Roger L. Gibson

Precise U–Pb titanite age constraints on the emplacement of the Bushveld Complex, South Africa

The c. 2.06 Ga Bushveld Complex intrudes the Transvaal Supergroup, South Africa. Calcsilicate xenoliths within the mafic phase of the Bushveld Complex (Rustenburg Layered Suite, or RLS) preserve calcsilicate xenoliths with high-temperature (c. 1200°C) mineralogies that were later metasomatized by hydrous, retrograde (c. 600–700°C) fluids whose timing has been unconstrained. New U–Pb isotope data from newly grown titanite within a completely retrogressed xenolith indicates that retrogression occurred at 2058.9±0.8 Ma. This suggests that retrogression was due to hydrothermal circulation associated with the cooling RLS, and not slightly later Bushveld-related granites. The new data place a tight constraint on the minimum age of emplacement of the RLS.

High-δ13C Paleoproterozoic carbonates from the Transvaal Supergroup, South Africa

Marbles from the Transvaal Supergroup (Kaapvaal craton, South Africa) record two positive carbon isotope excursions to average δ 13 C carbonate values of ∼+6‰ to +8‰ (relative to V-PDB), separated by carbonates with normal-marine δ 13 C values (∼0‰ ± 2‰). These elevated values have been preserved through metamorphic devolatilization. Together with an already documented high- 13 C excursion in overlying rocks, three separate positive carbon isotope excursions occurred on the Kaapvaal craton in the interval 2.43 to 1.93 Ga. It is not yet clear how many of these excursions reflect global rather than local processes.

The age and thermal evolution of the Vredefort impact structure: A single-grain UPb zircon study

Abstract Single-grain UPb SHRIMP analyses of a heterogeneous zircon population from a small granite body in the Archean basement core of the Vredefort Dome indicate an age of 2017 ± 5 Ma for the granite, which is identical, within errors, to the previously reported age determinations for the Vredefort impact event. Petrographic evidence from the granite indicates that it crystallized after the impact shock event, but some doubt remains whether it is a true impact melt, a decompression melt generated during rapid uplift of the crater basement following the impact event, or a melt generated during an earlier, 2.05–2.06 Ga, granulite facies anatectic event that was still at least partially molten at the time of the Vredefort impact event. Rare zircon xenocrysts displaying planar microdeformation features and giving discordant Archean ages are overgrown by polycrystalline zircon rims with UTh compositions similar to that of the main ∼2.02 Ga zircon population, but with discordant 207Pb/206Pb ages, indicating more recent Pb-loss.

Thermal-metamorphic signature of an impact event in the Vredefort dome, South Africa

Pseudotachylitic breccias and shock deformation features related to the 2.02 Ga formation of the Vredefort dome by meteorite impact were overprinted by a static metamorphic event, the intensity of which decreased from granulite-facies ( T ≥700 °C) in the center of the dome to greenschist-facies ( T ≤400 °C) around its margins. Geobarometric estimates of 0.2–0.3 GPa for the metamorphic parageneses indicate some 8–11 km of erosion since the impact event. The strong lateral thermal gradient implied by these P-T results is attributed to the combined effects of differential uplift of mid-crustal rocks heated along a pre-impact geotherm and increased shock heating of the target crust toward the center of the impact structure. We suggest that the exceptionally high grade of metamorphism in the center of the dome may, in part, reflect an elevated regional geothermal gradient of ∼25 °C/km in the target crust due to lingering thermal effects related to the 2.05–2.06 Ga Bushveld magmatic event.

Archean crustal structure of the Kaapvaal craton, South Africa – evidence from the Vredefort dome

Abstract Crystalline Archean basement rocks in the core of the Vredefort dome present a profile through a substantial part of the middle and lower crust of the Kaapvaal craton. Previously, this profile has been subdivided into two terranes with allegedly distinct lithologies and tectonometamorphic histories that were juxtaposed along a crustal-scale Late Archean brittle–ductile thrust zone. Lithological and structural mapping across the dome indicates, however, that the basement lithologies share a common polyphase tectonic history culminating in high-grade metamorphism and melting at ∼3.1 Ga. No evidence was found of the postulated tectonic terrane boundary, but the alleged boundary does coincide with a 1–2 km wide transition zone between upper amphibolite facies migmatitic gneisses and more restitic granulite facies gneisses. The implications of these results for Archean regional tectonic models for the Kaapvaal craton are discussed.

Mid-crustal granulite facies metamorphism in the Central Kaapvaal craton: the Bushveld Complex connection

Abstract The numerous greenstone remnants which occur as inclusions in the Archaean gneissic granitoid basement exposed in the core of the Vredefort Dome display evidence of very high grades of granulite facies metamorphism and an unusually complex three-stage metamorphic history: (1) in metapelites, migmatites and refractory restites were produced in a mid-crustal (±0.5 GPa) M1 anatectic event, at temperatures in excess of 900°C (in the restites); (2) in the migmatites, the peak metamorphic assemblages are variably overprinted by a high-grade retrograde reaction which involved crystallizing anatectic melts (M2); (3) both the peak metamorphic assemblages and the retrogression textures are cross-cut by pseudotachylitic breccia veins which mark a shock metamorphic event associated with the formation of the Vredefort Dome; and (4) the peak metamorphic assemblages and the M2 retrogression textures are overprinted by new generations of remarkably fine-grained, post-shock, high-grade phases which crystallised at a pressure some 0.25 GPa lower than that of both the peak metamorphic conditions and the retrogression (M3). These data indicate an anticlockwise P-T evolution where mid-crustal anatexis most probably resulted from the intraplating of Bushveld Complex-related ultramafic magmas at 2.06 Ga. A period of approximately isobaric cooling and retrogression was followed by the Vredefort Catastrophe, which resulted from the impact of a large meteorite into the terrane at 2.02 Ga. This shock event halted the natural mid-crustal metamorphic progression prior to the terrane cooling below 650°C, and resulted in the rapid exhumation of the terrane by some 9 km. In the process, it provided a window into the deep levels of the central Kaapvaal craton not seen elsewhere in the region.

