Geochemistry and zircon U–Pb ages of the Paleoproterozoic ultramafic rocks of the Mbi Valley, Boali area, Central African Republic

This paper investigates the geochemistry of bulk rock and infers the petrogenesis of ultramafic rocks in the Boali Precambrian terrane in Mbi Valley, in the North of the Central African Republic (CAR). The studied rocks comprise coarse primary olivine and orthopyroxene relics (dominant phase), magnesio-hornblende, magnetite, and antigorite. Whole-rock analysis indicates low SiO 2 (average of 43.14 wt%) and high MgO (19.84–26.98 wt%) contents and their Mg number (Mg#) ranges from 74 to 82. They display high Ni (526–865 ppm), Cr (1500–3680 ppm) contents. AFM (Na 2 O-K 2 O)-FeO-MgO) and ACM (Al 2 O 3 -CaO-MgO) ternary diagrams have revealed that the studied samples correspond to arc-related ultramafic cumulates. Chondrite-normalized REE plots display an increasing trend from La to Sm (Ce N /Sm N : 0.74–1.81), weak negative to no Eu (Eu/Eu* = 0.72–1.05) and strong negative Ce (Ce/Ce* = 0.33–0.98) anomalies. Primitive mantle normalized of multi-element diagrams exhibit LREE enrichment and large ion lithophile elements (LILE) relative to high field strength elements (HFSE), and notable negative anomalies in Nb. This suggests the generation of the parent melt by slab dehydration and wedge melting processes. In addition, incompatible trace element composition and ratios assumed that the source magma had an enhanced mantle source associated with a prominent influence of continental crust. Metasomatism of mantle wedge by plate-dehydrated, LILE-rich fluids and the incorporation of sediments derived from subduction explain the enhancement of the source. Integrated major and trace element compositions jointly with the tectonic reconstruction of this region and LA-ICP-MS U–Pb data on zircon constrain the emplacement age at ca. 2099 Ma in a continental margin arc setting involving subduction of an oceanic plate beneath the continental lithosphere, dehydration of the slab and mantle wedge melting. This result intimate that the remnants of Paleoproterozoic oceanic crust or subduction event and subsequent basins closure extended from the Congo craton in Cameroon to CAR and NE Brazil.