Reactions among sodic, calcic, and ferromagnesian amphiboles, sodic pyroxene, and deerite in high-pressure metamorphosed ironstone, Siphnos, Greece

Sodic actinolite, ferri-winchite, cummingtonite, crossite, and riebeckite occur in a metamorphosed ironstone from Siphnos, Greece, containing in addition deerite, quartz, magnetite, almandine, and aegirine-augite. Salient petrographic features are the following: idio-blastic sodic actinolite cores occur in large prisms of cummingtonite; cummingtonite crystal margins are replaced and overgrown by homoaxial sodic amphibole and, less abundantly, by sodic actinolite-winchite; internal S surfaces of garnet porphyroblasts are composed of aegirine-augite, deerite, and quartz; schistose rock matrix is composed predominantly of sodic amphibole, aegirine-augite, deerite, magnetite, and quartz; and deerite is partially replaced by magnetite, quartz, sodic amphibole, and ferri-winchite, and, later, by cron-stedtite and hematite.

These relationships are explained in terms of model hydration reactions consuming cummingtonite, quartz, and components of aegirine-augite and producing sodic and calcic amphibole. Three reactions, all proceeding toward dehydration, can be written between deerite and other minerals in the rock. A calculated P-T phase diagram shows that all observed reactions can be explained by a metamorphic path of isothermal decompression, in agreement with independent geologic evidence.

The occurrence in one rock of sodic, calcic, and ferromagnesian amphiboles requires an unusual whole-rock composition but not necessarily high pressure–even though this rock experienced P-T conditions within the stability fields of deerite and jadeite + quartz. The coexistence of cummingtonite and aegirine-augite at modest metamorphic temperatures does, however, require high pressure. Despite considerable compositional variability among the amphiboles, the results of microprobe analysis are indicative of the extent of mutual solubility between Ca, Na, and (very roughly) Fe on M(4), for conditions of approximately 15 kbar and 460°C. Ferri-winchite or sodic actinolite (depending on P-T) and riebeckite or crossite (depending on μ_FeA1−1) are alternates respectively in a stable three-amphibole paragenesis.