U-PB DATING OF DETRITAL ZIRCON FROM THE NEOPROTEROZOIC (?) FOND DU LAC AND HINCKLEY SANDSTONE FORMATIONS NEAR DULUTH, MINNESOTA

Laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) at Washington State University was used to determine U-Pb ages for detrital zircons (100+ dates/formation) from the Fond du Lac and Hinkley Formations located in east-central Minnesota. The Fond du Lac arkosic sandstone lies unconformably on the Keweenawan volcanic rocks and inter-flow sediments of the Midcontinent Rift (ca. 1100 Ma), and has a gradational contact with the overlying Hinckley sandstone.

The Fond du Lac Formation contains zircons from 1011 to 2897 Ma with major populations at 1085, 1210, and 1335 Ma. The Hinckley Formation displays a large array of zircon ages from 1053 to 3047 Ma with significant populations at 1170 and 1445 Ma. Other important populations include 1315, 1685, 2080, 2665, 2715, and 3045 Ma. These ages do not support the idea that local Midcontinent Rift (MCR) volcanic rocks were the primary source of detritus as suggested by paleoflow indicators and proximity. The presence of similar Mesoproterozoic zircon populations supports published evidence that the Hinckley sediments were derived from reworking the underlying Fond du Lac Formation. However, the presence of older zircons (>1900 Ma) in the younger Hinckley Formation, indicates a shift in sediment provenance during the deposition of the two formations. Some zircon grains between the ages of 1500 and 1000 Ma were likely derived from local sources (e.g. MCR and anorogenic granites), but others may have been derived from more distal sources (e.g. Grenville province). The youngest date recorded in the stratigraphically lower Fond du Lac Formation (1010.8±12.1 Ma) suggests that the maximum depositional age of both formations is Neoproterozoic.