The Geology and Petrography of Crater Lake National Park, 1902
LIGHT-COLORED GRANOPHYRIC SECRETIONS.
These are not to be considered secretions in the sense that this term is frequently used in describing the accumulations of older minerals that form dark-colored inclusions in igneous rocks, which is the sense in which the word is used in describing above the dark-colored secretions. But, rather, these light-colored secretions appear to represent local crystallizations or differentiations of the same minerals that are to be seen in the dacites in general, but so aggregated together as to appear in totally different structural relationships. They have not been found in place or inclosed in dacite, but only as loose fragments or bombs on the surface.
Plate XVIII.—THIN SECTIONS OF SECRETIONS IN DACITE.
FIG. A.—Vitrophyric dacite and secretion in the same, from Llao Rock. Magnified 48 diameters. Specimen No. 102. A photomicrograph in while light. On the left is the dacite, consisting of a glass with a multitude of augite microlites. On the right is the secretion, composed of a felt of plagioclase laths and slender prisms of reddish-brown hornblende, which appears nearly black in the figure. The interstices of this felt are filled with light-brown glass. The lack of a sharply defined line of junction between the rock and the secretion is characteristic. See pages 105 and 110.
FIG. B.—A photomicrograph in white light. Magnified 20 diameters. Specimen No. 2011b. Illustrates the abundance of reddish-brown hornblende in one of the dark-colored secretions. See page 129.
FIG. C.—A light-colored granophyric secretion from the dacitic ejectamenta. Magnified 20 diameters. Specimen No. 151. Shows a dark-colored brown glass and spongiform mantles of sanidine around plagioclase. See page 136.
FIG. D.—A photomicrograph in polarized light with crossed nicols. Magnified 86 diameters. Shows an enlarged portion of fig. C, giving an individual plagioclase grain with mantle of spongiform sanidine. The black portions are glass.
No. 146 is a small fragment about an inch square that formed part of a conglomerate overlying the small dacite flow immediately above Grotto Cove. Two other fragments from this same conglomerate have already been described. These are No. 139, a pumice fragment containing much hornblende, and No. 145, one of the dark-colored secretions. This association would suggest the same source as that of the dark-colored secretions. In the hand specimen this appears to be a fairly coarse grained, holocrystalline rock of very light color and composed of a whitish, glassy appearing, granular aggregate dotted with small, dark-colored grains. It is quite brittle and crumbles in the fingers. In thin section the rock is seen to consist mainly of plagioclase, which constitutes perhaps one-half of the whole or a little less, in addition to which there are hypersthene, augite, hornblende, and a very little biotite. All these ferromagnesian minerals together form but a small part of the whole. They are perhaps a little more abundant than is usual in the dacites of Crater Lake, but not markedly so. The rest of the rock forms a sort of groundmass either surrounding these older minerals or filling the spaces between them wherever the phenocrysts are close enough to touch each other. This groundmass consists in part of a deep brown and perfectly clear glass, but mainly of a beautifully sharp and characteristic granophyric or micropegmatitic intergrowth of quartz and sanidine. The presence of quartz could be demonstrated by observations in convergent polarized light, in which it gives the positive uniaxial cross, as well as by the more readily observed properties characteristic of the mineral. In the case of sanidine the demonstration is not complete, but is based upon the following observations: It is perfectly clear and colorless and contains no inclusions except minute air cavities; both the refractive power and the double refraction are less than in the adjacent quartz; its index of refraction is less than that of the surrounding balsam; in convergent polarized light it gives a biaxial image; it is entirely free from twinning striation or from zonal structure; this sanidine usually forms a mantle around the plagioclase crystals, so that either the outer part of the sanidine mantle is intergrown with quartz in granophyric fashion or, more usually, this granophyric intergrowth begins close to the plagioclase edge, with little or no free mantle growth visible; the contrast between the plagioclase and the sanidine mantle is usually rather sharp; the plagioclase very frequently shows no twinning striae, but the concentric zonal structure is always visible, whereas the sanidine mantle is entirely free from all twinning and extinguishes simultaneously. Becke’s method for determining the index of refraction, as compared with that of Canada balsam, by focusing sharply on the edge of a crystal in contact with balsam and then observing the movement of the white band upon raising or lowering the focus, was found very applicable in studying this sanidine mantle. Where a crystal of plagioclase was broken and in contact with balsam it was invariably found that the portion of the crystal that showed twinning or zonal structure had a higher index of refraction than balsam, while the outer, untwinned, and unbanded portion was lower in refractive power than balsam. The same method is equally applicable in this case in distinguishing between sanidine and quartz.
