LASER 40AR/39AR THERMOCHRONOLOGY OF SINGLE CRYSTALS FROM PALEOPROTEROZOIC TO ARCHEAN ROCKS OF THE NORTHERN WYOMING PROVINCE, BIG SKY OROGEN, AND GREAT FALLS TECTONIC ZONE: GILETTI’S LINE REVISITED

The most ancient portions of the lithosphere provide the earliest record of continental assembly and establishment of a stable crustal geotherm. We have undertaken a laser ⁴⁰Ar/³⁹Ar study of single crystals from exposures in southwestern Montana comprising Archean lithologies of the northwestern Wyoming craton (WC; Henrys Lake Mtns) that are tectonically overprinted by Paleoproterozoic deformation and metamorphism of the Big Sky orogeny (BSO; Ruby and Highland Mtns), and from the Paleoproterozoic arc associated with the BSO (Little Belt Mtns) located in the Great Falls Tectonic Zone (GFTZ). The southeastern margin of the BSO overprint approximately coincides with a line drawn by Giletti (1966) to demarcate rocks to the northwest with K/Ar and Rb/Sr mineral ages less than ~1.7 Ga from those of the WC to the southeast that yielded older ages ranging to the Archean. In the present study with the ANIMAL facility, hornblende, muscovite, and biotite from the WC yield single crystal ⁴⁰Ar/³⁹Ar ages interpreted to reflect a 2.5 Ga event followed by cooling through their respective partial retention temperatures for ⁴⁰Ar, with initial average rates > 1°C/m.y. that decreased to about 0.1 °C/m.y. by about 1.5 Ga. In the Ruby and Highland mountains of the BSO, initial ⁴⁰Ar retention in coarse hornblende and muscovite occurred as early as ca. 1.8 Ga, followed by cooling at rates generally similar to those inferred for the WC. The arc-related rocks within the Little Belt mountains also cooled following a ca. 1.77 event, yet at rates 2-3X greater than inferred for the Ruby and Highland mountains, consistent with a shallower crustal setting. The youngest ages determined in this study do not correlate with any particular event, and instead seem primarily functions of the relative grain size (diffusion dimension) for individual crystals and the slow cooling history. Our interpretations for the new ⁴⁰Ar/³⁹Ar data are very similar to interpretations of U/Pb cooling ages and modeling results of Blackburn et al. (2011) for rutile from xenoliths derived from deep portions of the Medicine Hat block. These results collectively indicate that, following the ca. 1.8 Ga BSO, the WC and GFTZ shared a common exhumation history that concluded with long residence times (ca. 1 Ga) in the middle crust across broad domains from the cores to the flanks of the orogens.