Experimental investigations of blueschist-greenschist transition equilibria: pressure dependence of Al₂O₃ contents in sodic amphiboles - a new geobarometer.

These experimental studies, together with those of Holland (A.M. 65-129), yield the following conclusions: 1) the blueschist-facies metamorphism may occur at shallower depths than previously suggested. For example, jadeite-bearing glaucophane schist may be stable at 6 kbar, 300oC instead of 8 kbar; 2) depending on the bulk composition, both blueschist- and greenschist-facies assemblages may be stable over a considerable P-T range and may form during one-stage metamorphic recrystallization. Coexistence of both facies assemblages in the same outcrop or in a single specimen has been documented in many blueschist terrains in addition to those described above. Two stages of recrystallization, with greenschist-facies after blueschist-facies metamorphism or vice versa, are common in many blueschist terrains and have been used to explain such observations. Distinction of one-stage vs two-stage metamorphism can only be done by textural and structural investigation of greenschist-blueschist rocks; and 3) for a given rock composition, the composition of sodic amphibole of the buffered assemblage sodic amphibole + actinolite + epidote (+ albite + chlorite + quartz) varies systematically with P; hence it can be used as a geobarometer. P estimates for metabasites in Ward creek of Franciscan, Mikabu of Sanbagawa, Lake Wakitipu of New Zealand and Ouegoa of New Caledonia, using the proposed geobarometry are in agreement with those derived from sodic pyroxene geobarometry. -J.M.H.