Mineralogy and petrology of the Chester pluton

A twelve square mile area in the Charlotte belt located just east of Chester, S.C. was mapped and found to contain a differentiated gabbroic pluton completely surrounded by metagabbro country rocks. Diabase dikes, granites, quartz monzonites, and granitic gneisses of the Charlotte belt were also observed in the study area. Field relations indicate the granitic gneisses, the oldest rocks observed, were intruded by a gabbro; that gabbro has since been metamorphosed and intruded by granites and gabbroic rocks of the Chester pluton. The form of the Chester pluton appears to be a sill that has been subsequently exposed in cross section. Igneous activity ended with the intrusion of a Mesozoic age diabase dike.

Rock types determined from modal data and listed from an apparent bottom to top (east to west) sequence include: olivine gabbronorite, olivine leucogabbronorite, pyroxene hornblende olivine norite, gabbro, and gabbronorite. Variations in rock type are associated with variations in color and grain size, all of which follow an east to west trend. Trends in mineral chemistries include variations in plagioclase compositions from An₈₂ in the olivine gabbronorite to An₄₆ in the gabbronorite; variations in mole percent Mg/Mg+Fe. in orthopyroxenes and clinopyroxenes (listed respectively) from .84 and .78 in the olivine gabbronorite through .80 and .73 in the gabbro, to .75 and .67 in the gabbronorite. Olivine compositions in all gabbroic rocks cluster around Fo80 Textural observations of intercumulus pyroxenes, cumulus olivines and plagioclases in the olivine gabbronorite and olivine leucogabbronorite, and finer grained, anhedral plagioclases and pyroxenes crystallized from residual liquid in the gabbro and gabbronorite, suggest differentiation occurred by crystal settling. Variations in volume percent plagioclase plus olivine from over 85% in the olivine gabbronorite to less than 50% in the gabbro supports this interpretation.

Clinopyroxenes and orthopyroxenes in the olivine leucogabbronorite contain sets of Fe-Ti oxide rodlets, with those in the augites oriented parallel to [010] of augite. X-ray diffraction and electron microprobe data indicate most rodlets are composed of titanomagnetite, and a few are composed of ilmenite. Single crystal photographs of (Okl) in augite contain weak reflections which exhibit two-fold symmetry and an 18.95Å spacing parallel to, but not coincident with [010] of augite. This 18.95Å spacing is much larger than any known in titanomagnetite spinels and may reflect the occurrence of an ordered spinel structure, or, an as yet, unknown phase.