High Alumina Metamorphic Rocks of the Kings Mountain District, North Carolina and South Carolina

Citation

Potter, Donald Brandreth (1954) High Alumina Metamorphic Rocks of the Kings Mountain District, North Carolina and South Carolina. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/4PF2-GN68. https://resolver.caltech.edu/CaltechTHESIS:02142018-102218444

Abstract

The Kings Mountain district is located in the center of the Piedmont province at the North Carolina-South Carolina border. The district consists of two areas which are underlain by high alumina metamorphic rocks including kyanite quartzite and sillimanite quartzite. The southern area covers about 35 square miles and lies just southeast of the city of Kings Mountain; the northern area covers about 20 square miles and is located 15 miles northeast of the city of Kings Mountain. The two areas are separated by the Yorkville granodiorite. A detailed geologic map was made of the southern area; in the northern area only the high alumina quartzites and the contact of the Yorkville granodiorite were mapped. The detailed mapping of the southern area serves as a basis for most of the discussion in the dissertation.

In both areas, and especially in the southern area, kyanite quartzite and sillimanite quartzite form prominent ridges and hills. In the southern area the long, craggy ridges and hills of Crowders Mountain, The Pinnacle, Kings Mountain, and Henry Knob rise abruptly above the gently rolling Piedmont surface; these hills accurately reflect the outcrop pattern of the kyanite quartzite that underlies them.

Kyanite quartzite occurs as well defined beds and lenses in a thick sequence of high alumina metavoloanic and metasedimentary schists, manganiferous schist, and magnetiferous quartzite. The high alumina nature of the schists is indicated by the presence of abundant white mica and one or more of the following accessory minerals: andalusite, chloritoid, kyanite, and staurolite. This sequence of high alumina schists and quartzites, manganiferous schist, and magnetiferous quartzite lies with apparent unconformity on a thick sequence of metamorphosed volcanic, sedimentary, and igneous rocks which include quartz-mica schist, oligo­clase tonalite, hornblende gneiss, and biotite gneiss and schist.

The entire sequence of rocks was folded along a N40E-trending axis; the outcrop pattern of the high alumina quartzites and manganiferous schist indicates the presence of a major anticline and syncline that probably developed at this time. A later deformation accompanied the emplacement of the Yorkville granodiorite. This large body was intruded along a north­ trending zone into the east limb of the N40E-trending anticline. In addition to apparently warping the earlier major structures, this deformation pro­duced numerous small steeply plunging folds, strong flow cleavage and lineation. These last three structural features bear no consistent relation to the major folds.

A major metamorphism accompanied the intrusion of the Yorkville granodiorite, producing sillimanite in the high alumina sediments near the granodiorite and kyanite in the high alumina sediments farther away. The rocks that occur as septa in the granodiorite, and those adjacent to the granodiorite contact belong to the amphibolite facies. These rocks grade westward over a distance of 2 to 3 miles into rocks of the albite-epidote amphibolite and greenschist facies. Andalusite occurs in schist adjacent to kyanite quartzite and sillimanite quartzite and it commonly occurs with sillimanite in quartzite, but not commonly with kyanite in quartzite. Some of the puzzling distributions of these polymorphs may reflect subtle com­positional differences in the parent sediments. The three polymorphs were seen together in a few thin sections. The paragenetic sequence, deduced from the study of three thin sections, probably indicates polymorphic trans­formations resulting from a single metamorphism rather than a series of metamorphisms.

The following evidence suggests that high alumina quartzite in this district is of metasedimentary origin: high alumina quartzite occurs as well defined thin beds that can be traced up to three and one half miles along strike; many outcrops of high alumina quartzite exhibit compositional layering (i.e., kyanite quartzite is interlayered with stauro­lite quartzite, and with non-kyanitic magnetiferous quartzite); high alumina quartzite beds occur in a conformable sequence of high alumina metasedimentary and metavolcanic schists. It is suggested that the high alumina quartzite beds are metamorphosed beds of sandy or silty clay; these beds probably represent one stage in the deposition of fine grained clayey elastic sediments.

No evidence was found to support the view of Smith and Newcombe (1951) that the kyanite at Henry Knob developed by hydrothermal introduction of alumina. The present study indicates that kyanite in the kyanite quartzite here, as throughout the district, is of metasedimentary origin.

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