Additional Aspects of Arc-Related Metallogeny
In the foregoing chapters, the most important types of deposits found in largely post-Paleozoic, arc-related tectonic settings were examined. These younger arc terranes are clearly more tractable to tectonic analysis than their older counterparts, where a
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Additional Aspects of Arc-Related Metallogeny
In the foregoing chapters, the most important types of deposits found in largely post-Paleozoic, arc-related tectonic settings were examined. These younger arc terranes are clearly more tractable to tectonic analysis than their older counterparts, where a combination of deeper erosion levels and later orogenic events render plate reconstructions more tenuous. There are, however, many important metal deposits in older orogenic belts that must be included in any meaningful synthesis of the relationships of metal deposits to plate tectonics. Furthermore, there are a number of metal deposits in post-Paleozoic arc systems that do not fall into the tectonic subdivisions of arcs dealt with in Chapters 1,2, and 3. This latter group will be considered prior to a discussion of the metallogeny of older arc-related metal deposits.
4.1 Metal Deposits Related to Forearc Felsic Magmatism In modern convergent plate boundary environments, the arc-trench gap is an important area of sediment accumulation. Thick sequences of sparsely fossiliferous turbidites accumulate in this zone, and igneous rocks are typically restricted to slices of ophiolite that become tectonically incorporated in the sedimentary prisms during subduction-related imbricate thrusting along the inner trench wall. The deeper levels of these sequences in older arc systems are characterized by low temperature-high pressure metamorphism (blueschists). The forearc terranes of southern Alaska, southwest Japan, and western Sumatra contain granitic intrusions emplaced between the magmatic arc and trench. The Sanak-Baranofbelt (Hudson 1983), which extends 2000 km along the curvilinear southern margin of Alaska, is perhaps the prime example ofthis type offorearc magmatism. The plutons throughout the belt have intruded an upper Mesozoic-lower Tertiary accretionary prism that lies close to the present-da y continental margin. The stocks consist mainly of biotite granodiorite and biotite granite, and many are elongate parallel to the regional structural trends of the strongly deformed host rocks. Age dating ofthese intrusive rocks (see Hudson 1983 and references therein) indicates emplacement ages of the range 47-51 Ma in the eastern segment and 58-60 Ma in the western segment. The origin of such granitic magmas in areas generally characterized by low heat-flow is puzzling, but Marshak and Karig (1977) and Delong and Schwarz (1979) have suggested that the subduction of spreading ridges associated with triple junction plate configurations would cause sufficient local heating of the base of the forearc sedimentary pile to induce melting. Hudson et al. (1979) F. J. Sawkins, Metal Deposits in Relation to Plate Tectonics © Springer-Verlag Berlin Heidelberg 1990
Metal Deposits Related to Forearc Felsic Magmatism
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report petrologic and strontium isotope data from granodioritic plutons in the Chugach Mountains of Alaska that strongly indicate an anatectic origin for the biotite tonalite, biotite, granodiorite, and granite intrusio
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