Metasomatic enrichment of Proterozoic mantle south of the Kaapvaal Craton, South Africa: origin of sinusoidal REE patterns in clinopyroxene and garnet
Metasomatic enrichment of Proterozoic mantle south of the Kaapvaal Craton, South Africa: origin of sinusoidal REE patterns in clinopyroxene and garnet
复制标题
南非卡普瓦尔克拉通南部元古代地幔的交代富集：单斜辉石和石榴石中正弦稀土元素模式的起源
DOI：
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发表时间：
2016
影响因子：
3.5
通讯作者：
C. Class
中科院分区：
文献类型：
--
作者：
A. Roex;C. Class
摘要
地幔橄榄岩捕虏体已从四个金伯利岩侵入体，梅尔顿沃尔德，希伯伦，Uintjiesberg和马克特，侵位通过中元古代纳马夸-纳塔尔带，沿着Kaapvaal盆地南部边界。虽然许多捕虏体严重蚀变，成分单斜辉石，石榴石和金云母是新鲜的，并已分析了电子探针的主要元素和激光烧蚀ICP-MS的微量元素。单斜辉石的原始地幔标准化REE丰度都是强烈的LREE富集，并显示出一系列的模式，包括均匀的MREE-HREE倾斜（这里称为“正常”），正弦曲线和驼峰正弦曲线模式。HREE丰度极低（Yb = 0.3-0.06 × PM）。REEN模式在共存石榴石显示了类似的模式范围。当标准化原始地幔值，微量元素模式在一些单斜辉石表现出强烈的相对亏损的Rb-Ba，Ta-Nb和Ti，与一些样品也相对亏损的Zr-Hf。这些微量元素的特征是无法区分的单斜辉石和石榴子石从橄榄岩从邻近的地幔。通过地球化学贫化橄榄岩基质的富集交代熔体的反应性多孔流动的数值模拟可以解释所观察到的REEN模式的全部范围。Rb-Ba、Ta-Nb和Ti的相对贫化可归因于金云母从结晶熔体中的早期结晶。某些单斜辉石中Zr-Hf的相对亏损需要锆石在近端交代组合中结晶，或由碳酸岩熔体交代。模拟结果，再加上缺乏单斜辉石与耗尽，甚至部分富集REEN模式，表明所有的单斜辉石已模态引入到最初高度亏损方辉橄榄岩原岩通过交代作用。单斜辉石Sr、Pb同位素组成的变化范围表明了不同成分的熔体对区域交代作用的影响。强烈的HREEN消耗被解释为反映在早元古代初始熔体消耗的影响，与熔融延伸到尖晶石稳定领域需要一个海洋领域，并再次晚在中元古代（纳马夸造山运动）。叠加的不相容元素富集表明，随后的多个富集事件上升的碱性熔体成分类似金伯利岩或超镁铁质碱性lamebrre，可能与中生代羽流上涌下的区域，重新引入单斜辉石到枯竭的元古宙harzburgelite原岩。
Abstract
Xenoliths of mantle peridotite have been sampled from four kimberlite intrusions, Melton Wold, Hebron, Uintjiesberg and Markt, emplaced through the Mesoproterozoic Namaqua-Natal Belt, along the southern border of the Kaapvaal Craton. Although many of the xenoliths are heavily altered, constituent clinopyroxene, garnet and phlogopite are fresh and have been analysed by electron microprobe for major elements and by laser ablation ICP-MS for trace elements. Primitive mantle-normalised REE abundances in clinopyroxene are all strongly LREE enriched and show a range of patterns including uniformly MREE–HREE sloped (referred to here as ‘normal’), sinusoidal and humped sinusoidal patterns. HREE abundances are extremely low (Yb = 0.3–0.06 × PM). REEN patterns in coexisting garnets show a similar range of patterns. When normalised to primitive mantle values, trace element patterns in some clinopyroxenes show strong relative depletion in Rb–Ba, Ta–Nb and Ti, with some samples also being relatively depleted in Zr–Hf. These trace element characteristics are indistinguishable from those found in clinopyroxene and garnet from peridotites from the adjacent cratonic mantle. Numerical modelling of reactive porous flow of an enriched metasomatic melt through a geochemically depleted peridotite matrix can account for the full range in observed REEN patterns. The relative depletion in Rb–Ba, Ta–Nb and Ti can be accounted for by an early crystallisation of phlogopite from the percolating melt. The relative depletion in Zr–Hf in some clinopyroxenes requires either zircon to crystallise in the proximal metasomatic assemblage, or metasomatism by a carbonatitic melt. Modelling results, together with the absence of clinopyroxene with depleted or even partially enriched REEN patterns, suggest that all clinopyroxene has been modally introduced through metasomatism into an initially highly depleted harzburgitic protolith. The range in Sr and Pb isotopic composition of the clinopyroxenes indicates regional metasomatism by melts of various compositions. The strong HREEN depletion is interpreted to reflect the effect of initial melt depletion in the early Proterozoic, with melting extending into the spinel stability field requiring an oceanic realm, and again later in the Mesoproterozoic (Namaqua Orogeny). The superimposed incompatible element enrichment indicates subsequent multiple enrichment events by rising alkaline melts similar in composition to kimberlite or ultramafic alkaline lamprophyre, possibly related to Mesozoic plume upwelling beneath the region, that reintroduced clinopyroxene into the depleted Proterozoic harzburgite protolith.
影响因子：
3.5
作者：
Girnis, A. V.;Bulatov, V. K.;Hoefer, H. E.
通讯作者：
Hoefer, H. E.
吉遍金伯利岩田（纳米比亚）下方的岩石圈地幔：古老和年轻成分的混合——来自 LuâHf 和 SmâNd 同位素系统学的证据
影响因子：
3.8
作者：
Luchs T;Brey G.P;Gerdes A;Höfer H.E.
通讯作者：
Höfer H.E.