ULTRAMAFIC BELTS IN THE TUGALOO TERRANE: HESS REVISITED

Hess related belts of ultramafic rocks in orogenic systems to boundaries between different masses of rocks. Hess viewed a belt of ultramafic rocks as making the closure of an ocean basin. Modern views of plate tectonics related the emplacement of ultramafic rocks to plate margins. The purpose of this paper is to examine a pair of ultramafic rock belts in the southern Appalachian orogen as first recognized by Hess and evaluate these rocks based on tectonic models.

The two belts of ultramafic rocks recognized by Hess in the southern Appalachian occur in the Blue Ridge and Piedmont provinces. The Blue Ridge belt is better defined with over 100 bodies of ultramafic rocks extending from Georgia, through North Carolina , and into Virginia. The Piedmont belt is not as well defined, but can be traced from Georgia, through the Carolinas and into Virginia. The rocks are typical Alpine-type ultramafic rocks and have been metamorphosed along with the enclosing rocks by several different periods of recrystallization.

The recognition of terranes and their geologic history as revealed by zircon geochronology has revolutionized the study of Appalachian tectonics. The Tugaloo Terrane extends from Georgia, through the Carolinas into Virginia and includes the eastern Blue Ridge and the Inner Piedmont provinces. Both belts of ultramafic rock occur within the Tugaloo Terrane and these are the subject of our study.

Recognition of the protoliths of the meta-ultramafic rocks is done using the normative mineralogy of the rocks. Moderate to high grades of metamorphism recrystallized ultramafic rocks into olivine and pyroxene- bearing rocks, similar to the original igneous rock. Olivine and pyroxene-rich rocks of the Blue Ridge have long been characterized as dunites and pyroxenites. Indeed, our results show dunite, harzburgite, orthopyroxenite are the common protoliths in the Blue Ridge belt of ultramafic rocks. Protoliths of ultramafic rocks in the Piedmont belt are predominantly gabbros with lesser amounts of orthopyroxenite.

The occurrence of two belts of ultramafic rocks within the same terrane have interesting implications for the interpretation of the history of the Tugaloo Terrane. Perhaps it is two terranes. Perhaps the Piedmont belt is related to a magmatic arc, while the Blue Ridge belt is related to a true terrane boundary. Or?