Wherever the quartz or the sanidine comes in contact with the brown glass base it breaks up at the edge into an irregularly lobed fringe, with the roundish lobes interwoven or intergrown with the glass in a way suggestive of the granophyric intergrowths themselves. This interweaving of glass with the fringed edges of the crystals is so much more beautifully developed in other specimens whose description follows that a detailed description of this phenomenon will be postponed for the present. The quartz is not present except in these granophyric aggregates. It incloses air cavities similar to the sanidine.
The plagioclase in this rock does not have as sharply developed forms as in the dacites. This is true even when the surrounding sanidine mantle is disregarded. Sections suitable for determining the extinction angles were not found, but such extinction angles as were noted indicate a basic plagioclase. This is further corroborated by the fact that the plagioclase even at the edge has an index of refraction greater than Canada balsam, i.e., greater than 1.540. A few inclosures of brown glass with gas bubbles and also small apatite prisms were noted.
The ferromagnesian minerals all have poorly developed forms. Hypersthene and augite occur mostly in small prismatic grains, some singly, but more commonly in more or less parallel arranged groups, together with magnetite and hornblende and with dark greenish-brown biotite. The hornblende has a greenish-brown color and occurs quite abundantly in longish granular or prismatic individuals. Biotite is scarce, but its presence is the more noteworthy as it appears to be entirely wanting in the dacites proper.
The writer has not seen a full description of rocks similar to this, but Professor Rosenbuscha makes brief reference to several quite analogous occurrences in Iceland, the original descriptions of which are not at the time of preparing this paper accessible to the writer. In these Icelandic rocks, which occur as inclosures in basalt and in loose fragments, are to be seen a similar mineral aggregation with plagioclase crystals surrounded with orthoclase mantles and granophyric growths.
aMikroskopische Physiographie, 3d edition, vol. II, 1896, p. 590.
Among the Crater Lake rocks collected in 1883 there are five specimens, Nos. 147, 148, 149, 150, and 151, which are labeled as being ejected volcanic fragments from the top of the divide between Sand Creek and Anna Creek, near the south rim of Crater Lake. These five specimens are closely analogous to No. 146, just described, and in one or two cases practically identical in all essential points. No. 149, for instance, is very light colored, distinctly granular, and very friable. It differs slightly in color, in that the feldspars have a distinctly pinkish color. On the other hand No. 148 is very much darker and looks as though it were composed about equally of a white granular feldspar and of black glass.
Under the microscope the practical identity of all of these specimens with No. 146 is very evident. There are the same marginal growths of sanidine around the plagioclase crystals, and the same granophyric intergrowths of sanidine with quartz. The deep-brown glass interweaving with the sanidine is also present, but in much greater abundance in two or three of the specimens. The most striking difference between the two occurrences is in the fact that both hornblende and biotite are totally lacking here. Owing to better prepared thin sections, as well as to the opportunity to study sections from a number of similar although not quite identical specimens, some of the features not very clearly observed in No. 146 can be presented here with greater fullness and certainty.
The plagioclase does not, upon the whole, appear to be quite so basic as is the same mineral in the dacites. Twinning striations are abundant and disclose the presence not only of the albite law but also of the pericline and Carlsbad laws. But sections suitable for determining the maximum symmetrical extinction angles do not seem to be common, which is doubtless due to the fact that the rather large average size of the crystals does not allow many of them in any one thin section to be cut in the right direction. The largest extinction angle measured on a section cut at right angles to the albitic twinning plane was 27° central part of the crystal. As all of the plagioclase has a well-defined zonal structure the margin of the crystal is necessarily much more acid. Usually the sections that give symmetrical extinctions give very small extinction angles, even on the interior of the crystal. This is so often the case as to indicate that at least some and probably most of the plagioclase is oligoclase. This supposition is further borne out by the fact that such crystals, when broken so as to have Canada balsam in contact with the plagioclase, are seen to have an index of refraction almost identical with that of the balsam, which would indicate a feldspar about on the border of oligoclase and andesine. While the index of refraction thus indicates that some of the plagioclase, even in the interior of the crystal, is hardly more basic than oligoclase, in all cases where the observation was made the outer portion of the plagioclase has this degree of acidity. It seems more than likely that there are two kinds of plagioclase crystals present, a fairly basic and not very abundant variety and a more commonly developed and presumably younger variety. But in any case all the plagioclase crystals have the mantle of sanidine which, where a glass base is very abundant, or wherever this glass base comes into contact with this mantle, is very remarkably fringed or lobed, so that the roundish feldspar lobes appear to be interwoven with the glass in a most intricate way but which, in the absence of a surrounding glass, forms with quartz a beautifully distinct and rather coarsely developed granophyric aggregate. While these marginal growths around the plagioclase crystals may be seen around every individual plagioclase they do not necessarily entirely surround the crystal, nor does the fringing mantle have constant width. In this respect the mantle is very irregular and even erratic. As above stated, the interweaving of the brown glass with the fringed sanidine gives almost exactly the impression of the granophyric growth when seen in polarized light, except that a portion of the intergrowing substance remains extinguished when the specimen is rotated. Even in white light this resemblance to granophyric growths is often pronounced whenever the glass has an unusually light color, and this is the case in those specimens in which the glass base is not abundant (149), or in which the section is unusually thin. Further, this resemblance is made still more marked by the fact that at times the glass that appears interwoven with the feldspar breaks up into rectangular or otherwise polygonal forms with more or less parallel arrangement. Or, to express it in another way, the sanidine margin develops into a skeleton growth with the brown glass base filling the interstices. The variations of structure that are produced in these marginal growths, owing to constantly varying amounts of glass and the consequent appearance and disappearance of real granophyric growths, fairly baffle description. It would require a large number of drawings or photographs to give a fair conception of these almost fantastic structures. An effort has been made to reproduce some of the most striking effects in fig. E of Pl. XVII (p. 128), figs. C and D of Pl. XVIII (p. 132), and figs. A and B of Pl. XIX (p. 138).
