CA. 626 MA TO 619 MA PEN BRANCH/DEEP ROCK VOLCANIC ARC TERRANE UNDERLYING THE U.S. D.O.E. SAVANNAH RIVER SITE, SOUTH CAROLINA

More than 10,000 m of basement cores have been recovered from 57 deep wells at SRS. These cores provide the only known samples of basement terranes southeast of the Fall Line in central SC. We define four distinct units in the basement beneath the Coastal Plain: (1) Crackerneck Metavolcanic Complex (MVC) (greenstones and felsic tuffs; greenschist-facies conditions), (2) Deep Rock Metaigneous Complex (MIC) (mafic to felsic volcanic and plutonic rocks; lower amphibolite–facies conditions), (3) Pen Branch MIC (amphibolites, garnet amphibolites, garnet-biotite schists, and gneiss), and (4) Triassic Dunbarton Basin.

All metaigneous rocks have calc-alkaline fractionation trends, consistent with formation in subduction-related arc terranes at convergent margins. Zircon U-Pb crystallization ages of ca. 626 Ma to 619 Ma, however, show that Deep Rock and Pen Branch complexes do not correlate with the younger Carolina terrane (570–535 Ma) or Suwannee terrane (ca. 550 Ma). The Deep Rock and Pen Branch MIC’s may be a continuation of Proterozoic basement that forms the older infrastructure of the Carolina arc. The contact between the Crackerneck MVC (=Persimmon Fork Fm?) and the Deep Rock and Pen Branch MIC’s thus may be equivalent to the Uwharrie Fm -Virgilina sequence angular unconformity. Crackerneck MVC is separated from Belair belt exposures by a strand of the Eastern Piedmont fault system, and a narrow belt of high grade rocks. The contact between the Pen Branch and Deep Rock MIC’s is the Alleghanian (?) Tinker Creek nappe. We tentatively correlate Deep Rock and Pen Branch rocks with the Hyco Fm in southern VA and central NC. The Hyco Fm constitutes the infrastructure of the Carolina terrane in VA and NC, where it was affected by the Virgilina orogeny. Rocks of the Kiokee and Uchee belts may represent correlatives of the Pen Branch/Deep Rock arc variably remobilized during the Alleghanian orogeny.

The rocks of the Deep Rock and Pen Branch MIC’s may have formed the arc infrastructure of the Carolina Slate belt in SC, detached by later tectonic events, or may have formed the Late Proterozoic arc infrastructure at another location in the arc that has been moved into its current location by transcurrent motions. Limited age and isotopic data suggest that none of these rocks correlate with the Suwannee terrane of North FL and southern GA.