SHRIMP U-PB GEOCHRONOLOGY OF GRENVILLE GNEISSES AND GRANITOIDS, SHENANDOAH NATIONAL PARK REGION, NORTHERN VIRGINIA

An integrated program of field mapping, petrology, and U-Pb geochronology has led to a more comprehensive understanding of the origin of Grenvillian high-grade rocks, including felsic gneisses, charnockites, and leucogranitoids, in the region of Shenandoah National Park (SNP), northern Virginia. Because zircons from these rocks frequently contain multiple age components, determining the emplacement sequence of members of the suite requires high spatial resolution microanalysis (by SHRIMP), in conjunction with cathodoluminescence (CL) imaging. Zircon morphology is key to interpreting age data. Euhedral, prismatic grains (typically with concentric, oscillatory CL zoning) generally are interpreted as igneous in origin, whereas equant, multi-faceted grains (with patchy CL zoning) likely formed during amphibolite- to granulite-grade metamorphism. Some prismatic grains also display one or more overgrowths, usually distinguishable from the igneous component by lack of CL zoning and much lower Th/U. Rarely, metamorphic overgrowths are so extensive that the igneous precursors of the grains are nearly obliterated, existing only as small remnant cores. Subhedral to anhedral zircons from a garnetiferous granite exhibit multiple irregular CL zones present another challenge. Because these grains lack typical concentric CL zoning, it is impossible to determine which component (if any) represents the original igneous portion. A possible solution is provided by ages from co-existing monazite, although these grains also have multiple age zones. At the other extreme, nearly unzoned, undeformed, colorless zircons from the youngest sample mostly contain only one age component and are easily dated.

So far, 15 samples from the SNP area have been dated by U-Pb geochronology. Although few clear-cut relative age relationships have been found in the field, age data plus deformation fabric of each map unit allow subdivision of the basement: (1) Group 1, highly foliated, ~1185-1150 Ma; (2) Group 2, variably foliated, ~1120-1110 Ma; (3) Group 3, non-foliated, ~1080-1030 Ma. Although most of the zircons from our samples are complexly zoned, SHRIMP microanalysis of identifiable single-age zones enables detailed interpretation of the history of emplacement and metamorphism of these Grenvillian rocks.