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首页> 外文期刊>Journal of Petrology >Geochemistry of South African On- and Off-craton, Group I and Group II Kimberlites: Petrogenesis and Source Region Evolution
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Geochemistry of South African On- and Off-craton, Group I and Group II Kimberlites: Petrogenesis and Source Region Evolution

机译：南非第一和第二克拉通克拉通岩的地球化学：成岩作用和源区演化

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摘要

Bulk-rock geochemical compositions of hypabyssal kimberlites, emplaced through the Archaean Kaapvaal craton and Proterozoic Namaqua–Natal belt, are used to estimate close-to-primary magma compositions of Group I kimberlites (Mg-number = 0·82–0·87; ∼22–28 wt % MgO; ∼21–30 wt % SiO2; ∼10–17 wt % CaO; ∼0·2–1·7 wt % K2O) and Group II kimberlites (Mg-number = 0·86–0·89; ∼23–29 wt % MgO; ∼28–36 wt % SiO2; 8–13 wt % CaO; ∼1·6–4·6 wt % K2O). Group I kimberlites are distinguished from Group II by their lower Ba/Nb (<12), Th/Nb (<1·1) and La/Nb (<1·1) but higher Ce/Pb (>22) ratios. The distinct rare earth element patterns of the two types of kimberlites indicate a more highly metasomatized source for Group II kimberlites, with more residual clinopyroxene and less residual garnet. The similarity of Sr and Nd isotope ratios and diagnostic trace element ratios (Ce/Pb, Nb/U, La/Nb, Ba/Nb, Th/Nb) of Group I kimberlites to ocean island basalts (OIB), but more refractory Mg-numbers and Ni contents, are consistent with derivation of Group I kimberlites from subcontinental lithospheric mantle (SCLM) that has been enriched by OIB-like melts or fluids. Source enrichment ages and plate reconstructions support a direct association of these melts or fluids with Mesozoic upwelling beneath southern Africa of a mantle plume(s), at present located beneath the southern South Atlantic Ocean. In contrast, the geochemical characteristics of both on- and off-craton Group II kimberlites show strong similarity to calc-alkaline magmas, particularly in their Nb and Ta depletion and Pb enrichment. It is suggested that Group II kimberlites are derived from both Archaean and Proterozoic lithospheric mantle source regions metasomatized by melts or fluids associated with ancient subduction events, unrelated to mantle plume upwelling. The upwelling of mantle plumes beneath southern Africa during the Mesozoic, at the time of Gondwana break-up, may have acted as a heat source for partial melting of the SCLM and the generation of both Group I and Group II kimberlite magmas.
机译：通过古生的Kaapvaal克拉通和元古生的纳马夸-纳塔尔带放置的海底金伯利岩的块状岩石地球化学成分，用于估算第一类金伯利岩的近原岩浆成分（镁数= 0·82-0.87； 〜22–28 wt％MgO; 〜21–30 wt％SiO 2 ; 〜10–17 wt％CaO; 〜0·2-1–7wt％K 2 O）和II类金伯利岩（Mg数= 0·86-0·89; MgO约23-29 wt％; SiO 2 约28-36 wt％; CaO 8-13 wt％ ；〜1·6-4·6 wt％K 2 O）。 I组金伯利岩与II组的区别在于其较低的Ba / Nb（<12），Th / Nb（<1·1）和La / Nb（<1·1）但较高的Ce / Pb（> 22）比。两种类型的金伯利岩的独特的稀土元素模式表明，II类金伯利岩的交代程度更高，残留的斜py石和较少的石榴石。第一类金伯利岩的Sr和Nd同位素比和诊断微量元素比（Ce / Pb，Nb / U，La / Nb，Ba / Nb，Th / Nb）与大岛玄武岩（OIB）相似，但难熔镁含量更高数和Ni含量与从陆相岩石圈地幔（SCLM）衍生出的I类金伯利岩相一致，该地幔层已被OIB状熔体或流体所富集。源的富集年龄和板块的重建支持这些熔体或流体与非洲南部南部地幔柱（目前位于南大西洋南部）下的中生界上升直接相关。相比之下，克拉通II类和非克拉通II类金伯利岩的地球化学特征与钙碱性岩浆都表现出强烈的相似性，特别是在Nb和Ta贫化和Pb富集方面。建议第二类金伯利岩来自古生界和元古代的岩石圈地幔源区，这些源区是由与古代俯冲事件有关的熔体或流体交代的，与地幔柱上升无关。在冈瓦纳解体时，中生代南部非洲下的地幔柱上升，可能是SCLM部分融化以及I和II组金伯利岩浆形成的热源。

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  • 来源
    《Journal of Petrology》 |2006年第4期|673-703|共31页
  • 作者

    MEGAN BECKER and ANTON P. LE ROEX;

  • 作者单位

    DEPARTMENT OF GEOLOGICAL SCIENCES UNIVERSITY OF CAPE TOWN RONDEBOSCH 7701 SOUTH AFRICA;

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