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

LITHOLOGY

GENERAL FEATURES

The rocks comprising the Cloudy Pass pluton and associated stocks in the Glacier Peak quadrangle have been previously described by Ford (1959, p. 182-236, 242-244, 247-249; 1960) and those in the Holden quadrangle, by Cater (1969). Grant (1966, p. 207-235) has described the batholith exposed to the north. The summary given here stresses new data and interpretations from our additional work in the Glacier Peak quadrangle.

We distinguished and mapped two phases of the main pluton and of the Sitkum stock—a light-colored, predominantly adamellite phase and a dark-colored phase ranging from granodiorite to gabbro. Corresponding to our light-colored phase are the "leucocratic facies" of Cater (1969, p. 17), the "denterically altered phase" of Grant (1966, p. 219), and the "rocks of granodioritic, quartz monzonitic and granitic composition" of Ford (1959, p. 104). Corresponding to our dark-colored phase are the "labradorite granodiorite" of Cater (1969), the "main intrusive phase" (quartz diorite) of Grant (1966, p. 314), and the "Miners Ridge quartz diorite" of Ford (1959, p. 182).

The light-colored phase is confined to the northwestern border zone of the pluton and to the north end of the Sitkum stock, but the subsurface part of the batholith, which is thought to connect the stock with the main pluton, would presumably also be of this rock type. The one examined specimen of the Milk Creek stock, a body lying between the main pluton and the Sitkum stock, is mineralogically of the dark-colored phase (compare fig. 4C with fig. 4A); texturally, however, it is much like light-colored phases of the Sitkum stock, and it has the lowest color index of all the phases mapped as dark colored (fig. 4D).

FIGURE 4.—Modal composition of the Cloudy Pass batholith and associated rocks. Dots, from Ford (1959, p. 186-187, 243, 248); circles, this report. (click on image for an enlargement in a new window)

In the Holden quadrangle, the light-colored rocks occur as small irregular patches scattered in the dark-colored phase near the roof of the batholith and on ridges (Cater and Crowder, 1967; Cater, 1969, p. 17; Grant, 1966, p. 221). In the Dome Peak area, they are sporadically distributed along the margin of the dark-colored batholith (Grant, 1966, p. 218).

DARK-COLORED PHASE

Rocks in the dark-colored phase are medium grained and hypidiomorphic granular granodiorite, tonalite, quartz gabbro, and gabbro (table 2, fig. 4). Large plagioclase crystals are euhedrally and oscillatorily zoned from cores of andesine or labradorite to narrow, commonly anhedral rims of oligoclase (average range about An₅₀ to An₂₂). Cores of plagioclase in the White Chuck. stock are as calcic as An₇₈. We estimate the average composition of plagioclase to be andesine. In the Holden quadrangle, however, Cater (1969, p. 19) considered that the average plagioclase composition of what he called the normal phase (our dark-colored phase or core phase, was labradorite; thus, he called the rock a labradorite granodiorite (granogabbro). Anhedral intergranular quartz forms a continuous network between plagioclase and mafic minerals in places this network is an optically continuous single large crystal. Potassium feldspar is entirely intergranular.

TABLE 2.—Average modal composition of Cloudy Pass batholith and associated rocks

[Volume percentages determined on thin sections stained for potassium feldspar by using the method of Chayes (1966). Five hundred points per section; numbers in parentheses indicate range. For additional modes see Ford (1959, Pt 186-187, 243, 248)]

In the Glacier Peak quadrangle, we found hypersthene and clinopyroxene in only the small intrusions—in the White Chuck and Cool stocks and in dark-colored rocks of the Sitkum stock, where they occur primarily as small resorbed inclusions in plagioclase or as relicts in uralite. Ford (1959, p. 210) found pyroxene near the contacts of the main pluton northeast of Image Lake and north of Dusty Creek, and Libby (1964, p. 122) found hypersthene to be a "typical constituent" near the pluton margin north of the Holden quadrangle. In the Holden quadrangle, hypersthene and augite are most abundant near the eastern margin of the batholith (Cater, 1909, written commun.). The pyroxene is thus preserved in border rocks or small stocks which may have undergone rapid cooling before hydrous minerals of Bowen's reaction series could form. Plagioclase is in a transitional structural state in the small White Chuck stock, whereas it is in a mostly ordered and rarely transitional structural state in the larger Cool and Sitkum stocks—according to determinations made using methods and curves of Slemmons (1962)—and in the much larger main pluton of the batholith (Cater, 1969, p. 45). The transitional structural state suggests that the smaller masses cooled rapidly. Fuller (1925) ascribed the preservation of pyroxene in cupolas of the shallow Snoqualmie batholith to dehydration by degassing, and Cater ascribed the chilling of the eastern margin of the batholith to degassing, but we find no vesicles, vugs, or any other indication of degassing in the stocks.

