PROTEROZOIC GRANITE SYSTEMS OF THE CENTRAL UNITED STATES AND THEIR BEARING ON PROTEROZOIC CONTINENTAL RECONSTRUCTIONS

Although petrologic, geochemical, and isotopic studies on metavolcanic and metasedimentary rocks of orogenic suites are indispensible for understanding their ages and primary genesis, similar studies on syn-tectonic, post-tectonic, and "anorogenic" granitoids associated with these domains provide much of the same information. Furthermore, in many instances granitoid rocks may be all that remain of terranes which have been deeply eroded, they may be hinterland manifestations of orogenic belts developing at active margins, or they may be indicators of processes not preserved in supracrustal rocks. Proterozoic basement of the central U. S. consists of several distinct lithotectonic suites with granites, including: 1) the 1.9 to 1.8 Ga Penokean orogen of Wisconsin; 2) 1.8 to 1.7 Ga rocks of the northern Central Plains orogen; 3) 1.7 to 1.6 Ga rocks of the southern Central Plains orogen; 4) a distinct terrane of 1.47 Ga high-silica rhyolite to dacite with epizonal granites in the east-central midcontinent region; 5) a large suite of 1.48 to 1.42 Ga A-type granitic plutons which intrude Paleoproterozoic basement; and 6) a 1.37 Ga southern granite-rhyolite province in the south-central midcontinent region. These systems record a complex history of crustal accretion of juvenile terranes, Andean type continental margin magmatism, and partial melting of continental lithosphere. These suites also form part of a distinct set of temporal, spatial, and compositional characteristics for southern Laurentia that provide important constraints on possible Proterozoic continental reconstructions, both to the east and to the west. Thus, although there are many similarities with orogenic and granitic systems in Baltica and Amazonia, there are also significant differences which limit possible geometries. These same characteristics also place important constraints on possible correlations with other cratons such as in Australia. To date there are no exact matches between southern Laurentia and the other cratons that require close proximity, although events on all cratons can collectively define Proterozoic global tectonic systems 1.9 to 1.3 Ga.