ABSTRACT

Pegmatoid bodies predominantly consisting of calcite and apatite occur associated with metasediments within Precambrian high-grade metamorphic basement rocks in the Fazenda das Panelas area, central Bahia State, Brazil. Petrology, trace elements, stable and radiogenic isotope chemistry of the carbonate-phosphate intrusives have been studied in order to clarify their genesis. K-Ar data give approximately 2.0b.y. that is likely to coincide with the metamorphic recrystallization of the basement rocks which occurred during the Transamazonic tectonic event. Rb-Sr data give somewhat older ages. C and O isotope data on the intrusive carbonates are distributed within the values of marine carbonates of Precambrian ages. Trace element chemistry and REE patterns in mineral phases (calcite, apatite, micas) give intermediate figures between sedimentary marine and igneous (carbonatitic) materials. The pegmatoids are supposed to have derived from the crystallization of carbonate-phosphate liquids formed by partial melting of phosphate-rich layers contained in the sedimentary sequence. The enrichment in REE and in large charged elements is explained by interaction of the phosphate-carbonate liquid with fluids of metamorphic origin. A possible paleoenvironmental characterization of the source phosphate rocks is presented.