Diamondiferous lamproites of the Luangwa Rift in central Africa and links to remobilized cratonic lithosphere
Diamondiferous lamproites of the Luangwa Rift in central Africa and links to remobilized cratonic lithosphere
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DOI：
10.1016/j.chemgeo.2020.120019
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发表时间：
2020-12
期刊：
影响因子：
3.9
通讯作者：
Ntando S. Ngwenya;S. Tappe
中科院分区：
文献类型：
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作者：
Ntando S. Ngwenya;S. Tappe
在赞比亚东部卢安瓜谷地的Kapamba河沿岸发现了中生代含金刚石煌斑岩管，这是东非裂谷系约300-200 Ma old Karoo-age的前身分支。卢安瓜裂谷发育在一个重新激活的巨型剪切带之上，该剪切带穿过刚果-坦桑尼亚-卡拉哈里克拉通之间的元古代伊鲁米德带，因此它提供了一个罕见的早期克拉通裂谷演化的快照。新鲜火山岩的原生矿物学表明，它们代表了原始橄榄质煌斑岩和稍微演化的橄榄-亮色煌斑岩之间的连续体。矿物组成和进化趋势，如在大地云母和钾质黄铜矿中富集Ti-F时的强al耗尽，类似于环克拉通环境中的经典煌斑岩省（如怀俄明州的露西特山和澳大利亚的西金伯利油田）。然而，与Kaapvaal克拉通（原第2组金伯利岩）的橙色岩、东非裂谷的卡马辉岩型、北大西洋裂谷关键区域的超镁铁质煌斑岩有一些相似之处，这表明在非洲中南部卡鲁时代煌斑岩岩浆形成过程中，源区形成与构造过程之间存在复杂的相互作用。卡帕姆巴火山岩的主体成分属于“克拉通”低硅煌斑岩的范围，但与橙色岩有重叠，特别是与更进化的含白钨矿和含水晶石的橙色岩品种重叠。模拟表明，卡帕姆巴原始碱性岩浆的K2O含量为~ 6-9 wt%， K2O/Na2O比值为~ 1.6-6.2 (bbb10 wt% MgO)，证实了卢安瓜裂谷下地幔源岩浆活动的超经典性质。几乎不含co2，富h2o -f的Kapamba煌斑岩是富co2 - h2o的kamamuite /超镁质煌斑岩群成员所显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不同分支的Kapamba煌斑岩和kamafuuites类型代表了相似的k交代克拉通地幔域的熔融产物，但它们的形成是在裂谷发育的不同阶段（即早期裂谷和稍晚的裂谷）不同的挥发条件下形成的。对来自卡帕姆巴煌斑岩的橄榄石衍生的杂晶的温度估计表明，卢安瓜谷是一个中断的克拉通裂谷，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，深度约为180-200 km。橄榄石主要元素和微量元素组成支持太古宙地幔根的存在（高达92.4 摩尔%的橄榄石含量），并逐渐向其底部交代（例如，随着深度的增加，Ti-Cu含量增加）。对于非洲中南部，似乎有大量太古宙克拉通地幔域在强烈变形和花岗岩侵入的元古宙地体下“幸存”下来，这表明大陆地壳在碰撞或裂谷构造期间受到的影响比“稳定”的地幔岩石圈更强烈。
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., incipientversusslightly 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.