Geological Survey Professional Paper 604 On Batholiths and Volcanoes—Intrusion and Eruption of Late Cenozoic Magmas in the Glacier Peak Area, North Cascades, Washington

PETROLOGY OF THE GLACIER PEAK LAVAS
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MINERALOGY

QUARTZ

The ubiquitous occurrence of partially resorbed quartz crystals (fig. 37) makes a xenocrystic origin unlikely. In the lavas bearing the holocrystalline groundmass patches, resorbed quartz phenocrysts are commonly surrounded by a halo of holocrystalline material similar to the patches, which suggests that the quartz was reacting with the melt in situ. In a few flows, partially resorbed quartz is surrounded by reaction rims of small crystals of pyroxene. X-ray diffraction studies indicate cristobalite is present in the groundmass of the lavas, but it was not identified in thin section except in inclusions.

FIGURE 37.—Unusually abundant resorbed quartz phenocrysts (qp) in dacite from west side of Glacier Peak. Plane-polarized light. Specimen FC—96—61.

PLAGIOCLASE

Considerable attention has been given to plagioclase in the flows of Mount Rainier by Coombs (1936, p. 175-180). He described three habits of plagioclase, and his classification can be applied to the plagioclase in Glacier Peak lavas: (1) Large glomeroporphyritic clots, commonly welded together by a common rim zone, (2) smaller intermediate-sized phenocrysts composed of single crystals, and (3) tiny groundmass crystals down to microlite size.

The conspicuous plagioclase phenocrysts in individuals and clusters (fig. 38) generally range in length from 0.5 to 2.0 mm (millimeters). Intermediate-sized plagioclase phenocrysts in pilotaxitic rocks are from 0.1 to 0.25 mm long. Larger glomeroporphyritic clots, particularly prominent in gray lavas, are 3 to 5 mm across. In most flows, the plagioclase phenocrysts are polysynthetically twinned and show oscillatory zoning, but the overall compositional differences between adjacent zones is small. Cores contain as much as 20 percent more anorthite than rims, although in a few lavas, especially those of Disappointment Peak, reverse zoning has produced rims nearly as calcic as the core. Most of the plagioclase² in Glacier Peak flows is An_35-48.

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²Extinction angles in plagioclase were measured in crystals cut perpendicular to (001) and (010), and the compositions were inferred from curves for high-temperature optics constructed from data by Slemmons (1962). In practice, the extinction angle of a zoned crystal was measured when the whole crystal was at its darkest position of rotation, which gave a rough average composition. Structural state was found to he disordered (high-temperature) by universal-stage study of two specimens, according to the method of Slemmons (1962).

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FIGURE 38.—Oscillatorily normally zoned plagioclase phenocrysts both individuals and glomeroporphyritic clots in dacite. Crossed nicols. Same specimen as shown in figure 37.

PYROXENE

The predominant mafic silicate in most of the flows is hypersthene. It occurs as sparse euhedral phenocrysts as much as 2 mm long and as common but scattered crystals 0.1 to 0.3 mm long. More rarely it occurs as tiny prisms in the groundmass. Larger phenocrysts are here and there rimmed with clinopyroxene grains, and one crystal of hypersthene was observed with a clinopyroxene overgrowth. Measured 2V_z indicates the hypersthene has a composition of about En50—70 (total range of 2V_x: 50°-90°; n_z<l.73), using a curve of Deer, Howie, and Zussman (1963, v. 2, fig. 10, p. 28).

In strongly oxidized rocks, normal hypersthene, with Z parallel to the c axis or to the elongation of the grain, is irregularly zoned on edges and in cracks to a hypersthene with Y parallel to c. This oxidized(?) hypersthene has a higher birefringence, slightly stronger absorption, and a somewhat smaller 2V_x (40°-50°) than the normal hypersthene. Lacroix (1910, p. 765) described a similar hypersthene which he named B-hypersthene. Oxidized hypersthene with an unusually small 2V has also been reported by Lewis (1960).

Monoclinic hypersthene is absent in Glacier Peak lavas, although it has been reported from rocks of several other Cascade volcanoes (Mount St. Helens, Verhoogen, 1937, p. 284; Mount Rainier, Fiske and others, 1963, p. 88; and Mount Baker, Coombs, 1939, p. 1503). The lack of clinohypersthene in the rocks from Glacier Peak was first noted by Ford (1959, p. 272).

