Paleozoic and Mesozoic rocks of Stikinia exposed in northwestern British Columbia: Implications for correlations in the northern Cordillera
Stikinia is a tectonostratigraphic terrane in the Canadian Cordillera that formed in a volcanic arc environment during Paleozoic and Mesozoic time. Until now, the Paleozoic rocks that form a semicontinuous belt along its western margin (Stikine assemblage) were only recognized in a restricted area in northern British Columbia, between the Stikine and Taku river areas. In contrast, Mesozoic Stikinia rocks form an almost continuous belt that extends much farther to the north, leading some authors to question the nature of the unexposed Paleozoic basement north of the Taku River area. The following correlations have significant implications for tectonic reconstructions of the northern Cordillera because they suggest that Stikinia9s Paleozoic volcanic-sedimentary basement is more widespread than previously thought. On the basis of similar rock types and lithologic associations, six new U-Pb zircon dates, and the common intrusive relationship with 184–195 Ma plutons, the Stikine assemblage is correlated with the Boundary Ranges suite, a metamorphosed Paleozoic volcanic assemblage exposed in the Tagish Lake area, north of the Taku River and south of the Yukon–British Columbia border. The recognition of the Boundary Ranges suite and the Jurassic plutons that intruded it (Tagish Lake suite) as part of Stikinia has implications for the age and character of the Stikinia–Tracy Arm terrane boundary because the Boundary Ranges and Tagish Lake suites form the footwall of a major Middle Jurassic shear zone that carried the continental margin–like rocks of the Tracy Arm terrane in its hanging wall. This correlation also implies that the late Paleozoic basement to the Mesozoic Stikinia arc is not a continental margin assemblage, at least as far north as the British Columbia–Yukon border, and possibly farther. The Boundary Ranges suite, and therefore the Stikine assemblage, are also tentatively correlated with parts of the Yukon-Tanana terrane in Yukon (Aishihik Lake area), parts of the Taku terrane in southeast Alaska, and undivided metamorphic rocks in west-central British Columbia. Differences in the isotopic signatures of these rocks may reflect along-strike changes in the character of the basement rocks of the late Paleozoic Stikinia volcanic arc.