Evidence for high pressure fractionation of olivine-normative dikes from the Central Atlantic Magmatic Province in Georgia
Clark, Steven Daniel
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Evolved, highly olivine-normative diabase dikes of the Central Atlantic Magmatic Province (CAMP) crosscut 6 active quarries in the Georgia Piedmont. Chill zone and dike interior samples were collected to study the magmatic evolution of these dikes. Plagioclase phenocrysts (An60-An52 for the most evolved dike) dominate the chill margin mineralogy of these dikes with only trace olivine microphenocrysts (Fo71-Fo67 for the most evolved dike), some dikes having a chill margin modal plagioclase:olivine ratio of more than 100:1. These dikes have very evolved compositions, evidenced by low Mg# (44-55), Ni (66-118 ppm) and Cr (<5-43 ppm), consistent with olivine and chromian spinel fractionation. They are also characterized by clinopyroxene ranging to very Fe-rich compositions and high contents of bulk incompatible major and trace elements. These observations are rationalized in a model that includes high pressure (~0.7-1.2 GPa) fractionation of olivine, spinel, plagioclase and augite from the parent magmas of these dikes.