Geological Survey Bulletin 1359 Geology and Mineral Resources of the Northern Part of the North Cascades National Park, Washington

CHILLIWACK COMPOSITE BATHOLITH

The Chilliwack composite batholith consists of medium- to fine grained granitic rock of various composition that was intruded during the Tertiary. Rocks of the Chilliwack batholith underlie approximately half the study area and extend continuously from the north to the south boundary (pl. 1). This batholith originally was named the Chilliwack granodiorite batholith by Daly (1912, p. 534-535) after exposures around Chilliwack Lake just north of the Canadian border. Misch (1966, p. 140-141) recognized the batholith as composed of several plutons of different ages and amended the name to the Chilliwack composite batholith.

The various intrusions that make up the Chilliwack composite batholith are clearly younger than most of the other rocks in the area and intrude the Custer Gneiss, the metagabbro, the phyllite and schist of Ross Lake, the Hozomeen Group, the greenschist and phyllite of Mount Shuksan (fig. 10), and the Chuckanut Formation (fig. 9). Some of the rocks of the Chilliwack, however, are younger and some are older than the Skagit Volcanics and the Hannegan Volcanics. The Skagit and the Hannegan Volcanics are cut by dikes of quartz diorite. The Chilliwack, in one place, is cut by a 25-foot dike of Skagit Volcanics, and in several places the Chilliwack batholith is overlain by the Hannegan Volcanics (fig. 12B).

The batholith consists mostly of quartz diorite and granodiorite and has lesser amounts of quartz monzonite, diorite, gabbro, and alaskite. Differences in composition appear to be caused partly by differentiation within various plutons during cooling and partly by differences in the composition of the magmas.

Age differences between rock types of the Chilliwack composite batholith can be seen in many places. In general the diorite and gabbro bodies were the earliest formed and are commonly veined by the lighter quartz diorite and granodiorite. Inclusions of the diorite are also common in the lighter rocks (fig. 15B). In one place in the northwestern part of the area, banded diorite and gabbro grade into quartz diorite. On the south slopes of Mineral Mountain and on Easy Ridge, light-colored granodiorite intrudes quartz diorite, but on Bear Mountain the two rocks grade into each other. Quartz diorite is clearly younger than granodiorite in several other areas. On Pioneer Ridge intrusive breccia of the Hannegan Volcanics cuts granodiorite, but is in turn cut and metamorphosed by later quartz diorite. Similarly, on the west side of Middle Peak, metaporphyry intrudes an older granodiorite, but is metamorphosed by a younger quartz diorite. Dikes and small intrusives of alaskite cut the diorite, granodiorite, and quartz diorite in many places and are generally the youngest intrusive rocks. On the northwest side of the Chilliwack River northwest of the mouth of Easy Creek, however, alaskite grades into granodiorite. These relations indicate a complex history of multiple intrusions.

Although the various plutons that compose the composite Chilliwack batholith may have been intruded over a considerable time span, they are all believed to be late Tertiary in age. Field evidence that points to a Tertiary age is the intrusion of several of the plutons into the Chuckanut Formation of Late Cretaceous and Paleocene age. Isotopic age determinations of the intrusives likewise indicate a probable late Tertiary age. Misch (1966, p. 139-140) reported an age of 30 m.y. for biotite from the ridge between Perry and Silver Creeks and late Eocene ages for biotite and hornblende from several samples collected along the Skagit River. Biotite from two quartz diorite intrusives, believed to be part of the Chilliwack batholith, near Wahleach Lake, British Columbia, at least 20 miles north of the U.S. border, were dated at 18 m.y. (Baadsgaard and others, 1961, p. 697-698).

The earliest rocks in the Chilliwack batholith range from mafic olivine-pyroxene gabbro to mafic quartz diorite. The two largest bodies are each about 1 square mile in area. Olivine-pyroxene gabbro is exposed on Mount Sefrit, on the west border of the mapped area, and a mafic diorite stock containing 25 percent hornblende crops out northwest of the Chilliwack River and south of Copper Lake (1-1/2 miles south-southeast of Copper Mountain). Most of the other bodies are much smaller and consist predominantly of hornblende-rich or chlorite-rich diorite. A small body of crudely layered gabbro at Chilliwack Pass (between Ruth and Mineral Mountains) contains clinopyroxene, and gabbro south of Whatcom Pass contains hypersthene.

Quartz monzonite (fig. 13) of the batholith is a light-gray to pink rock with about equal amounts of orthoclase and plagioclase, 25-30 percent quartz, and as much as 8 percent biotite and hornblende. This rock underlies several square miles west of Ensawkwatch Creek along the north border of the area, but is found in only a few small scattered areas elsewhere. Granodiorite and quartz diorite (fig. 14) commonly can be distinguished in the field by the pinkish cast of orthoclase that in the granodiorite composes 10-25 percent of the rock and in quartz diorite only 2.5 percent. Granodiorite and quartz diorite both contain 10-25 percent quartz. Some rocks contain principally biotite, others principally hornblende, and some contain equal amount of both minerals. In some areas the primary dark minerals have been largely altered to chlorite. Accessory minerals of both rock types are commonly magnetite, zircon, apatite, and sphene.

FIGURE 13.—Well-jointed quartz monzonite exposed on the north side of the peak northwest of Pocket Lake on the west side of Ensawkwatch Creek.

FIGURE 14.—Sheared quartz diorite of the Chilliwack composite batholith forms the western buttress of Whatcom Peak to the northwest of Perfect Pass. Lighter colored granodiorite underlies mountain in left foreground.

Light-tan to pink fine- to medium-grained bodies of alaskite are found on Hagan Mountain north of Berdeen Lake and on the north side of Bear Creek (fig. 15A). The alaskite is composed of 30-40 percent perthite, 20-35 percent sodic andesine, and 25-35 percent quartz. Dark minerals, principally biotite, make up <1-3 percent of the rock. Other minor accessory minerals, noted in some alaskite, are chlorite, magnetite, muscovite, sphene, and zircon.

FIGURE 15.—Rocks of the Chilliwack composite batholith. A (top), Alaskite body (white) intrudes granodiorite on the south side of Bear Mountain. B (bottom), Inclusions of hornblende diorite (dark pieces) in granodiorite near Copper Lake, 1-1/2 miles south-southeast of Copper Mountain.