1. Structure and Petrography of Some Diabase Dikes in Central
South Carolina PRIVETT, DONALD R., Dept. Geology, University of
Tennessee, Knoxville, Tenn. Diabase dikes of questionable Triassic
age intrude metamorphic rocks and granites in the Piedmont
physiographic province north and west of Columbia, South Carolina.
Deep chemical weathering prevents accurate determination of the
size and orientation of the dikes. In general, they strike 20--40
NW., and at two outcrops in roadcuts dip steeply 80-90 NE. Widths
range from 2 to 300 feet, averaging 30 to 50 feet, whereas lengths
range from about 300 feet up to 7 miles. The dike rocks are olivine
diabases with ophitic-to-intergranular texture. Major mineral modes
range from 40-60 per cent plagioclase (An61), 13-36 per cent augite
(Ca43Mg46Fe11), 10-30 percent olivine (Fa31), and 1-5 per cent
magnetite-ilmenite. Accessory and secondary minerals present are
tremolite-actinolite, apatite, biotite, hornblende, and leucoxene.
Plagioclase is generally unaltered and occurs both as phenocrysts
and interstitial laths. Augite is altered to uralite along cracks
and gtrain boundaries, and olivine is always partly, sometimes
completely, altered to serpentine minerals and magnetite. Two dikes
sampled at measured intervals from contact to contact showed modal
increases in plagioclase from peripheries to centers, whereas
olivine-plus-pyroxene decreased. Olivine modal percentage varies
inversely with pyroxene mode. However, when considered separately,
the olivine-pyroxene modes fluctuate, showing random variation
across the dikes. No systematic transverse variation in the
compositions of the major minerals was noted.