PALEOZOIC MAKE-UP OF WRANGELLIA AND ITS RELATIONSHIPS WITH THE ALEXANDER TERRANE: IMPLICATIONS FOR NORTHERN CORDILLERAN TECTONIC EVOLUTION

Wrangellia makes up much of the northwestern margin of North America and has long been considered exotic to the northern Cordillera. Original descriptions of Wrangellia were based primarily on Triassic strata. However, little attention was given to the Paleozoic underpinnings of Wrangellia. New stratigraphic data and ages from southwest Yukon and Vancouver Island indicate similar Paleozoic geology underlying the Triassic strata. Furthermore new geological evidence from Paleozoic rocks in Yukon suggests that Wrangellia and the Alexander terrane evolved together and that Alexander partially forms the basement of Wrangellia.

Large ~363 Ma gabbro complexes have non-arc geochemical signatures and intrude both terranes in southwest Yukon. Early Mississippian (~352 Ma) volcanic rocks near the base of northern Wrangellia have back-arc to N-MORB geochemical signatures whereas Late Devonian through Early Mississippian volcanic rocks and subvolcanic intrusions in southern Wrangellia have predominately arc geochemical signatures. At higher stratigraphic levels, northern Wrangellia contains abundant Mississippian to Pennsylvanian arc volcanic and volcaniclastic rocks. Similar-aged arc/back-arc rocks interpreted to be the southern extension of the Skolai arc are found in southern Wrangellia.

We propose that the Late Devonian gabbros in southwest Yukon and southeast Alaska represent the initiation of extension through an arc located at the margin of Alexander. Extension progressed to the point of depositing basalts within a back-arc basin setting. Subduction reversal closed the back-arc basin and rejuvenated the northern Wrangellia arc in the Pennsylvanian leading to collision with Alexander. As previous workers identified, Pennsylvanian plutons emplaced after collision intrude the basement units of both terranes in eastern Alaska, southwest Yukon and Haida Gwaii. Subsequent exhumation of northern Wrangellia led to the deposition of Lower Permian conglomerates unconformably on top of the gabbro complexes.