Plutonic Belts of Central and Southern Alaska Range and Alaska Peninsula: ABSTRACT

Abstract

Potassium-argon mineral ages and reconnaissance mapping of approximately 30,000 sq mi of the central and southern Alaska Range and Alaska Peninsula indicate that there were 3 major plutonic episodes during the Mesozoic and Tertiary. The first began in the Early Jurassic (about 180 m.y. ago) and continued for about 25 m.y. No plutonic rocks older than Jurassic have been recognized. Plutons of Jurassic age form an arcuate belt about 600 mi long which roughly parallels the Talkeetna geanticline and Matanuska geosyncline, major tectonic elements of south-central Alaska. Jurassic plutonic rocks are largely diorite and quartz diorite with minor granodiorite. Late Cretaceous and early Tertiary plutons (83-55 m.y.) occur locally within this belt, but in the southern Alaska Range hese plutons characteristically form north-trending belts transverse to the earlier tectonic elements and locally extend out into what was probably a more stable area bordering the earlier tectonic features. Composition of these plutons ranges from diorite through granite, but granodiorite and quartz monzonite predominate. Isolated granitic stocks of this age also extend eastward into the central Alaska Range. The data suggest that this period of magma generation and emplacement may be separated into Late Cretaceous (70-85 m.y.) and early Tertiary (50-65 m.y.) plutonic episodes. Middle Tertiary plutons (34-41 m.y.) form a north-trending belt about 100 mi long in the central part of the southern Alaska Range. These rocks, characteristically granites and quartz monzonites, are flanked by m re mafic early Tertiary and Late Cretaceous plutons. Small plutons of middle Tertiary age also are present locally in the central part of the Alaska Peninsula. A still younger plutonic episode (25-30 m.y.), perhaps a later phase of the middle Tertiary episode, is represented by small isolated granitic stocks. The plutonic rocks of the central and southern Alaska Range and Alaska Peninsula are more silicic with decreasing age. End_of_Article - Last_Page 2502------------