40Ar/39Ar and K-Ar Geochronology and Tectonic Significance of the Upper Cretaceous Adel Mountain Volcanics and Spatially Associated Tertiary Igneous Rocks, Northwestern Montana

摘要

We report new 40Ar/39Ar and K-Ar dates from the Upper Cretaceous Adel Mountain Volcanics of northwestern Montana and spatially related Tertiary igneous rocks. The Adel Mountain volcanic field consists of about 900 square kilometers of lavas, associated volcaniclastic strata, and intrusions that lie astride the easternmost folds of the Montana disturbed belt of the Cordilleran fold and thrust belt. The Adel Mountain volcanic rocks have been intensely deformed by folds and thrust faults along their southwestern margin but are essentially undeformed to the east. Prior to isotopic dating, the age of the Adel Mountain Volcanics was the subject of debate, with age assignments ranging from Late Cretaceous to early Tertiary. Isotopic dates reported here demonstrate that the Adel Mountain Volcanics are clearly Late Cretaceous and that the volcanic rocks were probably emplaced during an approximately 2- to 3-million-year interval between about 76 to 73 mega-annum (Ma). The new dates from the Adel Mountain Volcanics are significant in that they provide a more refined and reliable age for the Late Cretaceous cratonic paleomagnetic reference pole for North America.