Clinopyroxene is most abundant as small phenocrysts 0.1 to 0.3 mm long, and small prisms in the groundmass of pilotaxitic and trachytic rocks. Aggregates of clinopyroxene, and more rarely hypersthene, which pseudomorph amphibole are common. In some rocks the pyroxene of these pseudomorphs occurs as large discontinuous skeletal crystals set in a matrix of plagioclase—a combination which looks deceptively like a small plutonic xenolith. Optic axial angles of the clinopyroxene generally lie between 54° and 58° (range 45°-64°), which indicates a calcic variety (Deer, Howie, and Zussman, 1963, v. 2, p. 132). Ford (1959, p. 258-271) indicated that most of the clinopyroxene in the Glacier Peak lavas is augite, and this is consistent with optical properties observed by us.

HORNBLENDE

Deep red-brown or dark-brown oxyhornblende phenocrysts (or opaque pseudomorphs after them) are conspicuously abundant in the dacite of the Disappointment Peak dome and in similar-appearing dacite flows which, for the most part, are scattered about the summit cone. In other flows, oxyhornblende, although present, is sparse. Oxyhornblende is commonly pseudomorphed by aggregates of clinopyroxene, hypersthene, magnetite (fig. 39), and rarely, actinolite.

FIGURE 39.—Magnetite pseudomorph of hornblende rimmed by pyroxene. Dacite from upper Vista Creek. Plane-polarized light. Specimen RWT—343—61.

Small amounts of brown hornblende (|| Z==strongest absorption; Z^ c>10°) rimmed with opaque granules occur in flows of all ages, whereas greenish-brown to olive-brown (Z) hornblende phenocrysts are relatively abundant in younger glassy flows. An exception to this is the cliff-forming flow which is shown as a valleyside clinging flow in the upper reaches of the Suiattle River, and which contains olive-green hornblende. No greenish hornblende has been found in the oldest ridge-capping flows.

Out of nine samples of dacite containing green hornblende, seven have a silica content of more than 64.0 percent, in contrast to the predominant range for silica of 62.0 to 65.5 percent in all the flows. The green hornblende is also abundant in some of the youngest pyroclastic rocks (for example, the tuff of the White Chuck River valley and the yellow pumice discussed later), which also have a silica content of more than 65.0 percent.

The crystallization of intratelluric hornblende in lavas of Mount Rainier has been ascribed to higher-than-normal water-vapor pressure (Fiske and others, 1963, p. 89). The higher pressure can occur if the water is trapped by rapid chilling. Williams (1942, p. 155) also found hornblende in rapidly chilled rocks at Crater Lake:

Hornblende andesite lavas are rare throughout the High Cascades. In the Crater Lake region, only two examples are known. Between Mount Mazama and Mount Shasta, only one occurrence is known, namely in the dome on the summit of Rustler Peak. On Mount Shasta, hornblende andesite is developed in the dome of Black Butte. On Mount St. Helens, the plugs (domes) invariably carry hornblende. In brief, hornblende andesite lava is almost confined to quickly chilled viscous domes erupted from parasitic vents.

On the other band, hornblende is extremely abundant in the basic scoria flows of Mount Mazama. It is also common in the dacite pumice and almost ubiquitous, though in small amount, among the glassy dacite domes and flows erupted from the Northern Arc of Vents. Yet among the holocrystalline, pilotaxitic dacites of Mazama, the mineral is scarcely ever present. Hence, rapid cooling appears to be necessary to prevent complete resorption of the mineral in magmas of shallow origin.

In general, these remarks apply also to the occurrence of hornblende at Glacier Peak; we find that hornblende is most abundant in the Disappointment Peak dome, in the younger glassy flows, and in rocks with a silica content greater than 64.0 percent. The higher silica content, by making the lavas more viscous, may also have hindered replacement of the hornblendes by opaque ore minerals.

OLIVINE

Olivine is sparsely present in most of the flows. Crystals are commonly euhedral and fresh appearing, but many are rimmed by an extremely fine-grained aggregate of pyroxene and opaques. Not uncommonly there is a concentration of plagioclase laths or a holocrystalline mesostasis of plagioclase around olivine crystals. More rarely the olivine is partially replaced by plagioclase and phlogopite and, in a few lavas, by a moderately birefringent green montmorillonoid. Optic axial angles (2V_z) for the olivine generally range from 88° to 94° (total range 82°-101°) which indicates a composition of Fo_76-88. Partially resorbed quartz and olivine occur together in some rocks.

