LATE CRETACEOUS AND TERTIARY PLUTONISM, WEST-CENTRAL MONTANA--PINS IN THE END OF COMPRESSION AND THE BEGINNING OF EXTENSION

The sequence of granitic plutonism and relations of plutons to thrusting in west-central Montana are refined by new Ar40/Ar39 dates for plutonic rocks of the Big Belt, Castle, and Little Belt Mountains, Montana. New cooling ages for hornblende from the Boulder Baldy and Mt. Edith plutons in the southern Big Belt Mountains range from 73.8 to 72.1 Ma. Although slightly older than recently published results of duBray and Snee (2002; USGS PP 1657), these new data are consistent with emplacement of these plutons between 74 and 69 Ma. New cooling ages for low-temperature cooling domains in K-feldspar from related plutons in the Big Belt Mountains range between 62 and 61 Ma. Compositionally these plutons are similar to the similar-age Sodic series (Tilling, 1973) of the Boulder batholith and Pioneer batholith. Ages determined for the quartz monzonite pluton in the Castle Mountains, about 27 km east of the Big Belt plutons are about 58 Ma, statistically younger than the Big Belt plutons. Plutons in both the Big Belt and Castle Mountains intrude thrust faults that juxtapose Middle Proterozoic rocks over Paleozoic and Mesozoic rocks and thus establish the minimum age of thrusting as about 74 Ma. The Boulder Baldy pluton contains xenoliths both of Middle Proterozoic sedimentary rocks and of Early Proterozoic crystalline rocks that form the basement for all rocks south and northeast of the Big Belt Mountains. Quartz monzonite to monzodiorite intrusive rocks of the Little Belt Mountains to the north and northeast of the Big Belt Mountains have ages that range from 54.2 to 50.0 Ma. Plutonic rocks of the Little Belt Mountains intruded east-trending faults that have been recurrently active since Late Proterozoic time, along and northeast of the southeast-trending Lewis and Clark tectonic zone. These plutons represent a distinctly different interval of magmatism of wide compositional range that affected a much broader area. The emplacement of these plutons marks the beginning of the extension-associated magmatic episode also represented by the Absaroka, Challis, and Lowland Creek Volcanics and nearby core complexes.