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The Geology and Petrography of Crater Lake National Park, 1902

 

PART II.

 

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BASALTS.

INTERSTITIAL BASALTS.

PORPHYRITIC-INTERSTITIAL BASALTS.

This type of basalts may be said to contain well-defined and usually abundant phenocrysts, always of olivine and generally also of plagioclase, hypersthene; and augite, in a groundmass that bears a more or less close resemblance to the interstitial basalts. As a rule, however, the groundmass is inclined to contain more glass and the minerals of the second generation have more sharply developed forms than is the case with the corresponding minerals that compose the interstitial basalts.

The lava of Desert Cone, immediately north of Red Cone, may be taken as the best example of this type of structure. Red Cone also is composed of basaltic lavas that belong to this type, but not entire so, as will appear later.


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The lava of Desert Cone is represented by three specimens—No. 167 from the north side, near the summit, and Nos. 168 and 169 collected close together about 1 mile farther south on the southern slope. All three are very much alike. In the specimen from the north side olivine is the only phenocryst; in the two from the southern slope, in addition to olivine, plagioclase, in a rather subordinate role, occurs as a phenocryst.

The olivine in these specimens is to be seen in very sharp idiomorphic crystals and also in crystal fragments. They are particularly well adapted to a study of the crystallographic forms and optical properties. At first glance some of the olivine crystals, especially the fragments, resemble augite somewhat, the color being a light yellowish green. Irrespective of the crystal form, however, one can usually distinguish both the crystals and the fragments by their unusually rough appearing surface and by their high interference colors, also by the absence of well-defined cleavages. They usually appear in rectangular or hexagonal shapes. The dominant forms are the brachydome (021) and a prism. In sections cut parallel to the macropinacoid this combination gives hexagonal outlines, the angle formed by the trace of the brachydome being not far from 80. Such a section also gives a positive bisectrix, with large optical angle (168).

In No. 169 the olivine phenocrysts have very sharply developed forms and show an extensive alteration to iron oxides. This alteration product occurs both as a broad rim and as finely granulated masses scattered throughout the crystal. In the very smallest crystals the olivine has been almost completely replaced; in the larger ones the unaltered olivine rarely composes more than one-half of the entire bulk and generally much less than one-half. This iron oxide alteration product consists sometimes of perfectly black, opaque material, which is presumably magnetite; in other cases the grains are slightly transparent and are of a deep red color, suggestive of hematite. Still more commonly both magnetite and hematite abound. In spite of the sharpness of the crystal forms the olivine phenocrysts are frequently penetrated by plagioclase laths, thus indicating a very basic feldspar. (See fig. L of Pl. XIV, p. 76.) These penetrating feldspar laths do not occur in the center, but only at the margin of the crystals; occasionally, also, one may see an olivine crystal impressing its form on a plagioclase phenocryst. This, however, is by no means as common as the other case. It would seem from this that the feldspars had commenced to crystalize out before the olivines had ceased to grow. Possibly they may have begun simultaneously with the olivines.

The only other phenocryst in this basalt is plagioclase, and even this is absent from one of the three specimens (167). It is by no means conspicuous nor very markedly different from the smaller feldspathic laths that make up the greater part of the groundmass. The shape is rectangular or long rectangular. The largest extinction angles noticed on sections cut perpendicular to the albitic twinning plane was 30 in No. 168 and 29 in No. 169. This does not indicate so extremely basic a feldspar as its relationships to olivine would suggest. Probably if the plagioclase phenocrysts were more numerous sections could be found showing larger extinction angles.

The groundmass appears to be nearly holocrystalline. It is composed of well-developed plagioclase laths of hardly more than microlitic proportions, augite in minute grains, or in both prisms and grains, and abundant magnetite in mostly very minute octahedrons and grains. Hematite powder is also abundantly developed in the groundmass of No. 169 as well as in the olivine phenocrysts. In addition to the above-named minerals hypersthene has been recognized in No. 168. It is entirely confined to the groundmass and occurs in but one generation. It forms very small prisms that measure 0.06 millimeter long by 0.01 millimeter wide and that very closely resemble similar prisms of augite. They are too small to show any pleochroism and are to be recognized only by their parallel extinctions and lower interference colors. In fact, it requires some little familiarity with these rocks before the distinction between the pyroxenes can be made.

In the southwest corner of the area covered by the Crater Lake map, and about half a mile south of the road, was collected a specimen of basalt, No. 170, that is closely connected with the interstitial basalts. In a groundmass composed of plagioclase laths, augite, hypersthene, and a little dust-laden glass occur perfectly developed comparatively large plagioclase phenocrysts that exactly resemble the andesitic plagioclases; also an occasional hypersthene, augite, and blood-red olivine crystal. The plagioclase is in broad crystals that show rectangular and six-sided outlines and, as in the andesites, zonal structure. The interior is crowded with glass inclosures, while the margin is free from the same, or else both the interior and the margin are clear, while the inclosures are confined to a narrow intermediate zone. One of these phenocrysts, cut perpendicular to the brachypinacoid, gave an extinction angle of 34.

The groundmass of this rock, were it free from plagioclase phenocrysts, would be identical with the interstitial basalts. The plagioclase laths are the dominant mineral, and appear to inclose the angular augite grains as well as the little glass present in the interstices formed by their intersection. Hypersthene, which occurs only very sparingly as a phenocryst, is more abundant in the groundmass, but as it is found here only in prismatic crystals and not in grains like the augite, it is properly not to be considered as belonging strictly to the groundmass, but rather to the older generation of crystals.

