Black Hills-Alberta carbonatite-kimberlite linear trend: Slab edge at depth?

Carbonatites and kimberlites (-48 to <= 46 Ma) occur along a 700-km-long, N40 degrees W linear trend of alkalic magmatism (similar to 55 to <= 46 Ma) that extends from the Black Hills of South Dakota and Wyoming, across eastern Montana to southern Alberta. Isotopic and trace-element concentrations point to a genetic linkage among alkalic igneous rocks, carbonatites, and kimberlites along the trend (group 1), with Sr-Nd isotopic ratios flanking bulk silicate earth. In contrast, alkalic rocks southwest of the N40 degrees W trend (groups 2 and 3) have significantly lower initial epsilon-Nd values and different trace-element contents (e.g. higher Ba, but lower REE, U, and Th). A tectonomagmatic model is proposed here to account for placement of the genetically related group I magmas along the N40 degrees W linear zone; the model suggests that the edge of the Kula plate was lodged in the mantle transition zone directly below the N40 degrees W trend by -50 Ma. Mantle material rising through the Kula-Farallon slab window may have been concentrated at the southwest edge of the Kula slab, focusing melts upward along the trend. Geochemical attributes of group 1 magmas along the trend can be explained by an extremely small partial melt of carbonated peridotitic mantle (0-1%). Initiation of extension at similar to 53-49 Ma is important because it may have stimulated mantle upwelling and decompressional melting, as well as facilitated intrusion of asthenospheric melts into the Wyoming Archean craton. Although groups 2 and 3 are thought to be largely lithospheric melts, their Sr-Nd isotopic compositions reflect a greater proportion of asthenospheric component after -50 Ma, following onset of extension in the west, (C) 2008 Elsevier B.V. All rights reserved.