ND ISOTOPES AND THE SOURCE OF SEDIMENTS IN THE MIOGEOCLINE OF THE CANADIAN CORDILLERA

Nd isotopes in elastic 600-75 Ma sedimentary rocks from the miogeoclin e of Alberta and British Columbia are used to iii constrain the import ance of proximal and distal North-American basement and juvenile Cordi lleran sedimentary sources; (2) document large-scale changes in proven ance; and (3) develop a North American miogeoclinal reference for Nd i sotopes as a baseline for continental input to Cordilleran terranes. I nitial epsilon(Nd) for Neoproterozoic to Upper Ordovician samples rang e from -14 to -22 and can be explained by derivation from proximal Pre cambrian basement. A positive shift of six epsilon(Nd) units occurs be tween Late Ordovician and Late Devonian time and persists until forela nd basin formation in the Late Jurassic. epsilon(Nd) for this 370-170 Ma period ranges from -6 to -9 and requires involvement of a more juve nile source. Cretaceous foreland basin sediments show extreme heteroge neity, with epsilon(Nd) from zero to -12. High values can be explained by incorporation of volcanic detritus, either from Triassic and Juras sic units in the Quesnellia terrane, or as airborne material from magm atic activity in the Coast Belt of the Cordillera. Sediments with nega tive epsilon(Nd) may have come from Proterozoic or lower Paleozoic mio geoclinal sediments of the fold and thrust belt, or from the Omineca B elt. Three hypotheses can explain this Ordovician-Devonian epsilon(Nd) shift. First, the detritus could represent a mix of Cordilleran juven ile components plus Precambrian basement. The epsilon(Nd) shift is app roximately coeval with Devono-Mississippian igneous and tectonic activ ity along the western margin of North America. Any western juvenile so urces should have ceased to supply sediments by mid-Mississippian time , so this hypothesis requires sufficient detritus in Devono-Mississipp ian time so that cannibalistic recycling maintained the same isotopic signature until the Late Jurassic. Second, the detritus was possibly t ransported across the craton from Appalachian sources, maybe a multi-s tep journey. The Appalachian belt (which includes Grenville-age baseme nt and sediments ultimately derived from it) provides the appropriate isotopic signature. It was the main mountain belt in North America dur ing most of the Paleozoic and was a major source of detritus to the cr aton. Third, detritus may have come from the Innuitian Orogen in the C anadian Arctic. A large amount of elastic material was shed southward from the Innuitian Orogen in mid-late Devonian time. This hypothesis r equires that sediment delivered from the North in Devono-Mississippian time was cannibalistically recycled in the miogeocline until Late Jur assic time; however, isotopic signatures of Innuitian Belt rocks and o f sediments derived from them are presently unknown.