A TALE OF TWO FABRICS: AN EBSD INVESTIGATION OF CRYSTAL POPULATIONS IN A DOLERITE DIKE

The texture and fabric of igneous rock are the sum result of the cooling and transport history of the system. In magmatic dikes, both cooling and transport rates can be significant, resulting in flow-aligned crystals and potentially a variety of crystals shapes. Electron backscatter diffraction (EBSD) is capable of identifying both phase and phase orientation, and as such, is well-posed for dike textural analyses. We present the results of EBSD analysis of the fine-grained center of a small dolerite dike (~20 cm wide). The dike is located in the Keystone Blue Quarry in Carlton, Georgia. It was generated as part of the broader Central Atlantic Magmatic Province (~200 Ma), associated with the breakup of Pangea. The dolerite is olivine-normative, with the dominant phase being plagioclase (An₆₄-An₅₀). Based on EBSD results, crystal preferred orientation (CPO) plots and stereological estimates of crystal shape for plagioclase provide differing results as a function of crystal size. The finer population of crystals are prismatic (L-I-S: 2.8-1.2-1), while coarse crystals are more tablet shaped (L-I-S: 3.2-2.3-1), and are often swallow-tailed. While both crystal populations display lineated fabric, differences in CPO indicate different responses to fluid flow. Of special note, the finer population of crystals displays a bimodal and orthogonal alignment. We interpret this to be the result of streamline-parallel crystal alignment as well as vorticity-alignment (i.e. “log-rolling”). The two crystal populations are likely a reflection of crystals brought in with the magma (coarse crystals) and crystals nucleated during transport. This suggests that cooling history affects the fabric development in magmas.