NEOPROTEROZOIC GEOLOGIC HISTORY OF EASTERN LAURENTIA: EVIDENCE FROM U-PB DATING OF ZIRCON FROM RHYOLITE CLASTS IN MOUNT ROGERS FORMATION CONGLOMERATES, SW VA, USA

The initial stages of breakup of the Rodinian supercontinent that led to development of the eastern Laurentian margin are preserved as ca. 760-750 Ma bimodal volcanic rocks and coarse clastic deposits of the Neoproterozoic Mount Rogers Formation (MRF). Located in the Blue Ridge of SW Virginia, the MRF overlies Mesoproterozoic basement metagranitoids (~1.3-1.0 Ga) along a nonconformity that underwent shearing during Paleozoic deformation. In the region, there is no rock record between ~1.0 to 0.76 Ga. In order to investigate this 'age gap,' clasts from MRF conglomerates were sampled for age determination and geochemical analysis.

The upper MRF is dominated by rhyolite flows and pyroclastic rocks. The lower MRF comprises bimodal volcanic rocks interlayered with coarse arkosic sandstone and polymict conglomerate. Within the sedimentary sequence, the presence of poorly sorted, compositionally immature sediments, rapid changes in lateral and vertical facies, and generally lenticular geometry suggest deposition as alluvial fan deposits. Polymict conglomerates are dominated by boulder- to cobble-sized clasts of rhyolite and granitoids, commonly assumed to be locally derived. Field and petrographic observations reveal several distinctive varieties of rhyolite and granitoid.

Laser ablation ICP-MS Zircon U-Pb analyses of ten rhyolite clasts and one granitoid clast yield ages (within error of each group) of ~780-775 Ma (4 clasts), ~760 Ma (4 clasts), and ~753 Ma (3 clasts). The granitoid clast shows a large degree of inheritance (grains between 1.2-1.7 Ga), however, 5 relatively concordant grains yield a Concordia Age of 779.5 +/- 11 Ma. The 760-753 Ma ages are consistent with ages of underlying and overlying rhyolites based on SHRIMP ages. However, rhyolite clasts older than 760 Ma demonstrate that a larger volcanic field of longer duration occurred and was uplifted and rapidly eroded. Intracontinental rifting of eastern Laurentia began 20 m.y. earlier than previously known.