MINERALOGY AND BULK-ROCK GEOCHEMISTRY OF OLIVINE-NORMATIVE DIKES IN GEORGIA AND SOUTH CAROLINA

A number of olivine-normative diabase dikes in Georgia, and one in South Carolina, have been analyzed, including a basanite dike from Elberton, GA. These dikes are all northwest-trending and believed to be associated with the Central Atlantic Magmatic Province (CAMP) formed during the separation of Laurentia from Gondwana at 200±3 Ma.

Fe-Mg silicates in the dike interiors show significant variation (e.g., augite to ferroaugite or pigeonite) indicative of fractional crystallization prior to and following emplacement. All of the dikes are exposed in active quarries, so that the chill zone and initial phenocryst population could be characterized and used to help infer the course of magmatic evolution. The basanite dike, which crosscuts Elberton granite, displays typical alkaline characteristics such as low SiO2, high alkalis, and high bulk Ba and Sr.

Several olivine- and nepheline-normative dikes in the Georgia Piedmont show evidence of prior olivine and chromian spinel fractionation, as evidenced by low bulk Ni and Cr abundances and low Mg# in phenocrysts. Based on bulk compositions, we expected to see abundant olivines in the chill zones. However, chill zones commonly contain abundant plagioclase with only rare fresh or altered olivine phenocrysts. Moreover, olivines in the Georgia Piedmont dikes are quite Fe-rich (Fo 0.27-0.73) and have Ni less than 0.12 wt.%. None of the Georgia Piedmont dikes that we analyzed contain Cr-rich spinel. Consequently, chill zone mineralogy is inconsistent with bulk rock compositions.

In contrast, an olivine-normative dike in South Carolina, near Columbia, is a more primitive magma with high bulk Ni and Cr. Olivine grains in the dike have relatively high Fo (0.67-0.89) contents compared to olivines (Fo 0.27-0.73) from the Georgia Piedmont dikes. In addition, olivine grains from the South Carolina dike had higher Ni (0.10-0.28 wt.%). The chilled margin as well as the interior of the dike had abundant chromian spinel along with titanomagnetite, but the chilled margin is particularly rich in chromian spinel. The chromian spinel commonly occurs as inclusions within olivine grains. Therefore, the parental magma for this dike did not experience significant fractionation of olivine and chromian spinel prior to intrusion into the upper crust.