14-1 Unraveling the Origins of "Other Terranes" in the North Carolina eastern Piedmont: Insights from new U-Pb Zircon Geochronology and Sm-Nd Isotopes

Session: New Research in the Appalachian-Ouachita Orogen: Integrated studies from the Foreland to the Hinterland

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Abstract:

In the NC eastern Piedmont, dextral shear zones of the Eastern Piedmont fault system (EPFS) divide second-order, peri-Gondwanan domains into the third-order, infrastructural Warren (Wt), Raleigh (Rt), Crabtree (Cbt), and Falls Lake (FLt), and suprastructural Spring Hope (SHt) terranes. The Lake Gordon mylonite zone (LGMZ) of the EPFS separates the FLt, Cbt, and Rt from the Wt and SHt to the east. These lithotectonic elements may be cognate members of ca. 633–528 Ma Carolinia, or may be displaced, exotic blocks from the northern Appalachians, possibly Ganderian(?) or Avalonian(?) rocks. Terrane correlations remain uncertain due to limited geochronological and isotopic datasets and unknown EPFS slip magnitudes.

We present LA-ICP-MS U-Pb zircon datasets from the Rt, Cbt, FLt, and SHt. These new ages highlight major terrane age differences across the LGMZ. Four Rt and three Cbt samples yield unimodal age distributions and weighted mean ages of ca. 580–555 Ma for the Rt and ca. 570–566 Ma for the Cbt. These ages are interpreted to be igneous crystallization ages. Three FLt samples have multimodal age distributions indicating sedimentary protoliths. These samples yield Maximum Depositional Ages (MDAs; youngest statistical population) of ca. 626–608 Ma, dominant age modes of ca. 2100, 1800, 1500–990, 610 Ma, and some ages >2.3 Ga. Three SHt samples have sedimentary protoliths, multimodal age distributions, MDAs of ca. 635–508 Ma, and dominant age modes of ca. 1400, 1200–1000, and 630–560 Ma. Notably absent are >1.5 Ga zircons. One SHt sample yields a unimodal age distribution and ca. 539 Ma weighted mean igneous crystallization age.

Whole-rock Sm-Nd isotopic data further distinguish these terranes. The Rt has positive initial εNd values whereas the Cbt has positive to slightly negative εNd. Positive εNd reflect juvenile crustal melts extracted from the mantle and are consistent with existing data from Carolinia. In contrast, the Wt (ca. 460–400 Ma) and FLt have negative εNd suggestive of an evolved cratonic sedimentary input and are consistent with detrital zircon signatures having Precambrian grains. The SHt has the greatest variations of positive to negative εNd indicating contributions from both evolved cratonic and juvenile sources. The Wt and SHt have isotopic and published age data consistent with Ganderian and possibly Connecticut Valley trough rocks.

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