Early Proterozoic magmatism in Yukon, Canada: constraints on the evolutionof northwestern Laurentia

Northwestern Laurentia, after cratonization at about 1.85 Ga, underwent a s eries of tectonic and magmatic events during the Proterozoic that were foll owed by separation of Laurentia from another landmass, probably Australia. The oldest magmatic event produced the Bonnet Plume River Intrusions (BPRI) , which intruded the Wernecke Supergroup as short dikes and small stocks. T he BPRI are hydrothermally altered tholeiitic diorites, gabbros, and subord inate anorthositic and syenitic rocks, with trace element signatures consis tent with a rift origin. Depleted mantle model ages range from 2.29 to 2.57 Ga and epsilon (Nd) values range from +0.7 to -1.7. An increasing crustal component is apparent in rocks with more evolved compositions. Four U-Pb zi rcon ages (1705.9 +/- 0.7, 1709.4 +/- 1.4, 1711.1 +/- 5.1, and 1713.6 +/- 1 2.7 Ma) indicate a Paleoproterozoic age for the BPRI. These dates constrain the age of the Wernecke Supergroup to greater than or equal to ca. 1710 Ma , making it the oldest supracrustal succession in western Laurentia, e.g., >240 Ma older than the Belt Supergroup of southeastern British Columbia and the northwestern United States. The Wernecke Supergroup was deposited in t he first rift basin to open along the western margin of Laurentia, but was later inverted by the pre-1.6 Ga Racklan Orogeny, an event possibly influen ced by transmission of compression from the Yavapai and Mazatzal orogenies in southern Laurentia. The Neoproterozoic southwestern United States - east Antarctica (SWEAT) reconstruction, which places Australia next to northwes tern Laurentia, is supported by linkages between Paleoproterozoic and Mesop roterozoic geological features in northwestern Canada and Australia.