Minette bodies and cognate mica-clinopyroxenite xenoliths from the Milk River area, southern Alberta: records of a complex history of the northernmost part of the Archean Wyoming craton

Minettes exposed in southern Alberta near the Milk River are the northern o utliers of the Eocene Sweet Grass Hills igneous complex of the Montana alka lic igneous province. These minettes often contain coarse-grained xenoliths of phlogopite + clinopyroxene +/- apatite. The parent magmas of the minett es were generated at pressures > = 17 kbar in equilibrium with clinopyroxen e + phlogopite +/- olivine. Fractional crystallization and mixing provided a spectrum of evolved minettes and cumulates, the latter of which were samp led by subsequent minette magmas as xenoliths. Two xenoliths were dated at 49.0 +/- 0.8 Ma and 52 +/- 1.7 Ma. The host dyke of the latter xenolith gav e an age of 50 +/- 0.3 Ma. The minettes and their xenoliths have overlappin g values of Sr-87/Sr-86(i), epsilon Nd-T, Pb-206/Pb-204, Pb-207/Pb-204, and Pb-208/Pb-204, similar to those of alkaline igneous rocks from farther sou th in the Montana alkalic igneous province. The Sweet Grass Hills lie north of the Great Falls Tectonic Zone, previously interpreted as a Proterozoic suture zone separating the Archean Medicine Hat block from the Archean Wyom ing craton to the south. Geochemical data for the Milk River minettes provi de evidence for a history of the mantle underneath the Medicine Hat block, similar to that found previously for mantle-derived rocks of the Wyoming cr aton, including a significant Proterozoic mantle enrichment event. Given th is similarity, we suggest that the Wyoming craton extends into southern Alb erta, and that the Great Falls Tectonic Zone does not represent a Proterozo ic suture of two Archean blocks.