摘要
应用500多个科马提岩的岩石化学分析,分辨出三种科马提岩类型,并探讨了不同类型科马提岩与上地幔的演化关系。
Some of the most remarkable rocks, especially in texture and genesis, are undoubtedly the komatiites, which together with komatiitics basalts and/or tholeiites constitute the komatiitic suites. However, there are differences in geochemistry between komatiites from different areas and/or ages, providing a direct evidence of the physical and chemical changes of the mantle. Nesbitt and Sun (1979) recognized two kinds of komatiites: aluminum-undepleted komatiites (Group Ⅰ) and alum num-depleted komatiites (Group Ⅱ), discriminated by the diagrams of Al_2O_3 vs. TiO_2 and CaO vs. Al_2I_3. The typical examples are from Abitibi, Canada and Barberton. South Africa, respectively. Recently, Ohtani et al. (1989) identified a new kind of komatiites: alumium-enriched komatiites (Group Ⅲ). The author documents that the Group Ⅲ komatiites not only exist, but also have high Al_2O_3/TiO_2 ratios caused by depletion of Ti instead of enrichment of aluminum.
Although most of komatiites or komatiitic suites are of Archean ages and occur in greenstone belts, some also occur in Phanerozoic ophiolitic suites, such as the Cape Smith belt (Early Proterozoic), Canada; Betts Cove (Ordovician), Newfoundland, Canada; Othris Mountains (Triassic), Greece and Gorgona Island (Tertiary), Columbia. So far, the relationships between the various kinds of komatiites, komatiitic basalts and tholeiites and their evolution throughout the geological time are not well-known, although that might he a key problem in the understanding of Archean and Phanerozoic crust-mantle evolution, This paper attempts to discuss these problems on the basis of the petrochemistry of over 500 samples selected from a database established by the author.
出处
《岩石矿物学杂志》
CAS
CSCD
1990年第4期320-330,共11页
Acta Petrologica et Mineralogica
