A PRELIMINARY ASSESSMENT OF THE AGE AND TECTONIC AFFINITY OF BASEMENT ROCK IN THE BEAVER DAM MOUNTAINS, UTAH

The Beaver Dam Mountains in extreme SW Utah expose ~50 km² of basement rock, including paragneiss, orthogneiss, para-amphibolite, intrusive rocks (intermediate to felsic) and pegmatite. In order to maximize information on the age and tectonic history of these rocks, a laser-ablation ICP-MS U/Pb study of three samples (garnet paragneiss, tonalitic orthogneiss, and leucogranite) was conducted.

Not surprisingly, mean ages of 1739±8.4 Ma and 1728±4.8 were obtained for paragneiss and orthogneiss, respectively, and are concordant with many rocks of the inner gorge of the Grand Canyon and small basement blocks in the Mineral Mountains ~175 km to the NE. A few grains with older apparent ages are found in both samples. In paragneiss, three apparent ²⁰⁷Pb/²⁰⁶Pb ages between 1800 and 1900 Ma occur, whereas orthogneiss has three apparent ²⁰⁷Pb/²⁰⁶Pb ages between 1760 and 1790 Ma. Zircons from leucogranite were largely discordant, but exhibit a node at ~1730 Ma.

Recent work¹ on the Santaquin Complex (~365 km NE) suggests that plutonism and peak metamorphism at Santaquin preceeded 1673±23Ma. ²⁰⁷Pb/²⁰⁶Pb ages on monazite, a prograde chronometer, from the Farmington Canyon Complex (FCC, ~475 km NE) range from 1644-1711 Ma, although a large orthogneiss body from the northern FCC has an age of ~1800 Ma. If the interpretation that older (>2.0 Ga) discordant FCC zircons are detrital or inherited is correct, then there may be no exposures of basement rock older than 1840 Ma (Elves Canyon pluton) in the 5 basement localities (Grand Canyon, Beaver Dam Mountains, Mineral Mountains, Santaquin Complex, Farmington Canyon Complex) between the Grand Canyon and Brigham City, Utah. The best evidence for any crustal material older >2.0 Ga in this region rests in the interpretation of discordant FCC zircons, although the FCC has traditionally been interpreted to lie north of the Cheyenne Belt.

¹Nelson, S.T., Harris, R.A., Dorais, M.J., Heizler, M., Constenius, K.N., and Barnett, D.E., 2002, Basement complexes in the Wasatch fault, Utah provide new limits on crustal accretion: Geology, v. 30, p. 831-834 and refs. therein.