Nd–Hf Isotope Systematics of Megacrysts from the Mbuji-Mayi Kimberlites, D. R. Congo: Evidence for a Metasomatic Origin Related to Kimberlite Interaction with the Cratonic Lithospheric Mantle

Pivin, Marjorie; Debaille, Vinciane; Mattielli, Nadine; Demaiffe, Daniel

doi:10.1007/978-81-322-1170-9_8

Cited by 4 publications

(1 citation statement)

References 74 publications

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“…Large variations in radiogenic εHf is common in cratonic domains, as the Lu-Hf system is very sensitive to long-term complex depletion of the lithospheric root (e.g. Pivin and Debaille, 2013). On the other hand, as Lu and Hf are not hosted in the same mineral phases (contrary to the two rare earths Sm and Nd that behave more coherently), it cannot be excluded that the lack of correlation in the εNd (i) vs.εHf (i) diagram actually reflects secondary processes, as discussed in Section 7.3.…”

Section: Assimilation/contamination Processes and Coexistence Of Satumentioning

confidence: 99%

The Neoproterozoic Upper Ruvubu alkaline plutonic complex (Burundi) revisited: Large-scale syntectonic emplacement, magmatic differentiation and late-stage circulations of fluids

Decrée

Demaiffe

Tack

et al. 2019

Precambrian Research

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The Upper Ruvubu Alkaline Plutonic Complex (URAPC) in Burundi consists of three separate intrusions, each with a specific emplacement age and petrological composition. Three main units are recognized: an outer unit with silica-saturated plutonic rocks (from gabbro to granite), an inner unit with silica-undersaturated plutonic rocks (feldspathoidal syenite with subordinate feldspathoidal monzonite and ijolite) and a carbonatitic body in the subsoil, known by drilling. The URAPC is quite large in size (∼24 km long and up to 10 km wide). It is considered to have been intruded syntectonically in an overall extensional context, thanks to the kilometric shear zones that accommodated its emplacement. Radiometric ages from literature range from 748 to 705 Ma and point to structurally-controlled magmatic differentiation followed by long-lived circulations of late-stage fluids postdating the emplacement of a part of the undersaturated rocks and the carbonatites. In the northwestern part of the outer unit, gabbro likely has been emplaced at a deeper structural level than the granite, which represents a more apical structural level of emplacement. This petrological, geochemical and isotopic (Sr-Nd-Hf) study concentrates on the processes that generated the URAPC: (i) fractional crystallization, evidenced by the chemical evolution trends of the major and trace elements, and by marked P, Ti and Ba anomalies in the trace element patterns; (ii) crustal assimilation/contamination, as shown by the wide range of Nd isotope compositions and the general increase of the Sr isotope ratios with increasing SiO 2 contents, and (iii) late-magmatic/hydrothermal alteration inducing an increase of the Sr isotope composition without changing significantly the Nd isotope composition. The isotopic data are consistent with an asthenospheric mantle source, though less depleted than the Depleted Mantle (DM), contaminated by the Subcontinental Lithospheric Mantle (SCLM). The silicate and carbonate magmatic series are cogenetic. The outer unit is clearly more contaminated than the inner unit, whereas the carbonatitic body could have evolved by liquid immiscibility. The URAPC lies within East Africa's Western Rift Valley, which is marked by 23 alkaline plutonic complexes. Their emplacement has been ascribed to reactivation of Proterozoic lithospheric weakness zones resulting from the breakup of the Neoproterozoic supercontinent Rodinia supercontinent. 2 billion years (Ga) (Phalaborwa, South Africa) to the present day (Oldoinyo Lengai, Tanzania), with a marked abundance peak pointing to the Neoproterozoic and the Lower Paleozoic (roughly 850-500 millions years (Ma); Midende et al., 2014). The tectonic context at the time of emplacement of the Upper Ruvubu Alkaline Plutonic Complex

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Section: Assimilation/contamination Processes and Coexistence Of Satumentioning

confidence: 99%

The Neoproterozoic Upper Ruvubu alkaline plutonic complex (Burundi) revisited: Large-scale syntectonic emplacement, magmatic differentiation and late-stage circulations of fluids

