EVIDENCE FOR NEOARCHEAN TO PROTEROZOIC CRUSTAL FORMATION AND MODIFICATION IN THE NORTHERN U.S. CORDILLERA

The nature of the Precambrian crystalline basement underlying the North American Cordillera in northern Idaho provides an important record for understanding the growth, assembly, and modification of the North American continent. We have examined Precambrian basement gneisses exposed in two areas in north central Idaho in what has been called the Clearwater and Priest River complexes. Crystallization ages of the basement gneisses in both complexes fall into two discrete periods in the Neoarchean and Paleoproterozoic. Neoarchean rocks range from 2.67 to 2.65 Ga, are broadly granitic in composition with juvenile Hf isotope compositions (ε_Hf(i) +2 to +4). Coupled with the lack of zircon inheritance, this indicates derivation from a depleted mantle source with little, if any, input of older crustal material. Paleoproterozoic orthogneisses are more abundant, range from 1876 to 1837 Ma, are also broadly granitic in composition, and show clear evidence for older crust in their genesis, evidenced by xenocrystic zircon cores and less radiogenic and more variable Hf isotope compositions (ε_Hf(i) -8 to +8). These data indicate the gneisses across the region were produced by formation of juvenile crust at 2.66 Ga; subsequent magmatism at 1.86 Ga involves a mix of juvenile and pre-existing (Neoarchean crust?) sources. There is no evidence of younger (<~1.86 Ga) Proterozoic magmatism in these gneisses. The broad geochronologic and geochemical similarity of gneisses--in both complexes--indicates these core complexes are probably part of one continuous crustal block, which we refer to as the “Clearwater Block”.

Within the Clearwater core complex, orthogneisses (and overlying paragneisses) have been through several metamorphic episodes. Garnet Lu-Hf ages range from 1.46 Ga to 1.06 Ga, with the best-constrained garnet ages defining two tectonothermal events at ~ 1.33 and 1.08 Ga. The 1.33 garnet age is mirrored by monazite U-Pb ages from similar lithologies. The 1.08 garnet age is largely absent in the monazite data. Eocene metamorphic ages, that are so prominent in the monazite data, are completely lacking in the garnet data. Collectively, these ages, particularly those of metamporphic events, need to be considered in models for the assembly of the North American continent in the Proterozoic.