67 Secretions in Andesites

The Geology and Petrography of Crater Lake National Park, 1902

 PART II.

HYPERSTHENE-ANDESITES.

SECRETIONS IN ANDESITES.

In a number of the andesite rocks at different localities are found inclosures or nodules that appear to be secretions of the more basic minerals. Sometimes these inclosures have more or less roundish form and are evidently inclosed in the sense that they are not literally formed in place. This is particularly evident in a specimen (97) collected in 1883 from the southern rim of the crater. The rock itself, of which no thin section has been made, has a nearly black groundmass—which is probably hyalopilitic—inclosing numerous and very conspicuous plagioclase phenocrysts. Embedded in this rock is a roundish mass 2 or 3 inches in diameter from which the andesite appears to have shrunk away in cooling as though from a foreign inclosure. The inclosure is blackish gray, compact, without phenocrysts, and slightly porous. It has a very basaltic look.

In thin section this specimen is fairly characteristic of most of the inclosures. It consists of a deep clear-brown glass in which lie in loosely felted arrangement slender and very sharply defined laths of plagioclase and equally slender but not always so sharply cut prisms of hypersthene and augite; also a little magnetite in the customary octahedral form. Hypersthene is more abundant than augite, but both are much more abundant than is usual in the andesites. The two pyroxenes appear to form about as large a percentage of the rock as does the plagioclase. Parallelism of growth between the augite and hypersthene is common. The length of these pyroxenes is often ten times as much as the width. The plagioclase is rather simply twinned with but two or three bands and shows large extinction angles. They have square cross section with diagonal extinction. Phenocrysts are entirely wanting.

Fig. A of Pl. XVI presents a photograph of this thin section in polarized light and Fig. Dof Pl. XVI a photograph of thin section of No. 9 in white light, showing both the secretion and the inclosing hyalopilitic andesite.

Analogous to the above are No. 9, from the path that leads from the camp ground down to the water’s edge; No. 16, from Kerr Notch, at the head of Sand Creek; and No. 75 from near the road on Anna Creek. These three specimens consist of small inclusions in rather glassy andesites, the first two of the hyalopilitic and the last of the dacitic type. The inclusions form only a small part of the thin section in each case. They do not, however, appear to be foreign to the rock. On the contrary, the junction of the secretion with the surrounding andesite is such as apparently to preclude the idea of an entirely separate origin. The clear glass of the section is exactly of the same color as that of the main rock and the junction is not sharp, but the ingredients seem to have grown from the secretion out into the surrounding rock mass. The first two contain phenocrysts of plagioclase, and to a less extent of hypersthene similar to the corresponding phenocrysts in the inclosing andesite. In some cases one of these plagioclase phenocrysts appears to belong about equally to the secretion and to the main mass. In No. 16 the inclusion greatly resembles an enlarged area like these one frequently meets with in the more glassy andesites of Crater Lake. where a deep and clear brown glass, free from the customary microlites, occurs, filling the space between two or three adjacent phenocrysts.

No. 98 is one of these inclusions from a large bowlder between Sun Creek and Sand Creek. The rock in which this is inclosed (76) belongs to the dacitic type of andesite, and has already been described. This inclusion contains large, characteristic plagioclase phenocrysts that are crowded with the unusual inclusions, and that present clear margins; it contains also phenocrysts of hypersthene. The glass is stained a very deep red with ferritic matter.

Nos. 99 and 100 are sections in the above-described hypocrystalline andesite (40) found at the spring near the camp ground. They both resemble No. 97 in being entirely without phenocrysts and in having the same felted structure and the same composition as that rock, but they differ materially in that crystallization appears to have progressed until but little glass remains, and this little is nearly colorless and not readily discernible. In fact, these two inclusions were at first taken for basalt inclosed in the andesite, and were it not for numerous intermediate stages between these and the more evident secretions the determination as basalt might hold. Other cases of secretions resembling basalts are mentioned in connection with the description of the dacites of Crater Lake.

plate16

Plate XVI.—THIN SECTIONS OF ANDESITE.

FIG. A.—Secretion in andesite from the southern rim of the crater. Magnified 48 diameters. Specimen No. 97. A photomicrograph in polarized light with crossed nicols. The secretion consists of a deep-brown glass in which lie well-defined laths of plagioclase and equally slender prisms of hypersthene and augite; also a little magnetite. The pyroxenes can not well be distinguished from the plagioclase in the photomicrograph. See page 96.

FIG. B.—Andesite of the hypocrystalline type, from Cathedral Rock. Magnified 20 diameters. Specimen No. 45. A photomicrograph in white light. The ragged black crystal is a brown hornblende almost completely altered through resorption to a black aggregate of magnetite and augite. The section also shows two generations of plagioclase. The large phenocryst on the right represents the oldest generation with glass inclusions. For description see page 83.

FIG. C.—Andesite from the water’s edge at the head of Sun Creek. Magnified 48 diameters. Specimen No. 73. A photomicrograph in polarized light with crossed nicols. Illustrates the third or holocrystalline type of andesite and is characterized by the presence of allotriomorphic patches of feldspar, which appear as light spots hint the photomicrograph See page 89.

FIG. D.—Andesite from the path leading from the camp ground to the water at Eagle Cove. Magnified 48 diameters. Specimen No. 9. Shows the junction between a hyalopilitic andesite on the right and a secretion in the same on the left. The secretion consists of a deep-brown glass and slender laths of plagioclase and prisms of hypersthense and augite; also a little magnetite. See page 96.

 

In No. 25, a hypocrystalline andesite from Wizard Island, is to be seen an inclosure that may with more probability be taken for basalt. This consists of small, irregular, and apparently isolated grains of augite, or possibly of hypersthene—the distinction is not certain in this case—that extinguish simultaneously over considerable areas. The spaces between these grains are filled with short and not very sharp plagioclase laths.

A rock collected by the writer from the bottom of the crater on Wizard Island (2017, 4 of the private collection of the writer) seems to throw some light on these secretions. In the hand specimen it consists of a rather compact and nonporous gray andesite, with irregular, lighter gray patches scattered promiscuously through it. In thin section these lighter patches are seen to be holocrystalline, with a groundmass composed of plagioclase, augite, and magnetite. The plagioclase is largely in the form of short laths, but between these laths the same mineral appears in allotriomorphic form, but not in poikillitic patches. The darker portions of the rock do not appear to differ from the lighter part, except that the allotriomorphic feldspathic part of the groundmass is replaced by a deep clear-brown glass. That several such alternations in the groundmass may occur within the area of a thin section makes it more likely that the apparent inclosures, in which brown glass is so very conspicuous, are only local differentiations in a common magma.

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