CONTRASTING LU-HF AND SM-ND AGES IN GARNETIFEROUS GNEISSES OF THE BLUE RIDGE PROVINCE, VIRGINIA: IMPLICATIONS FOR THE TIMING AND DURATION OF THE GRENVILLIAN OROGENY IN THE CENTRAL APPALACHIANS

In order to provide better constraints on the timing of Grenvillian metamorphism in the central Appalachians, we analyzed garnet-bearing Mesoproterozoic orthogneiss (n=2) and paragneiss (n=6) samples using combined Lu-Hf and Sm-Nd geochronology. The samples were collected along a 150 km transect in the northern Blue Ridge Province in Virginia. The orthogneisses have U-Pb zircon crystallization ages of ~1140 and 1100 Ma. The paragneisses have maximum depositional ages between about 1050 and 1020 Ma, based on the youngest detrital zircon populations. Zircon overgrowths and monazite suggest several metamorphic events between about 1050 and 960 Ma.

Lu-Hf data for garnet from all of these samples result in well-defined ages that fall in a relatively narrow time span (1043 ± 12 Ma to 1016 ± 4 Ma; weighted mean age of 1026 ± 10 Ma, 2 sigma). Sm-Nd garnet ages, determined on the same dissolutions as were used for the Lu-Hf analyses, also define a narrow age range (974 ± 11 Ma to 932 ± 5 Ma; weighted mean age of 959 ± 18 Ma), but are systematically and significantly younger than the Lu-Hf ages. The oldest Sm-Nd age is ~ 40 Ma younger than the youngest Lu-Hf age. Ages from all samples are robust; regressions have large spreads in parent/daughter ratios and low MSWD values.

It is not uncommon for garnet Sm-Nd ages to be slightly younger than corresponding Lu-Hf ages because: 1) strong partitioning of Lu into the core at the beginning of garnet growth; and 2) the higher closure temperature for the Lu-Hf system compared to Sm-Nd (e.g., Scherer et al., 2001). However, the very large and systematic differences observed in the Lu-Hf and Sm-Nd ages in our Blue Ridge province garnetiferous rocks are unusual. None of the garnets have significant Lu/Hf or Sm/Nd zoning, which likely indicates equilibration of the garnets subsequent to their growth. Therefore, differences in elemental partitioning during garnet growth cannot explain the age variations. Instead, we interpret the younger Sm-Nd ages as being due to differences in closure temperatures; the Lu-Hf system closed soon after garnet growth at ~ 1028 Ma whereas the Sm-Nd system closed at ~ 970 to 930 Ma. These age data require that the rocks remained at elevated temperatures and pressures for tens of millions of years, presumably deep within thickened crust, during the culmination of the Grenvillian orogeny.