Geochemistry of early precambrian carbonate rocks from the Brazilian Shield: Implications for archean carbonate sedimentation

Petrological and geochemical studies were carried out on early Precambrian carbonate rocks metamorphosed under granulite facies conditions from three areas of Bahia State (Brazil). Older rocks attributable to the Archean or lower Proterozoic consist of carbonates (mainly dolomite) and abundant, partially-serpentinized forsterite. Chemical data confirm their strong magnesian character and the almost complete absence of elements like Al, Ti, Na, K, Zr etc., which are normally found in the non-carbonate detrital part of impure carbonate rocks. Comparison with other carbonate rocks of similar silica content further emphasizes the scarcity of these elements. Samples of probably more recent origin belonging to migmatized complex of Central Bahia are more calcic in character and have a higher Sr and Ba content. The MgCO₃ solvus thermometer in calcites coexisting with dolomite was determined both by chemical and X-ray procedures. Temperature values for carbonates under granulite facies range between 472 and 640° C. These temperatures seem to represent quench or recrystallization temperatures. The general geochemistry of carbonate rocks suggests two main hypotheses for their formation: a) chemical precipitation of pure carbonates during the Archean and later silica enrichment by metasomatic reactions; b) chemical co-precipitation in the Archean or early Proterozoic of carbonates and silica from silica-rich sea water. In this case precipitation would have occurred locally owing to increasing CO₂ pressure (due, for example, to exhalative volcanism) or in limited evaporitic basins in areas of temporary stability bordering the continents.