A PRELIMINARY REPORT ON THE MESOZOIC DIABASE DIKES FROM GEORGIA

Numerous diabase dikes intrude the Piedmont in Georgia. They are part of the system of Mesozoic dikes, sills, and flood basalts that occur in eastern North America from Nova Scotia to Alabama. The petrography and geochemistry of three such dike exposures in central and eastern Georgia are the subject of this study. Field localities include dike outcrops near Sparta, Clark Hill Reservoir, and Macon. A diabase dike near Sparta intrudes the Pennsylvanian Sparta Granite and surrounding amphibolite facies metamorphic rocks of the Kiokee Belt. The dikes near Macon intrude amphibolite facies metamorphic rocks of the Charlotte Belt.

Low water levels in Clark Hill Reservoir provide rare exposures of diabase dikes that strike N25^oW across the NE striking foliation of greenschist facies metavolcanic and metasedimentary rocks of the Neoproterozoic to Cambrian Persimmon Fork and Richtex Formations (Carolina Slate Belt). The near vertical dikes range in width from < 1 m to > 10-m thick and limited exposure suggests discontinuous, en echelon segments.

At Clark Hill the texture is diabasic to subophitic with subhedral plagioclase (An_51-60) composing 50-60% of the rock. A range of An_29-54was determined in grains that exhibited oscillatory zoning. Anhedral to subhedral olivine make up 20-25% of the diabase and have a bimodal size distribution (grain diameter < .1 mm and 1.5-2.5mm). The larger grains exhibit zoning with serpentine-talc alteration on rims and in fractures. Anhedral clinopyroxene comprises nearly 20 % of the rock and opaques make up around 2 % of the rock. The mineralogy of the Sparta and Macon dikes is similar to that from Clark Hill except that pervasive alteration of olivine and pyroxene to chlorite, serpentine, and iron oxides is quite noticeable at Sparta.

Geochemically, the Macon and Clark Hill diabase resemble other Southeastern US diabases reported in South Carolina (Warner et al., 1985 and 1992) and Virginia (Cummins, 1987 and Cummins et al. 1992). The diabase is olivine normative and lies in the olivine normative group proposed by Weigand and Ragland (1970). The Sparta diabase, however, is comparable to the high-Fe quartz tholeiites from west central Georgia (Milla and Ragland, 1992).