LOCATION: The outcrops are just below the eastern abutment of Neal Shoals Dam on the Broad River, at the end of National Forest Road 304, Sumter National Forest. The Neal Shoals Dam is owned by South Carolina Electric and Gas Company.
DESCRIPTION: Below the dam, there are excellent outcrops of fresh rock and saprock swept clean by periodic overflow. The main outcrop area is about 100m long by about 60 m wide. The rocks range in composition from felsic to mafic. The oldest rocks are inclusions of hornblende gneiss, amphibolite and metagabbro in granite and diorite. Various granitoid rocks make up the majority of the exposure. At least two types of mafic dikes and numerous pegmatite dikes crosscut the older units. Some rocks have a well developed tectonic foliation, but others are essentially undeformed. Where foliation and layering are developed they typically strike north-northeast and dip steeply east-southeast, approximately parallel to the regional trend. The outcrops here show many features typical of the Charlotte belt intrusive complexes in this region, but elsewhere they are mostly exposed piecemeal in small saprolitic outcrops. Here they are well displayed in one small area.
For convenience in pointing out the features, the exposure is divided into three parts, northern, central and southern, but they are all interpreted to be aspects of a single intrusive complex. The northern part (Fig. 7) is mostly foliated biotite granite with numerous inclusions of mafic rocks, all of which are cut by pegmatite dikes. The mafic inclusions are mostly amphibolite and hornblende gneiss that are typically elongate and oriented parallel to the regional trend. Some inclusions have wispy and stringy ends, suggesting partial assimilation and mixing with the granite magma.
The central part (Fig. 7) of the outcrop is mostly a variety of mafic rocks with some granite, cut by a mafic dike and numerous pegmatite dikes. The largest mafic body is a fine-grained biotite amphibolite with porphyroblasts of hornblende and relict (?) phenocrysts of plagioclase. The mafic dike is about one meter thick and has strong ductile deformation along its contacts. Rotation of the foliation and layering in the adjacent granite and amphibolite indicate that the block north of the dike moved relatively eastward. The dike is a metagabbro, with mineral assemblages indicative of amphibolite facies metamorphism.
The southern part of the outcrop is mainly strongly foliated coarse-grained granite, with some mafic inclusions, that is cut by a fine-grained, foliated mafic dike and many pegmatite dikes. In the steep bank on the eastern side of the outcrop, colluvium as much a two meters thick unconformably overlies saprolite of the intrusive complex.
The rocks here are interpreted to have been emplaced during a single protracted sequence of magmatic events. The mineral assemblages indicate pervasive metamorphism under amphibolite facies conditions. There is considerable evidence for ductile deformation. The last major event, emplacement of the swarm of pegmatites must have taken place while temperatures were still high. At this outcrop, we are probably looking at the midcrustal underpinnings of the Charlotte belt magmatic arc.
Fig. 7. View northwest across Stop 4, Neal Shoals