DETRITAL ZIRCON U-PB GEOCHRONOLOGY OF THE NEOPROTEROZOIC JACOBSVILLE SANDSTONE IN THE FOOTWALL OF THE KEWEENAWAN THRUST, MICHIGAN, USA

Recent detrital zircon studies on the Neoproterozoic Jacobsville Sandstone in the upper peninsula of Michigan have revealed that the deposition of the Jacobsville postdates volcanism associated with the Midcontinent Rift (MCR). Moreover, these data indicate that the provenance of the Jacobsville was variable in space and time. We present detrital zircon U-Pb age results for the Jacobsville Sandstone in the footwall (south and east) of the Keweenawan Thrust. Samples were collected at the north end of Lake Gogebic (n=183), west of Hubbell (n=210), west of Traverse Bay (n=302), and at the Jacobsville building stone quarry (n=107). The Gogebic sample was collected most proximal to the Thrust. The detrital zircon age spectrum here consisted of more than 80% Grenville [Grenville orogeny is usually cited as starting at 1300 Ma] (980-1200 Ma) and MCR (1086-1120 Ma) age grains. The peak age at Gogebic is 1110 Ma. The Hubbell sample was collected about 500 m east of the Thrust. The peak ages at Hubbell are 1100, 1325, and 1450 Ma. About 40% of the Hubbell age spectrum was derived from the Midcontinent Granite-Rhyolite (MGR) province. MCR and Grenville grains each comprise about 25% of the sample. The Jacobsville quarry sample is located about 10 km east of the Thrust and is comprised of 55% MGR grains with a peak age between 1335 and 1448 Ma. The MCR derived grains account for 21% of the sample and have a peak age of 1101 Ma. These data indicate that MCR rocks in the hanging wall of the Keweenawan Thrust at Gogebic were an important sediment source of the Jacobsville footwall sandstones. The Hubbell and Jacobsville localities have a mixed provenance. While MCR-derived sediment is still prominent, these areas are dominated by distal easterly sources from the Grenville highlands and adjacent MGR rocks.