U-PB GEOCHRONOLOGY OF AN ARCHEAN SERPENTINIZED HARZBURGITE, SOUTHERN BIGHORN MOUNTAINS, WYOMING

The Archean core of the Laramide Bighorn Mountain uplift of Wyoming is among the largest and least studied Archean areas in North America. The Archean rocks of the Bighorn uplift can be divided into an older southern quartzofeldspathic gneiss terrane (~2950 Ma) and a younger (~2850-2950 Ma) northern “undeformed” granite-granodiorite batholith. The southern gneiss terrane is comprised largely of tonalite gneiss with lesser amphibolite and late mafic dikes. Ultramafic rocks in the Wyoming province are rare, and have been reported as xenoliths in granitic and gneissic bodies and as part of the ~2700 Ma Stillwater layered mafic intrusion in the Beartooth Mountains. A unique serpentinized harzburgite intrusion occurs along Highway 16 about 15 km west of Buffalo, Wyoming. The harzburgite is composed of orthopyroxene (bronzite), serpentine and accessory Fe-oxides with little outcrop fabric. It is texturally and mineralogically homogeneous. It is about 100 km in diameter and circular in map view, and it discordantly intrudes the host gneiss. LA-ICPMS U-Pb ages of zircons (n=12) extracted from this harzburgite yield a Concordia age of 2926 ± 11 Ma (MSWD = 5.5), and a weighted mean average age of 2919 ± 10 Ma (MSWD = 3.6). These zircons are all >90% concordant, euhedral, dark brown in color, and had U concentrations that ranged from ~50-300 ppm. Because of the lack of a deformational fabric, the harzburgite intruded after the main phase of deformation of the southern gneiss terrane. The harzburgite intrusion was broadly coeval with the emplacement of the earliest phases of the Bighorn batholith, the nearest exposures of which occur 20-30 km to the north.