Transcurrent shearing, granite sheeting and the incremental construction of the tabular 3.1 Ga Mpuluzi batholith, Barberton granite–greenstone terrane, South Africa

Abstract

Structural, petrographic and geochronological studies show that the tabular 3.1 Ga Mpuluzi batholith in the Barberton granite–gneiss terrane in South Africa was emplaced via a combination of external and internal processes. External structural controls are indicated by systematic variations in intrusive relationships and strain along the margins of the Mpuluzi batholith and are consistent with an emplacement of the granite in a dilational jog within a NE–ENE-trending system of dextral transcurrent synmagmatic shear zones. Internally, the Mpuluzi batholith is essentially made up of granite sheets. The structurally higher parts of the granite are made up of shallowly dipping sheets that are underlain by an anastomosing network of steeply dipping, variably deformed dykes and sheets. These granite sheets at lower structural levels intruded either into the actively deforming shear zones or into extensional sectors between and along the bounding shear zones. Multiple intrusive relationships and geochronological evidence suggests that granite sheeting and the assembly of the pluton occurred over a period of 3–13 Ma. The spatial and temporal relationship between deformation and magma emplacement reflects episodes of incremental dilation related to deformation along the bounding shear zones and granite sheeting. The transition to the mainly subhorizontal granite sheets at higher structural levels of the tabular Mpuluzi batholith indicates the intrusion of the granites during subhorizontal regional shortening, where the reorientation of the minimum normal stress to vertical attitudes at the shallow levels of emplacement allowed for vertical dilation and subhorizontal emplacement of the granite sheets.