AGE AND COMPOSITION OF PRECAMBRIAN MAFIC DIKES WITHIN THE WYOMING PROVINCE: A WINDOW INTO THE EVOLUTION OF THE SUBCONTINETAL LITHOSPHERE

The Wyoming province contains a variety of sub-provinces, including the Beartooth-Bighorn magmatic zone (BBMZ) and Montana metasedimentary province (MMP). These two sub-provinces, as well as others in the province, contain mafic to intermediate intrusions. Mafic dikes in the BBMZ have been dated by whole rock K-Ar, Rb-Sr, and U-Pb methods and subdivided into at least four primary age groups (i.e., 2.5, 1.9-2.2, 1.4-1.8, and 0.7-1.0 Ga). The dikes in the MMP, on the other hand, are difficult to place in distinct age groups, although dated using the same isotopic systems. These dikes are thought to be the only magmatic events that occur between the cratonization of the BBMZ and MMP at ~2.8 Ga and sedimentation during Paleozoic time. The source of this mafic magmatism has been proposed to be the result of mantle plumes, asthenosphere upwelling, and/or generated by decompression melting of the subcontinental lithosphere. In a structural sense, mafic dikes in these two terranes are probably related to extension external to the Wyoming craton at 2.1-2.0 Ga (Kenorland rifting) and 1.5-1.4 Ga (Belt basin). Mafic dikes from both terranes were studied to constrain the evolution of the mantle and sub-continental lithosphere using major and trace elemental data as well as radioactive isotopes (i.e., Sm-Nd, Rb-Sr, U-Pb, and Lu-Hf). Bulk compositions range from 47 to 57 % SiO2 with total alkali weight percentage ranging from 1 to 11. REE patterns, with respects to chondrites, show two distinct patterns of enrichment in LREE with depletion recorded within the HREE, possibly from separate fractionation events. Initial epsilon Nd values range from +1 to -24 and Sr 87/86 ratios from .7026 to .7147. Hf isotopes and U-Pb collected from zircons from the dikes will be obtained to further constrain the origin of these dikes and develop and age, respectively.