2005 Salt Lake City Annual Meeting (October 16–19, 2005)

Paper No. 9
Presentation Time: 3:30 PM


BELCHER, Richard W. and KISTERS, Alex F.M., Department of Geology, University of Stellenbosch, Private Bag X1, Matieland, 7602, South Africa, rwb@sun.ac.za

Intrusive relationships and geochemical data of the 3.1 Ga Heerenveen batholith in the Barberton granite-greenstone terrain in South Africa document the incremental assembly of the heterogeneous sheeted pluton through distinct and successively emplaced magma batches. Magmatic and solid state fabrics point to the emplacement of the 450km2 Heerenveen batholith during NW-SE directed, subhorizontal shortening, coeval with the last phase of deformation recorded in supracrustals of the Barberton greenstone belt. Significantly, different magma batches show distinct structural emplacement controls and earlier magma batches can be shown to have had a profound effect on the emplacement of subsequent magmas.

Several different intrusive styles can be identified that show systematic variations with time. The earliest intrusive phases intruded as discrete cm- to m-wide sheets along the predominantly shallow-dipping gneissosity in the underlying basement. This lit-par-lit injection complex provided the sites for the nucleation of steep, ENE- and N-trending dextral and sinistral transpressive shear zones respectively, that form km-scale, conjugate shear couples within and along the margins of the NE-trending batholith. The shear zones are intruded by shear-zone parallel, multiple granitic sheets that are almost invariably mylonitized indicating the synmagmatic timing of shearing. The shear zones are interpreted to have acted as conduits for the ascending magma. The centre of the batholith is composed of a homogenous granitic phase containing the same magmatic and solid-state fabrics recorded within the shear zones, but whose internal contacts are elusive. However, over up to several km around the shear zones, subvertical sheets intrude and brecciate the homogeneous phase and this combined with the presence of the structural fabrics suggests a possible genetic link with the synmagmatic shear zones. The final intrusive phases were emplaced late- to post-tectonic, occurring as randomly orientated granite sheets and plugs, and marks the final emplacement style recorded in the batholith. Presently no geochronological data for the Heerenveen batholith is available, but age data for an adjacent, genetically related batholith indicates construction occurred over a protracted period of between 3 and 10 Ma.