11: Generalized maps showing distribution, lithology, and age of Cenozoic igneous rocks in the Western United States
John H. Stewart, John E. Carlson, 1978. "11: Generalized maps showing distribution, lithology, and age of Cenozoic igneous rocks in the Western United States", Cenozoic Tectonics and Regional Geophysics of the Western Cordillera, Robert B. Smith, Gordon P. Eaton
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Two maps (Pls. 11-1 and 11-2, in pocket) have been prepared to show the general distribution, lithology, and age of Cenozoic igneous rocks in the Western United States. Plate 11-1 shows igneous rocks that range in age from 65 m.y. (start of the Cenozoic Era) to 17 m.y., and Plate 11-2 shows rocks that are 17 m.y. or younger. The 17-m.y. age is a geologically significant division point. This age is approximately the low point of the mid-Miocene lull in igneous activity that has been recognized in the Great Basin (McKee and others, 1970), in the Basin and Range province as a whole (Damon, 1971), in Colorado (Marvin and others, 1974), and in New Mexico (Chapin and Seager, 1975). In many regions, few, if any, rocks of this approximate age are known. A marked change in the character of volcanism also occurred at approximately this time. Rocks older than 17 m.y. are predominantly calc-alkalic types ranging in composition from rhyolite to andesite, whereas rocks younger than 17 m.y. are predominantly basalt, or bimodal assemblages of basalt and rhyolite (Christiansen and Lipman, 1972).
Igneous rocks shown on the maps are divided into four general categories: (1) rhyolitic, (2) andesitic, (3) basaltic, and (4) intrusive. Rhyolitic rocks consist of widespread welded tuffs as well as local flows and generally range in composition from rhyolite to quartz latite. Andesitic rocks consist mostly of flows and breccias of intermediate composition, although locally large volumes of more silicic volcanic rocks are included in this category on Plate 11-1 and locally large areas of basaltic rocks are included in it on Plate 11-2. Basaltic rocks include both alkali and tholeiitic flows. Intrusive rocks include all compositional types.
Eocene basalts in the Oregon-Washington coast ranges are not shown. These rocks consist of thick sequences of spilite and tholeiite erupted on the ocean floor in contrast to the largely calc-alkalic continental volcanic rocks shown on Plate 11-1. The basalts are omitted because they were erupted in a distinctly different geologic setting and because their inclusion on Plate 11-1 would mask the distribution patterns of the more widespread continental volcanic rocks. Also omitted are Laramide intrusive rocks in California, Nevada, Arizona, New Mexico, Colorado, and Montana. In the Basin and Range province, these intrusive rocks range in age from about 50 to 75 m.y. and have a modal age of about 65 m.y. (Damon, 1968). The inclusion of only those Laramide plutons younger than 65 m.y. is impractical as well as misleading in that it would show the distribution of only half of the igneous rocks formed during this particular pulse of igneous activity.
Between lat 35°30’ and 42°N in California, Nevada, and western Utah, the maps are based on data from Stewart and Carlson (1976) and Stewart and others (1977). Elsewhere they are based largely on the geologic map of the United States (King and Beikman, 1974). The generalized radiometric dates shown on Plate 11-1 north of lat 42°N are based on a compilation by Armstrong (this volume). In Colorado, they are from Marvin and others (1974). Elsewhere south of lat42°N they are based on unpublished compilations prepared by W. S. Snyder, W. R. Dickinson, and M. L. Silberman (published in part in a summary form, Snyder and others, 1976). Because of the relatively short time span involved (0 to 17 m.y.), no radiometric dates are shown on Plate 11-2.