AGE AND SIGNIFICANCE OF THE FALLS LEUCOGNEISS

The Nutbush Creek fault zone (NCFZ), in eastern NC and VA, is a dextral shear zone that separates the Raleigh terrane from the Carolina, Crabtree and Falls Lake terranes. The NCFZ is thought to be an Alleghanian structure based on correlation with the Eastern Piedmont fault system and on Rb-Sr dating of plutons within the fault zone. The Falls leucogneiss (Flg) is a distinctive lineated granitoid gneiss within the NCFZ. The protolith of the Flg has been variously associated with the Raleigh and Crabtree terranes. However, previous Rb-Sr and U-Pb data for the Flg were interpreted as indicating an Ordovician protolith crystallization age, thus calling into question its correlation with these late Neoproterozoic terranes.

Because an Ordovician age of the Flg would be anomalous for this region, we undertook a detailed U-Pb study of this unit. Three fractions of small (50x100µm), clear, equant zircon grains (3-20 grains each) are discordant with an upper intercept age of 543 ± 22 Ma, and a lower intercept indicative of recent Pb-loss. Highly metamict brown zircon grains are larger, and five fractions of these zircon grains (1-3 grain fragments each) are highly discordant, yielding an identical upper intercept of 545 ± 25 Ma, but with a lower intercept indicative of non-zero Ma Pb-loss. We interpret the upper intercepts (ca. 545 Ma) as the crystallization age of the Flg protolith. Ongoing work on extremely abraded zircons is designed to refine the upper intercept errors on this age. The zircon systematics of this rock are clearly complex, but suggest that previous Ordovician Pb-Pb and Rb-Sr ages may have resulted from combinations of large bulk fractions, non-zero Pb-loss and open-system behavior.

In Flg samples we have examined, we have not seen any strong evidence of alkalic affinity, and we feel it is unlikely that the Flg is related to Laurentian rifting. On the other hand, our new age for the Flg fits well with late Neoproterozoic ages of subduction-related units of the Carolina Zone on either side of the NCFZ.