The Petragenesis of some Archaean Volcanic Rocks from Southern Africa |
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| Authors: | HAWKESWORTH C J; O'NIONS R K |
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| Institution: | Department of Geology and Mineralogy Parks Rd.,Oxford, U.K. |
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| Abstract: | Major, rare earth and transition elements, have been determinedon a selection of volcanic rocks from greenstone belts in Rhodesia( 2.6 by) and South Africa (3.4 by). In Rhodesia two distinctseries can be recognized: a komatite-tholeiite series whichoccurs early in the greenstone belt evolution and apparentlygrades into a second, calc-alkaline, series at higher structurallevels. Peridotitic komatites reflect higher degrees of partialmelting than any Phanerozoic rocks so far observed and are thereforeused to place limits on the composition of their source. Atlower MgO contents they merge into low K tholeites which havesignificantly higher CeN/YbN and Rb/Sr ratios (at any MgO content)than those observed in modern ocean floor and island are environments.The calc-alkaline series is characterized by andesites whichexhibit a marked heavy REE depletion, but similar light REEand transition metal contents to the more evolved tholeiticrock types. The continuum of compositions from komatiites totholeiites and calc-alkaline andesites suggests that the bulkof greenstone belt volcanics could have been derived by differentialpartial melting, and polybaric fractionation of an essentiallyhomogeneous peridotite source. Late stage dacitic lavas andintrusions probably reflect melting of a more eclogitic sourceregion. The distinctive trace element geochemistry of Archaeanvolcanics, particularly the high Ni and low Yb values of thecalc-alkaline rocks precludes direct comparison with modernisland are associations. Rather the large decrease in liquidustemperatures (500 °C) with increasing structural heightwithin greenstone belts, coupled with the fact that most ofthe volcanics could have been derived from an essentially homogeneoussource, may suggest that greenstone belts developed in a riftingenvironment. It appears unlikely that the tholeiite/calc-alkalineassociation observed in the Archaean may be taken as an indicationof subduction at that time. |
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