IS THE WINDY-MCKINLEY TERRANE A DISPLACED FRAGMENT OF WRANGELLIA? EVIDENCE FROM NEW GEOLOGICAL, GEOCHEMICAL AND GEOCHRONOLOGICAL STUDIES IN WESTERN YUKON

Windy-McKinley Terrane (WMT) in western Yukon and eastern Alaska is one of the least well understood terranes in the northern Cordillera, due to poor exposure, difficult access and limited study. It has previously been correlated with either the Cache Creek or Slide Mountain terranes. Reconnaissance studies of the age, composition, and tectonic affiliation of the WMT in western Yukon over the past six years indicate that the WMT in this area mainly comprises argillites interlayered with massive mafic flows and breccias (greenstones), all of which are intruded by large bodies of gabbro. The eastern end of the terrane consists of massive to sheared harzburgite that previous workers have concluded represents an upper mantle tectonite. The WMT appears to form a shallow, E-W trending syncline that structurally overlies lithologies correlated with the Yukon-Tanana Terrane (YTT) along its southern and northern boundaries. Gabbros and greenstones of the WMT yield very consistent N-MORB and/or BAB compositions, and one gabbro body has given a ~229 Ma U-Pb zircon/baddeleyite age. Rare limestone within the argillite package has thus far not yielded conodonts; however detrital zircons (n=60) from a quartz sandstone within the argillite package provide major age peaks at 400-600 Ma, 900-1300 Ma, 1500-2000 Ma, and 2500-2800 Ma. The absence of Late Devonian and Mississippian zircons, and the abundance of 400-600 Ma grains suggests an affiliation with the Alexander Terrane rather than the YTT or North American miogeocline. The greenstones are thought to be extrusive equivalents of the Late Triassic gabbros. Geochemical signatures of greenstones and gabbros from the WMT are generally similar to those for Nikolai Group volcanic rocks of Wrangellia that now lie to the southwest across the Denali Fault. Existing data therefore suggests that the WMT may represent a flap of Wrangellia (mainly equivalent to the Nikolai greenstone and associated sedimentary and intrusive rocks) that was thrust across the future trace of the Denali Fault during mid-Mesozoic terrane amalgamation. Additional geochronology of igneous rocks and detrital zircon studies within WMT and Wrangellia are now underway to further test this hypothesis.