HAFNIUM ISOTOPIC ANALYSIS OF THE CHOPAWAMSIC ARC, APPALACHIAN PIEDMONT OF VIRGINIA

The 474 – 465 Ma Chopawamsic magmatic arc (CA) in the Piedmont of Virginia has been tentatively interpreted to be underlain by an unexposed, peri-Gondwanan, Mesoproterozoic basement. It is separated from the Laurentian-derived Potomac terrane to the west by the Chopawamsic fault system, which is interpreted to represent the main Iapetan suture, previously unidentified in the southern Appalachians. In order to better understand the magmatic nature and origin of the arc terrane, we present LA-ICP-MS Hafnium isotope analyses of single zircon crystals from six magmatic rocks in the CA. Analytical uncertainties for all samples in this study result in an average 2-sigma error of ± 0.98 εHf units.

Zircons from metavolcanic samples from the northern extent of the CA collected along Abel Lake (n=12 zircons, KSH-11-21, 465 Ma) and Chopawamsic Creek (n=8, KSH-11-29, 468 Ma) yield similar positive εHf values with ranges of 1.8 to 10.1 and 5.2 to 9.0, respectively. Outside of a lone value of -0.1, zircons from a metavolcanic sample from the southern extent of the CA collected along the South Anna River (z=21 zircons, KSH-84, 468 Ma) yield negative εHf values between -6.6 and -4.3.

Seventeen of 22 zircons from one sample of the Richland Run pluton (KSH-12-02, 474 Ma) in the CA yield εHf values from -1.7 to 1.7 while the other 5 zircons yield progressively negative values to as low as -22.6. Zircons from a diorite (z=15 zircons, KSH-12-32, 468 Ma) and a felsite (n=4 zircons, KSH-12-33, 469 Ma) which both intrude the Garrisonville mafic complex of the CA yield ranges of 1.3 to 3.9 (with one outlier at 10.1) and -4.9 to 0.7, respectively.

The heterogeneity of the εHf signatures of zircons analyzed in this study could result from multiple situations, but an origin above evolved crust is still supported for the Chopawamsic arc. The difference in Hafnium signatures for coeval metavolcanic rocks could be a result of a spatially heterogeneous basement complex. The εHf signatures of the intrusive rocks overlap but have distinct populations in each sample, and may reflect a temporal evolution in magma composition. Very negative εHf values in zircons from the Richland Run pluton may be inherited basement xenocrysts. The Hafnium results here will be synthesized with pending whole-rock Neodymium analyses.