AGE AND ORIGIN OF THE ALBEMARLE-NELSON MAFIC-ULTRAMAFIC COMPLEX IN VIRGINIA'S EASTERN BLUE RIDGE: RIFT-RELATED DIFFERENTIATED SILLS?

In central Virginia, a suite of altered and metamorphosed mafic to ultramafic rocks intruded Neoproterozoic metasedimentary rocks in the eastern Blue Ridge. In Albemarle and Nelson counties, soapstone has been extensively quarried from this complex since the late 19^th century. The age and origin of these soapstone deposits has remained enigmatic for more than a century. Our research utilizes detailed mapping, analysis of ~600 m of rock core (from 13 holes) obtained during an exploratory drilling project in 2021, petrological and chemical analysis, and U-Pb zircon geochronology to understand the genesis of these mafic-ultramafic rocks.

Mafic and ultramafic bodies are parallel to the NNE-SSW regional trend of the eastern Blue Ridge. Soapstone bodies form tabular, sheet-like bodies up to ~150 m thick that dip 45˚ to 65˚ SE with a concordant relationship to bedding in metasedimentary rocks of the Neoproterozoic Lynchburg Group. Ultramafic rocks include massive to foliated talc-chlorite-carbonate soapstone, serpentinite, and minor hornblendite with 38-45% SiO2 and Mg# >73. Normatively, most ultramafic rocks plot as lherzolite and olivine websterite. Gabbroic rocks range from 40 to 50% SiO2 and Mg# ~70.

Massive intermediate to felsic rocks (60-70% SiO2) are rare but present as localized layers at the top of the larger bodies and as thin dikes that intrude into the soapstone and overlying country rock. Coarse-grained granodiorite consisting dominantly of recrystallized plagioclase and quartz as well as hornblende and chlorite contain ~70% SiO2. Fine-grained felsites consisting of a plagioclase and quartz matrix with epidote and biotite range from 60 to 64% SiO2. All intermediate/felsic samples have Mg# <10.

U-Pb zircon geochronology of the granodiorite yielded a mean age of ~592 Ma. Samples of felsite contained primarily zircons of Grenvillian age (1.0 Ga to 1.2 Ga), but <10% of the grains contain rims with ages of ~576 ± 4 Ma. We propose that the felsites are primarily derived from melted Neoproterozoic country rock that incorporated rather than dissolved Grenvillian detrital zircons. Collectively, these geochronological data indicate that the mafic-ultramafic complex is 575 to 590 Ma and is related to Iapetan rifting in the Ediacaran.