Masters Theses

Date of Award

12-1993

Degree Type

Thesis

Degree Name

Master of Science

Major

Geology

Major Professor

Harry Y. McSween Jr

Committee Members

Lawrence A. Taylor, Lula C. Misra

Abstract

A 400 m.y.-old mafic pluton intrudes the Charlotte Belt at Rock Hill, South Carolina. The pluton is predominantly massive anorthositic gabbro, containing occasional pegmatitic anorthosite lenses. Troctolite comprises less than half of the portion of the pluton studied, and contains discontinuous layering features and anorthosite lenses. Both rock units contain areas of alteration to metagabbro, mostly located along fractures, but the pluton is not otherwise significantly affected by metamorphism. Neither field relationships, petrography, nor mineral chemistry reveal a clearly systematic distribution pattern for the anorthositic gabbro and troctolite units. Plagioclase compositions analyzed by electron microprobe in samples of either main lithology range from An 64 to An 83. Likewise, the range of olivine compositions analyzed is from Fo 77 to Fo 84. Two distinct pyroxene compositions are found: augite (En42Fs12Wo46) and bronzite (En79Fs20Wo1). The stratigraphic distribution of lithologies, crystal sizes, and mineral compositions was a focus of this investigation; however, no systematic modal or cryptic layering was discovered. Textural and modal differences between the units must be explained without classical fractional crystallization as a major mechanism; models used to explain similar phenomena elsewhere are considered. The Rock Hill pluton probably formed its two modally and texturally distinct units through a mechanism of alternating convection cells and density-currents, as suggested by Irvine (1987) for Skaergaard. Finally, the association of gabbro and metagabbro is discussed. Based on observations made in this investigation, the metagabbro is thought to have formed along fracture surfaces where water was available. This conclusion differs from previous hypotheses that gabbro and metagabbro represent distinct intrusions of different age.

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