Stop 5: Mafic metavolcanic rocks of Carolina terrane intruded by foliated biotite porphyry granodiorite, McClure Creek
LOCATION: The outcrops are located on McClure Creek approximately 200 m northwest of (behind) log home on Lancaster Road. Lancaster Road is off S-42-12, 0.5 mile north of the intersection of S-42-12 with S-42-23. (Fig. 9, Glenn Springs quadrangle)
DESCRIPTION: Here are observed mafic to intermediate metavolcanic rocks that are intruded by a biotite porphyry granodiorite. Volcanic rocks show porphyritic and amygdaloidal textures, and appear highly strained (Fig. 10). The contact with the granodiorite is gently dipping and near the water level in McClure Creek. The granodiorite is characterized by biotite clots that define a crude lineation and have dimensions approximately 2 cm x 1 cm x 2 mm. The granodiorite has a strong, almost gneissic fabric that parallels that in the mafic metavolcanic rock. Petrographic examination indicates that this is a composite planar fabric (so-called "S-C") and that the granodiorite (and its country rock) have been ductilely sheared (Fig. 11). It is suggested here that the granodiorite is intruded into a locally significant dextral shear zone (of late Precambrian-early Cambrian age) based on the asymmetric tear-drop shape of the pluton and the sudden "hard-right" turn (at the map scale) of the foliation as it enters the pluton.
Foliations strike northerly to north and slightly west. Dips are steep (>70deg.) and to the west. This is typical for rocks of the Carolina terrane in this area. The thin foliation or cleavage observed mesoscopically on weathered rock surfaces is also typical. In thin section or on some fresh surfaces in hand specimen, it may be seen that the mafic metavolcanic rocks have been completely recrystallized with the growth of relatively coarse, idioblastic amphiboles, and fine-grained, equant, polygonal grains making up the felsic phases. The origin of this recrystallization is obscure.
Dennis and Shervais (1995) present geochemistry for an amphibolite at this site (their "McClure Porphyry") that is an andesite. Dennis and Shervais (1995) report an ankaramite tuff (their 1512) upstream (south) of this site on McClure Creek. The area south of this stop to West Springs is a historic locus of gold exploration in this part of South Carolina as discussed by LaPoint (1995) in this volume. In this area it is not unusual to find mafic and ultramafic dikes cross-cutting the quartz-sericite alteration zones that are frequently an gold exploration target. This indicates that the alteration was broadly coeval with ongoing mafic and ultramafic volcanism accompanying arc-rifting as described by Dennis and Shervais (1991).
Dennis and Wright (1993, 1996) analyzed two fractions of zircon by the U-Pb method from the foliated biotite porphyry granodiorite at this stop. They interpret the upper intercept at 571+/-16 Ma as a crystallization age. To the south of this area in the Cross Anchor quad, Dennis and Wright (1993, 1996) dated another foliated diorite that also intrudes mafic metavolcanic rocks (at Stop 12). Four size fractions of zircon including an air-abraded fraction dated by U-Pb yield an upper intercept interpreted as a crystallization age at 579+/-4 Ma. Thus at least in these two areas, mafic metavolcanic rocks predate ca. 570-580 Ma, and the observed metamorphic fabric postdates ca. 570 Ma.
Fig. 8. Geologic map of a portion of the Glenn Springs and Jonesville quadrangles, showing the location of Stops 5, 6, 7. Light grey - mafic metavolcanic rocks; outlined areas within light grey - quartz sericite schist or silicification; no pattern - felsic metavolcanic rocks; dark grey - metapyroxenite-hornblendite; double bar random pattern - metagabbro; rippled gneiss pattern - Mean Crossroads metadiorite-diorite gneiss; random dot pattern - foliated biotite porphyry granodiorite; aligned crosses - undeformed, unmetamorphosed diorite; single bar random pattern - Bald Rock granite. Adapted from Dennis and Wright (1996).
Fig. 10. (left) Amygdaloidal greenstone on McClure Creek, Stop 5. Note subvertical foliation.
Fig. 11. (right) Photomicrograph of foliated biotite porphyry granodiorite, Stop 5. Note rounded plagioclase porphyryclast wrapped in biotite. Aggregates of biotite flakes elsewhere in this pluton define a composite planar fabric that is subvertical in orientation. Field of view approximately 25 mm.