REFLECTIONS ON OLIVINE AND PYROXENE-RICH ROCKS FROM THE EASTERN BLUE RIDGE OF NORTH CAROLINA

Olivine-rich (and less common pyroxene-rich) rocks of the eastern Blue Ridge (BR) attracted the attention of geologists for over 100 years. Corundum, soapstone, olivine, chromite, vermiculite and anthophyllite asbestos were mined from these rocks (olivine and soapstone are still mined) and the economic interest prompted geologic mapping at a variety of scales.

Hess wrote of the tectonic significance of linear belts of ultramafic rocks in orogenic systems and used the BR occurrence as an example of alpine ultramafic belts. BR ultramafic rocks are typically dunites, but lesser amounts of more pyroxene-rich rocks are also known as are associated talc chlorite amphibole schists and granofels.

Interpretation of orogenic systems in terms of plate convergence refocused interest on ultramafic rocks and the search for ophiolites. Reconstruction of the ultramaifc section in the BR is difficult because of the isolated occurrences (disrupted ?) and the lack of other bulk compositions (dunite makes up a small proportion of the ophiolite section).

Recognition of the metamorphic character of the Blue Ridge ultramafic rocks revealed a complex history of recrystallization. Several distinct periods of recrystallization record a decompressional cooling from high grade (upper amphibolite and granulite) to low grade conditions. Key to the preservation of the high grade mineral assemblages was the apparent scarcity of water during the cooling. Some mineral textures and structures (exsolved cores of some pyroxene crystals, layers and pods of chromite) may record premetamorphic conditions, but olivine and most of the pyroxene compositions are in equilibrium with the metamorphic mineral assemblage. Some mineral properties (oxygen isotopes in olivine, core compositions of chromite) do reflect premetamorphic conditions (mantle isotopic signature, suprasubduction zone setting) of the minerals. But it is difficult to be more explicit about the setting of the formation of these rocks without an idea of other associated rock types. There is some suggestion that the amphibole-chlorite schists and granofels associated with the olivine-rich rocks may represent the “missing” peridotites and pyroxenites.