ALKALINE DIKES AND SILLS OF GARRISON, MT: A POSSIBLE EXTENSION OF THE CENTRAL MONTANA ALKALIC PROVINCE

Petrological and chemical examinations indicate that a series of shallow dikes and sills on the northwestern side of the Boulder batholith were formed from the mixing of mafic alkaline and intermediate calc-alkaline magmas. The dikes and sills have been dated at 76 Ma and are contemporaneous with the Boulder batholith. They intrude into late Cretaceous Golden Spike Formation and the Colorado Group, and have been deformed in the Lewis and Clark shear zone near Garrison. The alkalic magma is chemically similar to the shonkinites of the Central Montana Alkalic Province to the east. The dikes and sills are mostly porphyritic, with titaniferous augite and plagioclase (labradorite) phenocrysts and a few olivine pseudomorphs. The groundmass is mostly plagioclase with minor biotite, apatite, potassium feldspar, quartz and allanite. The Adel Mountains Volcanics, just east of the overthrust belt, correlate with the dikes and sills and are the closest alkaline intrusive center to the study area. Mixing and mingling of a mafic alkaline magma with the calc-alkaline Boulder batholith magma has been recognized along the east and south margins of the batholith. The presence of similar alkalic magma over 65 km from the Central Montana Alkalic Province indicates that it may extend farther west than previously recognized, from the stable craton into the Cordilleran thrust belt.