| Summary: | Mafic dikes provide geochemical and isotopic insight into processes affecting mantle evolution. Two specific areas within the Wyoming Province, the Beartooth-Bighorn Magmatic Zone (BBMZ), and the Montana Metasedimentary Province (MMP), and one area within the Great Falls tectonic zone (GFTZ), the Montana Alkali Province (MAP), each contain mafic intrusions, primarily as dikes. One of three dikes sampled from the BBMZ has a U-Pb zircon age of 2.8 Ga. Geochemical and isotopic similarities for the other dikes suggest a Late Archean emplacement for all three dikes. No dikes sampled from the MMP yielded reliable, datable phases, however, geochronology and paleomagnetic data suggest that dikes were either emplaced at 1450 or 780 Ma. The MAP, a petrologic province, formed ~50 Ma ago within the GFTZ, a largely Proterozoic feature. Major and trace element geochemistry for all samples exhibit features, such as a relative depletion in high field strength elements, characteristic of modern convergent margin volcanism. Nd and Pb isotopic data, however, suggest that the geochemical features were derived from the source of the mafic magmas and that these signatures were established well before the time that the magmas formed. In particular, Sm/Nd isotopic data from whole rocks suggest metasomatic enrichment in the mantle in the BBMZ at ~3.4 Ga and ~2.0 Ga for the MMP, whereas Pb/Pb whole rock analysis suggests metasomatism of the mantle in the BBMZ at ~3.2 Ga, ~1.9 Ga for the MMP, and ~1.8 Ga for the MAP. The ages quoted above are based on secondary isotopic ratios and are only loosely constrained (i.e., the 3.2 Ga and the 3.4 Ga ?ages? from the BBMZ samples are not statistically distinct). Similarly, the 1.9 Ga and 1.8 Ga ?ages? for the MMP and MAP samples are also indistinguishable from each other. What is distinct, however, is that samples from the Proterozoic GFTZ (MMP and MAP) suggest that the underlying mantle was modified (metasomatized) at this time and has remained largely undisturbed. The mantle-altering events inferred for all three areas appear to be similar to modern day island arc settings. We propose that the evolution recorded in the isotopic data suggests that metasomatism of the dike sources occurred at different times throughout the area and we conclude that the source of this metasomatism is from subduction related processes similar to modern day island arc settings.
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