Rates of thermal equilibration at the onset of subduction deduced from diffusion modeling of eclogitic garnets, Yukon-Tanana terrane, Canada

International audienceWell-preserved eclogitic rocks near Faro (Yukon-Tanana terrane, Canada) underwent a prograde evolution from ∼510 °C and ≥ 1.1 GPa to 690 °C and ≥ 1.5 Gpa followed by near-isobaric cooling to ∼540 °C. A remarkable feature of the garnet porphyroblast cores is the presence of minute garnet “inclusions” of distinctly different composition. Preservation of a sharp compositional gradient at the interface between the host and the inclusion garnets and results of diffusion modeling indicate that the counterclockwise pressure-time evolution took place on a very short time scale of about 0.2 m.y. Minimum rates of heating (950 °C/m.y.) and burial (7 cm/yr) calculated along the prograde part of the path are in good agreement with values extracted from thermal models of newly initiated subduction zones

GSA Bulletin: Structural and kinematic evolution of the Yukon-Tanana upland tectonites, east-central Alaska: A record of late Paleozoic to Mesozoic crustal assembly: Discussion and Reply: Discussion

We welcome the efforts of Hansen and Dusel-Bacon (1998) in synthesizing sparse and apparently inconsistent structural data sets from widely separated areas of the poorly exposed Yukon-Tanana Uplands. However, we consider that credible extrapolation of such a synthesis southward to include a 300 × 800 km region of the Canadian Cordillera is unwarranted. We clarify here points of terrane definition and discuss the implications for tectonic models for the Yukon-Tanana terrane in the Canadian Cordillera. TERRANE AND ASSEMBLAGE CLARIFICATIONS The emplacement of allochthonous terranes (upper plate) over parautochthonous North America (lower plate) has been known for decades from the recognition of obvious klippen of Slide Mountain and YukonTanana terranes resting on North American strata (e.g., Sylvester allochthon and St. Cyr and Stewart Lake klippen, The metamorphic-structural focus of the Hansen and Dusel-Bacon study diverts attention from the fundamental geological criteria of terrane definition: rock types and protoliths, ages and contact relationships. Ideas of Hansen and Dusel-Bacon (1998) appear to be an outgrowth of those presented b