Just another drip: Re-analysis of a proposed Mesoarchean suture from the Barberton Mountain Land, South Africa

Abstract

Structural analysis of a proposed Mesoarchean suture located immediately to the southwest of the Barberton Greenstone Belt, South Africa – known as the Inyoni Shear Zone (ISZ) – reveals that the main, steeply dipping, NNE-striking fabric is distributed across only 1km width, is late (D3), and clearly overprints two earlier sets of fabric elements (D1, D2) that were originally oriented at right angles to the direction of proposed collision. Dating of a S2 foliated meta-trondhjemite is interpreted to indicate that D2 deformation was at, or younger than, 3238.2±0.9 Ma. The D3 high strain fabric of the ISZ is localised around tightly folded, kilometre-scale supracrustal rafts, but dissipates in all directions away from the rafts, along and across strike. The kinematic asymmetry of S3 shear foliations changes across opposing limbs of tight, upright F3 folds of supracrustal rocks in the ISZ, indicative of simple shear related to buckle folding of more competent supracrustal rocks in a matrix of less competent granitic gneisses. F3 buckle folding was highly non-cylindrical on steep to subvertical axes that plunge to the south in the northern part of the zone and to the north in the southern part of the zone, with nests of vertically plunging sheath folds along amphibolite/metagranite contacts in the central area. High-pressure (HP), garnet-bearing amphibolites are restricted to close association with the vertical D3 sheath folds and do not extend along strike.Combined, the data indicate that the ISZ is not the product of west-directed subduction, as previously suggested. Rather, the strain and kinematic data indicate that the ISZ is just another drip of relatively cool, relatively rigid greenstones into hot, ductile granitic gneisses, formed during partial convective overturn of the upper (greenstone) and middle (granitic) crust. Uplift of the HP rocks was along a late (D4), brittle–ductile mylonite zone associated with emplacement of the c. 3.1Ga Kees Zyn Doorns syenite.