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

(2019) PRECAMBRIAN RESEARCH. 325. p.150-171
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Abstract
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 (similar to 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 north-western 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 SiO2 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.
Keywords
LITHOSPHERIC MANTLE SOURCE, CARBONATITE COMPLEX, LIQUID IMMISCIBILITY, ISOTOPE CONSTRAINTS, SILICATE ROCKS, KIBARA BELT, BREAK-UP, EVOLUTION, SR, GEOCHEMISTRY, Neoproterozoic, Carbonate-silicate melt immiscibility, Fractional, crystallization, Crustal contamination/assimilation, SCLM, Sr-Nd-Hf, isotopes

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MLA
Decree, Sophie, et al. “The Neoproterozoic Upper Ruvubu Alkaline Plutonic Complex (Burundi) Revisited : Large-Scale Syntectonic Emplacement, Magmatic Differentiation and Late-Stage Circulations of Fluids.” PRECAMBRIAN RESEARCH, vol. 325, 2019, pp. 150–71, doi:10.1016/j.precamres.2019.02.023.
APA
Decree, S., Demaiffe, D., Tack, L., Nimpagaritse, G., De Paepe, P., Boulvais, P., & Debaille, V. (2019). The Neoproterozoic Upper Ruvubu alkaline plutonic complex (Burundi) revisited : large-scale syntectonic emplacement, magmatic differentiation and late-stage circulations of fluids. PRECAMBRIAN RESEARCH, 325, 150–171. https://doi.org/10.1016/j.precamres.2019.02.023
Chicago author-date
Decree, Sophie, Daniel Demaiffe, Luc Tack, Gerard Nimpagaritse, Paul De Paepe, Philippe Boulvais, and Vinciane Debaille. 2019. “The Neoproterozoic Upper Ruvubu Alkaline Plutonic Complex (Burundi) Revisited : Large-Scale Syntectonic Emplacement, Magmatic Differentiation and Late-Stage Circulations of Fluids.” PRECAMBRIAN RESEARCH 325: 150–71. https://doi.org/10.1016/j.precamres.2019.02.023.
Chicago author-date (all authors)
Decree, Sophie, Daniel Demaiffe, Luc Tack, Gerard Nimpagaritse, Paul De Paepe, Philippe Boulvais, and Vinciane Debaille. 2019. “The Neoproterozoic Upper Ruvubu Alkaline Plutonic Complex (Burundi) Revisited : Large-Scale Syntectonic Emplacement, Magmatic Differentiation and Late-Stage Circulations of Fluids.” PRECAMBRIAN RESEARCH 325: 150–171. doi:10.1016/j.precamres.2019.02.023.
Vancouver
1.
Decree S, Demaiffe D, Tack L, Nimpagaritse G, De Paepe P, Boulvais P, et al. The Neoproterozoic Upper Ruvubu alkaline plutonic complex (Burundi) revisited : large-scale syntectonic emplacement, magmatic differentiation and late-stage circulations of fluids. PRECAMBRIAN RESEARCH. 2019;325:150–71.
IEEE
[1]
S. Decree et al., “The Neoproterozoic Upper Ruvubu alkaline plutonic complex (Burundi) revisited : large-scale syntectonic emplacement, magmatic differentiation and late-stage circulations of fluids,” PRECAMBRIAN RESEARCH, vol. 325, pp. 150–171, 2019.
@article{8669060,
  abstract     = {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 (similar to 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 north-western 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 SiO2 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.},
  author       = {Decree, Sophie and Demaiffe, Daniel and Tack, Luc and Nimpagaritse, Gerard and De Paepe, Paul and Boulvais, Philippe and Debaille, Vinciane},
  issn         = {0301-9268},
  journal      = {PRECAMBRIAN RESEARCH},
  keywords     = {LITHOSPHERIC MANTLE SOURCE,CARBONATITE COMPLEX,LIQUID IMMISCIBILITY,ISOTOPE CONSTRAINTS,SILICATE ROCKS,KIBARA BELT,BREAK-UP,EVOLUTION,SR,GEOCHEMISTRY,Neoproterozoic,Carbonate-silicate melt immiscibility,Fractional,crystallization,Crustal contamination/assimilation,SCLM,Sr-Nd-Hf,isotopes},
  language     = {eng},
  pages        = {150--171},
  title        = {The Neoproterozoic Upper Ruvubu alkaline plutonic complex (Burundi) revisited : large-scale syntectonic emplacement, magmatic differentiation and late-stage circulations of fluids},
  url          = {http://dx.doi.org/10.1016/j.precamres.2019.02.023},
  volume       = {325},
  year         = {2019},
}

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