Carbonate-silicate inclusions in garnet as evidence for a carbonate-bearing source for fluids in leucocratic granitoids associated with granulites of the Southern Marginal Zone, Limpopo Compl

We present a study of carbonate-bearing polyphase inclusions in garnets from leucocratic granitoids intruding

metapelitic granulites of the Southern Marginal Zone (SMZ) of the Neoarchean Limpopo high-grade complex,

South Africa, during the post-peak stage (2710–2650 Ma; U-Pb ages for zircons and monazites). Ternary feldspar

thermometry suggests that the granitoid magma cooled from temperatures 800–900 °C at a pressure of ca.

6.5 kbar. Abundant CO2 fluid inclusions in quartz and T-XCO2 phase equilibria modeling via PERPLE_X imply ac-

tion of an essentially carbonic fluid in the granitoids. Cores of almandine-rich garnet grains from the granitoids

contain polyphase carbonate-bearing inclusions with a distinct negative crystal shape. The major carbonate in

the inclusions is a strongly zoned magnesite-siderite variety, whereas pyrophyllite is the predominant silicate

phase. Raman spectra of unexposed inclusions revealed a presence of CO2, as well as CH4 and H2O. The

carbonate-bearing inclusions coexist with larger polyphase inclusions composed of biotite, quartz, K-feldspar,

plagioclase, sillimanite, which are interpreted as relics of granitic melts. Modeling the mineral assemblage

preserved within the carbonate-bearing inclusions shows that their present mineral and chemical compositions

are a product of interaction of the trapped aqueous‑carbonic fluid with host garnet during cooling below 400 °C.

Despite strong modifications, the inclusions bear evidence for initial saturation of the fluid with Mg‑carbonate.

This is taken as an evidence for an origin of the fluids by devolatilization of the Mg-rich carbonate-bearing ultra-

basic greenstone rocks of the Kaapvaal Craton that were buried under the SMZ. Being generated at temperatures

between 650 and 700 °C, the fluid subsequently participated in anatexis and coexisted with the granite magma

during exhumation and interaction of the SMZ granulites with cratonic rocks.