Abstract

A belt of the Piedmont between Lincolnton, North Carolina, and Gaffney, South Carolina, is underlain by metamorphic country rocks, granitic rocks and albitic pegmatite, and Triassic diabase. The country rocks comprise quartzite, variably micaceous crystalline limestone and dolomite, impure limestone or carbonate-silicate rocks, fine-grained mica schists and gneisses, hornblende and hornblende-biotite gneisses, and scarce pyroxene, amphibole, quartz-tourmaline, and chlorite rocks. The noncarbonate rocks, except quartzite, have been mapped as pre-Cambrian (the Carolina and Roan formations). The quartzite, carbonate-silicate rocks, and crystalline limestone have been mapped as Cambrian (the Kings Mountain, Blacksburg, and Gaffney formations).

Textures are for the most part crystalloblastic, but some varieties, particularly the carbonate-silicate, show definite evidence of sequential rather than simultaneous crystallization of the minerals; cataclastic texture is but slightly developed. The silicate minerals in the carbonate-silicate rocks are mineralogically and texturally identical with those in the closely associated silicate rocks. Those in the carbonate-silicate rocks clearly have not been formed by the recrystallization of minerals previously granulated, and it is therefore concluded that those in the silicate rocks have not had such an origin.

The genetic relation between the carbonate and noncarbonate rocks thus implied was clarified by large-scale mapping near Kings Mountain. There, the quartzite occurs interbedded with west-dipping rocks of the other types but is most abundant in two distinct zones within the limestone. The upper or western limit of the limestone throughout the area is about equidistant from the irregular eastern margin of a large granitic body and occurs at some places above the upper quartzite zone and at other places below it. The carbonate-silicate rocks, largely equivalent to the Blacksburg schist, have been found throughout the limestone as sporadic layers and bodies of indefinite outline, but they are most abundant near the irregular upper limit of the limestone—the lower limit of conformably overlying hornblende gneiss sharply interlayered with mica schist.

The upper quartzite is thus in a zone of transition from carbonate to silicate rocks, and the relation of this zone to the margin of the granite indicates that the transition is of metamorphic origin. The layers of hornblende gneiss immediately above the limestone are strongly metamorphosed beds of the limestone series, but it is not known whether they originally contained all the elements necessary to form the silicate minerals. Evidence for the chemical transfer of material is the presence of hornblende, tremolite, and microcline porphyroblasts in the limestone and of tourmaline, albite, microcline, and spodumene in the silicate rocks only near pegmatite and granite. It is believed that the granite was the source of aqueous solutions that invaded the sedimentary rocks and effected their recrystallization or reconstitution. The granite is apparently of late Carboniferous age; the country rocks are older, but no evidence could be found for determining their actual age.

Thus, near Kings Mountain, mica schists and gneisses previously mapped as Carolina gneiss, and hornblende gneisses previously mapped as Roan gneiss, actually constitute the metamorphosed upper part of the Gaffney. Similar rocks in the Beaverdam Creek area apparently are stratigraphically higher. The micaceous rocks were derived by the metamorphism of shaly rocks, and the hornblendic rocks by the metamorphism of pure or impure calcareous rocks, regardless of the stratigraphic position of either type. Evidence in other parts of the Carolina Piedmont, according to the literature as well as reconnaissance observations by the writer, indicates that intrusive diorite and possibly recrystallized mafic volcanic rocks have been included in the Roan, and possibly recrystallized felsic volcanic rocks in the Carolina. The names Carolina and Roan, therefore, have lithologic but not stratigraphic significance, and the rocks associated with them probably will be divided eventually into formations on the basis of comparative genetic history rather than physical appearance.

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