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

STRUCTURE AND CONTACT RELATIONS

GENERAL FEATURES

In the Glacier Peak quadrangle, the Cloudy Pass pluton is clearly discordant (pl. 1). The contacts, though poorly exposed, appear abrupt and are steep except in local places north of the mouth of Canyon Creek.

The likely relatives of the main pluton south and west of Glacier Peak show both a southwesterly and a northwesterly alinement. The northwesterly elongation of the Sitkum stock and the presence of the White Chuck stock along its southeast projection indicate that emplacement was locally controlled by the layering and foliation of the host rocks. The Milk Creek stock, Sitkum Stock and small bodies and dikes in the Lost Creek area a belt trending northeastward toward the west border of the exposed batholith, approximately at right angles to the regional foliation. The Cool stock and several dikes as much as 50 feet thick that crop out on Sulphur Mountain and White Mountain are probably offshoots of the Cloudy Pass pluton, but they are not confined to this northeasterly belt and appear to be independent of the northwesterly structural grain.

These isolated stocks, dikes, and small bodies, which crop out to the southwest of the main Cloudy Pass pluton, are lithologically and chemically similar to it. The distribution of contact metamorphism and alaskite dikes related to the batholith, as discussed below, suggests that the batholith connects with them below the surface. Furthermore, lead-isotope ratios of the main Cloudy Pass pluton and the largest of these satellitic bodies, the Sitkum stock, are essentially identical (table 6) and are uniquely different from lead-isotope ratios of other Tertiary plutons in the northwest (compare table 6 with Davis and others, 1966, table 13, p. 172).

Most of the observed contacts of the stocks are steep, although we infer from limited exposures on Milk Creek that the Milk Creek stock may be the flattop of the batholith (section B—B', pl. 1). The parts of the Cloudy Pass batholith now exposed in the Holden quadrangle (fig. 2) are near its top, judging from the presence of the flat roof around Plummer Mountain. A zone of satellitic (Cater, 1969, p. 50) intrusive breccias and porphyry plugs extending down Phelps Ridge suggests a south-plunging nose which parallels regional foliation. This zone lies within the Chiwaukum graben, which extends southeast from the pluton (figs. 2 and 4). The presence of a flat roof has been established by Grant (1966, p. 208) north of the Glacier Peak area, where the pluton floors the valley of the South Fork of Agnes Creek and plunges beneath the schists and gneisses of Seven Sisters Ridge (fig. 2). Indeed, because the schist and gneiss northwest of Seven Sisters Ridge are thermally metamorphosed and hydrothermally altered, Tabor (1961, p. 180) suggested that the very large dike of similar age at Cascade Pass, for which Misch (1966, p. 141) reported a potassium-argon biotite age of 20 m.y. (Tabor, 1963, and fig. 60), is an offshoot of the Cloudy Pass batholith.

FIGURE 2.—Cloudy Pass batholith and associated rocks are determined by workers shown. (click on image for an enlargement in a new window)

CONTACT METAMORPHISM

Thermal metamorphic effects are variable adjacent to the pluton and stocks (fig. 3). Within the aureole, textures indicative of thermal metamorphism are recognized in the tonalite-gneiss, biotite-quartz-oligoclase schist, hornblende schist, and granitoid alaskite dikes that cut the schists and gneisses. In the medium-grained gneisses, these textures are much harder to recognize than in finer grained schists. On Grassy Point the grain size of granitoid tonalite-gneiss near the pluton has been reduced by cataclasis, and the gneiss is locally recrystallized and contains aggregates of biotite and hornblende in place of original single crystals. In this area the granitoid gneiss is also altered (chlorite and saussurite), and in one specimen, untwinned potassium feldspar replaces and cements the fragments. This potassium feldspar may have emanated from the batholith or may have formed during contact metamorphism by the redistribution and annealing of the microcline locally prominent in the granitoid gneiss. On the north side of the Canyon Creek and within 1,500 feet of the contact, this granitoid gneiss has lost its pronounced foliation and is mottled by patches and veins of recrystallized hornblende. Near the steep contacts of the White Chuck stock, the light purple hornfels schist of the aureole contains clinopyroxene or andalusite; one specimen of hornfels schist bears the unusual association of cordierite and spinel. Near the contact of the Cool stock, the enveloping tonalite-gneisses are highly hornfelsed. Contact metamorphism of the biotite gneiss adjacent to the small bodies in the Lost Creek area disappears within a few tens of feet from the contacts. The extensive brecciation and oxidation prominent nearby may be related to the underlying intrusive rocks that probably connect these small bodies. In the upper reaches of Milk and Pumice Creeks, the schists and gneisses in places contain small aggregates or poikiloblasts of biotite, which strongly suggest contact metamorphism; the wide extent of the aureole recognized in these areas suggests that the batholith lies close below. In the Dome Peak area, contact metamorphism of even greater intensity is common adjacent to the batholith (Grant, 1966, p. 235-246), and incipient hornfels extends to 4 miles west of the pluton.

FIGURE 3.—Distribution of thermal metamorphism around the Cloudy Pass pluton and nearby stocks.