Hercynian low-pressure-high-temperature regional metamorphism and subhorizontal foliation development in the Canigou massif, Pyrenees, France—Evidence for crustal extension

A detailed study has been made of the structural and metamorphic history of part of the Hercynian low-pressure-high-temperature regional metamorphic terrane exposed in the Canigou massif, France. Microtextural relations between, and compositional zoning within, porphyroblast phases in pelitic rocks indicate that the pressure-temperature-time ( P-T-t )trajectory followed by the massif during metamorphism involved decompression under prograde and retrograde conditions (clockwise P-T-t trajectory). The geometric relations between inclusion trails within these porphyroblasts and the structures in the adjacent matrix indicate that metamorphism was synchronous with the development of the regional subhorizontal foliation (S3). The S3 foliation formed in response to noncoaxial bulk strain. The absence of a marked hiatus in sedimentation in the Pyrenean region during Hercynian metamorphism and deformation and the widespread preservation of low-grade Hercynian metasedimentary rocks suggest that low- P -high- T metamorphism occurred in an extensional tectonic environment and that extension manifested itself in asymmetric top-to-the-northwest shear during D3.

Nature of the archean midcrust in the core of the Vredefort dome, Central Kaapvaal Craton, South Africa

Extreme uplift associated with the formation of the 2.02 Ga Vredefort dome has exposed a substantial cross section through the crystalline early Archean basement complex rocks of the Kaapvaal craton. The rocks comprise polydeformed high-grade tonalite-trondhjemite-granodiorite (TTG) gneisses, migmatites and late-tectonic intrusive granitoids that straddle the upper amphibolite-to granulite-facies transition. Field, petrographic and geochemical data indicate that compositional heterogeneity occurs on a local scale and reflects the migmatitic character of the rocks rather than crustal-scale layering as has been previously proposed. No evidence has been found to support exposure of either a melt-depleted, refractory, lower crust or an upper crustal batholithic granite layer; however, the immense volume of granitic leucosome in the rocks suggests that the exposed section represents an intermediate level between these two zones. Granitic leucosomes in the upper amphibolite-facies migmatites appear to be intrusive into the predominantly trondhjemitic host rocks, rather than of in situ derivation. Leucosome compositions in the granulite-facies migmatites are more variable, ranging from granitic and charnockitic to enderbitic, probably reflecting at least some local derivation. Leucosomes and small granitoid bodies show local-scale geochemical variation that can be explained in terms of variable amounts of melt segregation and migration, and fractionation of minerals such as K-feldspar within the melts.

Origin of large-volume pseudotachylite in terrestrial impact structures

Large-volume pseudotachylite bodies in impact structures are dike like and consist of angular and rounded wall-rock fragments enveloped by a microcrystalline and sporadically glassy matrix that crystallized from a melt. Knowledge of the formation of pseudotachylite bodies is important for understanding mechanics of complex crater formation. Most current hypotheses of pseudotachylite formation inherently assume that fragmentation and melt generation occur during a single process. Based on the structure of pseudotachylite bodies at Sudbury (Canada) and Vredefort (South Africa), we show that these processes differ in time and space. We demonstrate that the centimeter- to kilometer-scale bodies are effectively fragment- and melt-fi lled tension fractures that formed by differential rotation of target rock during cratering. Highly variable pseudotachylite characteristics can be accounted for by a single process, i.e., drainage of initially superheated impact melt into tension fractures of the crater fl oor.

Economic Mineral Deposits in Impact Structures: A Review

Many large meteorite impact structures throughout the world host mineral resources that are either currently mined or have the potential to become important economic resources in the future. The giant Vredefort-Witwatersrand and Sudbury impact structures underline this statement, because of their enormous resources in gold and uranium, and nickel, copper, and PGEs, respectively. In relation to impact, three basic types of ore deposits in impact structure settings have been distinguished: (1) progenetic (i.e., pre-impact) deposits that already existed in the target regions prior to an impact event, but may have become accessible as a direct result of the impact; (2) syngenetic (syn-impact) deposits that owe their existence directly to the impact process, and (3) epigenetic (immediately post-impact) deposits that result from impact-induced thermal/hydrothermal activity. In addition to metalliferous ore deposits related to impact structures, impact structure-hosted epigenetic hydrocarbon deposits are reviewed and are shown to make a major contribution to the North American economies. Non-metallic resources, such as minerals derived from crater-lake deposits, dimension stone, and hydrological benefits, may also be derived from impact structures, and the educational and recreational value of many meteorite impact craters can be substantial.