Monroe Uplift

Igneous rocks have been encountered in nearly 100 boreholes across the Monroe Uplift, including granite, syenite, pyroxenite, dolerite, basalt, nepheline syenite, phonolite, lamprophyres and pyroclastics. Nepheline syenites are usually altered but fresh phonolites and tinguaites have been described from a number of boreholes, for example the Perkins and White, Lee No. 1 Well in Washington County, Mississippi (Kidwell, 1951, p. 193). A pyroxenite with a little nepheline was encountered in the Myers No. 1 Well in Tunica County, Mississippi, at 2736-9 m (Harrelson and Bicker, 1979, p. 249), and monchiquites have been encountered in many boreholes. Pyroclastic rocks are generally more abundant than intrusives but have been little studied, although they appear to be essentially alkaline. A borehole drilled in the southern part of Humphreys County, Mississippi, cored igneous rocks continuously for 613 m including flows, hypabyssal rocks and pyroclastics (Sundeen and Cook, 1977). Rocks described in detail include a phonolite of 10% augite and 15% fresh euhedral nepheline phenocrysts in a matrix of 25% pyroxene, 10% nepheline, 15% clear analcime and a little plagioclase; and a biotite analcimite of somewhat altered aegirine-augite (25%), nepheline (2%), altered plagioclase (5%) and fresh brown biotite phenocrysts (5%) in a groundmass of nepheline, clear analcime and plagioclase.