THE DETRITAL RECORD OF NEOPROTEROZOIC TO CAMBRIAN ROCKS FROM THE SOUTHERN APPALACHIAN BLUE RIDGE: PROTRACTED RIFTING, RAPID UNROOFING, AND STABILIZATION OF THE MARGIN

The southern Appalachian Blue Ridge consists of Mesoproterozoic basement rocks of the French Broad massif that are nonconformably overlain by Cryogenian-Ediacaran metasedimentary and metavolcanic rocks of the Mount Rogers Formation (Zmr), glaciogenic rocks of the Konnarock Formation (Zk), and clastic rocks of the Unicoi (Zu) formation (base of the Chilhowee Group). The Zmr represents a period of failed rifting unrelated to final rifting of the eastern Laurentian margin at 570 Ma.

The lower Zmr contains bimodal volcanic rocks interlayered with arkoses and polymict conglomerates containing clasts of rhyolite, granitoid, and minor quartz and sedimentary rocks. The upper Zmr contains pyro-volcaniclastic rocks and rhyolite flows with reported ages between 759-750 Ma. U-Pb zircon ages of ten rhyolite clasts and one granitoid clast define three age populations at 780-770 Ma, 764-756 Ma, and 748-755 Ma. While the 759 Ma and 753-750 Ma rhyolite flows are preserved in the Zmr, older flows are only preserved as clasts in the lower Zmr.

Pooled ages of detrital zircon (n=950 analyses) from a composite section (21 samples) in the Mount Rogers area have peaks at 1170 Ma, 1040 Ma, and 760 Ma that correspond to Grenville (early and late magmatic suite pulses), and Mount Rogers volcanism respectively. An abundance of euhedral Grenville-age zircon grainsis interpreted to result from rapid and/or localized basement uplift, erosion, and deposition in the Zmr section., through the section, 1170 Ma grains are the dominant component in the Zmr and lower Zk, but become progressively less abundant relative to 1040 Ma zircon in the upper part of the section at the top of Zk. This suggests progressive unroofing of an age stratified igneous basement.

125 U-Pb detrital zircon ages from Zmr and Zk samples fall within 800-720 Ma and likely correspond to Mount Rogers volcanism. Best fit modeling of the Neoproterozoic detrital zircon age group using three peaks, yields groups at ~780-770 Ma, ~760 Ma, and ~748 Ma, suggesting at least three episodes of magmatism. Both U-Pb detrital zircon ages and igneous clast ages support a protracted rifting history that spanned at least 30 my.

By the uppermost Zk and Zu, ~760 Ma zircons are almost completely absent. Above the pre-Zu unconformity, detrital age populations have similarities to regional detrital zircon patterns.