Abstract

Geologic reports and maps that involve the Pinto Diorite indicate considerable confusion concerning the time of emplacement of the diorite in relation to other events in the Little Belt Mountains area. Reported ages of the diorite range from prior to the last regional metamorphism, about 1920 20 m.y. ago (Catanzaro, 1967; Catanzaro and Kulp, 1964), to Tertiary (Ross and others, 1955). Directly related to the question of the time of emplacement is the origin of gneissoid foliation in the Pinto Diorite.

Both radiometric and geologic data indicate that the diorite was emplaced after the last regional metamorphism and prior to deposition of the basal part of the Belt Series, which has been dated as 1325 15 m.y. ago (Obradovich and Peterman, 1968). K-Ar analyses by Catanzaro and Kulp (1964) suggest that the diorite was emplaced 1820 40 m.y. ago. Detailed field mapping shows that the diorite was emplaced mainly by dilation and that the gneissoid foliation is primary flow structure developed during injection.