LIGHT-COLORED PHASE

The predominant rock of the light-colored phase is adamellite; modal data is given in table 2 and figure 4. The light-colored phase is generally porphyritic with phenocrysts of euhedrally and oscillatorily zoned plagioclase and partly resorbed quartz. Plagioclase ranges from a maximum of An₅₅ in euhedral cores to An₂₀ in wide anhedral rims, but more commonly the crystals are normally zoned from andesine to oligoclase, the predominant composition being oligoclase. Inner zones of plagioclase crystals in two specimens have both ordered and transitional structural states.

The groundmass of the light-colored phase is of two types. One consists of fine-grained potassium feldspar, quartz, and subhedral to anhedral plagioclase in a xenomorphic aggregate (fig. 5). In the other type, the potassium feldspar and quartz are in micrographic or granophyric, commonly plumulose intergrowths (figs. 6, 7). Between plagioclase crystals, the quartz in either type locally forms continuous mesostasis of individual small grains or large optically continuous grains. Potassium feldspar is perthitic and most is orthoclase. Small optic axial angles (and crosshatch twinning observed by Ford, 1959, p. 198) indicate some potassium-sodium feldspar. In contrast to the light-colored rocks of the main pluton, micrographic or granophyric matrix is lacking in the light-colored phase of the Sitkum stock.

FIGURE 5.—Xenomorphic texture with potassium feldspar (K), quartz (q), and plagioclase (P) in light-colored phase of the Cloudy Pass pluton. Note quartz phenocryst (qp). Crossed nicols. Specimen RWT—297—61 from the northwest side of upper Vista Creek.

FIGURE 6.—Granophyric texture in light-colored phase of the Cloudy Pass pluton: granophyre (G), plagioclase (P). Crossed nicols. Specimen DFC—236—62 from the north side of Miners Ridge.

FIGURE 7.—Granophyric texture in light-colored phase of the Cloudy Pass pluton: quartz (white), potassium feldspar (black), and plagioclase (gray). Crossed nicols. Specimen RWT—40—62 from the south side of Dolly Creek.

There is considerable evidence that the light-colored rocks contained abundant residual solutions which were particularly rich in potassium feldspar components (fig. 8). Potassium feldspar and micropegmatite rim and partially replace plagioclase in intricate comb-like intergrowths (fig. 9) and occur as irregular patches in calcic cores of plagioclase. Late sodic plagioclase also replaces potassium feldspar. In places, potassium feldspar appears to replace quartz of the groundmass (fig. 10). It has not been observed as phenocrysts. The light-colored phase is generally much altered; biotite is commonly replaced by chlorite, and plagioclase is clouded with epidote and white mica. Miarolitic cavities containing drusy quartz and sparse pyrite or iron oxides are common in the light-colored phases of the Stikum stock. Ford (1959, p. 208) reported quartz-lined cavities in light-colored rocks from Miners Ridge. The intergrowths, replacement features, and alterations of the light-colored phase indicate the prevalence of residual solutions; the cavities, an environment of low lithostatic pressure.

FIGURE 8.—Intergranular, late-formed potassium feldspar (K) intergrown with sodic plagioclase rims (P) in light-colored phase of the Sitkum stock. Replacement confined to rims of more calcic plagioclase. Quartz (Q). Crossed nicols. Specimen DFC—136—61 from upper Pumice Creek.

FIGURE 9.—Comblike granophyre intergrowth rimming and partially replacing twinned plagioclase crystal in Cloudy Pass pluton. Crossed nicols. Specimen DFC—236—62 from the north side of Miners Ridge.

