This chapter presents a summary of the major, regionally developed, metamorphic episodes that affected Alaska throughout the evolution and accretion of its many lithotectonic terranes. Plate 4 (map and table showing metamorphic rocks of Alaska, 2 sheets, 1:2,500,000 scale) accompanies this chapter. The metamorphic scheme (Zwart and others, 1967) used for the map (Fig. 1, Table 1) is based on the occurrence of pressure- and temperature-sensitive metamorphic minerals. Regionally metamorphosed rocks are divided into four facies groups, each of which reflects a different grade of metamorphism. In order of increasing temperatures of crystallization, they are: (1) lamontite and prehnite-pumpellyite facies (LPP), shown on Plates 4A and 4B in shades of gray and tan; (2) greenschist facies (GNS), shown in shades of green; (3) epidote-amphibolite and amphibolite facies (AMP), shown in shades of orange and yellow; and (4) two-pyroxene (granulite) facies (2PX), which occurs only on the Seward Peninsula, shown in reddish brown. Where possible, the greenschist-facies and the epidote-amphibolite- and amphibolitefacies groups are further divided on the basis of pressure of crystallization into three facies series: high-, intermediate-, or low-pressure series. These facies series are indicated by an H, I, or L in place of the final letter in the symbol used for the facies group. High-pressure greenschist-(blueschist) facies rocks, and rocks metamorphosed under blueschist-facies conditions that evolved to intermediate- or low-pressure greenschist-facies conditions during a single episode, are shown in shades of blue. The metamorphic facies symbol for each episode is followed by a symbol showing the age of
Figures & Tables
The Geology of Alaska
You get a comprehensive overview of the geology, tectonic evolution, and mineral resources of Alaska and adjacent areas of the continental margin. Plates include state-wide maps showing geology, physiography, lithotectonic terranes, metamorphic rocks, igneous rocks, sedimentary basins, isotopic age data, neotectonics, isostatic gravity, magnetics, and metallic mineral deposits. Summaries of bedrock geology and geologic history are given for eleven large regions of Alaska and adjacent offshore areas. Twenty topical chapters synthesize data on metamorphic and igneous rocks; major onshore and offshore sedimentary basins; the paleomagnetics evidence for latitudinal displacements and rotations, glacial history and periglacial phenomena; and the occurrence, evolution, and potential of Alaska's vast resources of petroleum, coal, and metallic minerals. A summary chapter provides an overview and presents a possible model for Alaska's Phanerozoic evolution. The Geology of Alaska is the largest publication produced in the Decade of North American Geology program, a fitting tribute to this magnificent area.