Geochronology of the midcontinent region, United States: 1. Scope, methods, and principles

Abstract

Isotopic age determinations, geological data, and geophysical observations are used in interpreting the history of the Precambrian basement of the midcontinent region of the United States. Rb-Sr age determinations of whole-rock and feldspar samples afford the least ambiguous information. Mica ages by both Rb-Sr and K-Ar methods commonly are lower than corresponding Rb-Sr ages for whole-rock or feldspar samples, reflecting loss of daughter products during metamorphism. Rb-Sr ages, particularly of biotite, may be low as a result of incipient weathering. Most of the radiometric ages are in the range 1000–1700 m.y. Two orogenic cycles are recognized: the Black Hills orogeny, approximately 1700 m.y. ago, in the Black Hills, South Dakota, and in the subsurface of adjacent areas, and the Llano orogeny, approximately 1100 m.y. ago, in Texas. A number of igneous events are locally recognized in the range 1000–1600 m.y., but these do not appear to be related to structural trends or to orogenic belts. They are defined as periods of igneous activity. The age of the Precambrian rocks is progressively younger from north to south.