New SHRIMP Ages for Grenville Rocks of the Northern Blue Ridge, Virginia and Maryland: Redefinition of Mesoproterozoic Magmatic and Metamorphic Events

The northern Blue Ridge basement of Virginia and Maryland consists mostly of Mesoproterozoic granitic gneisses. These rocks were deformed and metamorphosed during several Proterozoic and Paleozoic tectonic events. Determination of emplacement ages of protoliths of the gneisses was first attempted by the conventional U-Pb zircon method, using handpicked multi-grain fractions and single grains. The dated zircons were the best-preserved grains from each sample. These analyses yielded slightly discordant to concordant arrays with precise concordia upper intercept ages, which suggested that magmatic activity occurred at about 1.14, 1.11, and 1.05 Ga (Groups 1, 2, and 3, composed of 5, 2, and 6 units, respectively). Because new geochronologic studies of Grenville rocks from Vermont to Georgia show that the Mesoproterozoic gneisses typically contain complexly zoned zircons that record multiple growth events, we re-dated 13 samples from the northern Blue Ridge utilizing the high spatial resolution of the sensitive high resolution ion microprobe (SHRIMP), guided by cathodoluminescence (CL) imaging.

CL images reveal that zircons from all samples are composite grains with igneous cores (oscillatory zoning) and one or more overgrowths (poorly zoned, or unzoned). In some zircons, the igneous component is predominant and overgrown by small, usually dark, irregular metamorphic rims. Other zircons consist primarily (or entirely) of metamorphic components, surrounding small, partially resorbed, cracked igneous cores; these handpicked, least damaged zircons provided the age of metamorphism, not magmatic crystallization. New SHRIMP data indicate that most of the granitic gneisses belong to Group 1, now defined as 1.18-1.14 Ga. Marshall Metagranite (1123 ± 6 Ma) is in Group 2, and only one sample, megacrystic metagranite (1051 ± 7 Ma), remains in Group 3. Based on analyses of 33 additional granitoid samples, the revised age groups extend to the southern Appalachians from Shenandoah National Park to the Great Smoky Mountains National Park.