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• W3113143722 endingPage "120019" @default.
• W3113143722 startingPage "120019" @default.
• W3113143722 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% K 2 O and had high K 2 O/Na 2 O 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 CO 2 -free, H 2 O-F-rich Kapamba lamproites present an extension of the geochemical continuum displayed by the members of the CO 2 -H 2 O-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/m 2 ) 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. • Mesozoic volcanics of the Luangwa Graben in Zambia are evolved olivine lamproites. • Leucite to analcime conversion masks the true ultrapotassic nature of lamproites. • Lamproites formed in K-enriched SCLM at H2O-F-rich oxidizing conditions >FMQ. • Lamproites precede the East African Rift and kamafugite eruptions caused by CO2-influx. • The Luangwa Rift formed on 180 km thick cratonic lithosphere with ≤42 mW/m2 heat flow." @default.
• W3113143722 created "2020-12-21" @default.
• W3113143722 creator A5068946311 @default.
• W3113143722 creator A5071436693 @default.
• W3113143722 date "2021-05-01" @default.
• W3113143722 modified "2023-09-24" @default.
• W3113143722 title "Diamondiferous lamproites of the Luangwa Rift in central Africa and links to remobilized cratonic lithosphere" @default.
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