EVIDENCE FOR HIGH PRESSURE FRACTIONATION IN A DIABASE DIKE NEAR ELBERTON GA

A vertical diabase dike striking N 10^o W is exceptionally well-exposed in an Elberton granite quarry where quarry walls as well as pavement surfaces expose the 6 m wide dike in three dimensions. Fresh samples of a chilled contact zone and the medium-grained dike interior reveal intriguing petrology. Although the interior of the dike is olivine-rich (~25%, Fo70-25), the chill zone contains only plagioclase microphenocrysts in a “micro-komatiitic” groundmass where the dendritic crystals are augite. In addition to olivine, the clinopyroxene (Mg# = 62 to 9) and plagioclase (An67-20) show extensive zoning in the dike interior consistent with closed system crystallization. Even though olivine is absent from the chill zone, the bulk composition (Mg# ~ 43), low Ni content (~75 ppm) and low Cr (~ 30 ppm) content attest to olivine and spinel fractionation prior to emplacement in the upper crust. The dike is slightly ne-normative and strongly ol-normative; compared to most other olivine normative dikes in the southeast, this dike is quite evolved. We suggest that the parent magma to the dike fractionated olivine and spinel in mid to lower crust prior to ascent – the resulting decrease in pressure changed the liquidus phases from olivine + spinel to plagioclase. Most puzzling is that the high Fe content (~14.5 wt % FeO) would have made the diabase magma quite dense.