40Ar/39Ar and K-Ar geochronology and tectonic significance of the Upper Cretaceous Adel Mountain volcanics and spatially associated Tertiary igneous rocks, northwestern Montana

Abstract

We report new 40 Ar/ 39 Ar 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-mill ion-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. The dates from the Adel Mountain Volcanics, as well as those from spatially related younger intrusions, also provide important constraints on the age of fold and thrust-belt deformation along the eastern margin of the Montana disturbed belt. Syntectonic deformation of the Adel Mountain Volcanics, as well as apparent folding and faulting of Tertiary quartz monzonite sills, indicates that contractional deformation clearly spanned the Late Cretaceous and may have extended to as young as the Paleocene/Eocene boundary at about 55.5 Ma. Elsewhere, posttectonic field relationships indicate that deformation may have ended prior to 60 Ma. Complexities in field relationships with respect to folds and faults shown by the early Tertiary intrusions as well as complications in the argon systematics indicate that these interpretations must be considered preliminary. Further field work and structural studies and additional high-precision geochronology are needed in order to place limits on the cessation of contractional deformation in this part of Montana. Unambiguously posttectonic dikes (47.5 Ma) that cut all deformed rocks and structures in the area indicate that disturbed belt deformation had clearly ceased by the early middle Eocene prior to the onset of widespread crustal extension in this part of the northern Cordillera.