Abstract

Inspired by phase relationships in the Precambrian Ashe Formation of northwest North Carolina, a petrogenetic grid is presented for metabasites containing quartz, feldspar, magnetite and one or more of the minerals hornblende, biotite, garnet, epidote, orthopyroxene, clinopyroxene and fayalite. The applicable range of conditions is from the amphibolite facies to the granulite facies. There are 26 subfacies in the grid, each represented by a distinct CFM topology (C = CaO–Al2O3 + Na2O + K2O, F = FeO−Fe2O3, M = MgO). Construction of the grid was guided by natural mineral assemblages. Some of the subfacies, particularly at low pressures, have not been identified in natural assemblages.

Many of the subfacies represent conditions under which melting may occur. The possibilities for melting are explored in the context of previous work by the writer (Abbott and Clarke, 1979; Abbott, 1981). The analysis shows that anatexis of high grade metabasites may produce distinctly peraluminous, granitic liquids.

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