Symplectitic cordierite-orthopyroxene-garnet assemblages as products of contact metamorphism of pre-existing basement granulites in the Vredefort Structure, South Africa, and their relations to pseudotachylite

Schreyer, Werner; Abraham, K.

doi:10.1007/bf00375446

Cited by 50 publications

(26 citation statements)

References 21 publications

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“…Schreyer (1983), Stepto (1990) and Fricke et al (1990) attributed these annealing effects to a hidden plutonic heat source. However, Grieve et af.…”

Section: Geological Settingmentioning

confidence: 96%

“…Schreyer 1983). The higher grades have traditionally been attributed to the contact aureole of a hidden pluton localized beneath the Dome (Nel 1927;Bisschoff 1982;Schreyer 1983;Stepto 1990). This perceived coincidence of the Vredefort Dome with a centre of high-grade contact metamorphism has led some workers to reject the impact hypothesis for the origin of the dome and to seek alternative, endogenous models in which the doming event, high-grade metamorphism and shock phenomena can be reconciled (e.g.…”

Section: Introductionmentioning

confidence: 93%

“…The amphibolite facies metamorphism in the collar supracrustal rocks has been linked with the last of these events, which is perceived by most workers as being related to a localized igneous heat source hidden beneath the dome (e.g. Bisschoff 1982;Schreyer 1983;Stepto 1990). Attempts to constrain the age of this event and, more specifically, its relation to the formation of the dome, by conventional geochronology have produced equivocal results.…”

Section: Geological Settingmentioning

confidence: 99%

“…The rocks in the outer parts of the basement core, comprising granitic to granodioritic migmatitic gneisses (the Outer Granite Gneiss; Stepto 1990) contain upper amphibolite facies assemblages (Nel 1927;Bisschoff 1982), whereas those in the centre of the dome, comprising leucogranulites and subsidiary paragneisses and mafic orthogneisses (Inlandsee Leucogranofels Terrane and Steynskraal Formation; Stepto 1990) contain granulite facies assemblages (e.g. Schreyer and Abraham 1978;Schreyer et al 1978;Stepto 1990). The amphibolite-granulite transition occurs approximately 8-10 km from the core+ollar contact (Figure 2a) (Bisschoff 1982;Stepto 1990).…”

Section: Geological Settingmentioning

confidence: 99%

“…The rocks exposed in the dome are, however, also remarkable in that they experienced higher grades of low P-high T metamorphism than equivalent stratigraphic horizons in the surrounding region apparently penecontemporaneous with the shock event (e.g. Schreyer 1983). The higher grades have traditionally been attributed to the contact aureole of a hidden pluton localized beneath the Dome (Nel 1927;Bisschoff 1982;Schreyer 1983;Stepto 1990).…”

Section: Introductionmentioning

confidence: 97%

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Low pressure‐high temperature metamorphism in the Vredefort Dome, South Africa: Anticlockwise pressure‐temperature path followed by rapid decompression

Gibson

Wallmach

1995

Geological Journal

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Pelitic metasediments of the Witwatersrand Supergroup in the Vredefort Dome contain textural evidence for a twostage metamorphic history. The pressure-temperature (P-T) path derived for the rocks indicates that they were subjected to a high geothermal gradient (~4 0°C km-'), mid-amphibolite facies, M la metamorphic event concomitant with thickening of the overlying upper crust. Peak temperatures of 570-600°C were reached during M l a at depths of 14-16 km. After initial isobaric cooling following Mla, the rocks experienced rapid exhumation during a high strain rate deformation event associated with the formation of the dome. This event led to the development of abundant pseudotachylite and a brittle cleavage in these rocks. These features are overgrown, together with the M la assemblages, by a low P M 1 b paragenesis comprising microporphyroblastic cordierite+biotite. Estimated P-T conditions during M l b were <3.5 kbar, z500-530°C.The two-stage P-T path is incompatible with existing contact metamorphic and diapiric models that have attempted to link the mid-amphibolite facies metamorphism with the formation of the Vredefort Dome. Instead, it indicates that the doming event occurred some time after the peak of a regional low P-high T metamorphic event, during cooling of the terrain. Given an age of 2.02 Ga for the doming event, the M l a event is attributed to a widespread magmaticthermal event on the Kaapvaal Craton that accompanied the formation of the Bushveld Complex at 2.05-2.06 Ga. The M l b event developed in response to the localized exhumation of deep parts of this terrain while the crustal geotherm was still elevated (z 30°C km-'). The association, elsewhere in the dome, of pseudotachylite with shock metamorphic features generated at pressures in excess of 20-120 kbar indicates that exhumation was accomplished by the impact of a large meteorite into the terrain.

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“…Schreyer (1983), Stepto (1990) and Fricke et al (1990) attributed these annealing effects to a hidden plutonic heat source. However, Grieve et af.…”

Section: Geological Settingmentioning

confidence: 96%

Section: Introductionmentioning

confidence: 93%

Section: Geological Settingmentioning

confidence: 99%

Section: Geological Settingmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 97%

See 3 more Smart Citations

Low pressure‐high temperature metamorphism in the Vredefort Dome, South Africa: Anticlockwise pressure‐temperature path followed by rapid decompression

Gibson

Wallmach

1995

Geological Journal

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Polymetamorphic Al, Mg-rich granulites with orthopyroxene–sillimanite and sapphirine parageneses in Archaean rocks from the Hoggar, Algeria

Kiénast

Ouzegane

1987

Geol. J.

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An orthopyroxene-sillimanite association occurs in AI-Mg rich granulites within Archaean aluminous metapelites at In Hibaou (NW Hoggar). Sapphirine is characteristically present in those rocks as a reaction product of a high pressure mineral association. Other reaction products include cordierite and a second generation of orthopyroxene, confirming the presence of two superimposed metamorphic events (M1 and M2). The application of mineral geothermometers and geobarometers to the observed parageneses in the two metamorphic events gives the following P-T ranges M1, 10 2 1 3 Kb -800-900°C; M2, 5 t 1.5 Kb -8OG700"C. These values are in general agreement with those deduced from associated pyrigarnites.

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Economic Mineral Deposits in Impact Structures: A Review

et al.

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Symplectitic cordierite-orthopyroxene-garnet assemblages as products of contact metamorphism of pre-existing basement granulites in the Vredefort Structure, South Africa, and their relations to pseudotachylite

Cited by 50 publications

References 21 publications

Low pressure‐high temperature metamorphism in the Vredefort Dome, South Africa: Anticlockwise pressure‐temperature path followed by rapid decompression

Low pressure‐high temperature metamorphism in the Vredefort Dome, South Africa: Anticlockwise pressure‐temperature path followed by rapid decompression

Polymetamorphic Al, Mg-rich granulites with orthopyroxene–sillimanite and sapphirine parageneses in Archaean rocks from the Hoggar, Algeria

Economic Mineral Deposits in Impact Structures: A Review

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