Textural relationships and the trace element chemistry of accessory minerals and garnet can provide the linkage between in situ SHRIMP ages and quantitative pressure-temperature data that is required to decipher complex polymetamorphic and polydeformational histories. Application of these methods to lower amphibolite facies rocks of the Stewart River area, Yukon (Canada) yields robust new constraints on the tectonic evolution of central Yukon Tanana Terrane (YTT). A TIMS U/Pb titanite age of 365-350 Ma is interpreted to date low-P metamorphism (M1) and D1 deformation associated with arc plutonism above an east-dipping subduction zone. Monazite inclusions in garnet porphyroblasts record a transition from low to high pressure ($9 kbar and 600°C) at c. 239 Ma. These data help to establish a c. 260-240 Ma tectonometamorphic event (M2-D2) reflecting intra-arc thickening during west-dipping subduction of Slide Mountain Ocean. Another transition from low-to high-P (M3-D3; 7.8 kbar and 595°C), dated by c. 195-187 Ma monazite, is interpreted to reflect the change from regional contact metamorphism during arc plutonism to internal duplication of YTT during initial collision of YTT with the North American craton. The Mt Burnham (north-eastern) region records a different history because of its proximity to later plutons and its late exhumation via extensional faulting. Monazite growth at 146 Ma dates $9 kbar metamorphism (M4), interpreted to reflect a previously unrecognized period of plutonism associated with auriferous quartz veins in the Klondike region. Monazite growth at 114-107 Ma reflects low-P (<4.6 kbar) contact metamorphism (M5) accompanying regional plutonism and extension.

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Amphibolite facies metasedimentary schists within the Yukon-Tanana terrane in the northern Canadian Cordillera reveal a two-stage, polymetamorphic garnet growth history. In situ U-Th-Pb Sensitive High Resolution Ion Microprobe dating of monazite provide timing constraints for the late stages of garnet growth, deformation and subsequent decompression. Distinct textural and chemical growth zoning domains, separated by a large chemical discontinuity, reveal two stages of garnet growth characterized in part by: (i) a syn-kinematic, inclusion-rich stage-1 garnet core; and (ii) an inclusion-poor, stage-2 garnet rim that crystallized with syn-to post-kinematic staurolite and kyanite. Phase equilibria modelling of garnet molar and compositional isopleths suggest stage-1 garnet growth initiated at 600°C, 8 kbar along a clockwise P-T path. Growth of the compositionally distinct, grossular-rich, pyrope-poor inner portion of the stage-2 overgrowth is interpreted to have initiated at higher pressure and/or lower temperature than the stage-1 core along a separate P-T loop, culminating at peak P-T conditions of~650-680°C and 9 kbar. Stage-2 metamorphism and the waning development of a composite transposition foliation (S T ) are dated at c. 118 Ma from monazite aligned parallel to S T, and inclusions in syn-to post-S T staurolite and kyanite. Slightly younger ages (c. 112 Ma) are obtained from Y-rich monazite that occurs within resorbed areas of both stage-1 and stage-2 garnet, together with retrograde staurolite and plagioclase. The younger ages obtained from these texturally and chemically distinct grains are interpreted, with the aid of phase equilibria calculations, to date the growth of monazite from the breakdown of garnet during decompression at c. 112 Ma. Evidence for continued near-isothermal decompression is provided by the presence of retrograde sillimanite, and cordierite after staurolite, which indicates decompression below~4-5 kbar prior to cooling below 550°C. As most other parts of the Yukon-Tanana terrane were exhumed to upper crustal levels in the Early Jurassic, these data suggest this domain represents a tectonic window revealing a much younger, high-grade tectono-metamorphic core (infrastructure) within the northern Cordilleran orogen. This window may be akin to extensional core complexes identified in east-central Alaska and in the southeastern Canadian Cordillera.

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Development of amphibolite-facies transposition fabrics in the northern Canadian Cordilleran hinterland occurred diachronously in the Permian-Triassic, Early Jurassic, Middle Jurassic to Early Cretaceous, and Early to mid-Cretaceous. Rocks tectonized in the Permian-Triassic and Early Jurassic were exhumed in the Early Jurassic, while rocks immediately to the northeast (toward the foreland) were not buried and heated until the Middle Jurassic to mid-Cretaceous. Early Jurassic to mid-Cretaceous emplacement of the Yukon-Tanana terrane on the North American continental margin, together with the imbrication of parautochthonous rocks, formed a foreland-propagating orogenic wedge. Cooler rocks in front of the wedge were progressively buried and metamorphosed to amphibolite facies from the Jurassic to mid-Cretaceous as they were underthrust into a spatially and temporally transient distributed ductile shear zone near the base of the overriding wedge. Rocks previously incorporated into this zone were displaced upward and exhumed through the combined effects of renewed underplating at depth and compensating extensional and erosional denudation above to maintain a critically tapered wedge. Extensional exhumation of the metamorphic hinterland in the mid-Cretaceous marked the collapse and end of orogen-perpendicular wedge dynamics in operation since the Early Jurassic. Rocks incorporated into the midcrustal shear zone in the Middle Jurassic to mid-Cretaceous were exhumed in the mid-Cretaceous along southeast-directed (orogen-parallel) extensional faults from beneath a supracrustal "lid" tectonized in the Permian-Triassic and Early Jurassic. Like the Himalayan orogen and eastern Alps, orogen-parallel extension developed in an orthogonal plate-convergent setting, simultaneous with, and bounded by, orogen-parallel strike-slip faulting that facilitated northwestward lateral extrusion of rocks normal to the direction of convergence.