The geochemistry and metamorphism of calc-silicate rocks, marbles and amphibolites in a protion of the Zambezi belt, Southern Zambia

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Munyanyiwa, H
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Calc-silicate rocks and marbles of Katangan age underlie a significant segment of the Zambezi belt in Southern Province. The calcsilicate rocks are represented by spectacularly banded calc-silicate gneisses, calc-silicate schists, massive calc-silicate rocks and a few mottled varieties. The marbles are uniformly calcite - and dolomite marbles. In the Mapanza area, minor amphibolites are sporadically distributed as lenticular bodies, bands and boudins within the marbles and calc-silicate rocks.The rocks were deformed and metamorphosed to amphibolite facies and later partly retrogressed to greenschist facies during Pan-African tectonothermal events. Amphibolite facies assemblages in the calcsilicate rocks include biotite + scapolite + quartz + calcite, quartz + actinolite + diopside + grossularite + sphene and quartz + plagioclase+ diopside + calcite + sphene. Those in the marbles include dolomite + tremolite, calcite + biotite + scapolite, dolomite and dolonite + diopside. The amphibolites are of basic igneous origin with a possible minor sedimentary component for the amphibolite boudins and bands. Their mineralogy is largely influenced by the bulk composition, with garnet occurring only in amphibolites with Fe/(Fe +Mg)>0.5. Geochemical data, Niggli parameters and plots of inter-element correlations for the calc-silicate rocks and marbles are presented. These support the conclusion that during metamorphism the major and trace elements were not significantly mobilized. Instead the variation in bulk chemical composition of the rocks and the resulting variations in equilibrium metamorphic mineral assemblages, are best explained in terms of original variations in the proportions of clays and other detrital phases intermixed with carbonate during deposition. Geothermobarometry is applied for the first time in this part of the Zambezi belt. Campositions of coexisting biotite and garnet in the amphibolite yield temperatures of 63O-660°C and compositions of coexisting plagioclase and hornblende in the amphibolites yield temperatures of 56C)-630°C and pressures of 5-6 Kbars for the peak of metamorphism.These values are in accord with intermediate pressure (Barrovian type) amphibolite facies metamorphism, and are consistent with the phase assemblages in the calc-silicate rocks, marbles and amphibolites based on published experimental data. Compositions of calcite coexisting with dolomite in the marbles yield maximum temperatures of 490-500°C, apparently reflecting re-equilibration following the peak of metamorphism.
Metamorphism (Geology) -- Zambia -- Southern Province , Geochemistry -- Zambia -- Southern Province