LATE CRETACEOUS AMPHIBOLITE FACIES METAMORPHISM IN THE SAPPHIRE MOUNTAINS, SOUTHWESTERN MONTANA CONSTRAINED BY U-PB DATING OF MONAZITE

U-Pb ID-TIMS geochronologic data from metamorphic monazite in amphibolite facies pelitic schists, Skalkaho region, Sapphire mountains, southwestern Montana, creates a regional link between Late Cretaceous metamorphism in the Bitterroot and Anaconda metamorphic core complexes (MCCs). The schists, which are interpreted to be equivalent to the St. Regis or Empire formations of the Belt Supergroup, contain an early metamorphic assemblage of garnet-kyanite-biotite-plagioclase-quartz. This is overprinted by cordierite-sillimanite-biotite and andalusite-cordierite-biotite assemblages. P-T constraints for the early metamorphism are constrained by phase equilibria modeling. Isotope dilution thermal ionization mass spectrometry (ID-TIMS) was used to date metamorphic monazite from these schists, revealing Late Cretaceous ages. These ages place peak metamorphism considerably earlier than “main-phase” intrusion and subsequent exhumation of the Bitterroot lobe of the Idaho Batholith, and are similar to those reported from other northern Cordilleran core complexes. Previous workers in the Bitterroot MCC identified a metamorphic event at 64-56 Ma using U-Pb geochronology of metamorphic monazite, which corresponds to the pulse of “main-phase” magmatism in the Bitterroot lobe of the Idaho batholith. However, high grade metamorphism in the Bitterroot has also been interpreted to be coincident with intrusion of syntectonic plutons 80-75 Ma. Recent data from the Anaconda MCC brackets peak metamorphism between 75-74 Ma based on U-Pb zircon dates of syntectonic sills and leucosomes, and biotite cooling ages of undeformed post-metamorphic sills. When integrated with previous results, these data imply that this early metamorphic event can be attributed to late Sevier-aged thrusting, which likely overprints earlier Sevier related regional metamorphism.

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