AGE OF LASSEN PEAK, CALIFORNIA, AND IMPLICATIONS FOR THE AGES OF LATEPLEISTOCENE GLACIATIONS IN THE SOUTHERN CASCADE RANGE

Lassen Peak is a large volcanic dome of late Pleistocene age at the so uthern end of the Cascades are. This dacitic dame has a complex phenoc ryst assemblage of mixed-magma origin and has been effectively dated f or the first time using the Ar-40/Ar-39 technique. Emplacement of the dome occurred after an advance of late Pleistocene glaciers from an ol der volcanic cone to the southwest and from a large volcanic plateau t o the east. Emplacement of the Lassen Peak dome before the latest majo r Pleistocene glaciation-correlated by previous workers with the Tioga glaciation of the Sierra Nevada-formed a high peak that intercepted s ome of the precipitation that previously had fed the plateau ice cap, forming a 10-km-long glacier from a cirque on the northeast side of th e peak. The weighted mean Ar-40/Ar-39 age of the dacite of Lassen Peak is 28.3 +/- 2.7 ka. Comparison of its remanent magnetic direction to that of sedimentary deposits at Mono Lake, California, indicates a cor relation to strata of 27 +/- 1 ka, The dacite of Kings Creek, which er upted from the site of Lassen Peak before a glacial advance that preda ted the Lassen Peak dome, overlies peat that has a U-Th-calibrated C-1 4 age of 37.6 +/- 0.2 ka. The dacite of Kings Creek yielded a Ar-40/Ar -39,ge of 32 +/- 17 ka. The paleomagnetism of this dacite is similar t o that of Mono Lake sediments of 35 +/- 1 ka age. Thus, a late Pleisto cene glacial advance probably began in the Lassen region between about 35 and 27 ka.