NEODYMIUM ISOTOPIC EVIDENCE FOR THE TECTONIC ASSEMBLY OF LATE ARCHEAN CRUST IN THE SLAVE PROVINCE, NORTHWEST CANADA

The ca. 2.7-2.5 Ga Slave Province is a "granite-greenstone" terrane comprising deformed sedimentary and subordinate volcanic belts extensively intruded by granitoid rocks. The Nd isotopic data are reported for 58 samples of supracrustal and granitoid rocks exposed along a 400 km, east-west, transect at 65-degrees-N across the structural grain of the province. Initial epsilon(Nd) values reveal distinctly different crustal sources in the eastern compared to the western parts of the province, as expected from tectonic assembly of the province through accretion of juvenile crust to older continental crust. Supracrustal sequences (ca. 2.71-2.65 Ga) from the central and eastern parts of the province have positive epsilon(Nd(I)) values (+ 0.3 to + 3.6), consistent with juvenile sources and formation remote from significantly older crust. Syn to late-deformation (ca. 2.63-2.60 Ga), mantle-derived diorites and related tonalites (type I) from the central and eastern parts of the province have similar initial epsilon(Nd) values (- 0.1 to + 2.7). In contrast, samples from the westernmost plutons, which intrude exposed pre-3.1 Ga crust have much lower epsilon(Nd(I)) values (- 1.0 to - 4.6) suggesting contamination of these magmas by older crust. The epsilon(Nd(I)) values of post-deformation granites (s.s.) (type II) also vary systematically across the province: values for granites west of longitude 110-degrees-30'W range from - 0.2 to - 5.3; those to the east range from + 0.6 to + 3.7. These data suggest mixed crustal sources dominated by Mid to Early Archean material (epsilon(Nd)-2.6 to - 17 at 2.6 Ga) for the western granitoid rocks and juvenile sources for the eastern granites. The Nd isotopic data are consistent with the geology of the province in that exposures of Mid to Early Archean crustal rocks, predating the principal 2.7-2.5 Ga orogenic event are restricted to the western part of the province. The asymmetric pattern defined by the Nd isotopic data indicates the presence of distinct crustal rocks beneath the Slave Province. Similar isotopic variations observed across Phanerozoic collisional orogens have been interpreted to reflect tectonic assembly of crust by accretion of juvenile crustal terranes to an older continental margin. This process may also have been an important mechanism in the cratonization of the Slave Province.