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The Middle to Upper Cambrian Heritage Group, exposed in the Heritage Range of the Ellsworth Mountains, contains interlayered basaltic lava flows and intrusive gabbroic stocks, dikes, and sills that preserve evidence of a pretectonic burial metamorphism. Three zones ranging from the upper prehnite-pumpellyite facies (zone I) through the pumpellyite-actinolite facies (zone II) to the greenschist facies (zone III) are correlated with stratigraphic depth of the mafic igneous rocks in the Heritage Group. A temperature of approximately 370°C is obtained for the boundary between the pumpellyite-actinolite facies and the greenschist facies on the basis of Fe/(Fe+Al) in epidote coexisting with pumpellyite + chlorite + actinolite + quartz. A geothermal gradient of approximately 23 to 24°C/km is estimated for the stratigraphic interval containing pumpellyite-actinolite facies assemblages.

Syntectonic recrystallization associated with the development of penetrative foliation in the metasedimentary country rocks is only sparsely developed in the prehnite-and/or pumpellyite-bearing mafic igneous rocks, whereas the more penetratively deformed igneous rocks lack evidence of subgreenschist facies conditions. This differential recrystallization may account for the variation in K-Ar whole-rock ages reported previously for the mafic igneous and metasedimentary rocks of the Heritage Group.

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