Abstract:
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.
