Geochemistry of coexisting depleted and enriched Paringa Basalts, in the 2.7 Ga Kalgoorlie Terrane, Yilgarn Craton, Western Australia: Evidence for a heterogeneous mantle plume event
Geochemistry of coexisting depleted and enriched Paringa Basalts, in the 2.7 Ga Kalgoorlie Terrane, Yilgarn Craton, Western Australia: Evidence for a heterogeneous mantle plume event
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DOI：
10.1016/j.precamres.2009.08.002
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发表时间：
2009-11
影响因子：
3.8
通讯作者：
N. Said;R. Kerrich
中科院分区：
文献类型：
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作者：
N. Said;R. Kerrich
贫化和富集的Paringa玄武岩形成了2720-2680Ma卡尔古利地体Kambalda序列上部玄武岩单元的最上部，并为了解成分和同位素异质软流圈提供了一个窗口。贫化玄武岩实例的成分限制为 Mg# 介于 61 至 57 之间，Ni 介于 166 至 73 ppm 之间。在稀土元素和原始地幔归一化图上，它们的特征是：（1）接近平坦的稀土元素模式，具有小范围的轻稀土消耗（（La/Sm）N1.01-0.76）； (2) Ce和Eu均无异常； (3) Nb/Th 比值为 8.7-12，略大于原始地幔值 8； (4) 相对于邻近的稀土元素，没有负的原始地幔归一化 P 或 Ti 异常。它们的成分与与科马提岩相关的新太古代拉斑玄武岩类似，εNd2.7Ga 值为+1.3 至+2.2，与大多数地壳未受污染的新太古代玄武岩和科马提岩一致。 Paringa 玄武岩的富集对应物的成分各不相同，其中 Mg# 76–53 和 Ni 391–73ppm。在稀土元素和原始地幔归一化图上，它们具有以下特征：（1）系统分馏的轻稀土元素[(La/Sm)N=2.1–3.1)]； (2) HREE 大部分为 ∼1，异常值为 1.6； (3) Ce和Eu均无异常； (4) 0.7-1.6的低Nb/Th比率； (5)原始地幔标准化P和Ti强负异常。 Epsilon Nd 值为-1.7 至-4.4； εNd 与 (La/Sm)Nor Nb/Th 比等地壳污染指数没有相关性。这组玄武岩岩组以前被解释为胶古岩或地壳污染的科马提岩。使用科马提石作为母液进行同化-分级结晶（AFC）建模。该模型中使用的污染物是当地的长辉石-英英英安岩 (TTD) 黑旗群 (BFG)、Yilgarn 克拉通的平均高钙花岗岩以及 Taylor, S.R.、McLennan, S.M. 的平均上、中或下大陆地壳。 （1985）。大陆地壳：其组成和演化。布莱克威尔科学出版物，牛津，312 页]。任何太古代地壳的平均成分都不符合 AFC 趋势，BFG 也不符合。高钙花岗岩确实符合趋势，现代上地壳平均成分也符合这一趋势，但前者的地幔 Nd 同位素成分已耗尽。在丰富的帕林加玄武岩形成过程中，大陆地壳同化模型存在许多问题，包括一系列 Mg#、Nb、P 和 Ti 异常的相干 REE 模式，这些异常不会随着 Th 或 (La/Sm)N 的增加而增加。富集帕林加玄武岩在成分上与富镁大陆溢流玄武岩不同，富镁大陆溢流玄武岩的特点是 Ti 和其他不相容元素含量较高，并含有分馏的重稀土元素、或来自洋内弧的栉云石、苦玄岩或中钾玄武岩。相反，这些玄武岩被解释为旧大陆地壳循环进入卡姆巴尔达层序科马提岩和玄武岩喷发地幔源的结果。这两种不同的成分和同位素类型记录了异质地幔柱，可能在裂谷克拉通边缘喷发。
Depleted and enriched Paringa Basalts form the uppermost part of the Upper Basalt Unit of the Kambalda Sequence in the 2720–2680Ma Kalgoorlie Terrane, and provide a window into compositionally and isotopically heterogeneous asthenosphere. Depleted examples of the basalts are compositionally restricted with Mg# between 61 and 57, and Ni between 166 and 73ppm. On REE and primitive-mantle normalised diagrams they are characterised by: (1) near-flat REE patterns with a small range of LREE depletion ((La/Sm)N1.01–0.76); (2) neither Ce nor Eu anomalies; (3) Nb/Th ratios of 8.7–12, variably greater than the primitive mantle value of 8; and (4) no negative primitive-mantle normalised P or Ti anomalies relative to neighbouring REE. They have compositions similar to Neoarchean tholeiitic basalts associated with komatiites, and ɛNd2.7Ga values of +1.3 to + 2.2, in keeping with the majority of crustally uncontaminated Neoarchean basalts and komatiites. Enriched counterparts of the Paringa Basalt are compositionally varied, with Mg# 76–53 and Ni 391–73ppm. On REE and primitive-mantle normalised diagrams, they feature: (1) systematically fractionated LREE [(La/Sm)N=2.1–3.1)]; (2) HREE mostly ∼1, with outliers to 1.6; (3) neither Ce nor Eu anomalies; (4) low Nb/Th ratios of 0.7–1.6; and (5) strong negative primitive-mantle normalised P and Ti anomalies. Epsilon Nd values are −1.7 to −4.4; and there is no correlation of ɛNdwith indices of crustal contamination such as (La/Sm)Nor Nb/Th ratios. This basaltic suite has previously been interpreted as either boninitic or crustally contaminated komatiite. Assimilation-fractional crystallisation (AFC) modeling was conducted using a komatiite as parental liquid. Contaminants used in this modeling were local trondhjemite-tonalite-dacite (TTD) Black Flag Group (BFG), average high-Ca granites of the Yilgarn Craton, and average upper-, middle-, or lower continental crust of Taylor, S.R., McLennan, S.M. (1985). The Continental Crust: Its Composition and Evolution. Blackwell Scientific Publications, Oxford, 312 pp.]. Neither any of the average Archean crustal compositions match the AFC trends, nor does the BFG. High-Ca granites do match the trends, as does average modern upper crustal composition but the former have depleted mantle Nd-isotope compositions. There are numerous problems with a model of assimilation of continental crust in the genesis of the enriched Paringa Basalts, including coherent REE patterns over a range of Mg#, and Nb-, P-, and Ti-anomalies that do not increase with Th or (La/Sm)N. Enriched Paringa Basalts are compositionally distinct from Mg-rich continental flood basalts that feature greater contents of Ti, and other incompatible elements, with fractionated HREE, or boninites, picrites, or medium-K basalts from intraoceanic arcs. Rather, these basalts are interpreted to result from recycling of older continental crust into the mantle source of the plume from which the Kambalda Sequence komatiites and basalts erupted. The two distinct compositional and isotopic types record a heterogeneous mantle plume, possibly erupted at a rifted craton margin.