Petrology and Geochemistry of Neoproterozoic Arc Plutons Beneath the Atlantic Coastal Plain, SRS, SC Page: 9 of 47

                                
thin-bedded wackes and siltstones, interbedded with mafic to felsic crystal-lapilli tuff and, less
commonly, amygdaloidal basalt (Shelley, 1988; Shervais and others, 1996). The only age
constraint for rocks of the Augusta terrane is the presence of a lower Paleozoic trilobite segment
(Maher and others, 1981).
The Augusta terrane is separated from the Kiokee belt by the Augusta fault, a late Alleghanian
(~275 Ma), low angle normal fault that has Augusta terrane rocks in its hanging wall (Maher,
- -   1978, 1979; Maher and others, 1991). The Augusta terrane is separated from a higher grade
terrane to the southeast by an aeromagnetic lineament (figure 2). This geophysically defined crustal
block, which includes the Graniteville granite, appears to extend southward toward the Savannah
River Site (figure 2).
Suwannee terrane: The Suwannee terrane, which underlies the coastal plain sediments of south
Georgia, Alabama, and northern Florida, has only been sampled by drill core (Heatherington and
Mueller, 1996; Heatherington and others, 1996; Mueller and others, 1994; Guthrie and Raymond,
1992). It consists of low-grade metavolcanic rocks (North Florida Volcanic Series) and intrusive
rocks of diorite to granodioirite composition (the Osceola granite of central Florida and others).
Reported zircon U-Pb ages include 552 Ma for a dacite metavolcanic of the North Florida Volcanic
Series, 551 Ma for the Osceola granite, and 625 Ma for a granodiorite in southern Alabama
(Heatherington and others, 1996; Mueller and others, 1994).
Whole rock geochemistry shows that the North Florida Volcanic Series and related intrusive rocks
formed in a magmatic arc along the margin of Gondwana (Heatherington and Mueller, 1996;
Heatherington and others, 1996; Mueller and others, 1994). Sr and Nd initial ratios indicate the
involvment of both Proterozoic and Archean lithosphere. Metavolcanic and granitic rocks of the
Suwannee terrane are age equivalent with the Carolina terrane, and may have formed in the same
convergent margin setting.
Savannah River Site terrane: Major lithologic units of pre-Cretaceous basement beneath the
Savannah River Site include the Crackerneck Metavolcanic Complex (subgreenschist facies
metavolcanic rocks), the DRB Metavolcanic Complex (epidote-amphibolite facies metavolcanic
rocks), the Pen Branch Metavolcanic Complex (amphibolite to lower granulite facies metavolcanic
rocks), the PBF plutonic suite, the DRB plutonic suite, and clastic sedimentary rocks of the
Triassic Dunbarton basin (figure 2). Metavolcanic rocks of the Crackerneck, DRB, and Pen
Branch volcanic complexes are described in Mauldin et al. (this volume). The inferred occurrence
of a Triassic-Jurassic mafic igneous complex south of the Dunbarton basin is based on
aeromagnetic anomalies (Horton et al., 1991); core data from wells C-7 and C-10 indicate that
much of this basement is "pinked" granite of the PBF plutonic suite. Younger, undeformed plutons

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