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首页> 外文期刊>Contributions to Mineralogy and Petrology >Diamondiferous lithospheric roots along the western margin of the Kalahari Craton--the peridotitic inclusion suite in diamonds from Orapa and Jwaneng
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Diamondiferous lithospheric roots along the western margin of the Kalahari Craton--the peridotitic inclusion suite in diamonds from Orapa and Jwaneng

机译：卡拉哈里克拉通（Kalahari Craton）西缘的菱形岩石圈根-来自Orapa和Jwaneng的钻石中的钙质包裹体套件

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摘要

The Orapa and Jwaneng kimberlites are located along the western margin of the Kalahari Craton and the prevalence of eclogitic over peridotitic diamonds in both mines has recently been linked to lower P-wave velocities in the deep mantle lithosphere (relative to the bulk of the craton) to suggest a diamond formation event prompted by mid-Proterozoic growth and modification of preexisting Archean lithosphere (Shirey et al. 2002). Here we study peridotitic diamonds from both mines, with an emphasis on the style of metasomatic source enrichment, to evaluate their relationship with this major eclogitic diamond formation event. In their major element chemistry, the peridotitic inclusions compare well with a world-wide database but reveal differences to diamond sources located in the interior of the Western Terrane of the Kaapvaal block, where the classical mines in the Kimberley region are located. The most striking difference is the relative paucity of low-Ca (<2 wt percent CaO in garnet) harzburgites and a low ratio of harzburgitic to lherzolitic garnets (2:1). This suggests that lithospheric mantle accreted to the rim of the Zimbabwe and Kaapvaal blocks was overall chemically less depleted. Alternatively, this more fertile signature may be assigned to stronger metasomatic re-enrichment but the trace element signature of garnet inclusions is not in favor of strong enrichment in major elements. For both mines the majority of lherzolitic and harzburgitic garnet inclusions are characterized by moderately sinusoidal REE_N patterns and low Ti, Zr and Y contents, indicative of a metasomatic agent with very high LREE/HREE and low HFSE. This is consistent with metasomatism by a CHO-fluid or, as modeled by Burgess and Harte (2003), a highly fractionated, low-volume silicate melt from the MORB-source. In both cases, changes in the major element chemistry of the affected rocks will be limited. In a few garnets from Orapa preferential MREE enrichment is observed, suggesting that the percolating fluid/melt fractionated a LREE-phyllic phase (such as crichtonite). The overall moderate degree of metasomatism reflected by the inclusion chemistry is in stark contrast to lithospheric sections for Orapa and Jwaneng based on mantle xenocrysts and xenoliths, revealing extensive mantle metasomatism (Griffin et al. 2003). This suggests that the formation of peridotitic diamonds predates the intensive modification of the subcratonic lithosphere during Proterozoic rifting and compression, implying that diamonds may survive major tectonothermal events.
机译：Orapa和Jwaneng金伯利岩位于卡拉哈里克拉通的西边缘，这两个矿山中隐伏性超过橄榄石化钻石的盛行最近都与地幔深层岩石圈中较低的P波速度有关（相对于克拉通的大部分）这表明钻石形成事件是由元古代中期生长和对太古代岩石圈的改造引起的（Shirey等，2002）。在这里，我们研究了这两个矿的橄榄岩化钻石，重点是后代物来源富集的样式，以评估它们与这一主要渐近性钻石形成事件的关系。在其主要元素化学中，橄榄岩包裹体与世界范围内的数据库相比具有很好的对比，但揭示了与位于Kaapvaal区块西部Terrane内部（金伯利地区的经典矿山所在）内部的钻石来源的差异。最显着的差异是低钙（石榴石中CaO含量低于2 wt％）哈茨伯格石的相对稀缺性和哈茨伯格与石榴石石榴石的比例低（2：1）。这表明，在津巴布韦和Kaapvaal区块边缘增生的岩石圈地幔在化学上总体上耗竭较少。或者，可以将这种更肥沃的签名分配给更强的交代重富集，但石榴石包裹体的痕量元素签名不支持主要元素的强富集。对于这两个矿山，大多数左斜沸石和哈密榴石石榴石的内含物都具有中等正弦形的REE_N模式和低的Ti，Zr和Y含量，这表明它是具有非常高的LREE / HREE和低HFSE的交代剂。这与CHO流体的交代作用或Burgess和Harte（2003）建模的，来自MORB来源的高度分馏的小体积硅酸盐熔体是一致的。在这两种情况下，受影响岩石的主要元素化学变化都将受到限制。在一些来自Orapa的石榴石中，观察到了优先的MREE富集，这表明渗流液/熔体将LREE的叶状相（如菱沸石）分馏。包裹体化学反应所反映的交代作用的总体适度程度与Orapa和Jwaneng的岩石圈剖面形成了鲜明的对比，后者是基于地幔异晶和异岩的，揭示了广泛的地幔交代作用（Griffin等，2003）。这表明，在元古生代裂谷和压缩过程中，橄榄岩钻石的形成要早于克拉通岩石圈的强烈变质，这意味着钻石可能在主要的构造热事件中幸存下来。

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