Petrogenesis of the Kabanga–Musongati layered mafic–ultramafic intrusions in Burundi (Kibaran Belt): geochemical, Sr–Nd isotopic constraints and Cr–Ni behaviour

Abstract

A succession of mafic-ultramafic layered intrusions forms an alignment in the boundary zone between the Kibaran belt and the Tanzania craton. The intrusions represent a continuous series of cumulate rocks. For instance, in the Mukanda-Buhoro and Musongati (MBM) contiguous bodies, the series starts with dunite and passes to lherzolite, pyroxenite, norite, gabbronorite and anorthosite on top. Cumulate textures are conspicuous in all rock types and cryptic layering characterises cumulus mineral compositions, thus evidencing fractional crystallization as a major differentiation mechanism. The increase of Cr in the ultramafic members of the series indicates that chromite was not a liquidus mineral in dunite and lherzolite rocks, thus unable to form chromitite layers. The high Ni-content of dunite seems to preclude the existence of conjugate Ni-rich sulphide deposits. The 87 Sr/ 86 Sr initial ratio is relatively constant and averages 0.7087, with some values up to 0.712 due to local assimilation. Fine-grained rocks from the MBM area are isotopically (Nd and Sr) similar to the MBM cumulates. Modelling their crystallization produces cumulus mineral compositions similar to those in the Musongati ultramafic rocks, which suggests a broadly picritic parental magma. On the other hand, finegrained rocks from the Nyabikere area are not related to the Nyabikere cumulates. Nd and Sr isotope ratios show that the MBM magmatism is related to an enriched source, possibly an old subcontinental lithospheric mantle. The Nyabikere dykes, as well as the Waga dykes, come from a depleted mantle source, as do the A-type granitoids occurring in the same boundary zone. Several lines of evidence point to two types of parental magmas, a picritic magma, and a more evolved magma, broadly similar to the Bushveld Main Zone magma.

Platinum group element mineralization at Musongati (Burundi): concentration and Pd-Rh distribution in pentlandite

“…Bandyayera, 1997). The Fo content in olivine ranges from 90% in the dunitic Buhinda unit over 88% in the lherzolitic/ harzburgitic Marumanga/Rubara units, down to 83% in the upper part of the pyroxenitic-peridotitic Geyuka unit (Duchesne et al, 2004). The Mg# in Ca-rich pyroxenes ranges from the Buhinda unit up to the anorthositic-noritic Mukanda unit (Duchesne et al, 2004).…”

“…2A). The Deblond, 1994;Duchesne et al, 2004). (B) SE-part of the KM alignment with A-type granitoids of the Gitega-Makebuka-Bukirasazi alignment (modified after Deblond & Tack, 1999;Duchesne et al, 2004).…”

“…The Fo content in olivine ranges from 90% in the dunitic Buhinda unit over 88% in the lherzolitic/ harzburgitic Marumanga/Rubara units, down to 83% in the upper part of the pyroxenitic-peridotitic Geyuka unit (Duchesne et al, 2004). The Mg# in Ca-rich pyroxenes ranges from the Buhinda unit up to the anorthositic-noritic Mukanda unit (Duchesne et al, 2004). Modal proportions of minerals and detailed Fo content in olivine and Mg# in pyroxenes can be found in detail in Deblond & Tack (1999) and Duchesne et al (2004).…”

“…In addition, the petrogenesis of the intrusions needs further elaboration, expanding on the work of e.g. Bandyayera (1997) and Duchesne et al (2004).…”

“…Secondly, the mafic zone or basic zone is subdivided into a noritic and a gabbronoritic subzone. From the bottom unit of the ultra-mafic zone up to the upper unit of the mafic zone, a progressive magmatic differentiation is observed (Duchesne et al, 2004). belt (Fig.…”