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A new model is proposed to explain the origin of enigmatic fine-grained granulite facies rocks sampled from the Moon, based on observations from the Vredefort dome, South Africa. The dome is the deeply eroded central uplift of the ~300-km-diameter Vredefort impact structure, in the dome, fine-grained granulites displaying poikilitic or granoblastic microstructures were formed by relatively slow cooling of shock [+ or -] friction melts derived at T > 1350[degrees]C. Slow cooling was achieved owing to the >7 km depth of burial of the rocks following the impact. At least some of the lunar granulitic impactites may also have formed by shock heating and slow cooling at deep levels within the central uplifts of large impact structures, without the need for additional heating by younger intrusive or impact melt bodies. Keywords: breccia, granulite facies, impactite, Moon, shock metamorphism.

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 [greater than or equal to] 700 [degrees] C) in the center of the dome to greenschist-facies (T [less than or equal to] 400 [degrees] 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 [approximately]25 [degrees] C/km in the target crust due to lingering thermal effects related to the 2.05-2.06 Ga Bushveld magmatic event.

Archean spherule layers represent the only currently known remnants of the early impact record on Earth. Based on the lunar cratering record, the small number of spherule layers identified so far contrasts to the high impact flux that can be expected for the Earth at that time. The recent discovery of several Paleoarchean spherule layers in the BARB5 and CT3 drill cores from the Barberton area, South Africa, drastically increases the number of known Archean impact spherule layers and may provide a unique opportunity to improve our knowledge of the impact record on the early Earth. This study is focused on the spherule layers in the CT3 drill core from the northeastern Barberton Greenstone Belt. We present highly siderophile element (HSE: Re, Os, Ir, Pt, Ru, and Pd) concentrations and Re‐Os isotope signatures for spherule layer samples and their host rocks in order to unravel the potential presence of extraterrestrial fingerprints within them. Most spherule layer samples exhibit extreme enrichments in HSE concentrations of up to superchondritic abundances in conjunction with, in some cases, subchondritic present‐day 187Os/188Os isotope ratios. This indicates a significant meteoritic contribution to the spherule layers. In contrast to some of the data reported earlier for other Archean spherule layers from the Barberton area, the CT3 core is significantly overprinted by secondary events. However, HSE and Re‐Os isotope signatures presented in this study indicate chondritic admixtures of up to (and even above) 100% chondrite component in some of the analyzed spherule layers. There is no significant correlation between HSE abundances and respective spherule contents. Although strongly supporting the impact origin of these layers and the presence of significant meteoritic admixtures, peak HSE concentrations are difficult to explain without postdepositional enrichment processes. [ABSTRACT FROM AUTHOR]

Magnetic signatures generated by meteorite impacts are analyzed based on the results of a study of the ca. 2 Ga Vredefort structure, which is a product of a large meteorite impact. It is suggested that postimpact thermal event caused by the reset remanent magnetism in the rocks is responsible for the magnetic anomaly and thermal metamorphism predating the formation of the magnetic signatures.

Comments on a study by R. J. Hart et al on about the Archaean crystalline basement rocks in the core of the Vredefort dome southwest of Johannesburg, South Africa. Geological setting of the Beta-1 ultramafics; connection between the Beta-1 peridotites arid geophysical data from the central parts of the dome; Alternative interpretations on its crust-on-edge profile of the crystalline basement rocks.

This article provides a first report of the fall of a meteorite over the Thuathe plateau of western Lesotho, approximately 9 km east of the capital Maseru, on 21 July 2002. The strewn field has been mapped, and the characteristics of stones recovered from various sectors in the strewn field are compared. Initial mineralogical investigations have established that this new meteorite is an H type ordinary chondrite, of petrologic type 4/5, and shock degree S2-S3. The name Thuathe has been accepted by the Nomenclature Committee of the Meteoritical Society for this new fall, as the major part of the strewn field extends over the Thuathe plateau. We believe that Thuathe is the first meteorite recovered from Lesotho. [ABSTRACT FROM AUTHOR]

K–Ar isotopic ages presented by Uysal et al. for illitic clay minerals from drill core samples were interpreted to date the Woodleigh impact event at 359±4 Ma, allegedly implicating Woodleigh in the Late Devonian mass extinction. However, only very equivocal evidence is presented by Uysal et al. to support a link between clay mineral paragenesis and impact-related features, and the K–Ar ages reveal a distribution that is essentially a continuum between 308 and 364 Ma. The ‘age’ computed by Uysal et al. is based on an average of the five oldest ages within this group, which has no geological or statistical basis. The stratigraphic age constraints considered by Uysal et al. to be consistent with this age are much weaker than acknowledged, and the impact could have been much older than mid-Devonian. The size of the Woodleigh crater is poorly constrained (and the subject of an ongoing controversy); Uysal et al.’s suggestion of 120 km diameter is probably overestimated by a factor of two, in which case a link to any mass extinction is unlikely. [Copyright &y& Elsevier]

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. [Copyright &y& Elsevier]