SHRIMP U-Pb geochronology of volcanic rocks, Belt Supergroup, western Montana: evidence for rapid deposition of sedimentary strata

New sensitive high resolution ion microprobe (SHRIMP) U-Pb zircon analyses from two tuffs and a felsic flow in the middle and upper Belt Supergroup of northwestern Montana significantly refine the age of sedimentation for thi s very thick (15-20 km) Middle Proterozoic stratigraphic sequence. In ascen ding stratigraphic order, the results are (1) 1454 +/- 9 Ma for a tuff in t he upper part of the Helena Formation at Logan Pass, Glacier National Park; (2) 1443 +/- 7 Ma for a regionally restricted porphyritic rhyolite to quar tz latite flow of the Purcell Lava in the Yaak River region; and (3) 1401 /- 6 Ma for a tuff in the very thin transition zone between the Bonner Quar tzite and Libby Formation, west of the town of Libby. Combining these ages with those previously published by other workers for ca. 1470-Ma sills in t he lower Belt in Montana and Canada indicates that all but the uppermost Be lt strata (about 1700 m) were deposited over a period of about 70 million y ears, considerably reducing the time span from longstanding estimates rangi ng from 250 to 600 million years. Calculated sediment accumulation rates be tween dated samples indicates rapid, but not unreasonable, values for early Belt strata, with decreasing rates through time. These ages also suggest t he inadequacy of previously published paleomagnetic data to resolve Belt Su pergroup chronology at an appropriate level of accuracy.