Decrée

Demaiffe

Tack

et al. 2019

Precambrian Research

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Origin of complex zoning in olivine from diverse, diamondiferous kimberlites and tectonic settings: Ekati (Canada), Alto Paranaiba (Brazil) and Kaalvallei (South Africa)

et al. 2018

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Compositional Variations in Primitive Kimberlite Melts and Entrained Mantle Cargo from a Global Survey of Trace Element Compositions in Kimberlite Olivine

Howarth

Giuliani

Soltys

et al. 2022

Journal of Petrology

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Olivines are the dominant phase of kimberlites and the majority of grains display distinct compositional zoning with xenocrystic cores surrounded by magmatic rims. Previous work has documented large variations in both core and rim major and minor element compositions in kimberlites globally, which has been related to variable entrainment and assimilation of sub-continental lithospheric mantle (SCLM) material. However, there is limited knowledge of trace element variations in olivine from kimberlites and it is unclear whether mantle assimilation has any effect on the trace element composition of kimberlite melts. To fill this gap, we present a global survey of olivine trace element compositions, along with previously reported major and minor element compositions, for samples representing the full spectrum of olivine compositional variations in kimberlites, including samples from Lac de Gras, Canada (Grizzly, Koala), Kimberley, South Africa (Bultfontein, De Beers, Kimberley Big Hole, Wesselton), Kaalvallei, South Africa (New Robinson, Samada), and Alto Paranaiba, Brazil (Limpeza-18, Tres Ranchos-04). Trace element concentrations of olivine cores can discriminate between those derived from the disaggregation of peridotitic material and those related to the megacryst suite. The megacrystic olivine cores exhibit a negative correlation between Al and Mn, which is absent in peridotite-derived cores, and are characterised by high concentrations of temperature dependent elements (e.g., Al, Na, V) as well as Zn, Ti, and Mn. Following pre-screening of cores for megacrystic and spinel peridotite derived grains, we applied the Al-in-olivine thermometer to assess the P-T equilibration conditions of cores in equilibrium with garnet and estimate the sampling depth of kimberlite magmas in the lithospheric mantle. Our results are consistent with predominant entrainment of deep lithosphere xenocrysts in highly diamondiferous compared to diamond-poor kimberlites. Temperature dependent elements display a gradational increase with depth due to higher T with Ca, Cu and to a lesser extent Zn and Ti being higher and Mg# lower toward the base of the SCLM, which is consistent with melt modification of the lower lithosphere. The Zn, Ti, Co, Mn, Li, Al, Cr, Na, and V concentrations of magmatic olivine rims display systematic variations that have a negative correlation with Mg# (whereas Cr is positively correlated). Lac de Gras olivine feature Mg-rich rims (Mg# >90) and low concentrations of these trace elements whereas the Fe-rich olivine rims (Mg# ~85) of the Kaalvallei kimberlites have higher concentrations of these elements, with the Kimberley and Alto Paranaiba kimberlites being intermediate. Direct correlations between average Ti, Zn, Co and Li compositions of olivine cores and rims suggests that the olivine rim (i.e., proxy for primitive melt) variations are related to variable assimilation of metasomatised SCLM and can be effectively used to track the composition of the lithospheric column that is traversed by kimberlite magmas. These observations further imply an intimate link between early proto-kimberlite melt, leading to formation of megacrystic olivine at the base of the SCLM, and the composition of kimberlite melts which entrain and assimilate these products. We conclude that lithospheric mantle assimilation has a major and previously overlooked influence on the trace element composition of kimberlite magmas.

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Nd–Hf Isotope Systematics of Megacrysts from the Mbuji-Mayi Kimberlites, D. R. Congo: Evidence for a Metasomatic Origin Related to Kimberlite Interaction with the Cratonic Lithospheric Mantle

Cited by 4 publications

References 74 publications

The Neoproterozoic Upper Ruvubu alkaline plutonic complex (Burundi) revisited: Large-scale syntectonic emplacement, magmatic differentiation and late-stage circulations of fluids

The Neoproterozoic Upper Ruvubu alkaline plutonic complex (Burundi) revisited: Large-scale syntectonic emplacement, magmatic differentiation and late-stage circulations of fluids

Origin of complex zoning in olivine from diverse, diamondiferous kimberlites and tectonic settings: Ekati (Canada), Alto Paranaiba (Brazil) and Kaalvallei (South Africa)

Compositional Variations in Primitive Kimberlite Melts and Entrained Mantle Cargo from a Global Survey of Trace Element Compositions in Kimberlite Olivine

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