence of chemical analyses the correctness of these determinations must be taken with some reserve. A general statement of the differences noted between the andesitic basalts and the andesites will be given at the close of the more detailed description. As compared with the foregoing basalts, these may be said to be distinguished, with one or two exceptions, by the presence of a hypocrystalline groundmass or of one with an evident glass base, as well as by the presence of well-developed phenocrysts of plagioclase.

Four somewhat distinct structural types may be recognized. They are designated as types A, B, C, and D.

ANDESITIC BASALTS, TYPE A.

This may be studied in three specimens (179, 180, 181) from Crater Peak, about 4 miles south of the lake, and in one specimen (182) in the extreme southwestern corner of the Crater Lake area. These are black to blackish-gray, very fine grained and mostly porous rocks that have, with the exception of No. 182, small, but rather conspicuous, plagioclase phenocrysts. The groundmass of these rocks consists of a brownish glass base clouded with black dust and very thickly crowded with prismatic augite microlites and with minute magnetite octahedrons, and containing microlitic crystals that consist of very long and slender plagioclase laths. These laths have straight sides, while the ends are sometimes cut squarely off, but are more often somewhat frayed. They inclose usually small amounts of the dusty looking, almost opaque, glass of the groundmass. The inclosures take the form of minute specks that are strung together parallel to the longest axis of the lath, so as apparently to divide the lath into two or more longitudinal sections. The laths are not numerous enough to interlace to any marked extent. The occurrence of these slender lath forms is the most distinctive feature of type A.

The phenocrysts are mainly plagioclase crystals which in No. 182 occur in broad-shaped laths, and are similar to and grade into the more slender and much smaller laths of the groundmass, and disclose almost no inclosures, except an occasional bit of brown glass containing crystallizations similar to those of the groundmass. The rocks from Crater Peak, however, contain in addition to rectangular or broad lath-shaped individuals also larger and stouter crystals that exactly resemble the older plagioclase phenocrysts of the typical andesites. These usually have clear margins and clear centers, but between the two have a narrow zone that is clouded with brownish glass inclosures. Some of the larger crystals also show rounded forms with embayments due to resorption, in which case the clouded zone follows the contours of the corroded crystal, being still separated from the edge of the crystal by a narrow clear margin. This case is exactly identical with what has already been described under the andesites.

Augite occurs among the phenocrysts in mostly small and inconspicuous and not very well-defined crystals, also in grains, and is not very abundant. Hypersthene appears to be entirely wanting in one thin section, while in the other three only eight individuals altogether were noted. The habit is prismatic, but the form is not distinct. Six out of the eight individuals have rims of augite in parallel position.

Olivine is fairly abundant in small crystals and grains and occurs occasionally in better-formed, larger crystals of the ordinary type. In No. 179, however, and to a less extent in No. 181, this mineral also occurs in a very unusual form. It is in long slender prisms that show squarish cross section. The elongation is parallel to the crystallographic axis a and corresponds to the axis of least elasticity. These slender prisms extinguish parallel and exhibit the usual refractive properties of olivine. A section cut parallel to the brachypinacoid disclosed a negative bisectrix with large optical axial angle, the axial plane being parallel to the elongation. In No. 181 one section, cut so as to show squarish cross section, disclosed in convergent polarized light a bisectrix with the axial plane in a diagonal direction. This observation, coupled with the fact that all squarish sections extinguish in the direction of the diagonal, indicates that the prismatic habit is due to the extension of the crystal parallel to the a axis and that the form is really the brachydome (021). These olivines show a slight corrosion, and have developed narrow resorption rims of magnetite. They often contain inclosures of glass that are black and nearly opaque with inclosed magnetite grains. Such glass inclosures are often of irregular shape, but commonly they are arranged in longish forms stretched parallel to the elongation of the crystal. These long inclosures do not extend the whole length of the crystal. They are more apt to be in the center of the crystal and, when seen in cross section, are square, the sides of the inner black square being parallel to the sides of the crystal. This appearance is very suggestive of the black squares to be seen in cross sections of chiastolites.

ANDESITIC BASALTS, TYPE B.

This is represented by three specimens collected 2 or 3 miles west and northwest of Red Cone. They are distinctly andesitic in appearance, being gray, rather compact rocks with numerous but small and inconspicuous plagioclase phenocrysts. These might readily be taken for andesites, but, aside from certain differences in structure that alone might not be decisive, their association with distinctly basaltic rocks may justify their being classed under the basalts.

Under the microscope the groundmass is quite different from type A. One specimen (183) is apparently holocrystalline, one (184) contains a very little, and the third (185) quite a perceptible amount of glass. The holocrystalline specimen has a groundmass composed of minute prismatic crystals of augite, octahedral magnetite, and abundant plagioclase. The plagioclase consists in part of distinctly recognizable laths, much smaller and less well defined than in type A, also a sort of residual feldspathic paste that does not show distinct form and can not be clearly demonstrated to have polysynthetic twinning. It has, however, a more or less undulous extinction, and resembles the feldspathic residual paste of some of the andesites. In the more glassy variety the augite is not so abundant, and the plagioclase appears clouded and indistinct because of the dusty-looking glass that impregnates the whole, but still can not be plainly recognized.

