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Post-Breakup Magmatism in the Northern and Western Gulf of Mexico

Abstract

Magmatism on a rifted continental margin can be classified as pre-rift, syn-rift, syn-breakup, and post-breakup, based on their timing relationships. Post-breakup magmatism, which occurs on a mature passive continental margin, is still enigmatic regarding its petrogenetic and tectonic mechanism. The northern Gulf of Mexico (GoM) region witnessed widespread igneous activity from Arkansas to West Texas during the mid- to Late Cretaceous (ca. 108 – 76 Ma). This igneous province consists of carbonatites, alkaline basalts, nepheline syenites, and phonolites. In eastern Mexico, a Cenozoic alkalic igneous province exists along the western GoM margin from Oligocene to Quaternary. Both igneous provinces are characteristically derived from the sublithospheric mantle after the Laurentia-Yucatan separation. Previous tectonic models include: 1) hotspot track; 2) edge-driven convection; 3) lithospheric reactivation; and 4) flat-slab subduction. These models, however, cannot fully explain the geochronology, geochemistry, paleogeography, and seismic tomography data for the northern GoM magmatism. We propose a “stagnant slab” model, based on an integration of spatial, lithologic, geochronologic, and geochemical data of the Cretaceous magmatic rocks, reconstruction of the North American plate and remnant Farallon slabs in the lower mantle, and re-interpretation of seismic tomography and electrical conductivity observations. In this model, tearing and sinking of stagnant Farallon slabs control the magmatism in northern GoM as well as those coeval kimberlites and carbonatites in Kansas, Nebraska, Saskatchewan, Alberta, and Northwestern Territories. Subducted oceanic slab tends to stagnate above, within, or below the transition zone, accompanied by decarbonation in the mantle transition zone. Our reconstruction suggests a flat-lying slab geometry at the onset of the magmatism. As the stagnant slab finally sinks into lower mantle, it generates upwelling and decompression melting of the carbonatitic melts. Thickness and composition of the overlying lithosphere further affects the ascending magma by chemical assimilation, and results in various magmatism from the GoM to NW Canada. This slab model also explains the occurrence of the Eastern Mexico alkaline province. This study highlights a profound plate-scale relationship between the post-breakup, intraplate magmatism and the subduction factory down to the transition zone.