Complexity of characterizing mafic-ultramafic remnants of Archean layered complexes in high-grade terranes: The scenario from Motloutse Complex, southwestern Zimbabwe craton

Abstract

Mafic-to ultramafic-rocks of Archean layered complexes in high-grade terranes typically occur as inclusions closely associated with supracrustal rocks in granitoid gneiss. The reconstruction of the original stratigraphy and sequence of events related to Archean layered complexes is complicated by the fragmental occurrence and effect of overprint event(s). The case study presented here highlight these issues from the Motloutse Complex terrane, southwestern Zimbabwe Craton. Peridotite-pyroxenite and gabbro-leucogabbro-anorthosite form separate bodies closely associated with amphibolite in the Lechana area. They occur as inclusions within ∼2.65 Ga tonalite gneiss. Amphibolite is massive, has a stratiform occurrence and is folded, with gabbro-leucogabbro-anorthosite intruding along the fold limbs. Rare amphibolite xenoliths occur in gabbro-leucogabbro-anorthosite and peridotite-pyroxenite. Modal and cyclic layering is well preserved in the mafic and ultramafic intrusive bodies. Anatectic and alkali-metasomatic overprints, the later related to shear deformation, locally affected the Lechana rocks. The late stage events are contemporaneous and is dated by the ∼2.0 Ga (2013 ± 18 Ma) U-Pb zircon overgrowth age. Amphibolite exhibit prominent granoblastic polygonal fabric defined by amphibole-plagioclase. Gabbro-leucogabbro-anorthosite are locally gneissic. The different mafic rocks are variably affected by overprint events, and contain garnet related to anatectic overprint and epidote related to metasomatic overprint. Peridotite-pyroxenite exhibits no metamorphic re-equilibration textures, but are serpentinized and host widespread magnetite-rich layers. Amphibolite and associated gabbro have conspicuous sulphide concentration (up to 5 modal%), and include pyrrhotite, chalcopyrite (replaced by covellite), pyrite and pentlandite. No magnetite or sulphide concentration occur with leucogabbro-anorthosite. Whole-rock geochemical characteristics of the amphibolites are similar to tholeiitic basalts, but that of the peridotite-pyroxenite and gabbro-leucogabbro-anorthosite indicate different lineage. The suggested model considers amphibolite as a stratiform meta-basalt, which was folded during an earlier deformation event. Based on the presence of meta-basalt (amphibolite) roof pendants on top of both mafic and ultramafic intrusions, it is argued that the peridotite-pyroxenite and gabbro-leucogabbro-anorthosite were emplaced into Archean oceanic crust. The mafic-ultramafic intrusions could either be part of a single Mesoarchean layered complex or represent two different suites. In view of the restricted occurrence, lack of interlayering, different pyroxene affinities (clinopyroxene in gabbro and orthopyroxene in peridotite-pyroxenite) and wide compositional gap between mafic and ultramafic intrusions, we favour the later scenario. Peridotite-pyroxenite, interpreted as part of a Mesoarchean layered ultramafic complex, intruded the mafic volcanics. Layered gabbro-leucogabbro-anorthosite also intruded into the basaltic rock, separate from the ultramafic complex. The intrusion of protolith of the tonalite gneiss then dismembered the mafic and ultramafic sequences. The results of the present study highlight the need for detailed evaluation of spatially associated mafic-ultramafic rock remnants in high-grade terranes; they need not be genetically related but could be part of different magmatic episodes. Such exercise has implications in properly understanding the geodynamic setting of formation of these Archean upper mantle-derived rocks; the current arguments polarised between two end-member settings – plate margin versus intraplate setting.