Zircon Ages of Felsic Volcanic Rocks in the Upper Precambrian of the Blue Ridge, Appalachian Mountains

Abstract

Five Zirconi samples from Pennsylvania, Virginia, and North Carolina yield discordant uranium-lead ages which suggest an original age of 820 million years and an episodic lead loss at 240 million years. The indicated age of lead loss is interpreted as the age of movement of the Blue Ridge thrust sheet.

The volcanogenic Mount Rogers Formation and the overlying glaciogenic Konnarock Formation: Two late Proterozoic units in southwestern Virginia

“…He concluded that the rhythmites were deposited in quiet water and reflected volcanic cycles, not glaciation. Rankin (1967Rankin ( , 1969 suggested a glacial origin for the laminated pebbly mudstones and associated muddy-matrix conglomerates (diamictites), a conclusion supported by Blondeau and Lowe (1972), Schwab (1976), and Miller (1986, 1989a.…”

“…Schwab (1976) reported very rare striated clasts in the diamictite; I observed none. Both Rankin (1969) and Schwab (1976) noted the absence of a striated pavement, but pointed out that the diamictite everywhere rests on coeval subaqueous sediments (also implied in the lodgment till interpretation).…”

“…The massive diamictite also forms a thick unit here interpreted to be in the Mountain City window on the slopes of Beech Mountain. Rankin (1967) suggested that the massive diamictite is a tillite and later (Rankin, 1969) suggested that the tillite might have been the result of alpine glaciation related to volcanic edifices. As discussed below, that latter idea no longer seems reasonable.…”

Global Geoscience Transect 20. Central Appalachians: Cratonic North America to the Atlantic Abyssal Plain

“…for the Crossnore Granite itself. Odom and Fullagar found that zircon U/Pb analyses reported earlier (Rankin et al, 1969) gave older ages (820 Ma) because of contamination from old Grenville gneisses that they assimilated.…”

“…Ultramafic rocks are confined to the sequence below the Catoctin, and a set of hornblende gabbros may also be confined to the preCatoctin sequence. Most of these rocks were originally included in the Crossnore plutonicvolcanic suite of Rankin and considered to be ~820 Ma (Rankin et al, 1969(Rankin et al, , 1973. Since then, additional isotopic dating indicates that the felsic peralkaline plutonic and volcanic rocks of the Crossnore type are older than -650 Ma and unconformably underlie the Lynchburg/Ashe formations (see section on Crossnore plutonic-volcanic suite).…”