LOCATION: The quarry is located off US 176, west of Pacolet, South Carolina. (Fig. 1, Pacolet quadrangle)
DESCRIPTION: The Pacolet granite ranges from porphyritic to equigranular in texture. The Vulcan Materials quarry offers examples of both (potassium feldspar) porphyritic and equigranular phases of the granite. This quarry shows very clearly diking and discrete faulting in the granite that would be obscure in small Piedmont bedrock exposures. The dikes comprise both fine-grained leucogranite or alaskite and quartz-potassium feldspar-biotite pegmatites. Mutual cross-cutting relations between both types of dikes suggests that both types of dikes are related to the intrusion of the Pacolet granite and that the pegmatites are not a distinct different event. The significance of this observation will be discussed below. An approximately east-west striking fault zone dipping about 50deg. south, and east-west striking vein arrays may be observed at the south end of the active workings (Fig. 2). These vein arrays seem to have a consistent geometry: veins dipping about 50deg. north are several times thicker (ca. 1 m) than those that dip about 20deg. south (10's of cm thick). The kinematic significance of this geometry is not clear. It probably does not have its origin in rotated tension gashes (Hudleston, 1989), which would indicate top-to-the-north, because there appear to be only two dominant orientations, rather than a continuous range of orientations. The simplest explanation may be that these vein arrays represent a conjugate set related to the stresses at the time of emplacement of the Pacolet granite. David Cosh the quarry foreman suggested that the frequency of veining or diking may increase with depth.
Mittwede and Fullagar (1987) reported an whole rock Rb-Sr isochron for the Pacolet granite of 383+/-5 Ma based on 8 points, a biotite mineral age of 292 Ma, and a initial 87Sr/86Sr ratio of 0.7046+/-2. No more than 1.5 km to the west of the quarry, and 100's of m from the western edge of the pluton, the central Piedmont suture separating Inner Piedmont from the Carolina terrane rocks is exposed. Quartz-potassium feldspar-muscovite (rather than biotite) pegmatites are typically observed near the trace of the central Piedmont suture in the three quadrangles south of here (Glenn Springs, Cross Anchor, Philson Crossroads; Dennis, 1991). Only on the northern margin of the pluton are rocks of the Pacolet granite interpreted by Mittwede (1989) to be sheared; this fabric is thought to be an extension of Alleghanian strike-slip motion on the Kings Mountain shear zone (Horton, 1981). Foliation attitudes in rocks adjacent to the pluton parallel the pluton margin, and typically dip 50-70deg. to the southeast. Dennis (1991, 1995) presents form surface maps that show that the regional metamorphic fabric predates the central Piedmont suture and that this fault crosscuts the foliation. The purpose of this discussion and this stop is to bring together data that bear on the relative timing of motion on the central Piedmont suture versus the date of the intrusion of Pacolet granite. Did the Pacolet granite predate or postdate motion on the central Piedmont suture? If the central Piedmont suture dips at ca. 25deg., the projected depth to contact with where the central Piedmont suture would be about 700 m beneath the quarry floor. Is the pluton decapitated or does the pluton postdate this fault? Dennis (1995) and Dennis and Shervais (1992) argue that the sequence of events was metamorphic fabric development in Carolina terrane rocks (ca. 570-538 Ma, Dennis and Wright, 1993), ductile motion on central Piedmont suture, intrusion of post-metamorphic granites and gabbros (ca. 383+/-5 Ma, e.g. Mittwede and Fullagar), west-vergent folding of foliation surfaces and retrograde strike-slip shearing on northeast-striking segments of the central Piedmont suture (Alleghanian, 300-280 Ma).
Most of the Pacolet granite outcrops in the Pacolet and Pacolet Mills quadrangles, however, to the south in the northern part of the Glenn Springs quad, a megacrystic porphyry was mapped by Dennis (1989) in erosional windows through the aluminous schists of the Battleground Formation. That megacrystic porphyry is interpreted to be a phase of the Pacolet granite and not a tectonic window through the Carolina terrane into the Inner Piedmont (Dennis, 1989). The contact is interpreted to be intrusive; fault rocks are not recognized.
Fig 2. a. View east at south end of Pacolet quarry showing fault dipping 50-60deg. south, steeply north dipping veins, and shallowly south dipping, thinner veins. b. View east at the north end of the quarry. Veins dipping 45deg. north are ca. 1 m thick. Veins dipping 20deg. south are 10's of cm thick. Exposed face is slightly less than 100 m high.