PALEOPROTEROZOIC METAMORPHIC EVENTS IN THE MONTANA METASEDIMENTARY TERRANE (MMT), NW WYOMING PROVINCE: CONSTRAINTS FROM GARNET LU-HF AND SM-ND GEOCHRONOLOGY

The Montana Metasedimentary Terrane (MMT) in the northwest Wyoming Province preserves evidence of a multistage tectonic history during the Paleoproterozoic, making it an excellent case for studying the formation and modification of Earth’s early crust. Rocks in the MMT have undergone metamorphism to upper amphibolite and to lower granulite facies at 1.8 to 1.7 Ga, with peak temperature and pressure at >600℃ and >8 kbar (Crenshaw, 2021). Previous studies have mainly focused on zircon and monazite U-Pb systems in this area; what these ages mean, however, is not always clear. Zircon and monazite are minerals that can withstand over a wide range of P-T conditions, so it is important to carefully determine whether they represent the timing of magmatism or metamorphism. The common garnetiferous rocks in this area make garnet be a suitable indicator mineral to study the metamorphic history in this complex metamorphic terrane because garnet commonly shows compositional zoning that can preserve direct petrologic context.

In this study, we present garnet Lu-Hf and Sm-Nd geochronology, in conjunction with garnet compositional mapping and trace-element profiles from six high-grade metaigneous samples. Inclusions in all samples, are dominated by quartz, with apatite and/or rutile/zircon. Three samples have a typical prograde garnet growth zoning pattern in which Mn is enriched in the core and decreases toward the rim. Most samples show gradual increases in HREE from rim to core and higher Lu in the core. Five samples yield garnet Lu-Hf dates from 1.8 to 1.7 Ga; the Sm-Nd ages of those same samples are consistent or slightly younger (<40 myr) than their Lu-Hf ages. These ages are attributed to a major tectonothermal event, the Big Sky orogeny, in the MMT. In addition, one leucogneiss sample from the Ruby Range in the west MMT yields a garnet Lu-Hf age of 2.2 Ga, with a Sm-Nd age of 1.8 Ga. This new Lu-Hf date indicates a period of crustal instability and underplating that is associated with continental rifting. The garnet samples provide important information about the Big Sky orogeny and offer a new age constraint for terrane rifting in the NW Wyoming Province. Overall, our study shows that garnet can be a valuable indicator mineral for studying complex metamorphic history.