Paper No. 207-7
Presentation Time: 8:00 AM-12:00 PM
DUKE, Genet I., Dept. of Geology and Geological Engineering, SD School of Mines and Technology, 501 East Saint Joseph St, Rapid City, SD 57701,, SINGER, Brad S., Department of Geology and Geophysics, Univ of Wisconsin - Madison, 1215 West Dayton Street, Madison, WI 53706, and DEWITT, Ed, U.S. Geological Survey, DFC, MS 905, Denver, CO 80225

The timing and pattern of Paleocene-Eocene magmatism in the northern Black Hills have been perplexing, in part because published K/Ar ages (between 62.1 and 38.3 Ma with average 2s uncertainties of ±3.5 Ma), have implied a protracted ca. 25 myr period of magmatic activity. We determined 40Ar/39Ar ages from twelve intrusions in the Black Hills by incrementally heating sanidine or amphibole using a CO2 laser, which substantially narrows the timing of intrusion. For example, sanidine from Devil’s Tower and Missouri Buttes phonolites yielded indistinguishable plateau ages of 49.04±0.16 Ma (2s) and 49.24±0.28 Ma, respectively (earlier K/Ar ages are 41.6 and 50.9 Ma), strongly suggesting that intrusion of phonolitic magma was coeval along a 5-km, N60W axis. From oldest to youngest, the 40Ar/39Ar plateau ages are as follows: South Dakota: (1) 58.01±0.58 Ma amphibole, Vanocker latite, (2) 55.16±0.25 Ma sanidine, Elk Mountain phonolite, (3) 55.03±0.15 Ma amphibole, Anchor Hill trachyte at Gilt Edge, (4) 54.66±0.19 Ma sanidine, Annie Creek phonolite, (5) 54.47±0.15 Ma amphibole, Cutting Stock trachyte margin, and (6) 54.32±0.14 Ma amphibole, Cutting Stock trachyte, center main stock; Wyoming: (7) 49.62±0.17 Ma amphibole, Tinton pyroxenite, (8) 49.24±0.28 Ma sanidine, Missouri Buttes phonolite, (9) 49.04±0.16 Ma sanidine, Devil’s Tower phonolite, (10) 48.05±0.39 Ma amphibole, Tinton trachyte, (11) 47.33±0.21 Ma amphibole, Hershey Creek latite, Bear Lodge Mts., and (12) 45.99±0.31 Ma, amphibole from Bear Lodge trachyte, near Warren Peak.

These data indicate that magmatic activity in the northern Black Hills occurred during three periods at »58 Ma, 55-54 Ma, and 49.6-46 Ma, and began with less alkalic magma intruded in the east with a westward progression that included more alkalic, silica-undersaturated magma. The geographic-age trend is not perfectly linear, but rather is defined by the clustering of ages in South Dakota and a younger cluster in Wyoming, with a temporal gap between the two areas of »4 myr. Petrologic and isotopic studies underway are aimed at better understanding the origin of this pattern.

2002 Denver Annual Meeting (October 27-30, 2002)
Session No. 207--Booth# 168
Rocky Mountains III: Post-Laramide Uplift and Erosion of the Rocky Mountains and Colorado Plateau (Posters)
Colorado Convention Center: Exhibit Hall
8:00 AM-12:00 PM, Wednesday, October 30, 2002

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