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Moll-Stalcup, E.J., 1987

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Bibliographic Reference

Moll-Stalcup, E.J., 1987, The petrology and Sr and Nd isotopic characteristics of five Late Cretaceous-early Tertiary volcanic fields in western Alaska: Palo Alto, California, Stanford University, Ph.D. dissertation, 310 p., illust., maps.

Abstract

Chemical and Sr and Nd isotopic characteristics of five of the Late Cretaceous and early Tertiary volcanic fields were studied in an attempt to determine if old continental crust having high 87Sr/87Sr (SIR) and low 143Nd/144Nd (NIR) underlies the Yukon-Koyukuk province. The Blackburn Hills, Yukon River, and Kanuti fields lie within the Yukon-Koyukuk province and the Sischu and Nowitna fields overlie Paleozoic and Precambrian metamorphic terranes to the southeast. The Nowitna field is chiefly andesite having SIR = 0.7044 - 0.7051 and NIR = 0.51256 - 0.51257. The andesites can be modeled as fractionates of mantle-derived basalts that assimilated continental crust. The Sischu field is chiefly rhyolite and dacite having high SIR (0.7079 - 0.7140) and low NIR (0.51246 - 0.51252), which suggests that old continental crust was involved in their genesis, either by direct high SIR (0.7079 - 0.7140) and low NIR (0.51246 - 51252), which suggests that old continental crust was involved in their genesis, either by direct partial melting or by large degrees of assimilation. The Blackburn Hills field consists of medium-K basalt, andesite, and rhyolite intruded by a small granodiorite pluton and has SIR = 0.7033 - 0.7052 and NIR = 0.51253 - 51290. Isotopic variations in some of the andesites and basalts are best explained by either fractionation of mantle-derived basalts concurrent with assimilation of the island-arc Koyukuk terrane, or metasomatism of an oceanic-island-basalt-type mantle by fluids derived from subducted sediments. Other Blackburn Hills andesites and Kanuti field dacites (SIR = 0.7043 - 0.7048; NIR = 0.51270 - 0.51284) may have formed by partial melting of enriched (shoshonitic) parts of the lower crust of the Koyukuk terrane. The Yukon River field is basalt, andesite, dacite, and rhyolite having SIR = 0.70374 - 0.70511 and NIR = 0.51270 - 0.51284, and much of its isotopic variation can be modeled by assimilation of seawater-altered oceanic crust during fractionation of basalt. Isotopic compositions of most felsic rocks from the Blackburn Hills field (SIR = 0.7038 - 0.7041) and dacites from the Kanuti volcanic field (SIR = 0.7043 - 0.7048) require little or no old continental crust in their genesis, suggesting that ancient crust does not extend beneath this part of the Yukon-Koyukuk province. However, the ultimate source of the shoshonitic lower crust of the Koyukuk terrane (SIR = 0.705, NIR = 0.5125) may be continental mantle, which may have been thrust under this part of the Yukon-Koyukuk province during arc-continent collision in the early Cretaceous.

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