Archean Boninite-Like Mafic-Ultramafic Rocks from the Lechana Layered Intrusion, Motloutse Complex, Botswana

Mafic to ultramafic rocks occurring in the Lechana area forms part of the Archean high-grade

Motloutse Complex terrane in eastern Botswana. Their occurrence as inclusions (displaced

fragments) within ~2.6 Ga tonalite gneiss indicates that they are older than ~2.6 Ga. The

ultramafic rocks – peridotite and pyroxenite – are widespread in the mapped area as layers,

with some layers rich in pyroxenite, others rich in peridotite, and a third type of layer made

up of peridotite with patchy pyroxenite domains. Anorthosite occurs to the northern part of

the Lechana area and is gneissic. Both the layered mafic (anorthosite) and ultramafic

(peridotite-pyroxenite) rocks are prominently associated with amphibolites. Together they

are considered as part of an Archean layered intrusion – referred here as the Lechana layered intrusion. In addition to olivine, orthopyroxene (ultramafic rocks) and plagioclase

(anorthosite), both the mafic and ultramafic rocks are characterized by the prominent

occurrence of primary magmatic amphibole implying hydrous conditions during their

formation. Amphibole mineral chemical characteristics indicate preservation of primary

magmatic compositions in the anorthosite, with minor effect of sericitization, while

amphiboles in the peridotite exhibit the effect of serpentinization. Isocon analyses applied to

the amphibole mineral chemistry helped to understand which elements were gained and

lost during hydrothermal alteration. In terms of whole-rock geochemical characteristics, all the studied mafic to ultramafic rocks are characterized by high SiO2 and MgO contents,

similar to boninites. Significantly, a peridotite sample with patchy pyroxenite domains gave

the unique chondrite-normalized U-shaped boninite-like REE pattern. Trace element modeling indicated the possible role played by crustal contamination to account for the

boninite-like signature. Similar primitive mantle normalized multi element and chondrite

normalized REE patterns of the layered mafic to ultramafic rocks indicate likely derivation

from a common parental magma. Amphibolites indicate formation in a transitional setting

with MORB-like and arc-like characteristics, while the layered mafic to ultramafic rocks

indicate formation in an active continental margin setting. Combining together, it can be

suggested that the original magma formed in a MORB setting and then moved to an arc

setting – much like in a suprasubduction setting. The peridotite-pyroxenite-anorthosite are

related in terms of differentiation of the original magma. Comparison with mafic to

ultramafic rocks occurring in the mineralized Selebi-Phikwe belt (within the high-gradePhikwe Complex terrane) helps to evaluate the Cr-Ni-V mineralization potential of the rocks

of the Lechana layered intrusion.