CHARACTERISTICS OF RETROGRADED ECLOGITE AND IMPLICATIONS FOR WIDESPREAD ECLOGITE FACIES METAMORPHISM IN THE EASTERN BLUE RIDGE, NORTH CAROLINA

Retrograded eclogite presents a unique opportunity to examine the extent and timing of high pressure metamorphism in the eastern Blue Ridge. The retrograde path recorded by eclogitic rocks in the Ashe Metamorphic Suite (AMS) allows us to identify mineralogic and petrologic characteristics associated with the transition of eclogite to amphibolite. These characteristics could make it possible to recognize early, very-high-pressure metamorphism in widespread AMS amphibolites. The retrograde assemblages also constrain more precisely the retrograde P-T path of the eclogite and will better define the uplift history of these rocks.

Petrography, microprobe analysis, zircon inclusion studies and U-Pb geochronology will determine the timing and extent of these eclogite facies conditions. Petrography has revealed several classic textures that develop during eclogite retrogression. The earliest retrograde texture developed as plagioclase exsolved from omphacite making the clinopyroxene less sodic at lower pressure. This is followed by the widespread development of symplectic intergrowths of plagioclase and amphibole both as coronas around garnets and within exsolved clinopyroxene. Symplectite developed in the exsolved clinopyroxene contains amphibole showing weak pleochroism and poorly developed cleavage as compared to the coronal amphibole. Microprobe analysis should reveal a distinct chemistry within each of those retrograde eclogite phases.

If the characteristic amphibole compositions from the retrograded eclogite are also found in AMS amphibolites, this may indicate eclogite facies conditions were widespread in the eastern Blue Ridge. In addition, mineral inclusions in zircons extracted from AMS amphibolites should provide additional information on the grade of peak metamorphism. Zircon U-Pb geochronology will show whether peak metamorphism of the AMS amphibolites occurred synchronously with eclogite metamorphism. The extent and timing of high-pressure metamorphism in the eastern Blue Ridge is essential for constraining tectonic models of this segment of the southern Appalachians.