ABSTRACT

The mineralogy, major element and trace element composition, and age were determined for two granites of the central Beartooth Mountains. The more extensive unit is a foliated, syntectonic, biotite granite which was determined by zircon U-Pb methods to be 2752 ± 14 Ma old. Elevated LIL-element abundances (Ba, Th, U, and La > lOOOx chondrites) and a high initial Sr isotopic ratio (0.704) suggest the existence of an enriched, probably crustal, component in the source of the biotite granite. As suggested by the presence of magmatic epidote, the biotite granite crystallized at P-T conditions (600°C, 6-8 Kb) corresponding to emplacement depths of about 20-25 km. These data imply a relatively low late Archean geothermal gradient of 25-30°C/km in the vicinity of the study area.

A poorly foliated, muscovite-bearing pink granite and a set of related granitic dikes intrude the biotite granite. This unit has an age of 2090 ± 72 Ma (whole rock Rb-Sr isochron), a previously unreported age in this area. An elevated initial Sr isotopic ratio (0.726) suggests the pink granite was derived from an isotopically enriched source. Comparatively low LIL abundances in the pink granite (Ba, La, Ce > 70x chondrites), along with a mineralogy that includes two micas and garnets, indicate that the source had a major sedimentary component with little, if any, involvement of the older biotite granite.