NEW INSIGHTS INTO THE CHARACTER OF THE MEDICINE HAT BLOCK (SOUTHERN ALBERTA, NORTHERN MONTANA) FROM WHOLE ROCK GEOCHEMISTRY, ZIRCON LA-ICPMS U/PB AND HF ISOTOPIC ANALYSIS

The Medicine Hat Block (MHB) of southern Alberta and northern Montana is a largely concealed province of Archean and Paleoproterozoic rock buried by younger supracrustal successions. The MHB was involved in a complex collision between the Archean Hearne and Wyoming provinces during the formation of the Great Falls Tectonic Zone (GFTZ), Trans Hudson Orogen (THO) and Vulcan Zone during the amalgamation of Laurentia. However, its role in this collision is poorly defined by limited high precision geochronolgy and geochemisty. Xenoliths recovered by the Geological Survey of Canada from Eocene minette dikes and boreholes in the MHB north of the Wyoming craton include a population of gneisses, granulites, monozites, tonalities, amphibolites, and diorites sampling the otherwise concealed MHB crust. Previous research on these samples yielded U/Pb ages from abraded, acid-washed bulk-zircon dissolution methods with ages ranging from ~1.70 Ga to 3.26 Ga. Sm/Nd model ages for the samples range from ~1.80 Ga to 3.48 Ga. This has been interpreted to represent a mixture between evolved Archean crustal sources (i.e. MHB-Wyoming crust) and Proterozoic magmas (i.e. GFTZ-THO).

New LA-ICPMS U/Pb age and Hf isotopic data from zircon separates from these sources improves understanding of the age and origins of the MHB crust. Due to the overlapping ages of the events recorded, models for reconciling the high angle junction between the GFTZ and THO require improved age and geochemical constraint provided by MHB 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 or the structural formation of the Proterozoic Belt-Purcell Basin.