The 100 km2 Horse Creek anorthosite complex, which was emplaced between 1770 ± 3 and 1755 ± 2 Ma, is the older of two anorthosite complexes located in the southern Laramie Mountains, southeastern Wyoming. The complex is composed of four lithologic units, which are in order of decreasing age the Ragged Top granodiorite, Horse Creek granite, Horse Creek anorthosite, and Horse Creek monzonite. Nd, Sr, and Pb isotopic compositions of all units indicate that they are dominated by sources such as contemporary depleted mantle and crustal rocks with short crustal residence ages. However, the involvement of a small amount of Archean material is required by the Pb isotopic data.
Both the Horse Creek anorthosite complex and the 1.43-Ga Laramie anorthosite complex were emplaced along the Cheyenne belt, which marks the suture between the Archean Wyoming province and Proterozoic Colorado province. The time of emplacement of the Horse Creek anorthosite complex coincides with late stages of the Medicine Bow orogeny, in which Proterozoic island arcs collided with the Wyoming province. The Horse Creek anorthosite complex was emplaced in a zone of local transtension formed as a result of oblique collision of the Colorado province along the southern margin of the Wyoming craton.