Syntectonic emplacement and deformation of the Heerenveen batholith: Conjectures on the structural setting of the 3.1 Ga granite magmatism in the Barberton granite-greenstone terrain, South Africa

The Heerenveen batholith in the Mesoarchean Barberton granite-greenstone terrain is a horizontal, tabular, and compositionally composite granite body belonging to the 3.1 Ga magmatic suite of granodiorites, monzogranites, and syenogranites, the granodiorite-monzogranite-syenogranite (GMS) suite, that has intruded large parts of the Kaapvaal Craton. In this paper, we present structural data of magmatic and penetrative solid-state fabrics of the Heerenveen batholith that was hitherto considered to be undeformed. The orientation and kinematics of conjugate sets of synmagmatic shear zones along the margins and within the Heerenveen batholith, the presence of regionally persistent magmatic and solid-state fabrics, and the progressive deformation of dikes and sills related to the pluton all point to the syntectonic emplacement of the granites during bulk coaxial, northwest-southeast subhorizontal crustal shortening. Both the timing as well as the strain recorded by the Heerenveen batholith and other plutons of the 3.1 Ga GMS suite in the Barberton granite-greenstone terrain indicate an emplacement during the late-stage D3 convergent tectonics that is responsible for the northeast-southwest–trending main structural grain within the Barberton greenstone belt. One of the characteristic features of the D3 deformation in the granite-gneiss terrain is the very pronounced localization of the bulk strain into melt-bearing structures such as synmagmatic shear zones and granite sheets. Variations in the D3 strain intensity recorded in wide parts of the Barberton greenstone belt may be related to this strain localization, reflecting the proximity or absence of syntectonic intrusions. The late-stage transtensional and/or extensional deformation of the granite-greenstone terrain shortly follows the intrusion of the extensive tabular granites. The transition from convergent to divergent tectonics may be related to the gravitational collapse of the crust above the upper crustal melt sheets. We speculate that the setting of the GMS suite in the Barberton granite-greenstone terrain may be a contractional continental back arc, related to collisional plate-margin processes in the central parts of the Kaapvaal Craton.