T153 Sigma Gamma Epsilon-Undergraduate Research (Posters)

Monday, 23 October 2017
9:00 AM - 6:30 PM
Halls 4EF

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

Paper number: T153-27
Program: Technical Sessions
Time: 9:00 AM – 6:30 PM

Authors

Abstract

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 Kmax fabric (n=18) suggesting vertical intrusion. The ultramafic dike preserves a Kmax 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.

Geological Society of America Abstracts with Programs. Vol. 49, No. 6

doi: 10.1130/abs/2017AM-305869

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