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

GEOLOGIC SETTING AND AGE OF INTRUSION AT THE LEVEL EXPOSED

The Cloudy Pass batholith and its associated rocks were first mentioned in the geologic literature by Youngberg and Wilson (1952, p. 5). The extent and nature of the batholith has been established by subsequent studies of the U.S. Geological Survey and of students from the University of Washington working under the supervision of Peter Misch (fig. 2). A description of the batholith and its structural setting has been published by Grant (1969). Of timely interest is Grant's discussion of transverse structural belts controlling ore deposition.

The metamorphic and granitoid rocks intruded by the batholith strike northwestward, forming the core of the North Cascades. They consist of steeply dipping biotite and hornblende gneisses and schists, granitoid gneissic plutons, minor quartizite, and scattered thin lenses of marble (Crowder and others, 1966; Cater and Crowder, 1967). The age of deposition of the metasedimentary rocks is unknown but has been estimated to be pre-Ordovician (Waters, 1932, p. 608) to pre-Late Jurassic (Misch, 1966, p. 113).

Unconformably overlying the metamorphic country rocks is the Upper Cretaceous and Paleocene Swauk Formation. These sedimentary strata do not lie on the Cloudy Pass batholith itself, but faults that have downdropped the Swauk in the Chiwaukum graben are cut off by the pluton (Cater and Crowder, 1967, and fig. 61). Radiometric age determinations (table 1) show that the pluton now exposed was intruded about 22 m.y. (million years) ago, in the early Miocene Epoch.

Most of the intrusive rocks of volcanic aspect (that is, aphanitic and porphyritic-aphanitic rocks) associated with the Cloudy Pass batholith are exposed in the Holden quadrangle and have been described by Cater (1969). These rocks occur in a border zone on the east side of the batholith (the complex of Hart Lake) and in porphyry plugs and intrusive breccias that pierce the batholith and its roof. In the Glacier Peak area there are a few intrusive breccias, but the principal batholithic rocks occur in the main pluton and in a host of granitoid stocks which we contend are cupolas.

In this report, the contiguously exposed part of the batholith is referred to as the Cloudy Pass pluton, and the term "batholith" is used for the entire assemblage of granitoid rocks, that is, for the pluton and stocks as well as their subsurface connections.

TABLE 1.—Potassium-argon and lead-alpha radiometric age determinations of the Cloudy Pass batholith and its thermally metamorphosed host rock.

[Analysis: Biotite, Harold Thomas, Richard Marvin, and John Obradovich; zircon, T. W. Stern]