ROCKS, STRUCTURES AND GEOLOGIC RELATIONSHIPS OF THE SPRING HOPE TERRANE, NORTHEASTERN NORTH CAROLINA PIEDMONT

The geology of the northeastern North Carolina Piedmont is gradually becoming clearer as mapping progresses under the STATEMAP program. The Neoproterozoic Spring Hope terrane constitutes a suprastructural volcanic arc component of the Carolina Zone. In this region, it is bounded to the west by the infrastructural Raleigh/Warren terrane along the Macon fault, and to the east it is separated from the Roanoke Rapids and Triplet terranes by the Hollister fault. Both faults show strong evidence of Late Paleozoic dextral motion; the Macon fault may have experienced earlier tops-to-the-east thrust motion as well. Numerous ca. 300-Ma granitic plutons intrude the region; they vary from undeformed to mylonitic.

Metasedimentary rocks having protoliths of volcanogenic siltstone, sandstone, mudstone and rare conglomerate dominate the Spring Hope terrane in this area. Features such as cyclic and graded bedding and current ripples suggest that they were deposited, at least in part, as turbidites. Metavolcanic rocks include metadacite and metarhyodacite as well as metabasalt and rare intermediate rocks. Chemically, these rocks are strongly bimodal, with felsic varieties more common than mafic. They originated as pyroclastic deposits and as flows. Plutonic rocks of the arc complex are extremely rare. Overall, rock characteristics suggest a relatively distal volcanic arc setting.

South-plunging map-scale folds include the Spring Hope synform, and most of the region has experienced chlorite- to biotite-zone greenschist facies metamorphism. In the northwestern part of the area, near the Macon fault, metamorphic grade rises to mid- to upper amphibolite facies, and near some of the shallower granitic plutons, Spring Hope rocks are at hornblende-hornfels grade.

Early Mesozoic dikes intrude the region; these include rhyolite porphyry in addition to the more common olivine diabase. Locally, siliceous breccia indicates late faulting, possibly also of early Mesozoic age.

Establishment of a definitive stratigraphy for the Spring Hope terrane has been elusive. Some key areas still await mapping, no fossils have as yet been found, and radiometric age-dates are sparse. New data support the conclusion that stratigraphic models proposed by Farrar (1985) and Carpenter et al. (1995) need to be reworked.