Nd isotopes and the source of sediments in the miogeocline of the Canadian Cordillera

Nd isotopes in clastic 600-75 Ma sedimentary rocks from the miogeocline of Alberta and British Columbia are used to (1) constrain the importance of proximal and distal North-American basement and juvenile Cordilleran sedimentary sources; (2) document large-scale changes in provenance; and (3) develop a North American miogeoclinal reference for Nd isotopes as a baseline for continental input to Cordilleran terranes. Initial ∈_Nd for Neoproterozoic to Upper Ordovician samples range from -14 to -22 and can be explained by derivation from proximal Precambrian basement. A positive shift of six ∈_Nd units occurs between Late Ordovician and Late Devonian time and persists until foreland basin formation in the Late Jurassic. ∈_Nd for this 370-170 Ma period ranges from -6 to -9 and requires involvement of a more juvenile source. Cretaceous foreland basin sediments show extreme heterogeneity, with ∈_Nd from zero to -12. High values can be explained by incorporation of volcanic detritus, either from Triassic and Jurassic units in the Quesnellia terrane, or as airborne material from magmatic activity in the Coast Belt of the Cordillera. Sediments with negative ∈_Nd may have come from Proterozoic or lower Paleozoic miogeoclinal sediments of the fold and thrust belt, or from the Omineca Belt. Three hypotheses can explain this Ordovician-Devonian ∈_Nd shift. First, the detritus could represent a mix of Cordilleran juvenile components plus Precambrian basement. The ∈_Nd shift is approximately coeval with Devono-Mississippian igneous and tectonic activity along the western margin of North America. Any western juvenile sources should have ceased to supply sediments by mid-Mississippian time, so this hypothesis requires sufficient detritus in Devono-Mississippian time so that cannibalistic recycling maintained the same isotopic signature until the Late Jurassic. Second, the detritus was possibly transported across the craton from Appalachian sources, maybe a multi-step journey. The Appalachian belt (which includes Grenville-age basement and sediments ultimately derived from it) provides the appropriate isotopic signature. It was the main mountain belt in North America during most of the Paleozoic and was a major source of detritus to the craton. Third, detritus may have come from the Innuitian Orogen in the Canadian Arctic. A large amount of clastic material was shed southward from the Innuitian Orogen in mid-Late Devonian time. This hypothesis requires that sediment delivered from the North in Devono-Mississippian time was cannibalistically recycled in the miogeocline until Late Jurassic time; however, isotopic signatures of Innuitian Belt rocks and of sediments derived from them are presently unknown.