Mineralogical evolution and REE mobility during crystallisation of ancylite-bearing ferrocarbonatite, Haast River, New Zealand
Mineralogical evolution and REE mobility during crystallisation of ancylite-bearing ferrocarbonatite, Haast River, New Zealand
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DOI：
10.1016/j.lithos.2015.01.005
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发表时间：
2015-02
期刊：
影响因子：
3.5
通讯作者：
A. Cooper;Alice K. Collins;J. Palin;J. Spratt
中科院分区：
文献类型：
--
作者：
A. Cooper;Alice K. Collins;J. Palin;J. Spratt
来自新西兰南韦斯特兰的层状阿尔卑斯岩墙群的铁碳酸岩由方解石-方解石-钠长石-赤铁矿组成，并含有以方解石-铁闪石-重晶石-独居石-钍石-钠长石-易解石-方沸石为主的间隙斑，解释为修改后的晚期分凝。铁碳酸盐岩中的主要碳酸盐是含铁白云石，其中含有蠕虫状和块状方解石斑块和更富铁的铁白云石。方解石被解释为出溶产物或副产品，与白云石和赤铁矿，在与热液流体的相互作用过程中的原生铁白云石的氧化。晚期富含斜长岩的偏析使87 Sr/86 Sr成分相对于寄主碳酸岩升高，这表明它们是从与寄主片岩平衡的流体中结晶出来的，但其中的稀土元素来自于铁碳酸岩的分馏。矿物脉纹表明这一阶段的结晶晚于铁白云石到白云石的交代。碳酸岩，从矿物组合，结构和碳酸盐地质测温推断，有一个复杂的再结晶的历史，从晚期，低温碳热液流体。所有碳酸盐岩相的凸向上球粒陨石标准化稀土模式富集Nd，并在轻稀土和重稀土相对亏损。La和Ce的贫化归因于碳酸盐岩初始分离过程中长英质岩浆的互补富集，或者更可能的是，随后La和Ce-Cl−络合物在低温含水流体中的流动性优先增强。
Ferrocarbonatites from the lamprophyric Alpine Dyke Swarm, south Westland, New Zealand are composed of dolomite–calcite-albite–hematite and contain interstitial patches dominated by calcite–ancylite–barite–monazite–thorite–albite–aeschynite–analcime, interpreted as modified late-stage segregations. The dominant carbonate in the ferrocarbonatite is a ferroan dolomite that contains vermicular and blocky patches of calcite and a more Fe-rich ankerite. The calcite is interpreted as the product of exsolution or the by-product, with dolomite and hematite, of the oxidation of primary ankerite during interaction with hydrothermal fluids. Late stage ancylite-rich segregations have elevated87Sr/86Sr compositions relative to host carbonatite suggesting they have crystallised from fluids that have equilibrated with host schist, but with the REE derived from fractionation of ferrocarbonatite. Mineral veining indicates this stage of crystallisation post-dated the ankerite to dolomite replacement.The carbonatites, as inferred from mineral associations, textures and carbonate geothermometry, have a complex history of recrystallisation from late stage, low temperature carbo-hydrothermal fluids. All carbonatitic phases have convex-upward chondrite normalised REE patterns enriched in Nd, and are relatively depleted in lighter LREE and in HREE. Depletion in La and Ce is attributed to either complementary enrichment in felsic magmas during initial separation of carbonatite, or more likely, to subsequent preferentially enhanced mobility of La– and Ce–Cl−complexes in low-temperature aqueous fluids.