Abstract

The Inyoni shear zone represents an important tectonic boundary between (i) the ca. 3.45 Ga high-pressure amphibolite facies, granite-greenstone domain south of the Barberton greenstone belt, termed the Stolzburg terrane, and (ii) the ca. 3.29 to 3.23 Ga rocks of the trondhjemitic Badplaas pluton to the west. The Stolzburg terrane is separated from the greenschist facies rocks of the rest of the Barberton greenstone belt by the Komati fault, which records >10 km uplift of the Stolzburg terrane relative to the lower-grade rocks of the greenstone belt at ca. 3.23 Ga. A number of studies within the Stolzburg terrane have documented high-pressure amphibolite facies metamorphism that occurred concurrently with exhumation, with the lowest apparent geothermal gradients documented in the Inyoni shear zone, where strong constraints on the age of metamorphism are most limited. In addition, different studies on Inyoni metamorphism have produced significantly different temperature estimates.

This study utilizes garnet Sm-Nd geochronology in combination with P-T modelling to directly date the metamorphism and re-evaluate the P-T conditions of the Inyoni shear zone. Two petrologically distinct samples produce similar P-T evolutions. A heterogeneous sample with both garnet-bearing and garnet-absent domains gives up-P evolutions reaching conditions of 550 to 675°C and 7 to 10 kbar, whereas a homogenous sample containing garnet and clinopyroxene produces a similar dominantly up-P evolution reaching peak conditions of 650°C and 8 to 10 kbar. Sm-Nd garnet ages of 3 201.6 ± 4.7 Ma (MSWD = 1.02) and 3 200.3 ± 5.3 Ma (MSWD = 0.44) were obtained from two samples of the homogenous garnet and clinopyroxene-bearing amphibolite. The Sm-Nd garnet geochronology provides accurate ages for the metamorphism of the Inyoni shear zone, with age results suggesting activity on the Inyoni shear zone may have continued after the regional metamorphism at ca. 3.23 Ga previously established by zircon U-Pb geochronology. However, 147Sm decay constant uncertainty leaves open the possibility that Inyoni garnet growth could have coincided with the previously recognized 3.23 Ga regional metamorphism.

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