Abstract

The Franklinian mobile belt of Arctic Canada and Greenland was formed by plate convergence of an unknown landmass with North America during Silurian and Devonian time. Clastic sedimentary units of the thick passive- to convergent-margin sequence seen in the Franklinian mobile belt have been characterized for Nd isotopes and trace element geochemistry. Samples of Lower Cambrian to Upper Devonian sedimentary rocks from Ellesmere, Bathurst, and Melville Islands show no trends in trace elements either in space or time, but do display an abrupt shift in ϵNd values at ca. 450 Ma. Samples older than Late Ordovician have initial ϵNd values of −17 to −25, consistent with derivation from Archean to Early Proterozoic shield terrane of Greenland and Canada. Silurian and Devonian samples have less negative ϵNd values of −5 to −13, requiring detrital input from a source with a significantly younger mantle extraction age. The shift to a younger provenance occurred first in condensed deep-water sediments, whose deposition predated the arrival of voluminous clastic materials. For the massive quantities of Silurian and Devonian sedimentary rocks that are present in the Franklinian belt, the only plausible ultimate source lies in the Caledonian orogenic belt, which extends into the Arctic region along the east coast of Greenland. Sediments from these Caledonian mountains, which were probably first shed around the time of the Ordovician–Silurian boundary, were deposited in the Franklinian trough at ca. 440 to 435 Ma. As deformation in the Franklinian orogen progressed, this material was in turn recycled during Middle to Late Devonian time into the large clastic wedge of the Franklinian foreland basin. This propagated southwestward to give rise to the thick Upper Devonian Imperial Assemblage of the northern Cordilleran miogeocline, as well as to thinner Devonian clastic rocks over a wide region of the miogeocline and continental interior, at least as far south as Alberta. In all these regions, our previous work identified an anomalously young ϵNd signature similar to that seen in the post–450 Ma Franklinian rocks. Clastic material with this signature continued to dominate miogeoclinal sedimentation in Alberta until Late Jurassic time. Thus, the Franklinian orogen was a throughway for voluminous sediments derived from Caledonian mountains to reach the western side of the North American continent.

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