Study on barite as an accessory mineral in eclogite from main borehole core of the Chinese Continental Scientific Drilling (CCSD).

Sr-rich barite exists as an accessory mineral at the ultrahigh-pressure (UHP) and retrograde metamorphic stage of eclogites front the 190 similar to 320m main-hole core of the Chinese Continental Scientific Drilling (CCSD), most of which is recognized in the symplectite of clinopyroxene (Cpx) and albite (Ab) formed by omphacite breakdown. It appears at least in three different occurrences at three metamophic stages: i.e., stage I UHP barite which is preserved in garnet with homogeneously high Sr content (similar to 45mol% SrSO4); stage II early retrogressive barite which is dispersing in the symplectite (Cpx + Ab) with greatly variable Sr-content (0mol% similar to 21mol% SrSO4); stage III amphibolite-facies barite occurs as the oxidized rim of pyrite with 2mol% similar to 5mol% SrSO4. Barite is probably formed by the fluid activity and the Sr-rich barite may be precipitated front the fluid with high Sr/Ba. Barite together with the associated minerals especially the sulfide indicates the redox state of the eclogite at different metamorphic stages: Oxidation of the UHP eclogite-facies metamorphism, reduction of the early retrogression and oxidation of the amphibole-facies retrogression.