Evolution of the Amphibolite‐Granulite Facies Transition Exposed by the Vredefort Impact Structure, Kaapvaal Craton, South Africa

Abstract

Geological mapping is integrated with high‐precision U‐Pb zircon and monazite geochronology to document in detail the nature and temporal evolution of the amphibolite‐granulite facies boundary exposed in the Vredefort impact structure of the central Kaapvaal Craton. Our study has revealed that the preimpact configuration of the amphibolite‐granulite facies boundary is best preserved in panels between impact‐related brittle faults and that past concepts of the boundary must be revised. A regional lithologic transition exists between amphibolite facies Outer Granite Gneiss (3.084–3.09 Ga) and a heterogeneous assemblage of granulite facies gneisses (ca. 3.1–3.3 Ga). Tabular deformed quartz syenite bodies, dated at $$3.084\pm .001$$ Ga, parallel the metamorphic transition and crosscut both amphibolite and granulite facies rocks. Although two episodes of late Archean deformation modified the boundary, it has retained its primary intrusive continuity since 3.08 Ga. Subsequent activity includes minor dolerite dikes, 2.2 Ga monazite growth, 2.02 Ga meteoritic impact, and episodic hydrothermal zircon growth events from 615 to 380 Ma. Our results suggest that the Vredefort amphibolite‐granulite facies transition can shed valuable insight into Archean crustal evolution and the nature of metamorphic boundaries in cratonic crust. The nature and paleodepth of the Vredefort metamorphic transition are compatible with the seismic characteristics of the unexposed crust of the central Kaapvaal Craton. The absence of major tectonic activity post–3.08 Ga in the Vredefort crystalline basement is consistent with the presence of a lithospheric mantle root that stabilized the central cratonic crust against subsequent disruption. The lithologic, metamorphic, and structural complexity of the transition zone between amphibolite and granulite facies rocks in the Vredefort crystalline basement may be typical of this metamorphic boundary in intracratonic crust.