- UW Department of Geology and Geophysics
The Kennedy dike swarm of the central and northern Laramie Mountains is the most extensive Precambrian dike swarm of the Wyoming province. The dikes occupy from 15 to 20% of a 100-km-wide exposure of Archean basement, and include diabasic, plagioclase-phyric, and peridotitic rock types. U-Pb zircon and baddeleyite data indicate that dikes were intruded at 2010 Ma. The dikes were deformed locally and metamorphic zircon replaced igneous baddeleyite to varying degrees during the collisional Medicine Bow orogeny that affected the southern margin of the Wyoming province between 1780 and 1760 Ma.
The geochemical compositions of the dike swarm exhibit a continuum with considerable overlap between diabase, plagioclase porphyry, and peridotite. Medium-grained rocks along the margins of peridotite are the best estimate of parental, mantle-derived magma compositions. The variations in major element compositions are the result of magmatic differentiation and crystal accumulation. Minor amounts of assimilation of felsic Archean crust, identified from Nd, Sr, and Pb isotopic compositions, have significantly affected the abundances of some trace elements including the rare earth elements.
The Kennedy dike swarm records Paleoproterozoic rifting of the southern Wyoming province. Identification of the rifted continental block rests in part on the recognition of a coeval dike swarm. The only well-dated dikes of similar age are the Lac de Gras dike swarm of the Slave craton. A tectonic reconstruction juxtaposing the southeastern Wyoming province with the northern Slave province is proposed.