Al-Fe³⁺ and Ca-Sr²⁺ epidotes in metagreywacke-quartzofeldspathic schist, Southern Alps, New Zealand

Al–Fe³⁺ and Sr²⁺-bearing epidotes are a common constituent in chlorite and biotite–albite zone metagreywacke–quartzofeldspathic schist in the Southern Alps of New Zealand. On the basis of electron microprobe backscattered electron scanning of complexly zoned grains five generations of epidote growth are recognized. First generation epidotes (Ps₂₉₋₂₅) are typically fractured as a result of cataclasis during D₁ deformation and presumably represent relics of prehnite–pumpellyite and/or pumpellyite–actinolite facies conditions of metamorphism. Second generation epidote (Ps₂₅₋₂₀) rims first generation epidote and replaces fragmented grains. Third generation epidote involved Sr-replacement, up to 8.5 wt.% SrO, of earlier grains along fracture planes and grain margins. Single grains exhibit a range of Sr ⇌ Ca replacement that can vary by up to 4 wt.% SrO. Fourth generation (post D₁) epidote (Ps₂₀₋₉) occurs as overgrowths on earlier generation epidotes and as continuously zoned grains. Within the lowest grade part of the biotite–albite–oligoclase zone fourth generation epidote cores (Ps₁₀₋₁₃) are overgrown by slightly more Fe-rich fifth generation rims (Ps₁₆₋₁₄) suggesting involvement of the epidote Ca₂Al₃Si₃O₁₂(0H) component in an oligoclase-producing reaction. In quartzofeldspathic lithologies epidote disappears at higher grades than the appearance of oligoclase.

Epidote compositions plot within the miscibility gap defined by Raith (1976) and a discontinuity between compositions of PS₂₀ and Ps₁₆ in chlorite zone rocks is attributed to the persistence of relic cores due to incomplete Fe³⁺ ⇌ Al diffusional exchange at low temperature and textural grade. With increasing grade there is a decrease in the range of zoning in individual epidote grains resulting in a more homogeneous population of epidote compositions with the most Al-rich epidotes becoming Fe³⁺-rich and the most Fe³⁺-rich epidotes becoming more Al-rich. The formation of Sr-epidote appears to be related to the release of Sr into the fluid phase from the breakdown of detrital plagioclase over the pumpellyite-clinozoisite isograd during D₁ deformation. Early-formed oligoclase in the biotite–albite–oligoclase zone resulting from the disappearance of epidote contains up to 0.4% SrO. Mobility of Sr (and Ca) is indicated by veins containing Sr-bearing epidote and calcite. The complex zoning of epidotes is a function of bulk composition, time, variation in fluid composition, temperature, pressure and deformation of the rocks.