Fluid compositions and local fluid buffering during the retrograde metamorphism of Al-bearing dolomitic calc-silicate granulites in the Limpopo Central Zone, southern Africa

Abstract

Calc-silicate granulites were examined to evaluate the fluid composition and retrograde metamorphic conditions in the Central Zone of the Limpopo Belt, southern Africa. Quartz deficient assemblages are characterized by minerals such as diopside, forsterite, spinel and/or magnesiohornblende and tremolite in the presence of calcite and dolomite. Although the granulites are Al-poor (Al 2O 3 is less than or equal to 1.0 wt.%) and dolomitic in composition, they include Al-bearing phases. Phase analyses for the assemblages in the two model systems CaO–MgO–SiO 2–H 2O–CO 2 and CaO–MgO–SiO 2–Al 2O 3–H 2O–CO 2 provide constraints on fluid compositions in the granulite facies and retrograde metamorphisms in the Limpopo Central Zone. In the presence of amphiboles, isobaric T–X(CO 2) phase relations suggest that high X(CO 2) conditions were established in the calc-silicate rocks of present study. The phase relations with tschermakitic amphiboles at 0.35 GPa restrict diopside-spinel occurrences in the presence of calcite, dolomite and forsterite within very-high X(CO 2) with low a(H 2O). The fluid compositions, X(CO 2), were effectively buffered by the mineral assemblages during granulite facies metamorphism to subsequent decompression and cooling stages. The presence or absence of retrograde magnesiohornblende and tremolite appeared to be controlled not only by infiltration of H 2O-rich fluid during retrograde metamorphism but also Al content in the local bulk rock compositions. The presence of the two-amphibole phases shows that the fluid compositions were locally buffered in the Al-bearing dolomitic granulites. Comparing the calculated X(CO 2) values in the present study area and in the Alldays area, a difference of retrograde hydration effects is observed.