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Abstract

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

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