THE PRECAMBRIAN CRYSTALLINE BASEMENT OF NORTHWEST LAURENTIA: CONSTRAINING THE FORMATION AND EVOLUTION OF NORTH AMERICA

The nature of the Precambrian crystalline basement underlying the North American Cordillera provides an important record for understanding not only western Laurentia, but also for the growth, assembly, and modification of the North American continent itself. The detailed nature of the Precambrian basement throughout much of the North American Cordillera, however, has proved difficult to document because of limited exposures due to thick sequences of cover rocks, subsequent magmatism and metamorphism, and major structural/tectonic rearrangement. One such region, which has been the focus of recent attention, is the Clearwater complex where U-Pb zircon dating of rocks has documented the existence of Precambrian basement rocks extending along a 115-km stretch of north central Idaho. The Clearwater complex now represents the largest exposure of Neoarchean and Paleoproterozoic crust known in the Northwest U.S. Cordillera.

The ages of all samples analyzed to date define two discrete periods in the Neoarchean and Paleoproterozoic. Neoarchean rocks have ages that range from 2670 to 2651 Ma with a mean age of 2.66 Ga. These orthogneisses are broadly granitic in composition and have a very restricted range of initial Hf isotope compositions (initial ε_Hf +2 to +4). The radiogenic Hf and the lack of inheritance in the zircons demonstrate derivation of these rocks from a depleted mantle source with little, if any, input of older crustal material.

Paleoproterozoic orthogneisses are more abundant throughout the region and range from 1876 to 1837 Ma with a mean age of 1.86 Ga. These are also broadly granitic in composition but range from leucocratic granite to biotite tonalite. In contrast to Neoarchean samples, Proterozoic rocks show clear evidence for involvement of older crust in their genesis, both in terms of the existence of xenocrystic zircon cores and less radiogenic and more variable Hf isotope compositions (initial ε_Hf -8 to +8).

Collectively, the ages and Hf isotope compositions of the Clearwater complex basement indicate that it was produced by two discrete periods of magmatism of relatively short duration: the first producing juvenile crust in the Neoarchean at 2.66 Ga; the other in the Paleoproterozoic at 1.86 Ga involving a mixture of juvenile and and pre-existing Neoarchean crustal sources.