Characteristics of shear deformation along the eastern and southern terrane boundaries of the Motloutse complex, eastern Botswana

The Motloutse Complex to the southwestern margin of Zimbabwe Craton is a medium-grade

terrane in northeastern Botswana. Some studies consider it as part of the Limpopo Complex

high-grade terrane between the Kaapvaal and Zimbabwe cratons, while others argue it as an

allochthonous terrane separate to the Limpopo Complex. A number of shear zones are

postulated to delineate the extents of Motloutse Complex. Except the northern margin (the

Shashe shear zone), the rest of the terrane boundaries of the Motloutse Complex are poorly

understood. The Magogaphate-Dikalate shear zones are argued to delineate the eastern

margin with the Phikwe Complex (sub-terrane of the Limpopo Complex). The western and

southern terrane extents of the Motloutse Complex are buried under younger cover rocks.

This study characterizes the nature, extent, and timing of shear deformation along the

eastern and southern terrane boundaries of the Motloutse Complex. Representative areas

from the eastern (Masikati hills), and southern (Radisele-Mogome) regions were selected. A

combination of geologic (field, petrographic), geochronologic (40Ar/39Ar amphibole-biotite),

and geophysical (aeromagnetic) data were utilized.

Neoarchean tonalite gneiss and Mesoarchean mafic (amphibolite-gabbro-leucogabbro anorthosite)-ultramafic (dunite-peridotite-pyroxenite) rocks constitute the dominant rock

units in the Masikati hills area in eastern Motloutse Complex. Four concomitant overprint

events variably affect the rocks, and include upright isoclinal folding, anatectic, shear

deformation and metasomatic components. Shear deformation is prominent along the

Abstract

Masikati hills. Kinematic indicators, including asymmetric porphyroclasts and S-C fabric,

indicate dominant right lateral sense of movement, with minor left lateral movement. The

extent of the shear zone (N-S trending Dikalate shear zone), including its branches, is

delineated in the aeromagnetic imagery. Application of 40Ar/39Ar geochronology on fabric forming amphibole dates the shear deformation along the eastern terrane boundary of

Motloutse Complex at 2013 ± 12 Ma.

The NW-SE trending Radisele-Mogome area, beneath the cover rocks, exposes two set of

rocks – tonalite gneiss-amphibolite-ultramafic rocks (part of the Motloutse Complex) to the

NE, and granodiorite-diorite gneiss-leucogranite (part of the Mahalapye Complex) to the SW.

This is well exemplified in the aeromagnetic image, with change in magnetic signature

between rocks in the NE and SW parts. Shear deformation and metasomatism variably

overprint rocks along the contact zone. Mylonitic deformation is prominent in the SW part.

Like the case of the eastern terrane boundary, asymmetric porphyroclasts and S-C fabric

indicate a dominant right lateral sense of shear movement. The extent of this shear zone

(northwest trending Sunnyside shear zone) and its branch is delineated in the aeromagnetic

imagery. 40Ar/39Ar geochronology on biotite from a sheared diorite gneiss from the SW

margin of the shear zone indicate an age of 1987 ± 8 Ma.

It is argued that both the eastern and southern terrane boundaries of the Motloutse

Complex terrane were active in the Paleoproterozoic, with shear deformation slightly earlier

along the Dikalate shear zone (~2.013 Ga), relative to the Sunnyside shear zone (~1.987 Ga).