2.69-2.68 Ga granulite facies metamorphism in the Wyoming Craton revealed by Sm-Nd garnet geochronology and trace element zoning, eastern Beartooth Mountains, Montana and Wyoming, USA

Abstract:

The Beartooth Mountains in Montana and Wyoming, USA, form an extensive exposure of Archean rocks of the Wyoming Craton and are dominantly comprised of a ~2.8 Ga granitoid batholith known as the Long Lake Magmatic Complex (LLMC). Contained within the LLMC are numerous m- to km-scale enclaves of metasedimentary granulites. P-T pseudosection modeling indicates that these granulites reached peak pressure-temperature (P-T) conditions of 800 °C, 7-8 kbar. This has previously been interpreted to result from contact heating with the LLMC. However, substantial field evidence from multiple localities suggests that the texturally dominant phase of HT metamorphism in the metasediments postdates LLMC emplacement. Further, Sm-Nd garnet (grt) dates from the metasediments are in the range ~2.69–2.68 Ga (‘bulk’ dates incorporating crystal cores and rims), ~100 Myrs younger than LLMC emplacement (based on U-Pb zircon ages, 1). Trace element zoning in grt suggests that these dates record the age of granulite facies metamorphism. Euhedral high-Ca overgrowths in Grt from a residual pelite are coincident with a high Eu spike, interpreted to result from plagioclase breakdown during partial melting. These overgrowths are also coincident with high Sm and Nd annuli, and we thus interpret the bulk grt date (2689±4 Ma) to record timing of the late stages of grt growth during migmatisation near peak T. Coupled with major element zoning, retention of Sm and Nd zoning in euhedral grt from the leucosome of another sample suggest that its bulk date (2681±1 Ma) also represents peritectic grt growth rather than subsequent diffusion. Grt from a lithology that did not experience melting records a date of 2686±1 Ma. Together, these ages indicate that granulite facies metamorphism persisted in the area for at least ~3 Myrs (inner bounds of the 2σ dates), ~100 Myrs after batholith emplacement. Limited evidence for this later event in the plutonic rocks is consistent with their experiencing little recrystallization at a peak T of ~ 800 ˚C. HT metamorphism of this age is also recorded in the Teton Range, ~200 km SW of the eastern Beartooths (2), and thus the mechanism for HT metamorphism may have affected a broad region of the Wyoming Craton.

1 Mueller et al. 2010. Precambrian Research, v 183, 1, p 70-88 

2 Frost et al. 2007. GSA Abstracts with Programs, v 39, 6, p 202