The role of fluid for diamond-free UHP dolomitic marble from the Kokchetav Massif

A diamond-free, dolomite-bearing UHP marble showing a banded structure occurs at Kumdy-kol in the Kokchetav Massif, northern Kazakhstan. Three centimeter-scale zones occur in this marble (sample no. Y665): zone A = Ti-elinohumite-bearing dolomitic marble; zone B = dolomite marble; and zone C = dolomitic marble lacking Ti-clinohumite and forsterite. The occurrence of phlogopite and garnet lamellae in diopside in zone C suggests UHP metamorphism. Large-area chemical mapping covering about two-thirds of the sample surface was conducted. Ti-Kα X-ray images clearly indicate that TiO₂ is concentrated in zone A. Ca-Kα and Mg-Kα images showed that zones A and C have similar modal compositions of carbonates, but they have strong contrasts in TiO₂ contents and mineral paragenesis. Mineral assemblages in each zone were analyzed in terms of the CaO-(MgO, FeO)-TiO₂-SiO₂ compositional tetrahedron. In zone A, the aragonite-Ti-elinohumite tieline was stable under UHP conditions, whereas it was unstable in zones B and C. This indicates that XCO₂ in zone A was lower than that in zones B and C. Infiltration of TiO₂-bearing aqueous fluid is implied on the basis of paragenetic relations, distribution of Ti-bearing phases and chemical zonation in Ti-clinohumite in zone A. The lack of microdiamond in this dolomite-bearing carbonate rock suggests extremely low-XCO₂ conditions, and implies the infiltration of an aqueous fluid during UHP metamorphism.