FIGURE 10.—Potassium feldspar (gray) partially replacing quartz (clear white) in light-colored phase of the Cloudy Pass pluton. Plane-polarized light. Specimen DFC—100—62 from the south side of Miners Ridge.

Ford (1959, p. 204-209) described and illustrated the micrographic textures of the batholith in detail. He stated (1959, p. 207-208) "there are all gradations from irregular micropegmatitic intergrowths in which the quartz feldspar ratio is highly variable to more regular intergrowths with a micrographic texture," and concluded, as did Grant (1966, p. 221 and 223) and Cater (1969, p. 20), that "these intergrowths in the Miners Ridge granitic rocks [part of our Cloudy Pass pluton] have a similar origin, namely, one of replacement [of crystals in earlier formed tonalite] rather than of primary magmatic crystallization." Grant (1966, p. 221) proposed that the replacement (on Fortress Mountain) caused mobilization of the early formed quartz diorite "resulting in small intrusive plugs of acid rock," our alaskite dikes (Grant, 1966, p. 221).

In the light-colored rock examined by us, quartz and the margins of plagioclase grains are replaced by potassium feldspar (figs. 8, 9, and 10), but prevalent textures (figs. 8 and 11) more strongly suggest that most of the quartz and potassium feldspar crystallized from residual melt between earlier formed plagioclase crystals. The similarity in the composition of light-colored rocks (and some of the alaskites discussed below) to the composition of rocks falling in the minimum melting trough of Tuttle and Bowen (1958, p. 55, and our fig. 20) supports the contention that these light-colored rocks formed by normal magmatic differentiation, not by replacement of an early formed phase by, introduced residual solutions. These solutions, rich in alkalies silica, and water should yield rocks of a diverse composition that would differ from the composition of rocks formed by separation of crystals and melt. A few rocks of diverse composition, testifying to the existence of residual siliceous solutions, do occur—for example, a quartz mass as on Miners Ridge (Grant, 1966, p. 223), and quartz veins with sulfides at the Glacier Peak Mines and near Crown Point in the Holden quadrangle (Cater, F. W., written commun., 1967). But the composition of the granitoid batholithic rocks is more limited, as would be unlikely if replacement by residual solution had been significant. We postulate that mafic minerals and plagioclase settled from a dark-colored phase of the magma essentially in place, so as to form an adamellite cap on the pluton. There is no evidence that the light- and dark-colored phase are separate intrusions where they are in contact in the main pluton and Sitkum stock, for the contacts are gradational. However, mixing of two magmas is suggested by banded and "marble cake" layers of light-, and dark-colored phases, in a few places in the Holden quadrangle. (Cater, 1969, p. 19). On the east side of the pluton, a large satellitic dike of dacite porphyry at Hart Lake was later intruded by the core of the rising batholith (Cater, 1969, p. 47-48), and if the dark-colored core magma moved upward and intruded a still hot plastic cap of light-colored adamellite differentiate, the distribution of the adamellite along the north side of the batholith (pl. 1) could be explained. The lack of an intrusive contact between the dark-colored phase and the adamellite could be ascribed to the hot and plastic state the adamellite.

FIGURE 11.—Most prevalent texture of light-colored phase of the Cloudy Pass pluton. Micropegmatite fills in and around subhedral plagioclase crystals. Crossed nicols. Specimen DFC—40—62 from Suiattle River Trail west of Canyon Creek.

ALASKITE DIKES

Small dikes, sills, and irregular masses of alaskite (light-colored granitoid rocks, composed predominantly of quartz and feldspar and generally with less than 5 percent mafic minerals) of diverse form and origin are widespread in the Glacier Peak area; for simplicity, we will use the term "dikes" to include them all. Crosscutting relationships between dikes indicate a wide range of relative and, in places, conflicting ages; but in general, dilation dikes and sharply bounded dikes cut schistose dikes and dikes with gradational contacts. In the Glacier Peak quadrangle, as elsewhere, the texture of the alaskites is highly varied and ranges from crystalloblastic and xenomorphic to rarely, hypidimorphic granular. Grain size is mostly fine to medium, but a few coarse-grained, zoned pegmatities also occur. In some dikes quartz is intergranular. Biotite (commonly altered to chlorite), epidote, sphene, and opaque minerals are generally present in amounts of 3 percent or less. Small garnets are scattered through some specimens. Potassium feldspar is scarce; where it occur it is commonly intergranular and replaces quartz and plagioclase. Most of the potassium, feldspar-rich alaskites are, composed of poorly twinned oligoclase-andesine (50 percent); quartz (35 percent), and potassium feldspar (15 percent).

