INTERPRETING SUBDUCTION POLARITY FROM ECLOGITE-BEARING SLICES IN ACCRETIONARY OROGENS—A CAUTIONARY NOTE FROM THE YUKON–TANANA TERRANE

The assumption that eclogites form in the down-going slab of subduction zones is commonly used to establish subduction polarity in accretionary orogens; however, tectonic erosion can produce high- and ultrahigh-pressure metamorphism of the overriding plate. Subduction erosion is a notoriously difficult process to recognize in the rock record, but it may play a fundamental—and underappreciated—role in HP metamorphism of continental crust. A potential candidate for HP metamorphism in a tectonic erosional setting is the Yukon-Tanana terrane (YTT) of the northern Cordillera. The YTT can be viewed as a crustal fragment that rifted from the Laurentian margin and subsequently developed a continental arc with 3 distinct volcanic successions in Late Devonian to Permian times. YTT experienced subduction on both sides before it was accreted back on to North America in the Mesozoic. Mississippian eclogites in YTT are Franciscan-type mélanges, but Permian eclogites embedded in quartzofeldspathic host rocks are together interpreted as coherent slices of continental crust. E.g., eclogites in the St. Cyr area form tabular bodies up to hundreds of meters long within felsic meta-igneous and metasedimentary rocks; eclogite and host rock share the same deformation fabrics, and white mica from the host rocks has high phengite content (3.30-3.48 Si p.f.u.) that signals HP metamorphism. New detrital zircon data from the host rocks confirm that the eclogite-bearing crust is indeed part of the YTT. A sample from Last Peak has a detrital zircon signature with Meso- and Paleoproterozoic peaks typical of the Snowcap assemblage, the Laurentian substrate of YTT. Five samples (1 from Ross River and 4 from St. Cyr) contain Precambrian, mostly Mesoproterozoic, zircons and have significant Paleozoic peaks that match the ages of volcanoclastic sedimentation of the YTT arc. The coherent slices of crust containing Permian eclogites are thus interpreted as pieces of YTT that were eroded from the arc during subduction of an oceanic lithosphere and re-accreted to the arc prior to the collision of YTT with Laurentia. A subduction erosion origin for eclogite formation challenges the assumption that all HP metamorphism forms in the subducting slab, and that subduction polarity in accreted terranes can be deduced from the mere presence of HP rocks.