A calderitic garnet paragenesis in granitic gneisses in the Su-Lu ultra high-pressure terrane, eastern China

Calderitic garnet occurs in Al-poor granitic gneisses of the Donghai area in the ultra high-pressure terrane of eastern China. Calderitic garnet-bearing samples reequilibrated under greenschist-epidote amphibolite facies conditions, which postdated the ultra high-pressure metamorphism. Such samples contain quartz, K-feldspar and albite as major constituents with subordinate amounts of aegirine-augite, Zn-rich biotite (ZnO=2.7 wt% maximum), titanite, hematite, allanite and zircon with garnet. A small muscovite crystal occurs as an inclusion in albite. The calderitic garnet occurs as a rim around andradite-spessartine garnet or as anhedral grains in a matrix of equigranular felsic minerals. The empirical formula of the calderite-rich part is (Ca1.69Mg0.02Mn1.16Fe2+0.10)(Fe3+1.58Al0.40Ti0.03)Si3O12. Calderitic garnet is stable in a Al-poor environment and probably cannot coexist with muscovite/paragonite and epidote as suggested by the idealized reactions of calderite+muscovite/paragonite → spessartine+K-feldspar/albite+quartz+H2O and calderite+epidote → grossular-spessartine garnet+hematite+quartz+H2O.