Nd- and Sr-Isotope Evidence for Proterozoic and Paleozoic Crustal Evolution in the Brooks Range, Northern Alaska

Abstract

Exposures of Proterozoic crust in Alaska are rare; however, isotopic data from younger rocks derived from unexposed basement suggest that Proterozoic (or older) crust may underlie large areas of northern Alaska. Late Proterozoic orthogneisses, Late Proterozoic to Cambrian metasedimentary rocks, and Devonian granitic orthogneisses in the western Brooks Range have Nd-and Sr-isotope compositions consistent with derivation, at least in part, from Middle to Early Proterozoic crust. Proterozoic metasedimentary rocks have calculated Nd crustal residence ages of 2.0 Ga and are intruded by Late Proterozoic metagranites with somewhat more radiogenic initial isotopic compositions than the metasedimentary rocks at the time of intrusion. Initial isotopic compositions of Devonian metagranitic rocks range from . The least radiogenic values are compatible with nearly pure crustal melts of a source with isotopic composition similar to what exposed Proterozoic crust, as sampled by the metasedimentary rocks and Late Proterozoic metagranites, would have had at 370 Ma. The range of observed isotopic compositions may be explained by mixing mantle-derived magmas, or melts of juvenile crust, with melts of Proterozoic crust in varying proportions. Nd isotope systematics of the Late Proterozoic metasedimentary rocks are identical to reported values from Early Proterozoic metaplutonic rocks in the Idono Complex and Kilbuck terrane in SW Alaska, and to clastic sedimentary rocks from the Belt Supergroup in Montana. Isotopic compositions of sampled metagranitic and metasedimentary rocks are not compatible with derivation exclusively from crust with the age and isotope characteristics of the Yukon-Tanana terrane. Very limited isotopic data from other continental domains within North America leave open the possibility of affinity with crust exposed in NW Canada, the Canadian Arctic Islands, or the NW Canadian Cordillera.