ASPECT RATIOS OF BLUE RIDGE METAULTRAMAFIC ROCKS AND THEIR SIGNIFICANCE

Metaultramafic rock bodies (MUMRBs) occur as scattered masses in the Ashe and Pumpkin Patch metamorphic suites in the Blue Ridge Belt of the Appalachian Orogen. A slightly bimodal distribution of anhydrous metadunite and metaharzburgite vs. hydrated chlorite and related schists is puzzling. MUMRBs may represent ophiolite fragments. If so, and Ashe and Pumpkin Patch mafic rocks represent oceanic crust, then apparently the Blue Ridge Belt contains ophiolite tops and bottoms, without middles, with dunites representing melt channels in subcrustal lithospheric ultramafic tectonite. Analysis of available MUMRB chemistry reveals that hydrated bodies are more calcic and aluminous than anhydrous bodies. At least for some bodies, Ca and Al metasomatism accompanied hydration. Whether other bodies that initially were more calcic and aluminous were more prone to hydration remains moot. Aspect ratios (AR) of the MUMRBs plotted against short dimension, long dimension, Al₂O₃, and CaO, reveal that AR increases with increases in hydration, alumina, and lime. ARs for anhydrous bodies are < 5, partially hydrated bodies are < 15, and hydrated bodies range up to 33. Anhydrous bodies have olivine LPOs and weak alignment with regional metamorphic fabric, whereas hydrated bodies have both mineralogic fabric-scale and strong body-scale SPOs. Rheological data indicate that the dunites are substantially stronger than chlorite schists. It follows that the chlorite schists exhibit substantially greater elongation and SPO fabrics than anhydrous bodies. Thus, aspect ratio serves as a proxy for strength. We hypothesize that Acadian deformation elongated the hydrated bodies during re-metamorphism, whereas Taconic deformation may have separated some hydrated parts of the bodies from the stronger and more rigid anhydrous metadunites and metaharzburgites initially metamorphosed during the Taconic Orogeny. Together with metamorphic grade of surrounding rocks, the AR and MUMRB mineralogy reveal a complex interaction of chemistry, hydration, metasomatism, and deformation.