88 Dark-colored Secretions

The Geology and Petrography of Crater Lake National Park, 1902

 PART II.

HYPERSTHENE-DACITES.

DACITIC EJECTAMENTA.

DARK-COLORED SECRETIONS.

These are found as ejected fragments together with pumice. They occur as extremely dark, almost black, very porous and rough feeling bombs, and contain a great abundance of black pyroxenes and black hornblende as well as of the glassy plagioclase. They have a wide distribution, as may be seen from the following list: No. 141 comes from the Pumice Desert to the north of Crater Lake; No. 142 from near the summit of Dutton Cliff to the south of the lake; No. 143 from the summit of Llao Rock; No. 145 is a fragment from a conglomerate that overlies a small sheet of dacite just above Grotto Cove; and 2011, b (private collection of the writer) forms part of a large bomb attached to the pumice specimen, 2011, a, above described. This last was collected just south of Llao Rock. In spite of the dark color of the rock the ferromagnesian minerals are quite conspicuous. This is particularly true of No. 2011, b.where hornblende is more abundant than in the other specimens and is recognized by means of the brilliantly flashing cleavages. This mineral occurs here in crystals that measure up to 10 millimeters in length and 2 to 3 millimeters in width.

Under the microscope in thin section these rocks are seen to contain a very dark brown, glassy groundmass that incloses but few augite microlites but is crowded with air cavities. In color and general appearance the groundmass closely resembles the glass inclusions to be seen on the plagioclase crystals of the hornblendic pumice above mentioned. It forms from one-third to one-half of the rock mass, the rest being composed of phenocrysts and fragments of phenocrysts.

The plagioclase crystals belong to the type described under the dacites as occurring in rather large, plump crystals, with numerous crystal faces. Rectangular sections are rare, so rare, in fact, that they are probably only chance sections of crystals of the type just mentioned. The plagioclase crystals show a strong tendency to assume idiomorphic forms except in so far as they occur in shattered fragments. but they are generally crowded with irregular brown glass inclusions so as to assume a thoroughly sponge-like appearance, similar to but much more pronounced than the spongiform crystals in some of the pumice fragments. These ramifying glass inclusions are not confined to the interior of the crystal, but come to the surface and often appear as embayments filled with brown glass exactly like and continuing the glass of the groundmass. Figs. A, B,and C of Pl. XVII are reproductions of some of these spongiform plagioclase crystals and give a fair idea of their appearance. The plagioclase is probably all basic, and, in some cases, demonstrably so. For instance in No. 143 a honeycombed plagioclase crystal, with zonal structure, and cut symmetrically to the plane of twinning gave an extinction angle of 47° center and 37° margin. This indicates an extremely basic anorthite, more basic, in fact, than the most basic of the plagioclases whose extinction angles were measured in either the andesites or basalts of Crater Lake. Not all of the plagioclase crystals show polysynthetic twinning, but most of them show either this twinning or a zonal structure, and no reason was apparent for considering any of them to be orthoclase. They contain inclusions of ores apatite, and hypersthene

The hornblende in these dark-colored secretions is partially idiomorphic and partially allotriomorphic, with the latter tendency decidedly the stronger; that is, crystal forms may be developed on part of a crystal while the rest has a very irregular outline. The observed forms are the customary unit prism and clinopinacoid and flat terminal faces. The color is mostly like that of the greenish-brown hornblende of the dacites, and the absorption is b > c > a. The customary colors in thin section for rays vibrating parallel to the vertical axis is a deep brownish green to greenish brown, in accordance with the thickness of the section. The absorption and pleochroism parallel to b are almost exactly like that parallel to c, with the absorption usually a little stronger parallel to bthan to c. In No. 145 this mineral appears to be unusually scarce and the color in rays vibrating parallel to c a deep brownish red. Those parallel to b can not be determined in this section. In No. 141 there is a peculiar mingling of the reddish with the greenish-brown colors. Most of the crystals belong to the greenish-brown variety, but others have both colors shading into each other on the same crystal. This shading of colors occurs on perfectly fresh crystals. In fact, all the hornblende crystals in these specimens are absolutely fresh, and do not show any tendency to resorption or to the development of black borders. The extinction angle of the hornblende, as measured on cleavage fragments from No. 143, is 8° to the vertical axis. The optical axial plane lies in the plane of greatest and least absorption; that is, in the plane of the clinopinacoid. A section of this mineral in No. 143 that gave a prism angle of 104°, as measured by the trace of the prism faces with the plane of the section, gave a bisectrix in the center of the field.

