THE SPRING HOPE TERRANE: LITHOSTRATIGRAPHY AND NEW AGE CONSTRAINTS

New STATEMAP geologic mapping and geochronology elucidates some characteristics of the Spring Hope terrane. In the Centerville and Castalia Quadrangles, poorly bedded greenschist grade metagraywacke and metasiltstone dominate, with minor meta-arenite and mudstone. Clastic textures are common; sedimentary structures include scour-and-fill and graded bedding. Sucrosic, granular quartz rocks are interpreted as pods of recrystallized hydrothermal quartz. A nearly concordant U-Pb zircon age of 628 +/- 1.7 Ma has been determined on a dacitic crystal tuff unit within a metagraywacke sequence along Sandy Creek in the southernmost Centerville Quadrangle. In the northern and western parts of the area, pelitic schist and two-mica semischist are amphibolite-facies equivalents of lower grade metasediments.

Metavolcanic rocks are strongly bimodal in composition, and have been overprinted by amphibolite or hornblende-hornfels facies metamorphism near Alleghanian granitoid plutons in the west and northwest. Mafic rocks are greenstone and locally amygdaloidal metabasalt. Felsic metavolcanic rocks include dacitic and rhyodacitic crystal and crystal-lithic tuffs. Metavolcanics also include distinctive bluish, gray, or white weathering, thinly layered and locally fissile fine-grained rocks having a strongly recrystallized quartz-feldspar mosaic matrix and local plagioclase phenocrysts. These dacitic rocks are interpreted as pyroclastics or lavas. Eight zircon fractions from an exposure of the dacite are discordant with an upper intercept age of 524.9 +/- 8.6 Ma. Among the youngest dates from the Carolina Zone, this age overlaps with the likely time of deposition of clastic units such as the Yadkin and Asbill Pond formations (Pollock, 2007; Hibbard et al., 2002).

Spring Hope rocks thus span some 100 Ma and may cluster into several age groups (cf. Hibbard et al., 2002, 2007), two of which may correspond to the Virgilina/Hyco and Albemarle sequences. They also support the idea that the Mill Creek orthogneiss (620 Ma; Goldberg, 1994) in the Flowers quad belongs to the Spring Hope terrane and not the Raleigh terrane. Details of stratigraphy require more work, but the Spring Hope terrane apparently represents a long-lived arc(s) that shared some history with better-documented sequences of the Carolina Zone