BIOTITE

One flow contains brown biotite of possible phenocrystic origin. The small crystals are partially resorbed and rimmed with an opaque mineral, green biotite, and chlorite. Biotite is relatively common as tiny pale-brown flakes in holocrystalline groundmasses and in the "devitrified" patches. It is associated with opaque minerals in pseudomorphous aggregates of pyroxene after amphibole. In a few flows it also fills tiny fractures along with quartz.

OTHER MINERALS

Opaque minerals, mostly magnetite, are abundant in the groundmass of all lavas and occur as inclusions in the mafic phenocrysts. In the red flows, the magnetite is oxidized to hematite. Small amounts of apatite (rarely manganoapatite) and zircon also occur.

INCLUSIONS

The most common inclusions are medium-grained gabbroic rocks a few millimeters across that are thought to be glomeroporphyritic clots. These inclusions are generally more mafic than the host. They consist of subhedral to euhedral zoned plagioclase, clinopyroxene and (or) orthopyroxene and rare olivine, all of which are generally about the size of the largest phenocrysts in the host rock. Some of the larger inclusions are locally subophitic (fig. 40). These gabbroic inclusions are not plutonic equivalents of their volcanic host, because small angular patches of glass or a mesostasis of sodic plagioclase and cristobalite (?) are common between the larger crystals; the glass and mesostasis represent interstitial melt trapped be tween the larger crystals, which indicates that the inclusions are accumulates.

FIGURE 40.—Medium-grained inclusion with subophitic texture in dacite on ridge north of Gamma Creek. Note small areas of intersertal texture between larger crystals at upper left, which indicate a volcanic affinity. Plane-polarized light. Specimen RWT—12—63.

The most distinctive inclusions are porphyritic, hyalo-ophitic, and diktytaxitic (fig. 41); they are thought to be cognate crystal accumulates torn from the walls of feeder conduits. They occur in rounded fragments as much as 6 inches across (much larger than the gabbroic inclusions) and are generally light gray. The inclusions are most abundant in the hornblende dacite of the Disappointment Peak dome. Though mineralogically like their host, these inclusions are commonly richer in mafic minerals but nevertheless contain considerable intergranular cristobalite. The plagioclase laths are about the same size as intermediate-sized phenocrysts in the enveloping lava, although this size range is rare in lavas of the Disappointment Peak dome, which contains the greatest abundance of diktytaxitic inclusions. Inclusions of similar appearance and origin are common in the lavas of Crater Lake (Williams, 1942, p. 134-135). The medium-grained gabbroic inclusions and glomeroporphyritic clots of plagioclase containing the largest phenocrysts (fig. 40) probably crystallized in a deep magma chamber. The porphyritic and diktytaxitic inclusions containing intermediate-sized phenocrysts may have been torn from the walls of a near-surface chamber or vent. The inclusions derived from near-surface chambers are larger than those from chambers at greater depth, perhaps because of less trituration. The larger inclusions may have been drained of interstitial liquid during times of magma withdrawal from the higher chambers; thus, they acquired a diktytaxitic texture.

FIGURE 41.—Diktytaxitic inclusion from dacite of Disappointment Peak dome. Plagioclase is considerably finer grained than that shown in figure 40. Plane-polarized light. Specimen RWT—130—61.

Rare small inclusions of volcanic rock that are distinct from the host probably are fragments of earlier lavas. Certain pre-Glacier Peak inclusions have been identified in only one flow, an obsidian just below the Cool Glacier which contains numerous fragments of the underlying quartz diorite and of nearby Gamma Ridge volcanic rocks.

ALTERATION

Considering the volcano as a whole, the alteration is spotty and probably resulted from fumarolic activity. Around the summit crater, lavas are stained red and yellow. Carbonate partly replaces the groundmass of a few rocks, and very fine grained green to brown montmorillonoid mineral partly replaces the groundmass and mafic phenocrysts in others. The montmorillonoid alteration is particularly prominent at an altitude of 5,300 feet in Vista Creek, where the rocks are covered with yellow and red-brown blotches and are somewhat brecciated. Similar alteration is detectable in thin sections from scattered places in lower Vista Creek, from near the Cool Glacier, and from the west side of Glacier Peak (north of Sitkum Glacier), where an inter-flow breccia is green.