NEOARCHEAN-PALEOPROTEROZOIC BASEMENT OF SOUTHWEST LAURENTIA FORMED ADJACENT TO THE MAWSON CONTINENT

Identifying Precambrian crust along the western edge of Laurentia, and constraining its origin, is key to understanding the Paleoproterozoic growth of Laurentia and adjacent crust within Proterozoic supercontinents. The Grouse Creek Block, in the northern U.S. Rocky Mountains, contains Neoarchean-early Paleoproterozoic basement and lies immediately west of the Archean Wyoming Province. U-Pb zircon ages, isotopic data, and major and trace element data from the Grouse Creek Block document: 1) 2.67 Ga tonalitic gneiss with an average initial εNd value of -1.5 and an average initial εHf (zircon) value of +3, indicating significant mantle involvement, 2) 2.47 Ga quartz monzonite with an initial εNd of -6 and an average initial εHf (zircon) value of -9, suggesting increased crustal input at this time, 3) ~2.57 Ga amphibolite and quartzofeldspathic gneisses with initial εNd of -6 to +4, indicating mixed mantle/crustal involvement,) 4) zircon xenocrysts from Eocene dikes and migmatites with age-concentrations at 2.67 Ga, 2.44 Ga and 1.76 Ga, and 5) 1.86 Ga amphibolite with an N-MORB trace element signature and an initial εNd value of +3. These new data, along with extant data, suggest that the Grouse Creek Block and other Neoarchean to late Paleoproterozoic terranes in southwestern Laurentia (e.g., Mojavia and Priest River) comprise an exotic composite terrane rather than an extension of the cratonic core of Laurentia. Similar magmatic ages; Nd, and Hf isotopic systematics; and trace element compositions suggest this crust developed adjacent to the Gawler Craton (Australia) in the Mawson continent and was accreted to western Laurentia in the late Paleoproterozoic.