SLAB-EDGE MODEL FOR KIMBERLITE-CARBONATITE MAGMATISM, MID-CONTINENT NORTH AMERICA

Cretaceous-Tertiary kimberlite-carbonatite magmatism in mid-continent North America extends along a N40°W linear trend from Louisiana to Alberta, and occurs in at least four different pulses (~109-85, 67-64, 55-52, and less than 50 Ma). The lack of spatial age progressions of magmatism consistent with motion of North America over a fixed hot spot, the presence of Sr-Nd-Hf-Pb isotopic and trace-element compositions that show a temporal evolution from lithospheric to asthenospheric melt-sources, and the orientation of the magmatic belt parallel to the western subduction margin of the North American plate, suggest that this linear zone is the surface expression of mantle melting related to the subduction system.

In this model, fragmentation of the Farallon and Kula plates likely opened up slab windows perpendicular to their convergence direction. Sheet-like mantle upwellings were induced along slab-window margins, and these upwellings underwent low-degree partial melting to produce highly alkalic magmas along the trend parallel to, but ~2000 km east of, the convergent margin. The N40°W trend may reflect melting associated with penetration of the mantle transition-zone by the downgoing oceanic plate(s). In addition, seismic tomography indicates that the torn Farallon slab currently is stalled in the mantle transition zone below the mid-continent, and an older slab is within the lower mantle farther to the east (Sigloch et al., 2008).

Isotopic compositions of the alkalic rocks along the N40°W trend show a genetic similarity and similar patterns with time. Within each center where there exist precise age and isotopic data, magmatism began with a component that was dominantly lithospheric, with increasing proportions of an asthenospheric component with time, followed by a final increase in lithospheric component at the end of magmatic activity. This pattern may reflect compressional stress followed by relaxation (and local extension) of the lithosphere, and then a final return to a compressional stress regime.