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The Timing of Late Cretaceous Alkalic Igneous Activity in the Northern Gulf of Mexico Basin, Southeastern Usa

Upper Cretaceous igneous rocks in the northern Gulf of Mexico basin are poorly understood due both to the paucity of material recovered from drillholes in the sedimentary basins and lack of geochronological data. Rocks from the alkalic igneous complexes at Magnet Cove and Granite Mountain (Arkansas), the Monroe Uplift, and the Jackson Dome (Louisiana and Mississippi), yielded 40Ar/39Ar ages ranging from 94 Ma to ∼65 Ma. High‐precision plateau ages reveal that the Magnet Cove complex (94 Ma) predates Granite Mountain by 4 myr. Igneous activity in the Monroe Uplift may have been contemporaneous with that at Magnet Cove and continued sporadically for ∼30 myr. A single specimen from the Jackson Dome indicates volcanism at ∼65 Ma, coincident with waning igneous activity at the Monroe Uplift. Based on geochemical and geochronological data, the source for volcanoclastic material in the Woodbine Formation (Arkansas) are rocks belonging to the Magnet Cove phase of magmatism. Cretaceous alkalic rocks of central Arkansas show a northeast progression in ages, from Prairie Creek (108 Ma) to Granite Mountain (90 Ma). This “track” then turns southeast through the Monroe Uplift to the Jackson Dome (∼65 Ma). The distribution of alkalic material in the Gulf of Mexico basin was controlled by reactivation of older structural elements, possibly by regional isostatic adjustment to increased sediment load in Late Cretaceous time.