TEMPORAL AND SPATIAL DISTRIBUTION OF CRYOGENIAN AGE IGNEOUS ROCKS IN THE CENTRAL APPALACHIAN OROGEN: A SINGLE HOT SPOT INDUCED MAGMATISM?

The Grenville basement of North Carolina and Virginia host over thirty late Precambrian volcanic/plutonic complexes. A-type granites and rhyolites dominate the igneous complexes, but compositional variations range from gabbros through syenites. Mineralogy, chemical composition and field data suggest emplacement of these igneous complexes in an extensional setting. In a new effort to better represent the geodynamic setting of these rocks, ion probe U/Pb ages as well as chemical data were gathered for eight of these igneous bodies. The U/Pb ages of zircons suggest two episodes of magmatism within this belt. An older event (739 to 745 Ma) includes White Oak Creek, Suck Mountain and Amisville plutons. The younger episode (613 to 694 Ma) includes Dillons Mill, Stewartsville, Mobley Mountain and Rockfish River plutons. We also note differences in both geochemical and zircon morphological properties of the two age groups. In contrast to the older group, the younger plutons are more metaluminous, lower in silica, higher in aluminum and phosphorous, have lower agpaitic index and less REE enrichment with significantly less feldspar and accessory mineral fractionation. Zircons from the younger plutons show few overgrowths and cores, and are dominated by {100} faces. The spatial distribution of the older group of plutons, coupled with our model of utilizing plume head arrival ages (765 to 754 Ma in the southern part of the region and 735 to 705 Ma in the northern part), suggests a continental plume track with a plate motion rate of ~2cm/yr. The younger ages are more enigmatic, but it is interesting to note that they lie predominantly east of the older axis of magmatism and are similar in age to the Fine Creek Mills pluton of the Goochland lithotectonic block. Removal of Alleghanian displacement through palinspastic reconstruction of the Blue Ridge basement in the region of our study locates the younger plutons in a belt coincident with those found in the Goochland lithotectonic block. New tectonic models for a time period that is transitional to the eruption of the Catoctin Igneous Province and the older event are required if the younger plutons represent a different hot spot/crust interaction.