GEOCHRONOLOGY FROM A CONTACT METAMORPHIC ASSEMBLAGE NEAR THE EDGE OF THE WRANGELLIA COMPOSITE TERRANE, TALKEETNA MOUNTAINS, ALASKA

Three new U-Pb zircon ICP-MS dates (63.0±2.1 Ma; 66.1±2.6 Ma; 68.3±2.6 Ma) from granodiorite plutons in south central Alaska constrain the timing of intrusion, deformation, and metamorphism in the western Kahiltna basin to late Cretaceous and early Paleocene. Mineral stretching lineations in the country rock and shape preferred orientation lineations in the plutons plunge gently (~25°) to the southwest. The plutons have magmatic foliations parallel to the foliation in the country rock suggesting that the plutons crystallized during regional deformation. The country rocks near the plutons contain sillimanite, garnet ± staurolite, and those farther from plutons contain andalusite ± garnet. Sillimanite helps define the mineral lineation present in the country rocks and the foliation is deflected, and is overgrown by andalusite suggesting that metamorphic mineral growth was synchronous with deformation. Geothermobarometry from a low variance mineral assemblage from the garnet + andalusite zone yields 550°C and 290 MPa.

The relatively low pressure implied by the presence of andalusite and supported by geothermobarometry show that these plutons were intruded at relatively shallow levels (~10 km). In addition, the presence of shallowly plunging mineral stretching lineations suggest that most of the deformation associated with pluton emplacement and final suturing of the Wrangellia Composite Terrane to North America was accommodated by strike-slip motion in the western part of the Kahiltna basin.