Structure, Stratigraphy, and Isotopic Composition of Rocks of Seward Peninsula, Alaska: ABSTRACT

Abstract

The Seward Peninsula consists principally of metamorphic rocks of Precambrian age, of less metamorphosed pelitic and carbonate rocks of late Precambrian age, and of thick carbonate rocks of Paleozoic age. These rocks are intermixed in extensive thrust plates of two ages: the earlier (eastward thrusting) is probably pre-middle Cretaceous, and the later (northward thrusting) is older than 74 m.y. Stocks and batholiths of granitic rocks, containing alkalic rocks locally, and gneissic phases intruded the older thrust plates, whereas stocks of biotite granite with associated tin and beryllium deposits intruded the younger thrust sheets. Extensive andesitic volcanic rocks on the eastern Seward Peninsula are of Late Jurassic to Early Cretaceous age; they grade upward into graywa kes and siltstones of Cretaceous age which are tightly folded. Tertiary rocks are coal bearing and deformed and crop out in small areas; they are most probably of late Tertiary age. Extensive volcanic fields of latest Tertiary to Holocene age cover large areas of the central and eastern Seward Peninsula. Marine terraces older than Sangamon are warped, and a range-front fault along the Kigluaik Mountains offsets moraines of Wisconsin age. Early attempts to date the Precambrian rocks by K-Ar and Rb-Sr dating of micas failed because of thermal End_Page 2502------------------------------ effects of the large Cretaceous intrusives. The rocks are currently being dated by Rb-Sr analyses of whole rocks. Thirteen dated samples indicate an age of 700 m.y. for the regional high-grade metamorphism of Precambrian rocks. A younger (Cretaceous?) thermal metamorphism was so severe, however, that even some whole-rock ages of Precambrian(?) granitic sills and dikes were lowered. These reset ages generally fall near the range of the Devonian; thus they may reflect the widespread Devonian orogeny of the Circum-Arctic basin rather than resetting by younger intrusives. Uranium-thorium ratios correlate well for Cretaceous intrusives, but not so well for Precambrian intrusives. End_of_Article - Last_Page 2503------------