Petrogenesis of Proterozoic Anorogenic Granites in the Central United States

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The objective of the proposed work is to investigate the origin and tectonic setting of 1.1 to 1.8 Ga anorogenic granites that make up a large part of the crust in the central United States. The study will employ isotopic variations in Nd, Sr, Pb and O, and trace element patterns to elucidate the nature of granite source regions, the process of magma genesis, and the evolution of the magmas. Anorogenic granites from a 600-1000 km-wide band that extends across the North American continent. They were emplaced during several episodes between 1.1 and 1.8 Ga, and have a major and trace element chemistry that distinguishes them from other types of granites. Suggested hypotheses regarding both the chemical and tectonic sources of anorogenic granites are exceedingly diverse and encompass antithetic extremes. A comprehensive isotopic and chemical data base would help constrain the possibilities. This study focusses on four complexes located in Wisconsin, MIssouri, and Texas. All have known U-Pb ages, and each is petrographically and chemically characterized. Specific plutons will be targeted for especially detailed sampling and study. This approach allows study of both details of magma genesis in a specific pluton, and the comparison of plutons among different crustal provinces and intrusive episodes. Both scales of observation are required to identify and explain the main features of anorogenic magma genesis.