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首页> 外文期刊>Contributions to Mineralogy and Petrology >Modal metasomatism in the Kaapvall craton lithosphere: constraints on timing and genesis from U-Pb zircon dating of metasomatized peridotites and MARID-type xenoliths
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Modal metasomatism in the Kaapvall craton lithosphere: constraints on timing and genesis from U-Pb zircon dating of metasomatized peridotites and MARID-type xenoliths

机译：Kaapvall克拉通岩石圈的模态交代作用：交代化橄榄岩和MARID型异岩的U-Pb锆石测年对时间和成因的限制

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Modal metasomatism in the Kaapvaal craton lithosphere is well documented in upper mantle xenoliths sampled by both group I (mainly late Cretaceous) and group II (mainly early Cretaceous to late Jurassic) kimberlites in the Kimberley area. The metasomatic style is characterized by introduction of K, H and large ion lithophile/high field strength (LIL/HFS) elements into the lithospheric mantle leading to the crystallization of hydrous potassic phases such as phlogopite and/or K-amphibole. Textures indicate that the hydrous phases either replace pre-existing assemblages in peridotites, forming the metasomatized peridotite suite (phlogopite-K-richterite-peridotites: PKPs) or crystallize from K-rich melts, forming the mica-amphibole rutile-ilmenite-diopside (MARID) suite of xenoliths. These K-rich assemblages become potential low melting source components for alkaline incompatible trace element enriched magmas. The timing of metasomatism and its temporal and possible genetic relation to kimberlite magmatism is poorly constrained because of the rarity of phases in the metasomatic assemblages suitable for precise dating. Here we present precise sensitive high resolution ion microprobe (SHRIMP) U-Pb formation ages of 88 +- 2 (1 #sigma# = 1 standard deviation) and 82 +- 3 Ma data for zircons from a K-richterite-phlogopite-bearing metasomatized peridotite (PKP) and a MARID xenolith respectively, sampled by a group I kimberlite. Both average PKP and MARID zircon ages are indistinguishable from emplacement ages of group I kimberlite in the Kimberley area dated at 83 +- 4 (2#sigma#) and 84 +- 0.9 Ma. One exceptionally old age spot of 102 +- 5 Ma from a PKP zircon provides evidence for modal metasomatism predating group kimberlite emplacement by several millions of years with minor resetting of the U-Pb isotopic system of most analyzed PKP zircons to a group I emplacement age. Detailed textural and mineral chemical analysis, including high energy X-ray mapping and analysis of fluid inclusion daughter crystals, indicates a complex reaction history for both PKPs and MARIDs. U-Pb zircon ages from this study combined with literature data and experimentally derived models for MARID formation are used to suggest that MARID-formation is concurrent and genetically related to both group I and II kimberlite magmatism in the Kimberley area. MARID and PKP zircon ages are also consistent with the idea first proposed by Dawson and Smith (Geochim Cosmochim Acta 41: 309-323 1977) that metasomatized peridotites may form from interaction of hydrous fluids expelled by solidifying MARID-type melts with peridotitic wall rocks.
机译：在金伯利地区，第一类（主要是白垩纪晚期）和第二类（主要是白垩纪早期至侏罗纪晚期）金伯利岩采样的上地幔异岩中，Kaapvaal克拉通岩石圈的模态交际作用得到了很好的证明。交代方式的特征是，在岩石圈地幔中引入了K，H和较大的离子亲石性/高场强（LIL / HFS）元素，导致含水钾相（如金云母和/或K闪石）结晶。纹理表明，水相替代了橄榄岩中预先存在的组合，形成了交代的橄榄岩套件（金云母-富钾钾橄榄石-橄榄岩：PKPs）或从富含钾的熔体中结晶，形成了云母-闪石金红石-钛铁矿-透辉石（ MARID）xenoliths套件。这些富含钾的组合物成为潜在的低熔点来源成分，可用于富含碱的不相容微量元素岩浆。由于交代组合中相的稀少性适合于精确测年，因此交代作用的时间及其与金伯利岩岩浆作用的时间和可能的遗传关系受到了严格的限制。在这里，我们提出了一个精确的高分辨率高灵敏度离子微探针（SHRIMP）U-Pb形成年龄，从含K-富铝石-金云母的锆石中获得88 +-2（1＃sigma＃= 1标准偏差）和82 +-3 Ma数据。由I类金伯利岩取样的变质橄榄岩（PKP）和MARID异岩。 PKP和MARID锆石的平均年龄与金伯利地区I组金伯利岩的进驻年龄没有区别，分别为83 +-4（2＃sigma＃）和84 +-0.9 Ma。一个来自PKP锆石的102±5 Ma的异常老年斑为模态交代作用比金伯利岩组早几百万年提供了证据，而大多数分析过的PKP锆石的U-Pb同位素系统仅略微重置为I组。 。详细的纹理和矿物化学分析，包括高能X射线映射和流体包裹体子晶体的分析，表明PKP和MARID的反应历史很复杂。这项研究的U-Pb锆石年龄与文献数据和MARID形成的实验推导模型相结合，表明MARID的形成与金伯利地区的I组和II组金伯利岩岩浆作用同时发生并在遗传上相关。 MARID和PKP锆石的年龄也与Dawson和Smith（Geochim Cosmochim Acta 41：309-323 1977）首次提出的想法一致，即交代化橄榄岩可能是由于固化MARID型熔体与橄榄岩墙体岩石而排出的含水流体相互作用而形成的。

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