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Plate XVII.—THIN SECTIONS OF MINERALS IN SECRETIONS IN DACITE.

FIGS. A, B, and C.—Magnified respectively 40, 25, and 15 diameters. All from Specimen No. 145. From a dark-colored secretion among the dacitic ejectamenta. Show spongiform plagioclase crystals inclosing deep-brown glass similar to that composing the groundmass from a dark-colored secretion among the dacitic ejectamenta. See page 126.

FIG. D.—Magnified 18 diameters. Specimen No. 144. From a dark-colored secretion among the dacitic ejectamenta. Shows a grain of hypersthene inclosing numerous plagioclase grains, and two irregular grains of olivine marked 0. See page 129.

FIG. E.—Magnified 50 diameters. Specimen No. 148. A light-colored, granophyric secretion among the dacitic ejectamenta. Shows crystals of plagioclase with fringes of spongiform sanidine containing inclusions of brown glass and resembling granophyric intergrowths. Illustrates a section as seen in polarized light with crossed nicols, the glass appearing black. See page 136.

FIG. F.—Magnified 10 diameters. Specimen No. 142. From a dark-colored secretion among the dacitic ejectamenta. Shows a large crystal of brownish-green hornblende inclosing crystals of hypersthene = H, plagioclase = P, and magnetite. See page 129.

 

The hornblende of these secretions does not contain inclusions of glass, but it is very apt to contain inclusions of the other phenocrysts so as sometimes to be filled with them, somewhat after the manner of inclusions in the hornblende of the crystalline schists. Fig. F of Pl. XVII gives one of these hornblende crystals containing inclusions of hypersthene, plagioclase, and magnetite. In other cases augite may be seen inclosed in hornblende. These minerals when thus inclosed may have idiomorphic forms, but they are more apt to be hypidiomorphic. At least their forms are not always so sharp as when they occur embedded in the glass base.

Hornblende occurs in much larger crystals than do the other minerals. It is very fluctuating in quantity, being very sparingly developed in No. 145, or at least in the thin section prepared from this specimen, and extremely abundant in No. 2011b. Fig. B of Pl. XV1II reproduces a portion of thin section of No. 2011b, showing the relative abundance of the hornblende. This is not a fair average of the whole section, but shows the hornblende somewhat more abundantly than in the rest of the section. The crystal form is unusually sharp in this rock.

Hyperstheine is by no means so abundant as hornblende, nor does it occur in so large crystals. It is more inclined to idiomorphic forms, but it also shows a tendency to contain inclosures of other minerals. Fig. D of Pl. XVII gives a section of hypersthene from No. 144 containing inclosures of plagioclase and two irregular inclusions of olivine. This is exceptional. As a rule, hypersthene is older than plagioclase and even in this case the irregular form of the plagioclase crystals indicates that their period of formation did not greatly precede that of the hypersthene. The optical properties of the hypersthene are in no respect different from what may be seen in the hypersthenes of the dacites.

Olivine is to be found very sparingly developed and always inclosed in either hypersthene or hornblende.

Augite is much less common than hypersthene. It has a pale—greenish color and but little pleochroism. It is younger than plagioclase and hypersthene and older than hornblende. It occurs both in roundish grains and in fairly developed crystals similar to those in the dacites.

The order of crystallization of the phenocrysts of these secretions is in general as follows: 1, ores and apatite; 2, olivine; 3, hypersthene; 4, plagioclase; 5, augite; 6, hornblende. At times the plagioclase appears to change places with hypersthene.

That these dark-colored bombs are in reality secretions from the dacitic magmama fairly well be established by the similarity of the mineral contents with the phenocrysts of the dacites. This is more especially true of hornblende, which, with possibly one minor exception, is absolutely foreign to the andesites as well as to the basalts of Crater Lake, but which occurs with the same colors and other properties, although very sparingly, in nearly all the thin sections of dacites studied. That these fragments were erupted at the same time as the dacites is indicated by their occurring with the pumice of the dacites and more immediately by the finding of a bomb in the midst of the pumiceous deposit south of Llao Rock consisting in part of pumice and in part of this black, glassy, hornblendic rock.

The chemical analysis of No. 143 will be found on page 140. In spite of the evident relationship of these dark-colored secretions to the dacites the analysis shows a marked difference. This is particularly noticeable in the large amount of lime and in the corresponding decrease in the alkalies. The analysis, in fact, bears a close resemblance to the analyses of the more acid basalts Nos. 158 and 189.

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