Characterising the Distinct Crustal Protoliths of Roberts Victor Type I and II Eclogites
Characterising the Distinct Crustal Protoliths of Roberts Victor Type I and II Eclogites
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描述 Roberts Victor I 型和 II 型榴辉岩的独特地壳原岩
DOI：
10.1093/petrology/egab090
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发表时间：
2021
影响因子：
3.9
通讯作者：
Sharp, Zachary D
中科院分区：
文献类型：
--
作者：
Hardman, Matthew F;Stachel, Thomas;Pearson, D Graham;Cano, Erick J;Stern, Richard A;Sharp, Zachary D
榴辉岩的起源是长期以来一直存在争议的地幔根居住。在南非典型的罗伯茨维克托金伯利岩产地，两类榴辉岩强烈的结构和地球化学特征导致了不同的成因模式。研究了采自罗伯茨维克托矿的一套新榴辉岩捕虏体。除测定了所有新样品的主量和微量元素组成外，还测定了34个榴辉岩中石榴石的~（18）O/~（16）O。基于地球化学和纹理特征，我们确定了一个大的套I型榴辉岩（n= 53）与以前的解释，这些岩石起源于变质的玄武苦橄质熔岩或辉长岩堆从大洋地壳，从熔融的亏损洋中脊玄武岩（MORB）地幔结晶。我们确定了一个较小的II型榴辉岩（n= 10）的基础上，在已发表的文献榴辉岩的地球化学和纹理相似性。我们推断它们的δ 18 O值范围非常低，结合它们的变化，往往非常低的锆铪（Zr-Hf）比和轻稀土元素贫化的性质，表明原岩起源于低压单斜辉石轴承海洋堆积形成的熔体，更缺乏不相容元素比N-MORB。这些组合物是来自残余地幔源的指示，经历了不相容元素的优先提取和分馏的Zr/Hf在以前的熔融。
The origin of the eclogites that reside in cratonic mantle roots has long been debated. In the classic Roberts Victor kimberlite locality in South Africa, the strongly contrasting textural and geochemical features of two types of eclogites have led to different genetic models. We studied a new suite of 63 eclogite xenoliths from the former Roberts Victor Mine. In addition to major- and trace-element compositions for all new samples, we determined18O/16O for garnet from 34eclogites. Based on geochemical and textural characteristics we identify a large suite of Type I eclogites (n= 53) consistent with previous interpretations that these rocks originate from metamorphosed basaltic-picritic lavas or gabbroic cumulates from oceanic crust, crystallised from melts of depleted mid-ocean ridge basalt (MORB) mantle. We identify a smaller set of Type II eclogites (n= 10) based on geochemical and textural similarity to eclogites in published literature. We infer their range to very low δ18O values combined with their varied, often very low zirconium-hafnium (Zr-Hf) ratios and light rare earth element-depleted nature to indicate a protolith origin via low-pressure clinopyroxene-bearing oceanic cumulates formed from melts that were more depleted in incompatible elements than N-MORB. These compositions are indicative of derivation from a residual mantle source that experienced preferential extraction of incompatible elements and fractionation of Zr/Hf during previous melting.