40AR/39AR LASER INCREMENTAL-HEATING AGES OF DEVIL'S TOWER AND PALEOCENE-EOCENE INTRUSIONS OF THE NORTHERN BLACK HILLS, SOUTH DAKOTA AND WYOMING

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.