PROTEROZOIC AND PALEOZOIC THERMAL HISTORY OF THE GRENVILLIAN BLUE RIDGE TERRANE, CENTRAL VIRGINIA, AS PRESERVED BY COMPLEXLY ZONED MONAZITES

Igneous and granulite-facies metamorphic rocks of the Blue Ridge Terrane in central Virginia clearly show Grenvillian ages (ca. 1000 – 1050 Ma), based on published traditional U-Pb zircon geochronology. However, these rocks were subjected to multiple younger thermal events during Paleozoic tectonic activity in the central Appalachians. Monazites within these high grade rocks preserve a record of this thermal history that can be deduced with electron microprobe dating. Monazites from garnet + biotite quartzofeldspathic gneisses near Roanoke, Virginia were analyzed for U, Th, Pb, and Y by electron microprobe. These monazites are low and essentially unzoned in Y (1000-2000 ppm) and U (600-2600 ppm). However, they are strongly zoned in Th, with Th-rich interior portions of grains that reach values as high as 15 wt% and Th as low as 1-2 wt% in marginal overgrowths. Incipient alteration of monazite to thorite + secondary silicate(allanite?) + apatite occurs along fractures within the monazite grains. Ages calculated from electron microprobe data span a wide spectrum from 450 Ma to over 1050 Ma. The majority of data clearly define a Grenville age (1001 ±20 Ma) for the apparent initial crystallization of monazite. The remainder of the data indicate partial reequilibration and age resetting during later thermal events, particularly along some (but not all) margins and along fractures. The youngest dates most likely correspond to age resetting as a result of the thrust transport of the Blue Ridge terrane during either the Taconian or Alleghanian orogenies or both. Absence of ages younger than about 450 Ma suggests that well known younger tectonic events such as the stacking of Alleghanian thrust slices is probably not recorded by the monazite. This implies that Taconian-age deformation took place at sufficiently elevated T and P, or in the presence of aqueous fluids, to allow monazite to be partially reset down to that age (ca. 460-480 Ma), whereas Alleghanian tectonic events apparently were not accompanied by conditions sufficient to allow resetting.