Although this rock presents an entirely different appearance from the basalt from Desert Cone, it must still be assigned to the type which has been designated as porphyritic-interstitial. It is in a way intermediate between the holocrystalline interstitial and the andesitic types.

Quite similar to this rock is No. 171, collected between Crater Peak and Sun Creek.

Still another development of the porphyritic-interstitial type of basalt is to be seen in part of the lava rocks of Red Cone. These are dark-gray, in one case red, dense, scoriaceous lavas. The phenocrysts are very inconspicuous, but may be made out under the pocket lens. They consist of yellowish to red olivine, yellow to black augite, and white plagioclase.

Under the microscope these appear to be decidedly porphyritic rocks with abundant and well-defined, though small, phenocrysts of olivine, augite, and plagioclase. The plagioclase is the most variable of the phenocrysts, as one specimen (172, collected about l mile southwest of the cone) contains almost no phenocrysts of this mineral. It occurs in rectangular, broad to narrow, lath-shaped crystals, the smaller of which graduate into the plagioclase of the groundmass. The ends of the laths are cut squarely off, as by a pinacoid, where the groundmass is distinctly glassy, but, in case the latter is more decidedly crystalline, the ends are somewhat frayed. They inclose at times a small amount of dusty-looking glass similar to that of the groundmass, but neither in shape nor in the appearance or arrangement of the inclosures do these plagioclase phenocrysts resemble those of the andesites or of the last-described basalt. Extinction angles in symmetrically cut sections, i. e., in sections perpendicular to the brachypinacoid, were measured as follows: 27, 30, 31, and 34. They are younger than both olivine and augite.

Of the pyroxenes augite is the only one of consequence. Of the four specimens studied three (174, 175, 172) contained no hypersthene and one (173) contained but one individual, which consisted of a roundish grain having a small core of hypersthene and the outer and larger part augite in parallel position. This growth of augite appears to be secondary and distinct from the more customary occurrence of augite growing around hypersthene crystals. Aside from this one case augite occurs in granular form, but even so impresses its form upon the plagioclase wherever it comes in contact. It is to be found both isolated and in nests with olivine. The color is usually greenish, but in No. 174 many of the grains have a brownish cast, or else they have greenish centers and shade into brown on the outside. It may be added that the hand specimen also discloses a few black augite phenocrysts, 2 to 3 millimeters long, that do not appear in the thin section.

Olivine is a very abundant constituent. It occurs in well-defined, sometimes in very sharply defined crystals, as well as in grains, the latter form being common where the olivine forms nests with augite and occasionally with plagioclase. In No. 174 this mineral occurs exactly as described for No. 169, one of the specimens collected on Desert Cone. It has the same clear-cut forms and has undergone the same alteration to hematite. In this case, however, the alteration has progressed still further. Not only have we the outer rim of almost opaque hematite, but the center of even the largest crystals is so thickly crowded with hematite as to leave very little clear olivine visible. (See Fig. M of Pl. XIV (p. 76.)

The groundmass of these rocks is abundant and mostly very distinct from the phenocrysts. It consists essentially of a colorless but dusty glass base that is crowded with augite microlites in the form of minute prisms and grains, also microlitic plagioclase laths and sharp magnetite octahedrons. The interstitial structure is not clearly brought out, but is produced in part by the plagioclase of the phenocrysts and in part by the plagioclase of the groundmass, there being no well-defined distinction between the two. The porphyritic structure, therefore, is much more in evidence than is the interstitial, and this rock may be considered in a sense as intermediate between the porphyritic interstitial and the more distinctly porphyritic basalts. It does not, however, bear much resemblance to that type of porphyritic basalts that are described as andesitic in these pages.

The chemical analysis of No. 173 will be found on page 161.

Two specimens, Nos. 176 and 177, collected about 3 and 4 miles, respectively, west of Red Cone, have been placed in this group of porphyritic interstitial basalts, although they are decidedly transitional between this group and the interstitial basalts proper. The greater part of the rock, as seen in thin section, constitutes a sort of holocrystalline groundmass similar to that of the more coarsely grained interstitial basalts. In this groundmass of plagioclase, augite, and magnetite occur some rather sharply defined phenocrysts of olivine, augite, and plagioclase. The two last-named phenocrysts occur also in granular form or in nests of grains, and grade off into the same minerals of the groundmass, so that no sharp line can be drawn in either case between the components of the groundmass and the phenocrysts. In the case of olivine, granular forms also occur, but these are always distinct from the groundmass in which no olivine occurs.

The olivine is slightly serpentinized, and is stained in places a deep red. It contains inclosures of magnetite. The better formed crystals of augite disclose the customary forms, namely, prism, two pinacoids, and flat terminal pyramids or dome faces. It is frequently twinned in double, triple, and multiple twins. The twinning plane is the orthopinacoid. It incloses olivine and magnetite, and, to a very slight extent, also plagioclase. The plagioclase crystals show more or less rectangular, but not very sharply cut, outlines. The period of development almost exactly coincides with that of the augite, as these two minerals often impress their form on each other. A few crystals contain minute glass inclosures, but usually they are free from inclosures, and do not bear any resemblance to the plagioclase phenocrysts of the andesites.

Hypersthene appears to be entirely wanting.

A very similar basalt, No. 178, from near the road south of Castle Creek, at the very edge of the district covered by the map, is also placed in this division.

 

 

 

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