NDLI: Metasomatic enrichment of Proterozoic mantle south of the Kaapvaal Craton, South Africa: origin of sinusoidal REE patterns in clinopyroxene and garnet

Content Provider	Springer Nature Link
Author	Roex, Anton Class, Cornelia
Copyright Year	2016
Abstract	Xenoliths of mantle peridotite have been sampled from four kimberlite intrusions, Melton Wold, Hebron, Uintjiesberg and Markt, emplaced through the Mesoproterozoic Namaqua-Natal Belt, along the southern border of the Kaapvaal Craton. Although many of the xenoliths are heavily altered, constituent clinopyroxene, garnet and phlogopite are fresh and have been analysed by electron microprobe for major elements and by laser ablation ICP-MS for trace elements. Primitive mantle-normalised REE abundances in clinopyroxene are all strongly LREE enriched and show a range of patterns including uniformly MREE–HREE sloped (referred to here as ‘normal’), sinusoidal and humped sinusoidal patterns. HREE abundances are extremely low (Yb = 0.3–0.06 × PM). REEN patterns in coexisting garnets show a similar range of patterns. When normalised to primitive mantle values, trace element patterns in some clinopyroxenes show strong relative depletion in Rb–Ba, Ta–Nb and Ti, with some samples also being relatively depleted in Zr–Hf. These trace element characteristics are indistinguishable from those found in clinopyroxene and garnet from peridotites from the adjacent cratonic mantle. Numerical modelling of reactive porous flow of an enriched metasomatic melt through a geochemically depleted peridotite matrix can account for the full range in observed REEN patterns. The relative depletion in Rb–Ba, Ta–Nb and Ti can be accounted for by an early crystallisation of phlogopite from the percolating melt. The relative depletion in Zr–Hf in some clinopyroxenes requires either zircon to crystallise in the proximal metasomatic assemblage, or metasomatism by a carbonatitic melt. Modelling results, together with the absence of clinopyroxene with depleted or even partially enriched REEN patterns, suggest that all clinopyroxene has been modally introduced through metasomatism into an initially highly depleted harzburgitic protolith. The range in Sr and Pb isotopic composition of the clinopyroxenes indicates regional metasomatism by melts of various compositions. The strong HREEN depletion is interpreted to reflect the effect of initial melt depletion in the early Proterozoic, with melting extending into the spinel stability field requiring an oceanic realm, and again later in the Mesoproterozoic (Namaqua Orogeny). The superimposed incompatible element enrichment indicates subsequent multiple enrichment events by rising alkaline melts similar in composition to kimberlite or ultramafic alkaline lamprophyre, possibly related to Mesozoic plume upwelling beneath the region, that reintroduced clinopyroxene into the depleted Proterozoic harzburgite protolith.
Starting Page	1
Ending Page	24
Page Count	24
File Format	PDF
ISSN	00107999
Journal	Contributions to Mineralogy and Petrology
Volume Number	171
Issue Number	2
e-ISSN	14320967
Language	English
Publisher	Springer Berlin Heidelberg
Publisher Date	2016-01-23
Publisher Place	Berlin/Heidelberg
Access Restriction	One Nation One Subscription (ONOS)
Subject Keyword	Mantle peridotite Trace elements REE patterns Metasomatism Geology Mineral Resources Mineralogy
Content Type	Text
Resource Type	Article
Subject	Geochemistry and Petrology Geophysics

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