Abstract

Seventeen metamorphosed ultramafic bodies in the Precambrian Ashe Formation exposed along a northeast-trending belt from near West Jefferson, North Carolina to near Floyd, Virginia in the Blue Ridge, were investigated. The mineral assemblages in individual samples constitute subgroups drawn from the larger group consisting of olivine, enstatite, tremolite, anthophyllite, antigorite, talc, chlorite, magnetite, and dolomite. Although the ultramafic bodies occur in the garnet, staurolite, and kyanite zones recognized in the country rock, no correlation exists between the mineral assemblages in the ultramafic rock and the metamorphic zones. Modal analyses and quantitative chemical analyses for Mg, Al, Ca, Fe, Ni, Mn, Cu, Co, K, and Zn lead to the recognition of two distinct types of metamorphosed ultramafic bodies based on the degree of metasomatic alteration. The metasomatic exchange with the country rock involves the loss of magnesium and nickel and the introduction of aluminum, calcium, iron, manganese, and potassium into the ultramafic bodies. The less altered bodies are more massive and contain relict olivine and rare relict enstatite plus tremolite, anthophyllite, chlorite, and antigorite whereas the more altered bodies are more strongly foliated with abundant tremolite and chlorite and less common relict olivine.

Textural evidence, notably the pseudomorphous partial replacement of tremolite by serpentine, combined with that provided by the geochemical study provides the basis for understanding the recrystallization history and the calculation of mass balance equations. A retrograde hydration of the ultramafic protolith produced the tremolite-chlorite-talc anthophyllite assemblage with relict olivine. In a second stage of this hydration serpentine replaced some of the tremolite and olivine. A later partial dehydration led to the more strongly foliated and, in some places, mylonitized tremolite-chlorite-talc schist or phyllon-ite.

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