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

Abstract

Carbonatites and kimberlites (~ 48 to ≤ 46 Ma) occur along a 700-km-long, N40°W linear trend of alkalic magmatism (~ 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°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 1 magmas along the N40°W linear zone; the model suggests that the edge of the Kula plate was lodged in the mantle transition zone directly below the N40°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 ~ 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.