LIMITATIONS AND UTILITY OF SM-ND GARNET GEOCHRONOLOGY OF HIGH-TEMPERATURE METAMORPHISM: AN EXAMPLE FROM THE WYOMING CRATON

The use of garnet (grt) as a high-temperature (HT) geochronometer has proven challenging, particularly with interpretation of bulk grt ages. Significant scatter in isotopic data can exist between grt fractions used for multi-point isochron ages, with several factors contributing to scatter in resultant ages, including: a) sampling of grt from multiple growth events (polymetamorphism), b) a long grt growth duration, and c) protracted high temperatures resulting in diffusionally-controlled differential age resetting. In any of these scenarios, unbiased sampling of grt fractions from a bulk collection of crushed porphyroblasts could result in high errors and high MSWDs. Coupling grt geochronology with models of major element and REE diffusion can elucidate: 1) relative timing of HT metamorphism and 2) what influence the aforementioned factors can have on resultant bulk grt ages.

In the eastern Beartooth Mountains of MT/WY, USA, roof pendants of metasedimentary granulites are contained within a 2.79-2.83 Ga granitoid batholith (Long Lake Magmatic Complex, LLMC). HT metamorphism (~800°C, 6-7 kbar) has been previously thought to result from contact heating by LLMC emplacement, however Sm-Nd data suggest garnet growth during a secondary HT event that postdated emplacement. Sm-Nd bulk grt isochron ages from eight lithologies range from 2673 ± 75 Ma (MSWD = 3605) to 2717 ± 12 Ma (MSWD = 43). Major element diffusion modeling on prograde grt zoning constrains a brief duration near peak T (probably < 2 Ma), thus eliminating long growth duration as a contributor to age scatter. Models estimating the initial cooling rate based on peak temperature, degree of age resetting, and crystal size suggest contact heating by the 2.79-2.83 Ga LLMC followed by diffusive resetting of bulk Sm-Nd ages by >40 Ma requires unfeasibly slow cooling. Grt ages thus likely record growth during a later HT event (postdating LLMC emplacement by at least 40 Ma), combined with some degree of age resetting during cooling from this event. Varying influence of the earlier, LLMC-related metamorphism is revealed by the spread in ages between differing lithologies. Grt geochronology on distinct, microsampled growth zones provides a more accurate assessment of grt growth periodicity, highlighting possible effects of slow cooling on intra-mineral age resetting.