Complex Peridotitic Xenoliths: Rare and Important Samples for Understanding the Lithospheric Evolution
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摘要: 地幔和/或下地壳中存在2类“复杂”橄榄岩捕虏体: 一类为“混杂”, 即“polymict”橄榄岩捕虏体; 另一类为相对常见的“复合”, 即“composite”橄榄岩捕虏体.复合橄榄岩捕虏体通常由橄榄岩和辉石岩/麻粒岩或由橄榄岩夹辉石岩脉2部分或多部分组成.岩相学上易于辨认, 有时手标本上即可辨认.而混杂橄榄岩捕虏体为各种橄榄岩和/或辉石岩来源的矿物和熔体混杂在一起组成的构造岩, 岩相学上很难辨认, 只有通过矿物组成分析来判断.混杂橄榄岩捕虏体仅见于南非Kaapvaal克拉通金伯利岩岩管中, 该类捕虏体来源于地幔塑性剪切带, 记录了流/熔体参与下的地幔变形和交代作用的全过程.复合橄榄岩捕虏体较常见, 出现在全球各类玄武岩和金伯利岩中.复合橄榄岩捕虏体有多种成因, 如地幔变形、壳幔过渡带样品、交代作用和堆积成因等等.这些复合橄榄岩捕虏体同样记录了岩石圈地幔和/或下地壳交代作用/改造过程的中间状态.因此, 这些罕见而重要的复杂橄榄岩捕虏体为大陆岩石圈动力学演化过程提供了最直接的样品.Abstract: Two types of "complex" peridotitic xenoliths exist in the earth mantle and/or lower crust: "polymict" peridotitic xenoliths and "composite" peridotitic xenoliths. Composite peridotitic xenoliths usually comprise two or more components of peridotite and pyroxenite/granulite or of peridotite with pyroxenite veins and can be easily identified in petrography, sometimes in hand samples. But the polymict peridotitic xenoliths are tectonites composed of minerals from various peridotites and/or pyroxenites and melts and brought together by mantle shear zone and difficult to identify in petrography, they must be classified by compositional analyses of constituent minerals. Polymict peridotitic xenoliths have only been found in kim-berlite pipes from the Kaapvaal craton, South Africa so far. These polymict peridotitic xenoliths originated from the mantle shear zone and recorded the whole evolutionary processes of fluid/melt-assisted mantle deformation and metasomatism. Composite peridotitic xenoliths are relatively common and occur in global basalts and kimberlites of all kinds. Composite peridol-itic xenoliths have a variety of origins such as mantle deformation, crust-mantle transitional zone sampling, metasomatism and accumulates, etc.. These composite peridotitic xenoliths also record the intermediate stage of metasomatism and modification processes of the lithospheric mantle and/or lower crust. Therefore, these rare and important complex peridotitic xenoliths provide the most direct samples for the geodynamic evolution of the continental lithosphere.
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图 1 南非金伯利地区金伯利岩携带的混杂橄榄岩捕虏体
视域宽约5 cm.图a为样品BD344, 紫红色大颗粒为镁铝榴石, 黑红色粒间颗粒为金云母, 翠绿色为透辉石, 黑色为钛铁矿, 其他为橄榄石和斜方辉石; 图b为样品BD2666, 紫红或粉红色为石榴石, 黑色为钛铁矿, 浅灰色为斜方辉石, 白色为橄榄石
Fig. 1. Kimberlite-borne polymict peridotitic xenoliths from Kimberley, South Africa
图 2 南非金伯利地区金伯利岩携带的混杂捕虏体的显微照片
视域宽: 图a为1.5 cm, b、c为1.3 cm, d为0.5 cm.(a)斜方辉石交代单斜辉石, 伴随有石榴石的形成(JJG1414);(b)斜方辉石交代单斜辉石, 斜方辉石的边缘部位有金红石存在(BD344);(c)斜方辉石交代单斜辉石(BD2394);(d)斜方辉石和金云母交代单斜辉石(BD2394).Di、Opx、Gt, Phl和Rut分别为透辉石、斜方辉石、石榴石、金云母和金红石的缩写.其黑白图见Zhang et al.(2001a)
Fig. 2. Micro-photographs of kimberlite-borne polymict xenoliths from Kimberley, South Africa
图 3 (a) 辽宁复县古生代金伯利岩岩区50号岩管中产出的复合橄榄岩捕虏体(F50-9229)的探针片扫描.(b)方城玄武岩中产出的复合橄榄岩捕虏体(FC-12)的探针片扫描
图a视域宽约2 cm, 深色部分为尖晶石/斜长石橄榄岩, 浅色部分为石榴石麻粒岩; 图b视域宽约1 cm, 捕虏体由堆晶橄榄岩(中心)和辉石岩(边缘)组成②
Fig. 3. (a) Electron probe slice scan of a composite peridot-itic xenolith (F50-9229) entrained in No.50 pipe of Paleozoic Fuxian kimberlite field, Liaoning Prov-ince.(b) Electron probe slice scan of a composite peridotitic xenolith (FC-12) entrained in Fangcheng basalts
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