Abstract

Potassium-argon mineral ages and reconnaissance mapping of approximately 65,000 sq km in south-central Alaska indicate that the Mesozoic and Cenozoic plutonic rocks in the region were emplaced during three discrete intrusive epochs. Most of the plutonic rocks are part of the Alaska-Aleutian Range batholith; the remainder appear as outcrops in isolated plutons northeast of the main batholith. Maximum and minimum concordant mineral ages on coexisting biotite and hornblende are used to mark the beginning and ending, respectively, of each intrusive epoch.

The oldest intrusive epoch, Early and Middle Jurassic, contains plutonic rocks emplaced between about 176 and 154 m.y. ago. Jurassic plutonism occurred along a magmatic arc at least 1,300 km long, extending from about 480 km southwest of Becharof Lake northeast to the Talkeetna Mountains. Aeromagnetic data suggest that the magmatic arc, which represents the roots of the arc portion of an early Mesozoic arc-trench system, also continues southwest into the Bering Shelf. The associated trench is thought to be represented by an imbricated mélange of ophiolite and submarine lava with associated chert and argillite that occupies a belt 140 km southeast of the magmatic arc. Clastic sediments more than 4.5 km thick occupy the 140-km-wide arc-trench gap.

Late Cretaceous and early Tertiary plutonic rocks, emplaced between about 83 and 58 m.y. ago, are found mainly in the northern part of the batholith and in isolated plutons to the northeast toward Mount McKinley. Middle Tertiary plutonic rocks ranging in age from 38 to 26 m.y. occur in two areas within the batholith and also in the Mount McKinley area. Although some of the Late Cretaceous and Tertiary plutons are associated with extrusive rocks, these plutons are largely post-tectonic, and magma was generated beneath both stable platform areas as well as beneath former eugeosynclinal regions in which deformation had essentially ceased.

Magma for the Jurassic plutonic rocks appears to have been generated along or above an early Mesozoic subduction zone. The region southeast of the zone is considered to represent a classic example of continental accretion of eugeosynclinal sediments caused by under-thrusting of oceanic crust beneath an island arc. Magma generation for the Cretaceous and Tertiary plutons in the northern part of the batholith does not appear related to the early Mesozoic subduction zone, for it would require that the zone shift inland, or toward the continent, from its position during the Jurassic and then shift away from the continent once again to its present position.

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