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SUMMARY OF THE GEOLOGY OF THE CHELAN 30-MINUTE BY 60-MINUTE QUADRANGLE, WASHINGTON

Pre-Tertiary metamorphic rocks in the quadrangle can be subdivided into five major tectono-stratigraphic terranes: (1) the Ingalls, equivalent to the Ingalls Tectonic Complex, (2) the Nason, (3) the Swakane, (4) the Mad River, and (5) the Chelan Mountains (SEE GENERALIZED GEOLOGIC MAP). We use the term terrane in much the same sense as Beck and others (1980, p. 454): "a fault-bounded geologic entity characterized by a distinctive stratigraphic sequence and or structural history differing markedly from those of adjoining neighbors." Although the rocks within each of the terranes described here have some unique features suggesting separate early histories, they share a similar Late Cretaceous metamorphic history.

The Ingalls Tectonic Complex is a Late Jurassic or Early Cretaceous ophiolite melange (Southwick, 1974, p. 399-401; Hopson and Mattinson, 1973, p.57; Miller, 1977, p. 468) that has been thrust onto and locally imbricated with pelitic rocks of the Chiwaukum Schist (Miller, 1980b). The numerous ultramafic pods and layers in the Chiwaukum Schist, the principal unit in the Nason terrane, suggest that the schist protolith might be a facies of the oceanic assemblage of the Ingalls but the two units could also have been part of separate terranes with different early histories.

The Swakane terrane is composed entirely of the Swakane Biotite Gneiss. The gneiss is a uniform metaclastite (?) or metadacite (?) containing rounded zircons more than 1,650 m.y. old. Its protolithic age is at least pre-Cretaceous and presumably Precambrian (Mattinson 1972, p. 3773; Tabor and others, in press). The Swakane is tectonically overlain by the Mad River terrane, a heterogeneous terrane of mostly schist, micaceous quartzite, some marble, and orthogneiss. Complex U-Th-Pb systematics of zircons from an orthogneiss suggest a mixture of Paleozoic or older zircons and Late Cretaceous or early Tertiary metamorphic zircons. The protolithic age of the metasedimentary and metavolcanic rocks of the Mad River terrane is probably Paleozoic or older. Sharply bounded alaskite and granite pegmatite dikes and sills intruded the Swakane and Mad River terranes during the Late Cretaceous metamorphism (Mattinson, 1972, p. 3778-3779).

The Tertiary Chumstick Formation and the Late Ten Peak pluton obscure most of the contact between the Mad River and Nason terranes. However, on the southern end of Chiwawa Ridge unusual low-grade schists of the Chiwaukum Schist are imbricated with the complexly deformed metagabbro and metadiorite body of the Mad River terrane. Numerous ultramafite pods in the schists and the sheared-out metadiorite suggest that this is the original suture between the terranes.

The Mad River terrane is separated from the highly contrasting migmatitic Chelan Complex of Hopson Mattinson (1971) within the Chelan Mountains terrane which is also locally migmatized. Considerable mylonitization along the contact between the Mad River terrane and the Entiat pluton (within the Chelan Mountains terrane) suggests that a once deep-seated fault now separates the two terranes.

The Chelan Mountains terrane is dominated by the Chelan Complex of Hopson and Mattinson (1971) which is composed of migmatite and gneissic to tonalite of deep-seated igneous and metamorphic (Waters, 1938; Hopson, 1955; Hopson and Mattinson, 1971). The leucosome of the migmatite yields Late Cretaceous U-Pb ages (Mattinson, 1972, p. 3778-3779). The massive, possibly anatectic, tonalite is also in large part Late Cretaceous in age as shown by both U-Th-Pb and K-Ar ages, although relict zircons in several units yield a discordant pre-Cretaceous age (see Mattinson, 1972, p. 3775). The protolith of the migmatite includes both sedimentary and volcanic rocks of possible Permian age and Triassic plutons, be represented by the essentially isochemically metamorphosed rocks of Twentyfive Mile Creek and the Dumbell plutons, exposed in the Holden and Lucerne 15-minute quadrangles to the north (Cater and Crowder, 1967; Cater and Wright, 1967, Hopson and Mattinson, 1971; Mattinson, 1972, p. 3778).

During an episode of Late Cretaceous regional metamorphism, all the terranes were intruded by deepseated tonalite to granodiorite plutons, including the Mount Stuart batholith, Ten Peak and Dirty Face plutons, and the Entiat pluton and massive granitoid rocks of the Chelan Complex of Hopson and Mattinson (1971). Tabor and others (in press) discuss probable pre-Late Cretaceous accretion of the terranes to North America.

The Duncan Hill pluton intruded rocks of the Chelan Mountains terrane about 45 to 48 m.y. ago (Cater and Wright, 1967). Accompanying this early Tertiary intrusion was local dynamothermal metamorphism, reflected in the gneissic root at the northern end of the Duncan Hill pluton exposed in the Holden quadrangle (Cater, 1982, p. 57) and zircon ages of deformed gabbro in the Mad River terrane. At about the same time fluvial arkosic sediment of the Chumstick Formation (Gresens and others, 1977, p. 100-108; Gresens and others, 1981) was deposited in a depression, now called the Chiwaukum graben (fig. 1). The Mad River, Swakane, and Chelan Mountains terranes were riddled by 45 to 50-m.y.-old granite porphyry and rhyolite dikes related to the Duncan Hill pluton and the Cooper Mountain batholith (Barksdale, 1975, p. 17-18), which crops out just northeast of the Chelan quadrangle. The outpouring of basalt lavas to the southeast of the quadrangle during the Miocene built up the Columbia River Basalt Group. These now slightly warped lavas lapped onto rocks of the Mad River, Swakane, and Chelan Mountains terranes.

Deformation, uplift, and erosion recorded in the rocks and deposits of the quadrangle continued into post-Miocene time. Quaternary deposits reflect advances of glaciers down the major valleys, a complicated history of catastrophic glacial floods down the Columbia River, the formation of lakes in the Columbia and Wenatchee river valleys by landslides and flood backwaters, and hillslope erosion by large and small landslides and debris flows.

general geololgic map thumbnail

Generalized geologic map of the Chelan Quadrangle.
Click here for enlarged version with explanation (72K)

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