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