Springer Online Journal Archives 1860-2000
Abstract Monomineralic domains of chlorite, corundum and Cr muscovite coexist over a kilometer scale within ultramafic schists of the Harare greenstone belt (2.73 Ga). This exotic lithological association includes the conjunction of some of the most aluminous (Al2O3∼88 wt%) and potassic (K2O∼10 wt%) rocks known. The paragenetic sequence developed from chlorite→corundum→corundum+ diaspore: Cr muscovite variably overprinted both the corundum and chloritite domains. Terminal stages were marked by sporadic production of andalusite+quartz, and finally margarite. Chlorite (Cr2O3=0.31–2.65 wt%), corundum (0.79–2.66 wt%), and diaspore are all Cr-rich varieties. The chromian (Cr2O3∼3.86 wt%) paragonitic muscovite incorporates up to 17% of the paragonite molecule, and significant Mg and Fe substitutions. The suite of rocks are characterized by ‘chondritic’ Ti/Zr ratios (−x=107), systematically enhanced Cr (up to 14000 ppm) and Ni (up to 1200 ppm) abundances, low levels of the alteration-insensitive incompatible elements Th, Ta, Nb. Chlorite, corundum and Cr muscovite represent progressive stages in the incremental metasomatic alteration of a komatiite precursor. Mass balance calculations, constrained by the isochemical behaviour of Ti, Zr and Hf reveal that the komatiite chloritite transformation involved volumetric contractions of ∼60% by hydrothermal leaching of Si, Fe, Mn, Ca and Na. Reaction of chloritite to corundum involved further volumetric reductions of ∼50% due to essentially quantitative loss of Si, Fe, Mn, Mg, K and Ca. Conversion of corundum to muscovite required additions of Si, K, Fe, Mn, Mg, Rb and Ba at 50–200% dilation. K, Rb, Ba, Li and Cs are enriched by up to 2×103 over background abundances in ultramafic rocks, and the suite is also enriched in B, Se, Te, Bi, As, Sb and Au. REE were extensively leached during chloritite-corundum stages, whereas LREE additions accompany development of muscovite. Ti, Zr, Hf and Al were all concentrated by selective leaching of mobile components, but absolute additions of Al accompanied development of the corundum domains due to Al precipitation in response to depressurization. Corundum (δ 18O=3.5–4.8), muscovite (δ 18O=6.7–7.5‰) and chlorite (4.5–5.6‰) are isotopically uniform and formed at 380–520° C from a fluid where δ 18O=5.6–6.9‰. The corundum is 18O depleted relative to either igneous or anatectic counterparts (δOcor=7.6–8.2), or to gibbsitic laterites (δ 18O=12–17). Previous genetic schemes involving metamorphism of exhalites or bauxite, or Si-undersaturation of magmas, can all be ruled out from the data. The chloritite, corundum, Cr-muscovite association represents sequential alteration products of ultramafic rocks by high temperature, low pH hydrothermal solutions carrying LIL-elements, and in which excursions of pH and/or degree of quartz undersaturation account for the mineralogical transitions. A deep level acid epithermal system, or fluid advection across steep inverted thermal gradients in a thrust regime could account for required hydrothermal conditions.
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