Abstract

Portions of the systems cummingtonite (Mg50)-actinolite (Mg50)-plagioclase (An0, An20, An40)-quartz-water have been studied within the melting range 700-850°C under a water pressure of 5 kbar and oxygen fugacities of the FMQ buffer. In the system Cum-Act-An0-Qz-H2O, cummingtonite coexists with a range of Ca-Na amphiboles, which themselves show continuous solid solution, but have a wide miscibility gap with respect to cummingtonite. At 700°C, mafic assemblages (coexisting with quartz, silica-rich liquid, and vapor) change as follows with increasing Act component: cummingtonite; cummingtonite + crossite + orthopyroxene; subcalcic amphibole + orthopyroxene; actinolite + Ca-clino-pyroxene + orthopyroxene. Cummingtonite is present for bulk CaO contents to 3.4 wt%, and Ca in cummingtonite and Ca-Na amphibole correlates with bulk Ca content. At temperatures above 750-800°C, amphibole-bearing assemblages are replaced by Ca-cli-nopyroxene + orthopyroxene. In the corresponding systems with An20 and An40 components, cummingtonite is always accompanied by actinolite or actinolitic hornblende. With more calcic plagioclase components, Ca-amphibole stability increases, and the various stability fields of the An0 system shift toward Cum-rich compositions.

Chemical compositions of amphiboles reflect their high temperatures of equilibration relative to metamorphic conditions, with either single-phase amphiboles lying within accepted miscibility gaps or coexisting amphiboles showing extensive mutual solid solution. Controls on miscibility of amphibole endmembers and the petrologic significance of amphibole pairs in metamorphic and volcanic rocks are discussed in the light of the experimental results.

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