Abstract

Nd isotopic analyses of Precambrian granitic rocks from the central United States were made to determine crust-formation ages and thereby to investigate the structure and evolution of central North America. Samples of anorogenic granites that have 1.4-b.y. crystallization ages yield model TDM ages of 1.77 to 1.98 b.y. that indicate a previously undocumented 0.4- to 0.6-b.y.-old precrystallization crustal history. These and previously published data approximately define the boundaries of a large segment of continental crust that was formed from the mantle during a relatively short interval ∼1.9 b.y. ago. Both 1.4- and 1.8-b.y.-old samples from the Penokean terrene of central Wisconsin yield distinctly older TDM model ages of 2.1 to 2.3 b.y. Approximately 1.1-b.y.-old samples from the Llano uplift in Texas yield crust-formation ages of 1.3 to 1.4 b.y. The data appear to delineate provinces with well-defined, nonoverlapping crust-formation ages that young to the south, indicating episodic formation of the North American continental crust during Proterozoic time.

The initial 143Nd/144Nd ratios of the 1.4-b.y.-old granites are varied (ϵNd = +4.8 to −2.0). Most of the samples analyzed probably represent crustal melts, but the existence of some high initial ϵNd values indicates an admixture of mantle-derived material. The isotopic patterns suggest that the 1.4-b.y.-old “anorogenic” plutonism may be an inland manifestation of subduction activity related to formation of Llano crust. The data also indicate that the Penokean terrane is not entirely 1.8-b.y.-old, mantle-derived material, nor simply reworked Archean crust. It may represent a mixture of Archean and juvenile mantle material added during the 1.84 Ga Penokean orogeny.

Patterns of continental growth deduced from this study emphasize the episodicity of crust formation. Construction of an apparent crustal growth curve for North America indicates that the average age of the continent is ∼2.2 b.y., which is significantly older than the average age of ∼ 1.5 b.y. derived from Rb-Sr studies.

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