The Contribution of Volume Change in Preserving High-P Metastable Assemblages

Incomplete retrograde reaction of sodic–calcic pyroxene occurred during the exhumation of Eocene eclogite and blueschist in NE New Caledonia. Sodic–calcic pyroxene is well-preserved in reaction relationships with paragonite and structurally late albite, in conflict with the prediction that it should transform efficiently during decompression to albite, paragonite, glaucophane and quartz. The microstructural distribution of albite accompanying crenulation cleavage development is consistent with albite growth having been restricted by space-filling requirements. In contrast, domains of high strain enable the accommodation of volume and fluid infiltration, increasing the size of the equilibration volume leading to the complete conversion of sodic–calcic clinopyroxene to albite. Despite decompression-driven dehydration, limited volume accommodation presents a plausible mechanism to metastably preserve high-P (up to 18 kbar) sodic–calcic clinopyroxene-bearing assemblages to lower-P conditions (~10 kbar) that should stabilise albite. Efficient retrogression is facilitated by dynamic recrystallisation inducing dilation.