AGE AND TECTONIC SIGNIFICANCE OF THE ELK HILL VOLCANIC COMPLEX AND PEGMATITE BELT, CENTRAL APPALACHIAN PIEDMONT, VIRGINIA, USA

The Elk Hill Volcanic Complex (EHVC) and Pegmatite Belt (PB) are two elongate fault-bounded blocks in central Virginia that lie between the larger western and eastern Piedmont bedrock tracts. The EHVC and the PB are known to be lithologically and structurally distinct from the surrounding Chopawamsic and Goochland terranes. Even though first described more than a century ago, very little modern geochemical analysis and no previous geochronological analysis have been used to elucidate their origins and roles in the tectonic evolution of the Appalachian orogen.

One sample, a granodioritic gneiss, was collected from the type area of the EHVC along the east side of the James River 5.9 km upstream of Cartersville. Nine individual zircons analyzed using ID-TIMS define an upper-intercept age of 331 ± 10 Ma. This age of the EHVC indicates it is not a part of the much larger, Ordovician Chopawamsic volcanic arc to the northwest. The EHVC, bounded by the Spotsylvania Shear Zone to the southeast may actually be an along-strike equivalent of syn-kinematic intrusive rocks (335-320 Ma) in the Hyco Shear Zone at the latitude of the VA-NC border. Both the Spotsylvania and Hyco Shear Zones are part of the larger Alleghanian Central Piedmont Shear Zone system which extends along the entire length of the southern Appalachian orogen. More geochemical and petrographic scrutiny is needed to evaluate this possibility.

Two samples from the PB were collected for TIMS analysis. One sample, a medium-grained garnetiferous granitic gneiss, was taken from just west of Mill Creek 2.7 km west of Hadensville. Eleven zircons from this sample yield an upper-intercept age of 423.7 ± 1.5 Ma. Five monazite grains from the same sample yield a concordant age of 325.2 ± 1.3 Ma. The second sample, a medium-grained leuco-granitic gneiss, is from an outcrop on the south side of the James River 3.4 km downstream of Columbia. Seven zircons from it define an upper-intercept age of 427.9 ± 4.3 Ma. Two zircons from this sample have ca. 1 Ga ages and are interpreted to be xenocrysts. These Silurian crystallization ages for the PB indicate it is not coeval with the EHVC or the Chopawamsic terrane. A similar age to the Siluro-Devonian Concord-Salisbury plutonic suite to the south in the large, peri-Gondwanan block of Carolinia invites further comparison.