CRUSTAL XENOLITHS FROM A BURIED PALEOPROTEROZOIC CONVERGENT MARGIN IN THE NORTHERN ROCKY MOUNTAINS

The Great Falls Tectonic Zone (GFTZ) separates the Archean Wyoming craton from the Hearne/Medicine Hat craton, and formed during the c. 1.8 Ga amalgamation of Laurentia. The Little Belt Mountains (LBM) in central Montana provide the only exposures of meta-plutonic rocks that formed within a ~1.86 Ga continental margin magmatic arc, apart from rare, km-sized exposures of thrust bound slices of basement in the Cordilleran trust belt. This arc was built on Medicine Hat lithosphere based on U-Pb ages of intercalated mafic gneiss, detrital zircons from meta-sediments, Hf isotopes in zircon, and whole-rock Nd isotopes. Crustal xenoliths from Eocene diatremes in the Missouri Breaks (northeast of the LBM) and Bearpaw Mountains volcanics reveal a similar record of crustal evolution. U-Pb ages of magmatic and xenocrystic zircons from granitic xenoliths fall in two distinct ranges: 2.46 – 3.35 Ga and 1.60 – 1.90 Ga. The younger age range overlaps with magmatism in the LBM and with the reworking of Archean crust in the GFTZ. The older age range correlates with published ages from the MHB. Lu-Hf analyses suggest that the zircons yielding ~1.73 – 1.88 Ga and ~2.46 – 2.53 Ga ages were derived from similar Archean crustal sources. Trace element analyses of the xenoliths show depleted HREEs, indicating that partial melting likely occurred in equilibrium with garnet or amphibole. Negative Eu and Sr anomalies indicate plagioclase removal from the melt during fractional crystallization, or remained as a residual phase. The Nb anomaly indicates formation in a subduction-modified environment. Whole-rock Nd and Pb isotopic geochemistry provide further constraints on the nature of the sampled crust. Overall, these data are consistent with documented magmatic and metamorphic events in the western GFTZ (1.86 – 1.73 Ga), and previously recognized events in the Medicine Hat block (2.53 – 3.28 Ga). These results support the hypothesis that the GFTZ formed as a result of both subduction of oceanic lithosphere (which produced suprasubduction magmatism along the southern margin of the Medicine Hat block) and subsequent continent-continent collision that produced crustal thickening. Post-orogenic collapse resulted in extensive mafic underplating that at least partially accounts for the widespread, seismically fast, lower crust beneath the GFTZ.