PALEOMAGNETIC AND GEOBAROMETRIC STUDY OF THE MIDCRETACEOUS WHITEHORSEPLUTON, YUKON-TERRITORY

This is the first of several Lithoprobe paleomagnetic studies underway to examine geotectonic motions in the northern Canadian Cordillera. E xcept for one controversial study, estimates for terranes underlying t he Intermontane Belt in the Yukon have been extrapolated from studies in Alaska, southern British Columbia, and the northwestern United Stat es. The Whitehorse Pluton is a large unmeramorphosed and undeformed to nalitic body of mid-Cretaceous age (similar to 112 Ma) that was intrud ed into sedimentary units of the Whitehorse Trough in the Stikinia ter rane. Geothermobarometric estimates for eight sites around the pluton indicate that postmagnetization tilting has been negligible since cool ing through the hornblende-crystallization temperature and that the pl uton is a high-level intrusion. Paleomagnetic measurements for 22 of 2 4 sites in the pluton yield a well-defined characteristic remanent mag netization (ChRM) direction that is steeply down and northwards. The C hRM direction gives a paleopole of 285.5 degrees E, 81.7 degrees N (d( p) = 5.3 degrees m d(m) = 5.7 degrees). When compared with the 112 Ma reference pole for the North American craton, this paleopole suggests that the northern Stikinia terrane has been translated northwards by 1 1.0 +/- 4.8 degrees (1220 +/- 530 km) and rotated clockwise by 59 +/- 17 degrees. Except for an estimate from the similar to 70 Ma Carmacks Group volcanics, this translation and rotation estimate agrees well wi th previous estimates for units in the central and southern Intermonta ne Belt. They suggest that the terranes of the Intermontane Belt have behaved as a fairly coherent unit since the Early Cretaceous, moving n orthward at a minimum average rate of 2.3 +/- 0.4 cm/a between similar to 140 and similar to 45 Ma.