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首页> 外文期刊>Contributions to Mineralogy and Petrology >Metamorphic re-equilibration and metasomatism of highly chromian, garnet-rich peridotitic xenoliths from South African kimberlites
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Metamorphic re-equilibration and metasomatism of highly chromian, garnet-rich peridotitic xenoliths from South African kimberlites

机译：来自南非金伯利岩的高铬，富含石榴石的橄榄石异岩的变质重新平衡和交代作用

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A history of decompression and metasomatism is preserved in a suite of highly chromian, garnet-rich peridotitic xenoliths from the diamondiferous Newlands and Bobbejaan kimberlites, South Africa. A high proportion of the garnets and chromites in these rocks plot in the diamond-facies fields on Cr2O3–CaO and Cr2O3–MgO wt% plots, and Cr-rich compositions are found in both the harzburgitic and lherzolitic fields. Petrographic evidence suggests that the earliest known mineralogies were those of olivine-bearing, garnet-rich rocks. These were modified by a decompression event that caused recrystallization of garnets and led to orientated spinel and pyroxene inclusions in garnet. Chemical zonation within garnet is divided into (1) external re-equilibration between garnet and matrix; (2) internal re-equilibration between garnet and its inclusions; and (3) metasomatically induced zoning between garnet core and a metasomatic rim. The compositional trajectories associated with zonations (1) and (2) in Ca–Cr plots may be closely modelled by means of sliding, garnet–spinel transition reactions whose slopes vary with bulk Ca composition; at intermediate Ca compositions, the trajectories closely match the slope of the lherzolite line or harzburgite/lherzolite boundary. The decreasing Cr/(Cr + Al) of the garnet in these zonations is in agreement with the evidence for decompression given by the petrographic recrystallization features, and overall decompression of probably 10–20 kb is indicated. We speculate on the age of these events, and consider the possibility of their association with major orogenic events documented by South African crustal rocks at 2.9–2.7 Ga, and events evidenced by peridotite-xenolith Re–Os model ages at 2.8–2.7 Ga.
机译：减压和变质作用的历史保存在一组来自南非含钻石的纽兰兹和波贝贾恩金伯利岩的富含铬的，富含石榴石的橄榄石异岩中。 Cr 2 O 3 –CaO和Cr 2 O上的金刚石相场中，这些岩石中的石榴石和亚铬酸盐比例很高。 3 –MgO的重量百分比图和富铬成分都在哈兹伯格期和伊兹沃利特期均被发现。岩石学证据表明，最早的矿物学是含橄榄石，石榴石的岩石。这些被减压事件所修饰，减压事件导致石榴石重结晶，并导致石榴石中尖晶石和辉石夹杂物取向。石榴石内部的化学区域划分为（1）石榴石和基质之间的外部重新平衡； （2）石榴石及其内含物之间的内部重新平衡； （3）石榴石核心与交代轮缘之间的交代诱发地带。可以通过滑动，石榴石-尖晶石过渡反应（其斜率随整体Ca组成而变化）来紧密建模与Ca-Cr地带中的分区（1）和（2）相关的组成轨迹。在中等的Ca组成下，轨迹紧密匹配锂铁矿线或哈兹伯格/锂铁矿边界的斜率。在这些区域中，石榴石的Cr /（Cr + Al）降低与岩石学重结晶特征给出的减压证据相吻合，表明整体减压可能约为10–20 kb。我们推测了这些事件的年龄，并考虑了它们与南非地壳岩石在2.9–2.7 Ga记录的主要造山事件以及2.8–2.7 Ga的橄榄岩-异种岩Re-Os模型年龄证明的事件相关的可能性。

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