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Layered Biotite Granulite

Geologic Formation in VA

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Lithology

Granulite

Gneiss

Geologic Age

Mesoproterozoic (1600 Ma – 1000 Ma ago)

Geologic Unit Label

Ygb

Leucocratic to mesocratic, segregation-layered quartzofeldspathic granulite and gneiss contain quartz, plagioclase (albite), microcline (includes assemblages with one alkali feldspar), biotite, ilmenite, and titanite; garnet and horn blende are commonly present. Accessory minerals include apatite and zircon. Epidote and white mica are ubiquitous secondary minerals. Relict pyroxene, largely replaced by actinolitic amphibole, occurs locally. Segregation layering is defined by alternating quartzofeldspathic and biotite-rich domains on the order of a few millimeters to centimeters thick. Quartz and feldspar are granoblastic; biotite defines a penetrative schistosity that crosscuts segregation layering. Migmatitic leucosomes composed of alkali feldspar and blue quartz cut segregation layering, and locally define attenuated isoclinal folds. This unit surrounds pods of layered pyroxene granulite (Ypg), and is cut by Grenville-age metaplutonic rocks including porphyroblastic biotite-plagioclase augen gneiss (Ybg) and alkali feldspar granite (Yal). The unit has been correlated with Flint Hill Gneiss (Yfh) (Evans, 1991), and may correlate with Stage Road layered gneiss of Sinha and Bartholomew (1984). These gneisses have been interpreted as derived from layered pyroxene granulite (Ypg) by retrograde hydration reactions (Evans, 1991).

— USGS

Taxonomy

Classified As

Geologic Formation