科研之友
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摘要
A breccia facies of mafic composition in the subaqueous Misema shield volcano building phase and Misema caldera of the Blake River caldera complex, Abitibi Greenstone belt, Canada preserves putative microbial ichnofossils in formerly glassy breccia fragments. These Neoarchean ichnofossils are comparable in morphology, distribution, and mineral association to similar textures found in other Archean cratons as well as Phanerozoic ophiolite successions and are interpreted to represent an early Earth subaqueous biosphere. However, a detailed volcanological facies analysis of the rocks that host these purported microbial ichnofossils is lacking from previous studies. The present field locality (Hurd property) is a 62 m-thick volcanic succession that displays two main lithological fades: (i) a massive to lobate facies and (ii) breccia facies. The 5-25 m-thick massive to lobate facies contains massive basalts that grade into lobate flows toward the top. The 1-15 m-thick breccia facies displays delicate volcanic textures that are preserved due to extensive silica precipitation, a low temperature hydrothermal process, and the subgreenschist metamorphic grade. Geochemically, the mafic flows are tholeiitic basalts with TiO(2) contents of 1-2% but high SiO(2), up to 71%, related to hydrothermal silica precipitation. Microbial ichnofossils are found in the glassy fragments of the breccia facies. The breccia fades that displays evidence of associated hydrothermal activity, as in modern seafloor and summit caldera smoker settings, provided a favourable environment for ancient biological interactions between glassy rocks and microorganisms during formation of the Abitibi Greenstone belt.
- 出版日期2010-5
