CLOSURE OF A PALEOPROTEROZOIC OCEAN RECORDED BY HF ISOTOPES IN ZIRCONS

Assembly of macro-continents by amalgamation of micro-continents invariably begins with ocean closure and ends with continent-continent collision. Tracking this sequence in ancient orogens is difficult because deformation makes use of the magnetic record in the rocks very difficult and current exposures often do not provide the temporal detail required. An alternative is to use the Lu-Hf systematics recorded in zircons preserved in both sedimentary and igneous rocks to track the interactions between oceanic and continental lithosphere as the orogen evolves. In western North America, the Great Falls tectonic zone (GFTZ) trends NE from Idaho to southern Saskatchewan and contains a complex record of the Paleoproterozoic closure of an ocean basin between the Wyoming craton and Medicine Hat block. Ocean closure began by ~1860 Ma as recorded by formation of the Little Belt arc (LBA). Dioritic to granitic metaigneous rocks exposed in the Little Belt Mountains (Montana) preserve the record of magmatism in the LBA from this time until ~1790 Ma. Trace element abundances (e.g., HFSE depletion) along with Pb, Nd, and Hf isotopic compositions of the LBA rocks support the model of arc formation by ocean closure with magmatism along the edge of the overriding Medicine Hat (Archean) crust. Once ocean closure was complete, continued interaction of the two micro-continents produced magmas with recognizably more negative initial Hf isotopic compositions. In the case of the closure of the Medicine Hat ocean there were also changes in the style of collision along the axis of the orogen. To the east, collision may have been highly oblique because the Medicine Hat block was not thrust over the much thicker crust of the Wyoming craton when the ocean closed, but in the western GFTZ Medicine Hat crust was thrusted at least 100 km over Wyoming crust at ~1780 Ma. This change is evident in the lack of Paleoproterozoic metamorphism along the northern margin of the Wyoming craton and the intense Paleoproterozoic metamorphism recorded in the Archean rocks of SW Montana. By combining zircons from presently exposed remnants of the LBA and both metaigneous and metasedimentary xenoliths extracted from diatremes of the Montana Alkali Province, we can “digitally” trace the evolution of the Great Falls orogeny from convergence to collision over a period of ~100 Ma.