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Bassett Formation - Biotite gneiss and granite gneiss. (VAZbg;0)

Geologic Formation in VA

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Lithology

Gneiss

Geologic Age

Neoproterozoic (1000 Ma – 542 Ma ago)

Geologic Unit Label

Zbg

Light-gray to black-and-white, fine to medium-grained, leucocratic biotite gneiss that is mostly segregation-layered, but locally is a medium-grained quartzfeldspar granofels. Contains interlayers of muscovite-biotite schist, quartz schist, and epidote quartzite. Mineral assemblages: (1) quartz + plagioclase + potassium feldspar + biotite + muscovite + magnetite-ilmenite + tourmaline ± kyanite ± epidote ± titanite ± hornblende ± garnet; (2) quartz + plagioclase + epidote + pyroxene. Porphyroclasts of zoned plagioclase in an equigranular, polygonal quartz-potassium feldspar groundmass and medium to thick bedding suggest a volcaniclastic protolith (Conley, 1985). Gneisses are migmatitic and cut by numerous granite dikes and sills near the contacts with the Martinsville igneous complex. Geophysical signature: potassium feldspar-bearing gneisses have positive radiometric, and generally flat magnetic signatures relative to adjacent amphibolite units. In the core of the Sherwill anticline (Campbell and Appomattox counties), the dominant rock-type is graded salt-andpepper metagraywacke, interbedded with lesser mica schist and graphite schist. This association bears lithologic affinity to the Lynchburg Group, which occupies the cores of structural domes to the west; this correlation has been made by several workers (Brown, 1958; Kaldy, 1977; Gates, 1987).

— USGS

Taxonomy

Classified As

Geologic Formation