Late Archean mantle composition and crustal growth in the Western Superior Province of Canada: Neodymium and lead isotopic evidence from the Wawa, Quetico, and Wabigoon subprovinces

Neodymium and lead isotopes are presented for late Archean rocks from the Wawa, Quetico, and Wabigoon subprovinces (Sp) in the Western Superior Province (Ontario, Canada). The isotopic compositions were determined on greenstone volcanic sequences, pre-tectonic (2.73-2.69 Ga) trondhjemite-tonalite-granodiorite (TTG) suites, metasedimentary rocks, and post-tectonic (2.69-2.67 Ga) dioritic to granitic plutons in order to characterize the mantle and crustal reservoirs involved in the evolution of the southern part of the Western Superior Province. Although derived from a depleted mantle, almost all the greenstone volcanism and pre-tectonic TTG suites record contamination by crustal material as old as 3.2 Ga. Neodymium and lead isotopic compositions of the pre-tectonic TTG can be modeled as products of the melting of amphibolitic crust (e.g., subducted basalts) contaminated by 1-10% of subducted sediments. Moreover, lead isotopes of TTG suites provide evidence that the isotopic compositions of these subducted sediments varied as a result of the addition of new ca. 2.7 Ga juvenile material, in agreement with neodymium isotopic differences recorded by sedimentary units of the Quetico Sp (50:50 mix of young:old crusts) and Wawa Sp (75:25 young:old). Neodymium and lead isotopes of the post-tectonic REE-enriched Archean sanukitoid suites define an isotopic trend which is distinct from that of the TTG suites and which is interpreted to reflect the importance of crustal assimilation in the evolution of these suites. Furthemore, such plutons found in the Wawa Sp have more juvenile neodymium and lead isotopic compositions and higher Nd contents than those intruded into the Quetico and Wabigoon Sp which have more crustal isotopic signatures and lower Nd concentrations. This geographical difference could be due either to differences in Nd contents of initial Archean sanukitoid magmas, lesser assimilation in the Wawa Sp or, more probably, because the Wawa crust is on average more juvenile than the Quetico and Wabigoon crusts in agreement with the fact that the geographical difference mirrors that recorded by metasedimentary rocks deposited in Quetico and Wawa Sp. Copyright (C) 1998 Elsevier Science Ltd.