Re-assessment of the thrust-accretion hypothesis for the southwestern Barberton greenstone belt, South Africa

A field and petrographic re-assessment of the SW margin of the Barberton Greenstone Belt (BGB) is combined with new zircon age and whole-rock Sm-Nd isotopic data. Most significantly, we recognize in the Theespruit Formation (TF) consistent facing directions of bedding to the northeast, distinct stratigraphic variations across strike, and moderate to low strain, which suggests the TF is an autochthonous stratigraphic succession. Rocks previously interpreted as tectonic slivers of basement orthogneiss were found to be little deformed, intrusive quartz-feldspar porphyry, or sheared felsic volcaniclastic sediment.
Singe zircon U-Pb dating of 14 TF felsic metavolcanic rocks yield consistent eruption ages of 3538-3522 Ma. We interpret a 3453 ± 6 Ma population of zircons previously analysed from an amphibolite-facies TF felsic schist as being of metamorphic, rather than detrital, origin, arising from emplacement of c. 3460 Ma felsic plutonic rocks that transect the KSZ and thus stitch the TF to the Komati Formation ~210 Ma earlier than the previously proposed thrusting event. Sm-Nd isotopic data of TF felsic rocks and associated komatiites yield an 3592 ± 120 Ma errorchron age, whereas data from three komatiite samples alone define an isochron age of 3510 ± 130 Ma, similar to the zircon ages. Low initial εNd(t) ~0.6 reflect a slightly depleted mantle source for the komatiites and predominantly isotopically similar mantle-derived material with short crustal residence time for the felsic rocks. Involvement of older crustal material is supported by ages of xenocrystic zircons and εNd(t) -1.4 in some felsites.
Kinematic data indicates that the Komati schist zone was an extensional detachment plane, active during partial convective overturn and exhumation of mid-crustal levels to the SW. Metamorphic mineral assemblages indicate an early anticlockwise P-T path for the TF, interpreted to result from isothermal burial of flanking greenstones during emplacement of the c. 3460 Ma felsic plutonic suite and the onset of doming.
The new data support autochthonous development of the TF as a c. 3530 Ma basement to the upper Onverwacht Group. A tectonic model of crust formation through plume-derived volcanism on a substrate of older granite-greenstone crust is consistent with all of the data. It is suggested that most of the structural and metamorphic features of the BGB can be explained by partial convective overturn of greenstone-over-granitoid layered crust as a result of extensive partial melting of the middle crust accompanying mantle-plume related, and possible convergent margin, tectonic events at ca. 3230–3216 Ma.