Crude estimates of the amount of alaskite in outcrops were made in the Glacier Peak, Holden, and Lucerne quadrangles (Crowder and others, 1966; Cater and Wright, 1967; Cater and Crowder, 1967). The data are not complete, because large areas are covered by younger volcanic rocks or by detritus and thick vegetation in valleys. For example, the linear concentration of dikes stretching to the northwest corner of the Glacier Peak quadrangle (fig. 12) could mapped along a well exposed ridge, but not in the valley bottom where bed-quartz and plagioclase. Most of the potassium feldspar-fined work change the pattern depicted in figure 12. Although it is difficult, to distinguish alaskite derived from the batholith from the abundant prebatholith alaskite, the older alaskite is definitely most abundant in biotite gneisses of the Glacier Peak-Lake Chelan area. In the Holden quadrangle, for example, replacement and secretion dikes of alaskite are particularly common in the Swakane Biotite Gneiss, of pre-Late Cretceous age. They were formed from the gneiss by metamorphic differentiation (Crowder, 1959, p 855-862). In contrast to their host rocks, the Cloudy Pass batholith and nearby stocks are rarely cut by alaskite dikes. One alaskitic granophyre dike was found cutting the light-colored phase on Grassy Point, and irregular bodies of alaskite (points in the granite field in fig. 4A) were found within the light-colored phase of the batholith on Miners Ridge (A. B. Ford, oral commun., 1965; Grant, 1966, p. 222). A few alaskite dikes containing vugs and miarolitic cavities cut dark-colored phases of the pluton in the Holden quadrangle (Cater, 1969, p. 20). The dikes of alaskite that cut the batholith are apparently related to it, and so are clearly different from the replacement and secretion dikes in the gneissic host rocks.

FIGURE 12.—Distribution of alaskite around the Cloudy Pass batholith. Data in Glacier Peak quadrangle from Crowder, Tabor, and Ford (1966); data in Holden quadrangle from Cater and Crowder (1967). (click on image for an enlargement in a new window)

Many alaskite dikes, however, are concentrated in country rocks near the Cloudy Pass batholith, especially north of Pumice Creek, between the forks of Milk Creek and along the Entiat fault (outlined areas in fig. 12). In these areas, irregular masses of alaskite grade locally into country rock intensely riddled by dikes. In upper Vista Creek, the amount of alaskite increases gradually away from the batholith contact through a zone a few hundred feet wide. Alaskite that appears to be the southern continuation of the Sitkum stock occurs above Baekos Creek and the White Chuck River (pl. 1), and south of Baekos Creek numerous sills of this alaskite penetrate the foliation of the host schist and are locally rich in unoriented inclusions of schist. Examples such as these suggest that some of the alaskite in the gneiss and schist of the host rock is genetically related to the batholith. The areas of alaskite, where particularly abundant in upper Milk and Pumice Creeks, are also thermally metamorphosed (compare fig. 3 with fig. 12). The alinement and concentration of stocks and dikes (satellitic bodies or cupolas) in those areas suggest that the batholith lies below.

Two generations of alaskite were seen along the eastern contact of the Sitkum stock east of the head of Chetwot Creek. Massive granodiorite of the stock grades within 2 feet into slightly gneissic alaskite with foliation parallel to the contact. The gneissic alaskite is cut by a dike of even-grained light-colored massive alaskite containing uralite with relicts of clinopyroxene. The alaskite dike is probably a satellite of the stock, but the affinity of the gneissic alaskite is uncertain.