The amount of quartz present appears to be inversely proportional to the amount of glass base. Where much glass is present it is not inclined to form granophyric growths but to occur in rather isolated, roundish, or irregular grains. Although it may have very irregular outlines, it does not break up into lobes or fringes at contact with the glass, as does the sanidine, at least not to any marked extent. In the more thoroughly crystalline specimens and therefore more thoroughly granophyric varieties, the quartz seldom occurs in separate grains. The sanidine is not absolutely confined to the granophyric margins but also occurs very sparingly, as does the quartz, in separate but irregularly formed grains.
Inclosures in the central portions of the plagioclase are not common. Glass inclusions appear to be entirely wanting, but not infrequently a crystal may contain small augite grains, or individual grains of magnetite or prisms of apatite. Minute air cavities, however, are very abundant, both in the plagioclase and in the surrounding fringes of quartz and sanidine.
It should, perhaps, be added that no additional reason can be given for the presence of sanidine in these specimens than those already given in the description of No. 146.
Plate XIX.—THIN SECTIONS OF BASALTS AND OF SECRETIONS IN DACITE.
FIG. A.—Light-colored granophyric secretion from the dacitic ejectamenta. Magnified 48 diameters. Specimen No. 148. A photomicrograph in white light. Shows the occurrence of dark-brown glass, plagioclase, spongiform sanidine, parallel growing prisms, and grains of hypersthene and augite, also a little magnetite. See page 136.
FIG. B.—Light-colored granophyric secretion from the dacitic ejectamenta. Magnified 86 diameters. Specimen No. 149. A photomicrograph in white light. Shows a bundle of parallel arranged prisms of hypersthene and augite, which can not be distinguished from each other in the photograph. See page 139.
FIG. C.—Magnified 48 diameters. Specimen No. 2029 (private collection of H. B. Patton). A photomicrograph in white light. Shows the interstitial structure of some of the basalts. See page 142.
FIG. D.—Basalt, Timber Crater. Magnified 48 diameters. Specimen No. 165. A photomicrograph in white light. Illustrates the fluidal-interstitial structure of some of the basalts. See page 149.
Hypersthene and augite are both very abundant. Neither of them occur in the customary idiomorphic crystals but either in perfectly irregular grains or—which is generally the case—in various shaped aggregates, or more especially in clusters or bunches of loosely aggregated and parallel-growing roundish and rather slender prismatic grains. Both of these minerals assume this same form and usually both of them occur together in parallel growths. All conceivable variations may be observed between isolated individuals of hypersthene or augite of the shape just mentioned and parallel aggregates of the same that contain hundreds of the small prismatic grains. Some idea of this occurrence may be derived from fig. B of Pl. XIX. Hypersthene seems to be the more abundant of the two. In color the hypersthene is not quite so dark as it is in the ordinary dacites of Crater Lake. Pleochroism, therefore, can not readily be seen in such sections as are necessary for proper study of the aggregates. The augite appears almost colorless in thin section. With these two pyroxenes there is constantly associated considerable magnetite, which occurs mostly in very minute grains inclosed in the individual pyroxene grains or clustered with them in these bunches.
The chemical analysis of No. 151 will be found on page 140, together with the other chemical analyses. While somewhat more basic than are the dacites proper, the analysis of this specimen will be seen to compare fairly closely with that of No. 114. The excess of lime may perhaps be attributable to the abundance of the pyroxene. It can hardly be due to the relative abundance of the feldspar, as the plagioclase present does not appear to be particularly basic, and the sum of the alkalies present is rather less than is the case with the dacite whose analyses are given.
In conclusion, we may say that the absolute freshness of these specimens would seem to preclude the possibility of a secondary origin for the granophyric growths.