A chilled margin of komatiite and Mg-rich basaltic andesite in the western Bushveld Complex, South Africa
A chilled margin of komatiite and Mg-rich basaltic andesite in the western Bushveld Complex, South Africa
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DOI：
10.1007/s00410-016-1257-5
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发表时间：
2016-05
影响因子：
3.5
通讯作者：
W. Maier;S. Barnes;B. Karykowski
中科院分区：
文献类型：
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作者：
W. Maier;S. Barnes;B. Karykowski
在南非联合剖面西部Bushveld杂岩下带底部的一个寒岩序列中，含有隐晶型富镁玄武岩安山岩和刺状结构的科马铁矿。玄武岩安山岩的平均组成为15.2% MgO、52.8% SiO2、1205 ppm Cr和361 ppm Ni，而科马地岩的平均组成为18.7% MgO、1515 ppm Cr和410 ppm Ni。这两种岩石类型的不可移动不相容元素浓度都很低（0.14-0.72 ppm Nb, 7-31 ppm Zr, 0.34-0.69 ppm Th, 0.23-0.27 wt% TiO2），但PGE含量（19-23 ppb Pt, 15-16 ppb Pd）和Pt/Pd比值（Pt/Pd 1.4）很高。锶和S同位素相对于大多数其他下带岩石显示出富集的特征。这些岩石可能代表了约20%的次大陆岩石圈地幔的部分熔融。这将与岩石的PGE含量相匹配。但该模型与岩石中SiO2、Fe、Na2O含量高，特别是K2O、Zr、Hf、Nb、Ta、Th、LREE、Rb、Ba含量低的特征不一致。另一种可能性是，这些寒带可能是源自软流圈的马马辉岩岩浆，经历了石英岩底的同化，并伴有橄榄石和铬铁矿的结晶。该模式与岩石亲石元素特征和Sr、S同位素特征一致。然而，为了解释岩浆中Pt和Pd的高含量，地幔中PGE的含量必须是目前PUM估计的两倍，可能是由于不完全平衡的晚期贴面成分。
A chill sequence at the base of the Lower Zone of the western Bushveld Complex at Union Section, South Africa, contains aphanitic Mg-rich basaltic andesite and spinifex-textured komatiite. The basaltic andesite has an average composition of 15.2 % MgO, 52.8 % SiO2, 1205 ppm Cr, and 361 ppm Ni, whereas the komatiite has 18.7 % MgO, 1515 ppm Cr, and 410 ppm Ni. Both rock types have very low concentrations of immobile incompatible elements (0.14–0.72 ppm Nb, 7–31 ppm Zr, 0.34–0.69 ppm Th, 0.23–0.27 wt% TiO2), but high PGE contents (19–23 ppb Pt, 15–16 ppb Pd) and Pt/Pd ratios (Pt/Pd 1.4). Strontium and S isotopes show enriched signatures relative to most other Lower Zone rocks. The rocks could represent a ~20 % partial melt of subcontinental lithospheric mantle. This would match the PGE content of the rocks. However, this model is inconsistent with the high SiO2, Fe, and Na2O contents and, in particular, the low K2O, Zr, Hf, Nb, Ta, Th, LREE, Rb, and Ba contents of the rocks. Alternatively, the chills could represent a komatiitic magma derived from the asthenosphere that underwent assimilation of the quartzitic floor accompanied by crystallization of olivine and chromite. This model is consistent with the lithophile elements and the elevated Sr and S isotopic signatures of the rocks. However, in order to account for the high Pt and Pd contents of the magma, the mantle must have been twice as rich in PGE as the current estimate for PUM, possibly due to a component of incompletely equilibrated late veneer.