Rb-Sr isotopic data document Alleghanian ages for the metamorphic event which formed the upper greenschist- to amphibolite-grade Raleigh belt and for the D3 deformational event which formed regional-scale folds and mylonite zones in the eastern North Carolina Piedmont. The Butterwood Creek granite, which straddles the eastern boundary between the Raleigh belt and the Eastern slate belt, has a whole-rock isochron age of 292 ± 31 Ma. Biotite ages indicate that the eastern, undeformed part of the pluton cooled rapidly after intrusion, whereas a younger biotite age of 251 Ma indicates slow cooling for the western, foliated portion of the granite which lies in the Raleigh belt. Other biotite ages within the Raleigh belt range from 247–238 Ma.
The thermal maximum for the Raleigh belt probably accompanied the intrusion of numerous granitic plutons during the time 315 to 285 Ma. The biotite ages indicate that the Raleigh belt remained at elevated temperatures and did not cool to the blocking temperature of biotite until 250–240 Ma. Rapid cooling of the eastern part of the Butterwood Creek helps define the eastern boundary of the Alleghanian thermal event, which corresponds to the eastern limit of the high-grade rocks of the Raleigh belt.
The S3 tectonic biotite foliation in the western part of the Butterwood Creek and in other Alleghanian plutons of the Raleigh belt, associated regional F3 folds, and the D3 Hollister mylonite zone which cuts the Butterwood Creek pluton formed between the time of intrusion of the Butterwood Creek at 292 Ma and regional cooling by 250–240 Ma.
This study further defines the narrow Alleghanian thermotectonic belt along the eastern Piedmont which is known as the Raleigh belt in Virginia and North Carolina and reappears as the Kiokee belt in South Carolina and Georgia.