Document Type

Thesis

Abstract

Late Proterozoic mafic and ultramafic rock bodies in the eastern Blue Ridge, Madison County, Virginia, are characterized by the presence of amphibole and plagioclase in mafic rocks and chlorite, actinolite, talc, and biotite in ultramafic rocks. Petrographic and chemical analyses reveal these rocks have solidified from a hornblende rich magma are hornblende gabbros and chlorite-actinolite schists respectively. Both types occur within Proterozoic granites and gneisses of the Blue Ridge basement complex and Neoproterozoic metasedimentary rocks of the Lyncburg Group and Mechum River Formation (MRF). Both are interpreted to preserve greenschist facies metamorphism, yet the hornblende gabbros have been previously classified amphibolites (Allen, 1963). The presence of hornblende/actinolite, plagioclase, epidote, and quartz in the hornblende gabbros and chlorite, actinolite, talc and serpentine in the chlorite-actinolite schists are characteristic greenschist facies assemblages and the rocks lack any true metamorphic texture that would be expected in an amphibolite. Field relations on Gallihugh Mountain reveal that intrusion of a mafic sill, followed by fractional crystallization of the magma and subsequent alteration and deformation, resulted in adjacent bodies of hornblende gabbro and chlorite-actinolite schist. This explanation must be further explored in Madison and surrounding counties to explain the mafic-ultramafic association. Tectonically, the rocks are rift-related and intruded ~ 700 Ma prior to the opening of the Iapetus Ocean. Bimodal magmatism including hornblende gabbros and A-type granites of the Robertson River Igneous Suite, and “Ar/>’Ar total gas ages on single hornblende crystals support this origin.

Date Awarded

2000

Department

Geology

Advisor 1

Christopher M. Bailey

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