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Grand Teton National Park Report

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The crystalline rocks that form the core of the Teton Range are part of the Wyoming Province, which is one of the oldest portions of North America. Study of the basement of the Tetons, coupled with the results of ongoing research in similar-aged rocks exposed elsewhere in Wyoming, will provide information on how the crust evolved in the early Earth in general and in the Wyoming province in particular. In 1999 the project involved two weeks of fieldwork in Grand Teton National Park and regions to the east, including the Gros Ventre Range, deep canyons of the Buffalo Fork River near Togwotee Pass, and outcrops of basement near Dubois, Wyoming. The main goals of the fieldwork were to complete the sampling of key units in Grand Teton National Park, and to determine whether or not the next nearest outcrops of basement (Gros Ventre, Togwotee Pass and Dubois regions) share the early geologic history preserved in the rocks of Teton National Park. This field work involved four faculty members from UW and a graduate student, who is doing the study as part of her MS thesis. Several months of laboratory analysis at UW have characterized the rocks through thin section, stained slabs, and whole rock geochemical and Nd, Sr, and Ph isotopic methods and produced preliminary U-Pb dates. The principal results from this year 's efforts are that the Teton basement rocks consist of large proportions of juvenile crust, the majority of the rocks formed over a relatively narrow time span from ~2.74 to 2.68 Ga, they were deformed at about 2.67 Ga, and that rocks exposed in the Buffalo Fork River to the east are shallow level equivalents to the deep rocks exposed in the Tetons. Based on these observations and measurements, we hypothesize that the basement rocks of the Tetons formed in an off­shore, island arc setting between 2.74-2.68 Ga, and they were accreted to the Wyoming province at about 2.67 Ga. Post-tectonic intrusion of distinctive peraluminous granites in both the Teton's (Mt. Owens quartz monzonite) and elsewhere in the Wyoming province at 2.55 Ga strengthens our interpretation of a shared history after 2.67 Ga. If this model for the basement rocks in the Teton's holds up, it will be the first case of crustal growth by lateral accretion for the Archean Wyoming province, and one of the earliest examples of plate tectonics style crustal growth documented from anywhere in the world. Plate tectonic growth has dominated the Earth 's evolution from ~2.5 Ga to the present, but it is unclear whether or not analogous processes operated before 2.5 Ga.