Title

Neodymium isotopic and uranium-lead geochronologic studies of southern Appalachian terranes

Date of Award

1998

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Earth Sciences

Advisor(s)

Scott D. Samson

Keywords

Neodymium, Uranium-lead, Georgia, South Carolina, North Carolina, Virginia, Geochronologic, Appalachian

Subject Categories

Geochemistry | Geology

Abstract

The magmatic and tectonic history of the Carolina terrane, and its possible relationships with other southern Appalachian terranes has been poorly understood because of a lack of reliable Nd isotopic and U-Pb geochronological data. The Milton terrane, which borders the Carolina terrane to the northwest, along the Hyco shear zone, has been considered the high-grade basement to the Carolina terrane. This hypothesis was tested by comparing the Nd isotopic compositions of these terranes. Milton terrane rocks analyzed have [Special characters omitted.] (600 Ma) values ranging from -6.5 to +6.1 and T DM ages ranging from 930 to 1690 Ma, whereas those from the Carolina terrane have [Special characters omitted.] (600 Ma) values ranging from +1 to +6 and T DM ages from 745 to 1040. These results indicate that the Milton terrane is composed of isotopically more evolved crust than the Carolina terrane, and are not compatible with models that interpret the Milton terrane as the high-grade equivalent of the Carolina terrane.

The timing of deformation and metamorphism along the Hyco shear zone, the northernmost segment of the central Piedmont shear zone, are constrained by new U-Pb zircon dates for pre-, syn-, and late-synkinematic plutons, which range from 335.4 [Special characters omitted.] 2.2 Ma to 319.6 [Special characters omitted.] 0.7 Ma and and a 40 Ar/ 39 Ar hornblende date of 322.9 [Special characters omitted.] 7.8 Ma. These new dates indicate that deformation and metamorphism associated with the Hyco shear zone were in their final stages by ca. 323 Ma and are consistent with dates that constrain the timing of deformation along other segments of the central Piedmont shear zone.

New U-Pb zircon dates for metavolcanic and plutonic rocks from the Virgilina sequence, the oldest part of the Carolina terrane, range from 633 +2.0/-1.5 Ma to 612.4 +5.2/-1.7 Ma. These new dates document that magmatism in the Carolina terrane had begun by 633 Ma and continued at least until 612 Ma, an interval of ca. 20 m. y. Deformation associated with the Virgilina orogeny is constrained between 612.4 Ma and 586 [Special characters omitted.] 10 Ma. The Nd isotopic character and U-Pb zircon systematics of the Virgilina sequence suggest that it probably was built on young oceanic crust. This is in sharp contrast with the Nd isotopic character and U-Pb zircon systematics of the Avalon and Cadomia terranes, which show clear evidence of an old, continental crustal component. Thus, the oldest part of the Carolina terrane probably was built on a different type of crust from Avalon and Cadomia and should be considered a distinct entity.

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