Zircon ages of felsic volcanic rocks in the upper precambrian of the blue ridge, appalachian mountains

Five Zirconi samples from Pennsylvania, Virginia, and North Carolina yield discordant uranium-lead ages which suggest an original age of 820 million years and an episodic lead loss at 240 million years. The indicated age of lead loss is interpreted as the age of movement of the Blue Ridge thrust sheet.

Ancient Granite Gneiss in the Black Hills, South Dakota

Granite gneiss, with an age of approximately 2.5 billion years, in the Black Hills, South Dakota, provides a link between ancient rocks in western Wyoming and Montana and in eastern North and South Dakota and Minnesota. The discovery suggests that early Precambrian rocks covered an extensive area in northcentral United States and were not restricted to several small nuclei.

Fission-Track Ages and Track-Annealing Behavior of Some Micas

The density of tracks from spontaneously fissioning uranium atoms in mica has been used to make quantitative measurements of geologic ages. In comparison with methods in which ages are derived from measurements of radioactive decay products, the fission track method is much simpler and is usually accurate for micas up to a few hundred million years old. Annealing experiments suggest that high ambient temperatures are responsible for track fading in the older samples.

Geochronology

In summary, it now seems possible to obtain accurate ages for many of the common rock types. When rocks have been affected by metamorphic episodes after the time of crystallization, it is possible to recognize this from the age values and, in some cases, to date both the time of crystallization and the time of metamorphism. Work is under way that will make it possible to draw accurate geochronological maps of the continents. An impressive regularity is already evident in the map for North America. Knowledge of the temporal and areal distribution of ages should have an important bearing on theories of the origin of continents and of mountain-building processes (50).