Detrital white micas from a sandstone unit within the Old Red Sandstone of southwest Wales have been analyzed with 40Ar/39Ar ultraviolet and infrared laser microprobes (UVLAMP, IRLAMP) to determine their source region. Single-grain fusion ages determined by the IRLAMP range from 430 ± 2 to 587 ± 3 Ma, and intragrain ages determined by the high-spatial-resolution UVLAMP range from 435 ± 9 to 875 ± 5 Ma. The UVLAMP technique reveals strong 40Ar/39Ar age zonation preserved within all the analyzed grains, and thus the single-grain fusion 40Ar/39Ar ages determined by the IRLAMP are artifacts of this zonation and represent an average of the old and young ages within each grain. Comparing the observed UVLAMP ages with calculated diffusion profiles for various thermal histories suggests that these detrital micas had a complex polymetamorphic evolution. A thermal event ca. 870 Ma was partly reset by a subsequent thermal event ca. 470 Ma, which is also complicated by Ar loss via a mechanism other than volume diffusion. Such detail as this from detritus allows for a fairly narrowed interpretation of the possible source region, which in this case is the Laurentian Northern and Central Highlands of Scotland. The UVLAMP technique also offers valuable insight into the complexities of dating detrital mineral grains and into the potential for misidentifying source regions and constructing misleading sedimentary systems, because the ages produced by 40Ar/39Ar dating techniques using either bulk-mineral separates or single-grain fusion will result in average ages from strongly zoned mineral grains. These data represent the first application of the 40Ar/39Ar UVLAMP to detrital minerals and provide insight into the potential for such a technique to investigate the thermal histories of now-eroded mountain belts.