Alaskite dikes that are definitely associated with the metamorphic rocks and (or) are at great distance from the pluton and its satellites are generally poor in potassium feldspar, as are the metamorphic rocks themselves (see Crowder and others, 1966; Crowder, 1959, p. 867). Alaskites with more than 10 percent potassium feldspar occur most commonly near the pluton and stocks (Crowder and others, 1966). Grant (1960, p. 239-246) described contact migmatites with introduced potassium feldspar adjacent to the pluton in the Ross Pass area (fig. 2). We therefore assume that most alaskite dikes with more than 10 percent potassium feldspar are related to the batholith.

In table 3, chemical analyses of four specimens (samples 13-16) of potassium feldspar-rich alaskite from the Glacier Peak area are given. Because two specimens (samples 15, 16) have compositions near the variation curves and the ternary minimum of rocks of the Cloudy Pass batholith (figs. 19, 20), they may have differentiated from the batholith. The other two specimens (samples 13, 14) differ (fig. 20); if they are not of prebatholith rocks, they may be of rocks that have been altered by residual solutions emanating from the batholith.

TABLE 3.—Composition of the Cloudy Pass batholith and associated rocks

[Samples, sawed from hand specimens; homogeneous and fresh, except as noted, but not collected specifically for chemical analysis and (or) statistical studies of composition. Locations of samples are shown on plate 1, except for sample 18. Oxides: Samples analyzed by X-ray fluorescence supplemented by methods described in U.S. Geological Survey Bulletin 1144—A. Analysis: Paul L. D. Elmore, S. H. Botts, Gillison Chloe, Lowell Artis, and H. Smith]

INCLUSIONS AND DIKES OF HORNBLENDE TONALITE PORPHYRY

Hornblende tonalite porphyry inclusions and dikes are confined to the light-colored phase of the main pluton and are particularly prominent on Miners Ridge. The inclusions occur in swarms, which suggests that they were derived from disruption of discrete larger bodies. Many are cut by thin dikes of adamellite. Characteristically, the inclusions (fig. 13) contain decussate brown hornblendes needles as much as 1 cm (centimeter) long in a fine-grained matrix. Partial digestion and reaction with the enclosing magma is suggested by some inclusions, where tonalite with hornblende needles grades into fine-grained tonalite without needles, and in other inclusions in which patches of medium-grained tonalite occur near the margins. The marginal patches contain coalescent grains of euhedrally zoned plagioclase similar to those in the dark-colored phase of the Cloudy Pass pluton.

FIGURE 13.—Hornblende tonalite porphyry. Specimen DFC—99—61 from an inclusion on the south side of Miners Ridge. The hornblende tonalite porphyry dikes are identical. Note variable grain size.

The tonalite porphyry also occurs in a dike northeast of Grassy Point near the margin of the batholith. The dike is 20 to 40 feet wide, cuts adamellite, and has sharp contacts and aphanitic margins; clearly it was intruded when the adamellite was solid and relatively cool. Another dike of fine-grained microporphyritic hornblende tonalite also crops out on the lower slopes of Miners Ridge.

The dike on Grassy Point is less altered than the other tonalite porphyries, and its mode (volume percentages) may be considered representative: quartz, 10; plagioclase, 57; potassium feldspar, 6; hornblende, 20; biotite, 2; opaque minerals, 3; and apatite and white mica, 1. The conspicuous hornblende prisms are commonly poikilitic and have irregular borders (fig. 14). Crystals are zoned from cores in which Z equals greenish brown to rims in which Z equals pale green. In one specimen, brown hornblende crystals broken by stretching along the c axis are healed with green hornblende. Most hornblende is partly replaced by brown biotite, and in many inclusions chlorite and epidote replace mafic minerals; plagioclase is saussuritized. The matrix of the porphyry is a tightly packed mesh of oscillatorily zoned, subhedral to euhedral plagioclase laths surrounded by anhedral quartz, potassium feldspar, and fibrous green hornblende (figs. 14, 15). Opaque minerals and apatite are liberally sprinkled throughout some inclusions but are rare in others. The angular spaces between plagioclase laths are locally filled with clinozoisite (fig. 15).

FIGURE 14.—Hornblende tonalite porphyry inclusion. Note ragged hornblende prisms, some with inclusions. Quartz and potassium feldspar (gray) fill spaces between tightly packed plagioclase laths (white). Plane-polarized light. Specimen RWT—58—62 from south side of Miners Ridge.

