Paper No. 5
Presentation Time: 2:40 PM
MEDICINE HAT BLOCK EXPOSED IN THE LITTLE ROCKY MOUNTAINS
GIFFORD, Jennifer N., Department of Geological Sciences, University of Florida, 241 Williamson Hall, Gainesville, FL 32611, MUELLER, Paul A., Department of Geological Sciences, University of Florida, 241 Williamson Hall, Gainesville, FL 32611-2120, FOSTER, David A., Department of Geological Sciences, University of Florida, Gainesville, FL 32611 and MOGK, David W., Dept. of Earth Sciences, Montana State University, Bozeman, MT 59717, giff4088@ufl.edu
The role of the Medicine Hat block (MHB) in the amalgamation of Laurentia is poorly defined, in spite of its critical structural position along the northern margin of the Great Falls Tectonic Zone (GFTZ). This block was caught up in the collisional zone between the Archean Hearne and Wyoming cratons during the Paleoproterozoic closure of an ocean. The MHB appears to be composed of Archean (2.6-3.1 Ga) and Proterozoic (1.75 Ga) continental crust based on limited data from drill holes and xenoliths, but is recognized primarily by its seismic signature and its influence on the geochemistry of younger igneous rocks. The Little Rocky Mountains (LRM) is the only Precambrian basement exposure in the northeastern area of the GFTZ, and potentially represents the only surface exposure of the Medicine Hat block. The LRM is composed of a Tertiary syenite core, with Precambrian metamorphic units uplifted along the margins of the dome. High precision geochronological and whole-rock geochemical data from the LRM are sparse. Limited geochronology from the region include K/Ar ages of 1.7-1.75 Ga, and Rb/Sr ages at c. 2.55 Ga, from quartzofeldspathic paragneisses. These data left the affinity of the LRM uncertain, either representing reworked Archean rock and/or Paleoproterozoic material generated during the subduction of oceanic crust and formation of the GFTZ.
New U/Pb ages of non-detrital zircons from the LRM range from 2.2 - 3.3 Ga, with prominent peaks between 2.6 - 2.8 Ga. Outliers clustering around 1.7 - 1.8 Ga are rare and likely reflect Paleoproterozoic reworking of older material. These ages are consistent with a MHB affinity for the LRM. Pb-isotope data define a 3.1 Ga model age, which suggests some influence of older Wyoming or MHB crust. The dominance of 2.6-2.8 Ga U/Pb ages suggests that the Paleoproterozoic igneous arc was constructed on pre-existing MHB crust. Models for reconciling the high angle junction between the GFTZ and Trans-Hudson orogen require the age and geochemical control provided by LRM samples. The data also provide insight into later geologic events potentially influenced by MHB crust reworked in the GFTZ, such as development of the Cenozoic Montana Alkali Province.
