矿床地球化学国家重点实验室知识库

An unusual feature of the magmatic Fe-Ti oxide deposits associated with layered intrusions in the Permian Emeishan large igneous province (LIP) in SW China is the association of the mineralization with olivine that is significantly more Mg-rich than those that occur elsewhere. A working hypothesis for the production of Mg-rich olivine is that significant assimilation of carbonate occurred during the formation of the Fe-Ti oxide deposits in the Emeishan LIP. This study provides several lines of supporting evidence from a coeval picritic dyke in the footwall of the Panzhihua layered intrusion at Zhujiabaobao. Country rocks to the Zhujiabaobao picritic dyke are brucite marbles. Olivine phenocrysts in the dyke have Fo contents varying from 79 to 91 mol %. The most Mg-rich olivine phenocrysts (Fo89-91) are present in the margins of the dyke and contain undissolved calcite. Olivine phenocrysts in the marginal sample show reversed zoning in Fo content and contain similar to 20% lower Ni than those with similar Fo contents in the coeval picrites emplaced in the nearby area. Olivine-hosted oxide inclusions in this sample are exclusively Cr-magnetite (calculated atomic Fe3+/Cr3+ > 1) instead of chromite or Cr-spinel (calculated atomic Fe3+/Cr3+ < 1) that characterizes the coeval picrites. Within the dyke olivine phenocrysts with different ranges of Fo content (Fo89-91 versus Fo78-83) have similar Mn contents but different Ni contents. The most Mg-rich olivine phenocrysts have Ni contents that are 20-30% less than those for the more Fe-rich olivine phenocrysts. Diopside crystals associated with the most Mg-rich olivine phenocrysts contain higher Fe2O3/FeO than those associated with the more Fe-rich olivine phenocrysts. The dyke samples with different olivine compositions have similar E >(Nd) values (1 center dot 4-2) but contrasting Sr isotope compositions. The sample with the most Mg-rich olivine (Fo89-90) has the highest initial Sr-87/Sr-86 ratio (0 center dot 7054), whereas samples with more Fe-rich olivine (Fo78-83) have lower initial Sr-87/Sr-86 ratios (0 center dot 7046-0 center dot 7049). The results of mixing calculations indicate that up to similar to 25 wt % assimilation of brucite marble by basaltic magma with a composition similar to that of the average of the coeval high-Ti basalts can readily explain the observed mineralogical and isotopic variations within the dyke. The results from the dyke show that carbonate assimilation by basaltic magma is an efficient way to produce the association of magnetite with olivine that is anomalously rich in magnesium.