CONTINENTAL INFLUENCE ON CANADIAN CORDILLERAN TERRANES FROM ND ISOTOPIC STUDY, AND SIGNIFICANCE FOR CRUSTAL GROWTH-PROCESSES

Nd isotopic data are presented for rocks of the terrane assembly that lies inboard of the Stikine terrane in the Canadian Cordillera of Brit ish Columbia and Yukon. These are, from most inboard outward: Cassiar, Kootenay, Slide Mountain, Quesnel, and Cache Creek terranes. They are regarded as documenting a transition from terranes whose evolution wa s closely tied to that of the North American continental margin out to far-traveled oceanic terranes. The results emphasize sedimentary rock s as indicators of tectonic position of the crustal fragments. Sedimen tary rocks of the Cassiar and Kootenay terranes show a strong connecti on to miogeoclinal sediment sources. Argillites of Pennsylvanian-Permi an age from a paleontologically controlled section in the Slide Mounta in terrane are also consistent with sediment sources in the North Amer ican miogeocline. Igneous rocks of the Slide Mountain, Quesnel, and Ca che Creek terranes show juvenile oceanic or are origins based on epsil on(Nd) values between +3 and +10, and are essentially identical with p ublished results. Cache Creek and Quesnel terranes also contain sedime nts with positive epsilon(Nd) values, suggesting a juvenile, ultimatel y volcanogenic, origin. Both terranes, however, also contain some Tria ssic and apparently Pennsylvanian-Permian sedimentary rocks with negat ive epsilon(Nd) values between -5 and -7, like those of Devonian to Ju rassic sedimentary rocks of the North American miogeocline. Possible e xplanations include proximity to sources of North American terrigenous sediment, expected in Triassic time, or very far-traveled fine-graine d sediment in the form of hemipelagic clay or eolian dust for older sa mples. The addition of a continental sedimentary component to wide are as of the Cordillera represents an important point of comparison to Pr oterozoic orogens, where this component affected the isotopic signatur es, but usually cannot be separately identified due to intense reproce ssing during orogenesis.