PALEOMAGNETISM OF AN ARCHEAN HARZBURGITE, BIGHORN MOUNTAINS, WYOMING, USA

A circular harzburgite intrusion (1 km diameter) intrudes 3000 Ma gneisses in the hangingwall of the Laramide Bighorn uplift in Wyoming. The harzburgite is composed of pristine orthopyroxene (bronzite), clinopyroxenite, olivine and accessory Fe-oxides with little outcrop fabric other than joints. A preliminary U-Pb zircon (n=12) Concordia age of 2926 ± 11 Ma (MSWD = 5.5), and a weighted mean average age of 2919 ± 10 Ma (MSWD = 3.6), has been determined. The harzburgite is crosscut by a hydrothermally altered ultramafic dike (N20°E, 90°, 1 meter wide) with no zircons recovered. Anisotropy of magnetic susceptibility (AMS) was used as a proxy for magmatic intrusion and the harzburgite preserves a sub-vertical K_max fabric (n=18) suggesting vertical intrusion. The ultramafic dike preserves a K_max fabric (n=19) that plots along the great circle of the dike and is difficult to interpret. Alternating Field (AF) demagnetization for the harzburgite yielded a paleopole of 177.7 longitude, -14.4 latitude (α₉₅=3.4). There is a two-component magnetization preserved in the harzburgite which suggests a younger Cretaceous chemical overprint that may indicate a 90° clockwise vertical axis rotation of the Clear Creek thrust hangingwall, a range-bounding, east-directed thrust fault responsible for uplifting the Bighorn Mountains in Eocene the Laramide Orogeny.