GEOLOGIC EVOLUTION OF PROTEROZOIC ROCKS, EASTERN AND SOUTHERN MIDCONTINENT OF UNITED STATES

Upper crustal Proterozoic rocks in the eastern and southern midcontinent of the United States are part of the Transcontinental Proterozoic province and consist mainly of epizonal to mesozonal granite and related rhyolite, immature to mature sedimentary rocks, and widespread late diabase dikes and sills. Major element, trace element, and isotopic ratios of the felsic and mafic igneous rocks record the geologic and tectonic environment in which they formed. Felsic igneous rocks in most of the region have mainly A-type chemical affinities and accumulated in a within-plate tectonic environment. Toward the south in the Arbuckle Mountains and adjacent regions, 1,400-1,350 Ma granitoids have a calc-alkaline geochemical signature, suggesting they formed within a magmatic arc along the southern margin of Laurentia. Late diabase dikes and sills have geochemical characteristics of continental within-plate tholeiites.

The main granite-rhyolite igneous suite implies magmatism associated with extension in Middle Proterozoic time, probably during the development of regional and local continental basins within zones of subsidence. Consistent with this interpretation is the presence of basinal Proterozoic sedimentary rocks that accumulated beneath or within the granite-rhyolite sequence, sheet-like mature sedimentary deposits, and immature sedimentary strata that are associated with basaltic and diabasic rocks that formed as rifting caused further extension.

Prominent and thick seismic layered units in parts of Illinois, Indiana, Ohio, and Panhandle Texas support the presence of Proterozoic basins in the midcontinent. The layering probably reflects these basinal strata and the diabase dikes and sills. The layered sequence may be analogous to thick accumulations of Proterozoic supracrustal rocks that are notable in reconstructions of the Proterozoic North Atlantic shield.