Geochronology and the Archean of the United States

Abstract

Archean rocks of the United States represent more than a billion years of earth history. Major episodes of igneous and metamorphic activity are recognized in the intervals of 3,700 to 3,500 m.y. 2,900 to 2,800 m.y., and 2,750 to 2,600 m.y. Additional events are identified between 3,100 and 3,000 m.y., but the extent of rocks of this age is not clear although ages in this range are now common in gneiss belts of the Superior Province. A younger period of granite emplacement (2,550 to 2,400 m.y.) is suggested by ages in Wyoming, but these require confirmation from additional dating.The late Archean episode of greenstone-granite formation (2,750 to 2,600 m.y.) was a major crust-forming event in both the Lake Superior region and the Wyoming age province. The abundance of ages of 2,800 m.y. and greater confirms the presence of an extensive sialic crust at this time. It follows that the greenstone belts formed on or adjacent to segments of this older crust.The presence of gneisses of early Archean age has been demonstrated conclusively at only two localities in the Lake Superior region--southwestern Minnesota and northern Michigan. Whether these very ancient rocks were originally of limited extent, or were more extensive but have been so modified by later metamorphism that their detection is difficult, is uncertain.Where not affected by younger metamorphism or tectonism, Archean terranes attained crustal stability in the latest Archean or earliest Proterozoic (2,400 to 2,600 m.y. ago). A granitic terrane in the northern Bighorn Mountains was not affected by significant thermal events after 2,700 m.y. even though major igneous activity and metamorphism occurred to the south approximately 100 m.y. later. Stability of this block prior to the development of abundant greenstone-granite complexes also supports the existence of a thick sialic crust by the end of middle Archean time.Much of the Archean terrane of the Lake Superior region was involved in the early Proterozoic Penokean orogeny, and the effects of this have greatly complicated geochronologic studies of the older rocks. Similar involvement of Archean rocks in early Proterozoic tectonism occurred marginally to the Wyoming age province. Vertical tectonism in middle Proterozoic time resulted in differential uplift of major blocks of Archean rocks in central and southern Wyoming.A connection between the Archean rocks of the Lake Superior region and those in the Wyoming age province in the buried basement of the northern Great Plains remains to be demonstrated. Both terranes can be projected substantial distances in the subsurface, but additional dating of basement samples is required to establish firmly a possible continuity between these exposed terranes.