Late Mesoproterozoic mafic magmatism in the southwestern U.S. diabase province is expressed as diabase dikes, sills, sheets, and flows. Previous radiometric ages range from 1140 Ma to 1040 Ma. We used high-precision thermal ionization mass spectrometry to date baddeleyite in diabase from four localities in Arizona to obtain 206Pb/238U dates of 1080 ± 2 Ma, 1080 ± 3 Ma, 1088 ± 3 Ma, and 1094 ± 2 Ma. We also obtained single-crystal laser-ablation and ion microprobe ages on zircons from two localities in New Mexico that indicate a wider geographic extent of this diabase province. The samples have SiO2 ranging from 46.6 to 50.1 wt%, Mg# from 67 to 83, and εNd ranging from +4.7 to -1.4. A compilation of previously published ages of silicic rocks in the same age range suggests that mantle-derived magma induced crustal anatexis resulting in silicic magmatism, and this bimodal event forms a large igneous province that stretches 1500 km from east to west and 500-1000 km from north to south. Because some of the ca. 1.1 Ga plutonism extends outside the United States into northern Mexico, we suggest renaming this event as the Southwestern Laurentia large igneous province (SWLLIP). Magmatism in the province from 1094 to 1080 Ma occurred largely after the end of the Grenville orogeny. Two models that are not mutually exclusive are proposed: (1) lithospheric delamination following the Grenville collision; and (2) arrival of a mantle plume beneath south-central Laurentia, which spread beneath the lithosphere, with a northward-heading portion causing Keweenawan magmatism (at the boundary with the Superior craton), and a southward-heading portion creating the Southwestern Laurentia large igneous province. Other large igneous provinces have been previously correlated to these events, but the 1075 Ma Warakurna large igneous province in Australia is too young, and the 1110 Ma events in the Amazonian Congo and Kalahari cratons are too old. © 2014 Geological Society of America.