106 Andesitic Basalts Type A

The Geology and Petrography of Crater Lake National Park, 1902





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.

 << previousnext >>