ECLOGITE FORMATION BY SUBDUCTION EROSION OF THE OVERRIDING PLATE

Over half of the active subduction zones on Earth are experiencing tectonic erosion in a process whereby parts of the overriding plate are removed and transported down the subduction channel. Subduction erosion is a notoriously difficult process to recognize in the geologic record, but it may play a fundamental – and underappreciated – role in eclogite facies metamorphism of continental crust. In addition, subduction erosion provides a mechanism to develop high- and ultrahigh-pressure metamorphism in the overriding plate of collisional orogens. One potential candidate for HP metamorphism in an erosional setting is the Yukon-Tanana terrane of the Canadian Cordillera. The Yukon Tanana terrane has been interpreted as a continental arc built on a peri-Laurentian substrate that experienced subduction on both its west and east sides prior to accretion to North America in the Mesozoic. The arc is the overriding plate to subduction of oceanic crust in current models. Although some of the Mississippian eclogites in Yukon-Tanana are clearly Franciscan-type melanges, Permian eclogites are interpreted as coherent pieces of arc crust because they are interleaved with their quartzofeldspathic host rocks on a sub-meter scale and both lithologies share the same fabrics, which is suggestive of pre-metamorphic intrusive contacts. These Yukon HP rocks are the medium-temperature type, formed at 550-700 °C, and thus too hot to form in a typical accretionary wedge. Exhumation rates also appear to be too fast for Pacific-type eclogite; they are more on the order of the cm/year rates of continental subduction zones. These anomalous features suggest that subduction erosion is a possible mechanism for formation of the central Yukon eclogites, and one that can explain their origin in the overriding plate. A subduction erosion model for eclogite challenges the assumptions 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.