Development of granitic magmas by melting of continental crust is documented by field studies in migmatite terranes (e.g., Kenah and Hollister, 1983; Johannes and Gupta, 1982), petrologic and mineralogic studies (e.g., White and Chappell, 1977; Speer, 1981; Wright and Haxel, 1982), geochemical and isotopic studies (e.g., Ben Othman and others, 1984; Kistler and others, 1981; Arth, this volume), and experimental studies (e.g., Clemens and Wall, 1981; Wyllie, 1983a). Although the processes that lead to melting of continental crust are complex (e.g., Leake, 1983), where present, these events are obviously an important element in the geologic evolution of a region. One expected result of such an event is the regional emplacement of felsic granitic rocks.
In this chapter, any granitic rocks whose composition can be inferred to be predominantly derived from sialic crustal materials—even juvenile crustal materials as is the case for some accretionary parts of southern Alaska—are considered to be the products of crustal melting. However, generally accepted criteria for recognizing such granitic rocks, such as high initial 87Sr/86Sr, peraluminous composition, restite minerals or inclusions, and relations to metamorphism and migmatization, are generally not available for most granitic rocks in Alaska. For this reason, and because in some settings the distinctions between mantle and crustal processes are blurred (e.g., Leeman, 1983; Wyllie, 1983b; Ben Othman and others, 1984), a simplistic approach is taken here. The two principal criteria used to distinguish Alaskan granitic rocks whose origin may have been dominated by crustal melting processes are: (1)
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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.