FIGURE 15.—Clinozoisite (C) filling space between plagioclase laths (P) in hornblende tonalite porphyry inclusion. Plane-polarized light. Specimen DFC—99—62 from the south side of Miners Ridge.

In the Glacier Peak quadrangle, no inclusions or dikes of tonalite porphyry have been found outside the light-colored phase of the batholith, although similar porphyry forms a stock just south of Holden and forms dikes in the Entiat Mountains (Crowder, 1959, p. 864-865; Cater and Crowder, 1967). Grant (1966, p. 227-229) reported nonporphyritic (?) hornblende diorite inclusions in the main (dark-colored) phase of the batholith in the Dome Peak area; he considered them to be derived from hornblende gneiss country rocks.

In major-element and trace-element composition, the tonalite porphyry (sample 2, table 3 and fig. 19) is similar to the mafic phases of the Sitkum stock. The chemical composition and the occurrence of the tonalite porphyry in only the light-colored margin of the batholith in the Glacier Peak area strongly suggest that the porphyry is related not only to the batholith but to the light-colored phase.

The origin of the hornblende tonalite prophyry inclusions could have been as follows: The light-colored phase of the western border of the batholith was still hot and plastic, though sufficiently solidified to crack, when tonalitic magma of the still molten core intruded newly formed joints to form dikes; slightly later movement of the batholith then disrupted the dikes and formed inclusions. Ford (1959, p. 233) argued that, if the inclusions of hornblende tonalite porphyry were of early formed rock of the batholith, they should contain pyroxene or at least uralite. As has been shown, most of the pyroxene in the core rocks has been converted to hornblende; if the core magma containing pyroxene were injected to form the tonalite porphyry dikes, the pyroxene would have been similarly converted. The porphyry dike with chilled margins, which occurs on Grassy Point near the border of the pluton, may have been intruded either after the period of disruption or into the more solid light-colored border which was not disrupted (fig. 26). The hydrous residual solutions concentrated in the enveloping light-colored phase may have diffused into the drier tonalite and promoted the development of conspicuous hornblende prisms and variable grain size. The fibrous green hornblende, biotite, chlorite, saussurite, and epidote that the dikes and inclusions now contain may be the final reaction products of the hydrous milieu. Filter pressing of the light-colored cap or border by the intruding core of the pluton might well have occurred at this time with resultant separation of the interstitial melt from the adamellite to form alaskites.

INTRUSIVE BRECCIA, DIKES, AND SMALL MASSES

The country rock is pierced by tonalite dikes and intrusive breccia in and near the southwest-trending zone between the Suiattle River and Red Mountain thought to be underlain by the batholith. Associated with the breccia are small and poorly exposed masses composed predominantly of tonalite. Direct proof that the intrusive breccias, tonalite dikes, and small masses of tonalite are offshoots of the Cloudy Pass batholith is not found in the Glacier Peak area. Such intrusions do, however, cluster near the edge of the batholith or in the roof, and we know of no other pluton with which they might be associated.

The dikes on Sulphur and White Mountains are fine- to medium-grained hypidiomorphic granular tonalite. Some dikes contain clinopyroxene and resemble the dark-colored phases of the batholith and the Sitkum stock. The contacts of the small masses of tonalite (and some minor quartz gabbro) with intrusive breccia on Grassy Point and elsewhere were not observed. Some of the small tonalite masses also resemble dark-colored phases of the batholith, but many are finer grained. On Grassy Point and near the Cool stock, because the hornblende and biotite in the small masses of tonalite have been statically recrystallized, they may be akin to rocks with similar recrystallization textures that are described by Grant (1966, p. 211) as the early pyroxene diorite phase of the batholith. They also resemble the more granitoid phases of the "outer layer of the Hart Lake complex" and the "porphyry plugs" (Cater, 1969, p. 12 and 23) near Plummer Peak and on Phelps Ridge. Occurring with the small tonalite masses are porphyritic-aphanitic rocks, many of which are so highly altered to chlorite, calcite, epidote, and rare zeolites that their original character is obscure. Float from the nose of Vista Ridge, a location over a mile away from the lateral margin of the batholith but perhaps near its roof, is very similar to dacite capping the pluton at Cloudy Peak (Cater, 1969, p. 13).

