Magnesiocummingtonite In Metadunites from the Spruce Pine Mining District, Western North Carolina

Magnesiocummingtonite (Mgcumm) occurs with tremolite (Tr) and anthophyllite (Antho) in amphibolite-grade metaultramafic rocks in the Spruce Pine Mining District of western North Carolina. Host rocks are metadunites and metaperidotites. Mineral assemblages are consistent with Evans= 1977 petrogenetic grid and include olivine + enstatite + Tr + Mgcumm along with chlorite and chromite. Local enrichment in enstatite (disrupted orthopyroxenite dikes ?) produces assemblages of olivine + enstatite + Mgcumm. Talc replaces Tr and Mgcumm in the olivine-rich assemblages, but talc is stable with amphibole in the areas enriched in enstatite. Amphiboles also occur in talc chlorite schists (altered peridotites ?).

Mgcumm forms large bladed crystals in the olivine-rich assemblages. It also forms acicular overgrowths on tremolite. Similar textural relations are noted in the talc chlorite schists. A fine-grained amphibole in some of the schists is seen to replace Mgcumm and this amphibole is thought to be Antho.

Mgcumm has a lower mg# than coexisting silicate phases. Mgcumm in olivine-rich assemblages has a mg# of 86-91, coexisting Tr varies from mg# 96-98. Amphibole compositions are more Fe-rich in the talc chlorite schists (Mg cumm mg# 81-87, Tr mg# 91-96). Calcium contents of the Mgcumm are low, less than 1 wt. %. Manganese contents of the Mgcumm are higher (0.2 to 0.4 wt. %) than MnO contents of Tr (< 0.1 wt. %). Microprobe compositions of Mgcumm and Antho(?) are indistinguishable.

The Mgcumm is a common phase in the Spruce Pine area, but it is not found in metaultramafic rocks to the south (higher grade) or north (lower grade). The author overlooked the coarse grained Mgcumm in his early (pre-probe work)studies of these rocks. Gentle coaching by Bernard Evans resulted in the eventual recognition of Mgcumm in these rocks.