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New Ages on Some Old Rocks

ome of the oldest rocks in the Appalachian Mountains are exposed in the Virginia Blue Ridge. At the Geological Society of America’s annual meeting in November, Richard Tollo from George Washington University and John Aleinikoff from the U.S. Geological Survey report new isotopic age dates from the northern Virginia Blue Ridge.

The pair analyzed zircon grains from a suite of igneous and metamorphic rocks that crop out in the core of the Blue Ridge anticlinorium. Zircon (ZrSiO₄) is a common accessory mineral that forms during the solidification process in granitic rocks. A small amount of uranium (U) commonly substitutes for zirconium (Zr) in the zircon crystal lattice. Over time uranium isotopes undergo radioactive decay and form lead (Pb) isotopes. By measuring the ratio of particular Pb isotopes to U isotopes, geologists are able to determine the solidification age of the rock.

Chronology of Grenvillian Events in the northern Virginia Blue Ridge Province

Tollo and Aleinikoff report evidence for distinct episodes of igneous activity in the Blue Ridge. The early event includes granitic and charnockitic (granite containing the mineral pyroxene) rocks that formed between 1,155 Ma (million years ago) and 1,145 Ma. Over the next 70 million years the only magmatic activity in the northern Virginia Blue Ridge is associated with the emplacement of a 1,110 Ma granite near Marshall in Fauquier County. A second major episode of igneous activity and deformation occurred between 1,080 Ma and 1,050 Ma. The Old Rag granite, exposed in Shenandoah National Park, formed during this later event at 1,065 ± 6 Ma. The formation of these igneous rocks is related to a period of tectonic activity and mountain building known as the Grenvillian Orogeny. This mountain building event resulted from plate collision along the eastern margin of North America and affected rocks from Texas to northeastern Quebec.

Zircon grains from high-silica charnockite exposed at Mary's Rock in Shenandoah National Park imaged under cathodoluminescence radiation. Note concentric oscillatory compositional zoning typical of crystal growth from igneous melts. The zircon cores yield an age of 1,158± 14 Ma and corresponds to a period of widespread magmatism throughout the Blue Ridge.

The rims yield an age of 1,053 ± 11 Ma and are associated with a second episode of magmatism, metamorphism, and deformation. Individual grains range from 100 to 500 microns in length.

Tollo R. P., and Aleinikoff, J. N., 2001, Timing of Grenville-age magmatism and deformation, Blue Ridge province, central Virginia, Geological Society of America Abstracts with Programs, vol., 33, no. 2, pg. 7.

Tollo R. P., and Aleinikoff, J. N., 2001, Petrologic and temporal characteristics of Grenville-age orogenesis, Blue Ridge province, Virginia Geological Society of America Abstracts with Programs, vol., 33, no. 7, pg. 28.

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