Continental Influence on Canadian Cordilleran Terranes From Nd Isotopic Study, and Significance For Crustal Growth Processes

Abstract

Nd isotopic data are presented for rocks of the terrane assembly that lies inboard of the Stikine terrane in the Canadian Cordillera of British 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 was closely tied to that of the North American continental margin out to far‐traveled oceanic terranes. The results emphasize sedimentary rocks as indicators of tectonic position of the crustal fragments. Sedimentary rocks of the Cassiar and Kootenay terranes show a strong connection to miogeoclinal sediment sources. Argillites of Pennsylvanian‐Permian age from a paleontologically controlled section in the Slide Mountain terrane are also consistent with sediment sources in the North American miogeocline. Igneous rocks of the Slide Mountain, Quesnel, and Cache Creek terranes show juvenile oceanic or arc origins based on ϵNd values between +3 and +10, and are essentially identical with published results. Cache Creek and Quesnel terranes also contain sediments with positive ϵNd values, suggesting a juvenile, ultimately volcanogenic, origin. Both terranes, however, also contain some Triassic and apparently Pennsyl‐vanian‐Permian sedimentary rocks with negative ϵNd values between –5 and –7, like those of Devonian to Jurassic sedimentary rocks of the North American miogeocline. Possible explanations include proximity to sources of North American terrigenous sediment, expected in Triassic time, or very far‐traveled fine‐grained sediment in the form of hemipelagic clay or eolian dust for older samples. The addition of a continental sedimentary component to wide areas of the Cordillera represents an important point of comparison to Proterozoic orogens, where this component affected the isotopic signatures, but usually cannot be separately identified due to intense reprocessing during orogenesis.