Abstract

Nd-isotopic compositions and Sm, Nd, Li, K, Rb, Sr, Ba, Ni, and Cr abundances are reported for 25 shale samples from the Canadian Shield (late Archean Abitibi greenstone belt and the mid-Proterozoic Labrador Trough) and from the Quebec Appalachians (lower Paleozoic Humber Zone). The chemical and isotopic characteristics of the samples are used to monitor the rate of generation and the compositional evolution of continental crust through time. The Nd crustal-residence ages record preferential time of continental growth around 2.7 and 1.7 Ga. The Nd model ages of the Appalachian shales do not record evidence for the formation of large crustal volumes through mantle extraction since 1.3 Ga. Consequently, crustal recycling was the dominant process taking place at their source areas in the Grenville Province.The trace-element distributions of shales show systematic trends as a function of time: Li, K, Ba, Sm, and Nd contents regularly increase in the post-Archean record; in comparison, the Cr and Ni contents reached a maximum towards the end of the Archean and regularly decreased thereafter. These observations could reflect two classes of processes: (a) the development of infracrustal K-rich granitoid magmatism at the expense of mantle-derived Na-rich magmatism, which dominated the Archean period; or (b) differential erosion effects, which reduced the sampling of the old, smooth crustal parts in comparison to the younger, recycled segments. In the latter case, (b), shale formation involved components whose nature and respective proportion changed after Archean time.

You do not currently have access to this article.