A Nd Isotopic Study of Early Proterozoic Crustal Growth in Southern Wyoming

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Sm-Nd isotope systematics will be applied to Precambrian metasedimentary and metaigneous rocks in and adjacent to the Cheyenne Belt (CB), an ancient crustal suture zone in southern Wyoming, constrain models for the genesis of Proterozoic continental crust and for the accretion of these crustal segments to the margins of preexisting Archean continental cratons. The Cheyenne Belt represents a rare example of a well exposed Archean continental margin in North America which is apparently not directly mantled by a large segment of "mixed" Proterozoic crust, that is, crust produced in the Proterozoic that intermixed with Archean crustal material. The proposal will center on determining whether the lack of such crust at the southern margin of the Wyoming craton reflects the fact that such crust was never accreted in this region (unlike other segments of Archean cratonal margins) or whether tectonic movement along the CB has removed and mixed crust that may have originally been present. Three separate, but interrelated projects will be undertaken 1) Sm-Nd isotope systematics of Proterozoic miogeoclinal sedimentary rocks north of the CB. The purpose will be to use the temporal variations in the Nd isotopic compositions of these sediments to determine whether Proterozoic island arcs composed of newly formed juvenile or "mixed" crustal material initially approached the ancient continental margin, 2) Sm-Nd isotopic systematics of para-and orthogneisses in tectonic blocks within the CB itself, to determine if these rocks represents remnants of "mixed crust that might have initially accreted against the southern margin of the Wyoming craton, and 3) the isotope systematics of Proterozoic igneous and metasedimentary rocks of the Colorado province south of the CB, to determine if "mixed" crust is present in the northernmost portions of this terrane, and to determine if the metasedimentary rocks record the isotopic characteristics (Archean or "mixed" crust) of the crust against which this terrane initially impinged. The results of all three projects will be integrated with existing geologic and tectonic data in an attempt to construct a model for the Precambrian tectonic evolution of the southern margin of the Wyoming Archean craton.