GEOCHEMICAL DATA PROVIDE KEYS TO THE FORMATION OF MAFIC COMPLEXES AND SMALLER AMPHIBOLITE BODIES IN THE CAT SQUARE TERRANE, EASTERN INNER PIEDMONT, NORTH CAROLINA

The eastern Inner Piedmont Cat Square terrane (CSt) consists of a Siluro-Devonian assemblage of metagraywacke and pelitic schist, with mafic and ultramafic bodies, accreted during the collision of Laurentian terranes and the Carolina superterrane during the Neoacadian orogeny. Numerous isolated mafic and ultramafic map-scale bodies occur in the CSt in Georgia and North and South Carolina, as documented by reconnaissance and detailed geologic mapping. The majority of map-scale mafic and ultramafic bodies, including the Hammett Grove mafic-ultramafic and Turnersburg ultramafic complexes, are located in the eastern portions of the CSt near the Kings Mountain shear zone and Central Piedmont suture.

Recent detailed geologic mapping of sillimanite I and II grade metamorphic rocks west of the Newton window (Nw), approximately 70 km northwest of Charlotte, NC, has identified eight previously unrecognized mafic complexes and amphibolite bodies. Mafic complexes consist of undifferentiated metagabbro, metadiorite, and amphibolite, which contain plagioclase (An_45-65) ± hornblende ± biotite ± clinopyroxene ± orthopyroxene ± quartz ± garnet ± olivine ± opaques. These rocks are weakly to moderately foliated and medium- to coarse-grained with a granoblastic to schistose texture. Map patterns suggest these bodies may have been dismembered during accretion and development of map-scale sheath folds during Neoacadian deformation.

Whole-rock geochemical data of CSt mafic complexes west of the Nw and amphibolites from the Brushy Mountains suggest these bodies represent vestiges of oceanic crust formed in a back-arc basin setting, characterized by mixed N-MORB and calc-alkaline volcanic-arc signatures. Enrichment in the compatible and mobile LIL elements (Cs, Rb, Ba, K, Pb) likely indicates post-crystallization alteration. Values for immobile HFS elements (Nb, Ce, Zr, Hf, Sm, Ti, Y) are similar to normalizing values for MORB. Samples from separate map bodies of mafic complexes near the Nw have similar geochemical signatures, supporting a post-formation dismemberment. These data support the hypothesis that metasediments of the Cat Square terrane were deposited on oceanic crust in a remnant ocean basin.