Abstract

Plutonic rocks in the eastern Alaska Range were emplaced in Late Pennsylvanian time (282 to 285 m.y. B.P.) and during two distinct intervals in Cretaceous time (105 to 117 and 89 to 94 m.y. B.P.) Development of a large plutonic-metamorphic complex, consisting of diorite and quartz diorite intimately associated with banded gneiss and other metamorphic rocks, apparently occurred during Late Triassic to Middle Jurassic time (163 to 199 m.y. B.P.). A smaller plutonic-metamorphic complex is Miocene in age (17 m.y.).

The younger Cretaceous plutons are recognized only in the regionally metamorphosed Devonian and older terrane north of the Denali fault. Plutons of the older Cretaceous and Pennsylvanian events are restricted to Pennsylvanian and younger terrane south of the Denali fault and are associated with coeval volcanic rock assemblages. The major plutonic-metamorphic complex is also restricted to the terrane south of the Denali fault and may relate to collapse of an upper Paleozoic volcanic arc in Triassic time followed by syntectonic magmatism in Jurassic time. The Miocene plutonic-metamorphic complex may reflect the time of initial movement along the Denali fault.

Porphyry copper deposits are associated with the Cretaceous plutons south of the Denali fault. The source of the copper may be subjacent copper-rich basalt flows (Nikolai Greenstone) of Triassic age.

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