OUTCROP AND MICROSTRUCTURAL EVIDENCE OF DEFORMATION HISTORY IN THE WEBSTER-ADDIE ULTRAMAFIC BODY (BLUE RIDGE THRUST COMPLEX, NORTH CAROLINA)

The Webster-Addie body (W-A) is part of a terrane interpreted to be a remnant of an early Paleozoic accretionary melange preserved within the Alleghanian Blue Ridge thrust complex of western NC. In the study area, upper amphibolite facies mineral assemblages help define a penetrative foliation in more pelitic lithologies that is attributed to the Ordovician Taconic collision. W-A is weakly boudinaged within this foliation, which are together folded by a km-scale doubly plunging antiform. This presentation is part of an ongoing study and focuses on structures defined by and cross-cutting the ultramafic mineral assemblage.

Foliation in the W-A is defined by cm- to dm-scale compositional layering, and locally, weak grain shape orientation. It is generally oblique to foliation outside the body and is comprised of olivine ± clinopyroxene ± orthopyroxene indicating a higher grade than surrounding rocks. Serpentinization along grain boundaries is ubiquitous and nonsystematic minor faults (with serpentine and talc slickensides) and antitaxial veins (filled with serpentine or chlorite) are also common. The minor faults and veins do not continue into adjacent rocks, nor is there evidence of hydrothermal alteration and related structures outside of W-A. These observations suggest that W-A largely records a deformation history preceding the regional foliation-forming event.

Four samples with the least alteration and fracturing (dunites from near the NE fold closure, and a websterite and olivine websterite from the SE fold limb) were chosen for microstructural and electron backscatter diffraction (EBSD) analyses. Grain size ranges from 0.25-2mm with most 0.5-1mm. Evidence of penetrative ductile recrystallization includes equant grains with straight grain boundaries commonly forming triple junctions. Rare larger grains help define foliation and exhibit undulatory extinction and subgrains. No primary igneous structures are observed. These observations suggest that these are ultramafic ultramlyonite gneisses possibly formed during deformation over 1000°C. Given the regional geologic history and W-A’s chemical affinity with rocks formed in new oceanic lithosphere, we speculate that mylonitization records deformation related to mid-ocean ridge spreading.