2015 GSA Annual Meeting in Baltimore, Maryland, USA (1-4 November 2015)

Paper No. 305-2
Presentation Time: 9:00 AM-6:30 PM


DRAGOVIC, Besim1, GUEVARA, Victor1, CADDICK, Mark J.2 and BAXTER, Ethan F.3, (1)Geosciences, Virginia Polytechnic Institute and State University, 4044 Derring Hall, Blacksburg, VA 24061, (2)Department of Geosciences, Virginia Polytechnic Institute and State University, 4044 Derring Hall, Blacksburg, VA 24061, (3)Department of Earth and Environmental Sciences, Boston College, 140 Commonwealth Ave, Chestnut Hill, MA 02467, dragovic@vt.edu

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.