Corundum-Fuchsite Rocks in Greenstone Belts of Southern Africa: Petrology, Geochemistry, and Possible Origin |
| |
| Authors: | SCHREYER W; WERDING G; ABRAHAM K |
| |
| Institution: | Institut für Mineralogie, Ruhr-Universität Postfach 102148, D-4630 Bochum, W.-Germany |
| |
| Abstract: | Unusual corundum-fuchsite rocks with Al2O3 contents up to 89per cent and Cr2O3 values up to 2.8 per cent were investigatedfrom two localities in Zimbabwe and Transvaal. They form lenseswithin volcano-sedimentary series of different metamorphic gradesand are closely associated with metamorphic ultramafics. In Zimbabwe the corundum contains up to 3.8 wt. per cent Cr2O3,and fuchsite up to 3.7 per cent Cr2O3. Coexisting minerals areandalusite ( 2 per cent Cr2O3), chlorite (3.2 per cent Cr2O3),complex margarite solid solutions, tourmaline (4.9 per centCr2O3), dispore (1.1 per cent Cr2O3), and rutile (1.9 per centCr2O3); gersdorffite (NiAsS), wehrlite (BiTe), and native bismuthare occasional opaque accessories. The minerals from the Transvaallocality are generally poorer in Cr2O3. Important parageneticdifferences are the lack of diaspore, tourmaline and margarite,the occurrence of kyanite (0.9 per cent Cr2O3) instead of andalusite,exsolution bodies of complex CrFeAl-oxides in rutile, and theappearance of biotite and plagioclase. Both biotite and fuchsitemay be rich in Ba. Critical mineral assemblages indicate that the Zimbabwe rockswere metamorphosed at temperatures not greatly exceeding 400°C and at pressures below 3.5 kb, those from Transvaal near600 °C at or above 5 kb. The textures suggest that the extremeAl-enrichment did not occur during metamorphism but essentiallyprior to it or at least in its early stages. Major and minor element analyses of the rocks from both localitiesshow that they are strongly enriched in the elements Al, Cr,B, V, and As, and locally also in K, Rb, Ni, Sb, Bi, and Te,whereas they are depleted in Si, Mg, Fe, Mn, Na, Ca, S, Cu,Zn, Ga, Sr, and Y. During their formation a strong Al/Ga fractionationmust have taken place leading to exceptionally low Ga/Al ratios. Three modes of primary origin are discussed. (1) Formation ofa low-iron bauxite in a reducing Archaean atmosphere is consideredunlikely, ly on geochemical grounds (very high B-contents; aberrantCr/Ni ratios; low Ga and Y), partly because similar rocks arefound in a non-Archaean formation of New Zealand. (2) Metasomatismin connection with early metamorphic serpentini-zation of theultramafic country rock does not seem impossible but would haveto be utterly different from the commonly observed rodingitizationand other metasomatic zones surrounding serpentinites. (3) Amodel is proposed for premetamorphic postvolcanic exhalativealteration of ultramafic komatiitic lavas, during which theelements B, K, Rb, As, Sb, Bi, Te were deposited from the solutions,while Al, Cr, Ni, and V were concentrated as immobile remaindersof the original rock, and Mg, Si, Fe, and Ca were largely dissolvedand transported away. The mineralogy of these alteration productsmay have been governed by aluminous sulphate minerals like alunite,KAl3SO4]2 (OH)6, which, during subsequent regional metamorphism,broke down to form, with the remaining silica, fuchsite andAl2SiO5, and without silica, diaspore and corundum, while sulphatewas carried away by the metamorphic solutions. |
| |
| Keywords: | |
| 本文献已被 Oxford 等数据库收录！ |
|
