U-PB AGES OF DETRITAL ZIRCONS IN QUARTZ ARENITES OF THE ORDOVICIAN BLOUNT MOLASSE, SOUTHERN APPALACHIANS, USA

Detrital zircons from medium grained to pebbly to locally conglomeratic quartz arenites of the Ordovician Blount molasse of Virginia, Tennessee, and Georgia have been analyzed by laser ablation ICP-MS to obtain their U-Pb radiometric ages. The principal age clustering in all samples is of Grenville-age zircons, approximately 0.9-1.5 Ga, which is similar to previously reported ages of many detrital zircons in Paleozoic sandstones of the central and southern Appalachians. A few zircons are much older, with a few small groups in clusters between 1.9 and 2.6 Ga. The sandstones from which all of the zircons were obtained are the Walker Mountain Sandstone in Virginia, the middle sandstone member of the Bays Formation in Tennessee, and the Colvin Mountain Sandstone in Georgia. Each of these sandstones is interbedded with redbeds and associated K-bentonites, with the sandstones and redbeds being derived from tectonic highlands that were uplifted during the Taconic Orogeny, and the K-bentonites being altered tephras generated by explosive volcanism along Taconic magmatic arcs. The detrital zircon data, combined with thin section analysis, indicates that the source terrane consisted of older Paleozoic sedimentary rock with associated plutonic and metamorphic rock, but little or no volcanic rock.