Abstract Mesozoic diamondiferous lamproite pipes occur along the Kapamba River within the Luangwa Valley of eastern Zambia, which is a ca. 300–200 Ma old Karoo-age precursor branch to the East African Rift System. The Luangwa Rift developed above a reactivated mega-shear zone that cuts through the Proterozoic Irumide Belt between the Congo-Tanzania-Kalahari cratons and thus it provides a rare snapshot of early-stage cratonic rift evolution. The primary mineralogy of the fresh volcanic rocks suggests that they represent a continuum between primitive olivine lamproites and slightly more evolved olivine-leucite lamproites. Mineral compositions and evolutionary trends, such as the strong Al-depletion at Ti F enrichment in groundmass phlogopite and potassic richterite, resemble those of classic lamproite provinces in circum-cratonic settings (e.g., the Leucite Hills of Wyoming and the West Kimberley field in Australia). However, there are some similarities to orangeites from the Kaapvaal craton (formerly Group-2 kimberlites), type kamafugites from the East African Rift, and ultramafic lamprophyres from a key region of the rifted North Atlantic craton, which implies a complex interplay between source-forming and tectonic processes during Karoo-age lamproite magma formation beneath south-central Africa. The bulk compositions of the Kapamba volcanic rocks fall within the range of ‘cratonic’ low-silica lamproites, but there is overlap with orangeites, in particular with the more evolved leucite- and sanidine-bearing orangeite varieties. Modelling of the process by which most of the original leucite was transformed into analcime suggests that the primitive alkaline magmas at Kapamba contained ~6–9 wt% K2O and had high K2O/Na2O ratios between ~1.6–6.2 at >10 wt% MgO – confirming the ultrapotassic nature of the mantle-derived magmatism beneath the Luangwa Rift. The virtually CO2-free, H2O-F-rich Kapamba lamproites present an extension of the geochemical continuum displayed by the members of the CO2-H2O-rich kamafugite / ultramafic lamprophyre group. Hence, we suggest that the Kapamba lamproites and the type kamafugites, located within separate branches of the East African Rift System, represent melting products of similar K-metasomatized cratonic mantle domains, but their formation occurred under contrasting volatile conditions at different stages during rift development (i.e., incipient versus slightly more advanced rifting). Temperature estimates for peridotite-derived olivine xenocrysts from the Kapamba lamproites suggest that the Luangwa Valley is an aborted cratonic rift that retained a relatively cold (≤42 mW/m2) lithospheric mantle root down to ~180–200 km depth during the Mesozoic. Olivine major and trace element compositions support the presence of an Archean mantle root (up to 92.4 mol% forsterite contents) that is progressively metasomatized toward its base (e.g., increasing Ti Cu contents with depth). For south-central Africa, it appears that significant volumes of Archean cratonic mantle domains ‘survived’ beneath strongly deformed and granite-intruded Proterozoic terranes, which suggests that the continental crust is more strongly impacted during collisional or rift tectonics than the ‘stabilizing’ mantle lithosphere.

中文翻译：

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中非卢安瓜裂谷的含金刚石菱镁矿与重新活动的克拉通岩石圈的联系

摘要 赞比亚东部卢安瓜河谷内的卡潘巴河沿线出现中生代含金刚石的菱镁矿。东非裂谷系统的 300-200 Ma 古老的卡鲁时代前体分支。卢安瓜裂谷发育在一个重新激活的巨型剪切带之上，该带穿过刚果-坦桑尼亚-卡拉哈里克拉通之间的元古代伊鲁米德带，因此它提供了早期克拉通裂谷演化的罕见快照。新鲜火山岩的主要矿物学表明，它们代表了原始橄榄石菱镁矿和稍微进化的橄榄石白榴石菱镁矿之间的连续体。矿物成分和演化趋势，例如在地块金云母和钾质富锂辉石中 Ti F 富集的强铝耗竭，类似于环克拉通环境中经典的菱镁矿省（例如，怀俄明州的 Leucite Hills 和澳大利亚的 West Kimberley 油田）。然而，与来自 Kaapvaal 克拉通（以前的 Group-2 金伯利岩）的橙岩、来自东非裂谷的类型 kamafugites 和来自北大西洋裂谷的关键区域的超镁铁质有一些相似之处，这意味着源-非洲中南部卡鲁时代菱铁矿岩浆形成过程中的形成和构造过程。Kapamba 火山岩的整体成分属于“克拉通”低硅钾镁矿的范围，但与橙石重叠，特别是与更进化的含白榴石和含闪锌矿的橙石品种重叠。对大部分原始白榴石转化为方沸石的过程进行建模表明，Kapamba 的原始碱性岩浆含有约 6-9 wt% K2O，并且在 >10 wt% MgO 时具有高 K2O/Na2O 比率在约 1.6-6.2 之间 -证实了卢安瓜裂谷下地幔源岩浆活动的超钾质性质。几乎不含 CO2、富含 H2O-F 的 Kapamba 菱镁矿代表了富含 CO2-H2O 的 kamafugite/超镁铁质菱镁矿组成员所展示的地球化学连续体的延伸。因此，我们认为位于东非裂谷系统不同分支内的 Kapamba 菱镁矿和 kamafugites 型代表了相似的 K-交代克拉通地幔域的熔融产物，但它们的形成发生在裂谷发育不同阶段的不同挥发性条件下（IE，初期与稍微更高级的裂谷）。对来自 Kapamba 菱镁矿的橄榄岩衍生的橄榄石异种晶体的温度估计表明，卢安瓜河谷是一个废弃的克拉通裂谷，在中生代期间保留了一个相对寒冷 (≤42 mW/m2) 的岩石圈地幔根部，深度约 180-200 公里。橄榄石的主要和微量元素组成支持太古代地幔根部（高达 92.4 mol% 镁橄榄石含量）的存在，该根部逐渐向其基部交代（例如，随着深度增加 Ti Cu 含量）。对于中南部非洲，似乎有大量太古代克拉通地幔域在强烈变形和花岗岩侵入的元古代地体之下“幸存”下来，