No direct information about the age and composition of rift-related igneous activity associated with the Late Jurassic opening of the Gulf of Mexico exists because the igneous rocks are deeply buried beneath sediments. Three salt diapirs from southern Louisiana exhume samples of alkalic igneous rocks; these salt domes rise from the base of the sedimentary pile and overlie an isolated magnetic high, which may mark the position of an ancient volcano. Three samples from two domes were studied; they are altered but preserve relict igneous minerals including strongly zoned clinopyroxene (diopside to Ti-augite) and Cr-rich spinel rimmed with titanite. 40Ar/39Ar ages of 158.6 ± 0.2 Ma and 160.1 ± 0.7 Ma for Ti-rich biotite and kaersutite from two different salt domes are interpreted to represent the time the igneous rock solidified. Trace element compositions are strongly enriched in incompatible trace elements, indicating that the igneous rocks are low-degree melts of metasomatized upper mantle. Isotopic compositions of Nd and Hf indicate derivation from depleted mantle. This information supports the idea that crust beneath southern Louisiana formed as a magma-starved rifted margin on the northern flank of the Gulf of Mexico ca. 160 Ma. These results also confirm that some magnetic highs mark accumulations of mafic igneous rocks buried beneath thick sediments around the Gulf of Mexico margins.