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. 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.
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Posted August 2005 |
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