The contacts of intrusive dacite breccia are sharp, and the wallrocks are locally shattered and altered. On Grassy Point, the intrusive breccia surrounds inclusions of shattered country rock as much as several hundred feet across and is capped by a jumble of tightly packed but partly rotated fragments of fresh hornblende schist having little or no matrix. The intrusive breccias consist of locally alined angular fragments of aphanitic rock and lesser amounts of schist, gneiss, and granitoid rock in a greenish matrix (fig. 16).

FIGURE 16.—Intrusive dacite breccia. RWT—352—61 from the north side of Grassy Point.

The matrix of the breccias ranges from porphyritic aphanitic to protoclastic (fig. 18). In a monolithologic breccia near the Cool stock, the matrix consists entirely of ground-up and hydrothermally altered country rock (fig. 17). Phenocrysts and porphyroclasts are plagioclase and partly resorbed quartz that still show some crystal faces. The aphanitic parts of the matrix, and aphanitic clasts in it are dacite judging from the quartz phenocrysts and the andesine composition of plagioclase microlites. The numerous plagioclase microlites in both the matrix and the aphanitic clasts are commonly flow alined and occur in a mass of chlorite, opaque granules, low birefringent material (feldspar?), and a little glass.

FIGURE 17.—Intrusive breccia near the Cool stock. Clasts or hornblende diorite and alaskite in protoclastic matrix. Specimen RWT—183—62 east of the Cool Glacier.

FIGURE 18.—Cataclastic, highly altered matrix of intrusive breccia. Plane-polarized light. Specimen RWT—313—61 west side of Dolly Creek.

Most of the intrusive breccias are intensely altered: the matrix is an indistinct mass of chlorite, epidote, and leucoxene; mafic minerals in the matrix or clasts are reduced to aggregates of chlorite and epidote; the plagioclase is saussuritized. In places, potassium feldspar forms an irregular mesostasis and embays the margins of plagioclase and lithic clasts and so appears to be a late addition to the rock.

Intrusive dacitic breccias in upper Milk Creek and near Mica Lake differ from the breccias just described. The Milk Creek breccia grades from a slightly protoclastic dacite to dacite charged with inclusions of country rock. The surrounding country rock is shattered and iron stained. The breccia near Mica Lake consists mostly of rotated blocks of country rock schist cut by thin dikes of pulverized schist and rhyodacite. Vuggy quartz veins and seams of pyrite cut the breccia at Milk Creek, and vuggy veins of quartz and calcite invade the breccia at Mica Lake. In the Milk Creek mass, crystal fragments and phenocrysts of quartz and plagioclase occur in a xenomorphic matrix of quartz, biotite, muscovite, and (or) scattered tiny euhedral hornblende crystals; there is no flow structure and the rock appears to have been statically recrystallized. The rhyodacite dikes which cut breccia at Mica Lake (sample 17, table 3) are similar to the Milk Creek mass, but they lack the protoclasis and bear abundant reticulated biotite instead of hornblende.

The intrusive breccias clearly originated under low lithostatic pressure when clasts of country rock, and earlier solidified magma, were explosively broken and transported by magma and gas in a diatreme. During the process, the breccia fragments were pulverized and altered. The intrusive breccias of the Glacier Peak area had an origin similar to that of some of the intrusive breccias to the east, particularly those on Phelps Ridge (Cater, 1969, p. 33). Many of the Phelps Ridge intrusive breccias were mobilized largely by gas rather than gas-magma mixtures, for matrices consist of highly pulverized and altered material and lack the abundant flow-alined plagioclase crystals and inclusions of aphanitic rocks common in most intrusive dacite breccias near Glacier Peak.

Although none of the intrusive breccias cut the batholith, they were probably emplaced after it cooled at the present erosional level. The breccias are not cut by alaskite dikes, which appear to be late derivatives of the main pluton. In the section on the early episode of volcanism, we discuss the possibility that the intrusive breccias of Glacier Peak area erupted onto the solidified and eroded batholith.