NEW SHRIMP AND CA-TIMS GEOCHRONOLOGIC CONSTRAINTS ON AND ɛHf DATA FROM <685 MA EPICLASTIC VOLCANIC SANDSTONE ON OXFORD MOUNTAIN, POCATELLO FORMATION, SOUTHERN BANNOCK RANGE, SE IDAHO

New mapping along the Oxford Ridge segment of the southern Bannock Range in SE Idaho has resolved the stratigraphic placement of the previously dated volcanic sandstone and lapilli-tuff within the Neoproterozoic Pocatello Fm. This so-called Oxford Mountain volcaniclastic unit of the Scout Mountain Member of the Pocatello Fm lies above up to 200 m of variably volcanic diamictite in the eastward-dipping limb of the Oxford Ridge Anticline. The diamictite is gradational to underlying metabasalt of the Bannock Volcanic Member.

The Oxford Mountain volcaniclastic unit consists of up to 60 m of dacite-trachyte bearing volcanic diamictite, volcanic sandstone, and lapilli-tuff. The unit is laterally discontinuous over 5.5 km of strike outcropping above the low-angle Clifton Canyon Fault.

SHRIMP U-Pb zircon dating has revealed a complex range of dates with components at ca. 709, 702 and 686 Ma. LA-MC-ICP-MS Lu-Hf analyses of these same zircons yields initial ɛHf values in the range +1 to -11. This indicates that these igneous zircons crystallised from a source that has a significant Paleoproterozoic to Archean crustal component.

Five new samples along strike of the Oxford Mountain volcaniclastic unit were analyzed using high-precision (~0.1%) CA-TIMS techniques. The zircon grains are 50-200 µm, equant to subprismatic and are euhedral to subrounded. CL images show narrow to broad oscillatory zoning patterns with common sector zoning suggesting that all grains are magmatic. From each sample, six grains without inherited cores were selected and analyzed. All together these samples yield age components of 709, 702, 695, 690 and 685 Ma. The northernmost sample yields a weighted mean age of ca. 685 Ma for five out of six grains. The older 25 zircon grains from these samples are interpreted to represent either preexisting volcanic grains that were reworked into the deposit or xenocrystic material entrained during eruptions.

685 Ma is a robust maximum age of deposition. A protracted volcanic history from 709 to 685 Ma is required.