Portal de Periódicos da CAPES

Geochemistry of marbles and calc-silicate rocks in the Pan-African Zambezi belt, Zambia

1988; Elsevier BV; Volume: 38; Issue: 3 Linguagem: Inglês

10.1016/0301-9268(88)90001-0

1872-7433

Hubert Munyanyiwa, Richard E. Hanson,

Geochemistry and Geologic Mapping

Amphibolite-facies marbles and calc-silicate rocks make up an important part of the supracrustal sequence in the Pan-African Zambezi belt of Zambia. Major- and trace-element geochemistry shows that dolomite marbles in the sequence were derived from nearly pure dolostones, calcite marbles were derived from impure limestones, calc-silicate schists were derived from calcareous shales, and massive calc-silicate rocks were derived from calcareous sandstones. A pronounced compositional banding in abundant calc-silicate gneisses, now folded and transposed, reflects original interbedding of carbonate-rich and pelite-rich sediments. Trends in major and trace elements in the calcite marbles and calc-silicate rocks are controlled largely by original variations in the amount of silicate phases admixed with CaCO3 during deposition, and these compositional trends have been preserved into the amphibolite facies. Dolomite was present in significant amounts only in the protoliths to the dolomite marbles, and played little or no role in the evolution of the calcite marbles and calc-silicate rocks. Correlation of most major and trace elements with Al2O3 indicates that an alumina-bearing phase made up a major fraction of the material admixed with CaCO3. The use of the Niggli parameter al-alk reveals that the Al2O3 was carried largely by clays and detrital micas. Other components contributed by the phyllosilicates include SiO2, TiO2, FeO, Na2O, K2O, Ba, Rb, Cr, Ni and Zn. Sr was added with the carbonate fraction during deposition, and detrital zircon may have been an important carrier of Zr and Y. Some of the massive calc-silicate rocks are geochemically distinct from the other calc-silicate rocks, suggesting that admixture of other detrital phases, such as quartz and feldspar, played a more important role in governing element contents in these rocks. Evidence of significant alkali metasomatism is shown only by some of the massive calc-silicate rocks, in which Na2O replaced K2O.