Phenocrysts are abundant and well formed. They are plagioclase, hypersthene, augite, and, in the case of No. 183, olivine. The plagioclase is characteristically andesitic in type, showing zonal structure and containing inclosures of hypersthene, magnetite, and glass in its accustomed distribution. Hypersthene and augite are in sharply defined crystals, except where they occur in nests, which is commonly the case. They have the color and pleochroism common to these minerals in the andesites. They both contain inclosures of colorless glass with air cavities, also of magnetite. Parallel growths of augite around hypersthene allies two of these (184 and 185) to the basalts. The olivine of No. 183 is not abundant. It occurs mostly in grains and in rounded forms with magnetite rims. Magnetite occurs in the customary octahedral crystals.

ANDESITIC BASALTS, TYPE C.

This is, perhaps, the least andesitic in appearance and the least deserving of being classified as a distinct type of all the basalts of this region. The two specimens that belong here (186 and 187) were collected from the basaltic rocks in the extreme southwestern part of the mapped area, about 1 mile from No. 182. As these two specimens bear a closer resemblance to this rock, perhaps, than to any other one, they may be looked upon as a facial development of No. 182. These are blackish-gray, very dense rocks with a basaltic rather than andesitic appearance, being apparently free from phenocrysts. In thin section, however, they are seen to contain numerous but small phenocrysts of plagioclase with broad lath-form, suggestive of a basaltic rather than an andesitic type. In addition to these there are only a few olivine and augite (but no hypersthene) phenocrysts in roughly idiomorphic or in granular form. The groundmass consists of a feldspathic paste thickly crowded with roundish, roughly prismatic or irregular augite grains, and with magnetite. Glass appears to be wholly wanting.

ANDESITIC BASALTS, TYPE D.

This type is preeminently andesitic, either through the development of plagioclase phenocrysts or through the presence of an abundant glassy base. The rocks here represented are confined to the lava flows to the northwest of the lake. Nos. 188 and 189 come from the eastern slope of the second summit north of Desert Cone, No. 190 comes from the summit of the first hill north of Desert Cone, and No. 191 about half a mile south of this same summit and not far from the saddle between that hill and Desert Cone. Nos. 192, 193, and 194 were collected at, or not far from, the summit of Bald Crater, and Nos. 195 and 196 from the nearly flat region west of Red Cone.

These basalts are dark to black, dense and partly vesicular rocks with a more liberal development of minute feldspar phenocrysts than is the case with most of the other andesitic basalts. In spite, however, of the distinctly andesitic type, these rocks when examined under the microscope are seen to be almost entirely free from the larger, stout plagioclase phenocrysts that are so characteristic a feature in most of the andesites, and that may also be seen occasionally in some of the above-described basalts. Certainly they are free from all resorption phenomena and from the intermediate clouded zone characteristic of the larger plagioclase phenocrysts of Nos. 179, 180, and 181 of type A. In form they are usually rectangular or broad lath shaped, and they vary in size from one-half millimeter or even one millimeter down to the microscopic dimensions of the groundmass laths. There is, in fact, no sharp line to be drawn between the plagioclases of the groundmass and of the phenocrysts. They vary greatly in the character and amount of inclosures. At times the centers are thickly crowded with irregular glass inclusions; at times these glass inclusions are scattered or even altogether missing. Less commonly one may note inclusions of hypersthene or of augite.

While plagioclase is by far the most abundant phenocryst, the pyroxenes are also abundant. These are usually well developed, especially in No. 196, and have the customary properties already frequently described. With the exception of the three specimens from Bald Crater (192, 193, 194) both augite and hypersthene are present. In these three specimens hypersthene could not be found. On the other hand, olivine, as is usually the case where hypersthene is absent or scarce, is much more abundant than is the case with the other basalts of type D. The pyroxenes, however, fluctuate greatly in amount and in definition. In Nos. 188, 189, 190, and 191 hypersthene is the more abundant, the augite being almost confined to the groundmass. Olivine is never entirely wanting, but it is conspicuous only in the three rocks just referred to. In these it is not always easy to distinguish between the olivine and the very light green augite, as the color is almost alike in the two. This difficulty is greater in sections of olivine that are cut nearly perpendicular to one of the bisectrices, as in this case the interference colors are no higher than in augite. The most conspicuous form developed is the brachydome (021), in addition to which there occurs either a prism or a pinacoid. One section cut perpendicular to this brachydome gave in the center of the field a positive bisectrix with large optical axial angle. The trace of the sides measured 82°, which is about one degree larger than is given for the angle of the brachydome (021).

The groundmass of these basalts contains not a little glass which is either brown or is colorless, but rendered brown by the presence of dust-like globulitic material. This brownish glass is thickly crowded with minute augite prisms and, to a much less extent, with magnetite and with plagioclase microlites, the last named being, in fact, almost absent from the Bald Crater basalt.