MORPHOLOGY AND SHRIMP U-PB AGES OF ZIRCON AND MONAZITE FROM HIGH-GRADE ROCKS OF THE WILMINGTON COMPLEX, DELAWARE

The Wilmington Complex (WC) of Delaware is composed of high-grade rocks of arc affinities. Previous efforts to determine conventional U-Pb ages of WC rocks using multigrain fractions of zircon were hampered by the common occurrence of metamorphic overgrowths. We have used the sensitive high resolution ion microprobe (SHRIMP) for in situ spot analyses of zones within complex zircons to measure the different age components. Several WC lithodemes as defined on the new Bedrock Geologic Map of the Delaware Piedmont were dated. Intrusive units with ages interpreted as the time of igneous crystallization include: 1) Christianstead Gneiss (485±7 Ma), 2) Brandywine Blue Gneiss (476±6 Ma), and 3) Barley Mill Gneiss (468±9 Ma). Zircons from these rocks are elongate and prismatic with concentric, oscillatory zoning in cathodoluminescence (CL); they are interpreted to be igneous. However, based on our new age data, some elongate zircons formed during local partial melting accompanying Silurian granulite facies metamorphism. Ages of metavolcanic units are: 4) Windy Hills Gneiss (481±4 Ma, with Grenville-age detrital zircons), 5) Faulkland Gneiss (482±4 Ma), and 6) Rockford Park Gneiss (476±4 Ma). Four samples from WC contain both elongate and equant zircons. On the basis of CL zoning and age, equant zircons are interpreted as detrital, igneous, or metamorphic in origin. The Arden Pluton (434±4 Ma) cuts the older igneous rocks and their internal metamorphic foliation. Concordance of the time of emplacement of the Arden Pluton with metamorphic zircon and monazite ages (Faulkland Gneiss 428±8 Ma; Brandywine Blue Gneiss 428±4 Ma; Rockford Park Gneiss 432±6 Ma) suggests that magmatism was associated with regional-scale metamorphism, although the nature of this Silurian event is unclear. The new ages for WC are 20-30 m.y. older than ages from the supposedly correlative James Run Formation in Maryland. However, they are quite similar to ages from arc units within the Taconide and Bronson Hill sequences in VT and NH.