Low pressure‐high temperature metamorphism in the Vredefort Dome, South Africa: Anticlockwise pressure‐temperature path followed by rapid decompression
作者:
R. L. Gibson,
T. Wallmach,
期刊:
Geological Journal
(WILEY Available online 1995)
卷期:
Volume 30,
issue 3‐4
页码: 319-331
ISSN:0072-1050
年代: 1995
DOI:10.1002/gj.3350300311
出版商: John Wiley&Sons Ltd
关键词: anticlockwise P‐T path;decompression texture;low P‐high T metamorphism;meteorite impact;pseudotachylite;Vredefort Dome
数据来源: WILEY
摘要:
AbstractPelitic metasediments of the Witwatersrand Supergroup in the Vredefort Dome contain textural evidence for a two‐stage metamorphic history. The pressure‐temperature (P‐T) path derived for the rocks indicates that they were subjected to a high geothermal gradient (≈40°C km‐1), mid‐amphibolite facies, Mla metamorphic event concomitant with thickening of the overlying upper crust. Peak temperatures of 570–600°C were reached during Mla 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 Mla assemblages, by a low P M1b paragenesis comprising microporphyroblastic cordierite+biotite. Estimated P‐T conditions during Mlb were<3.5 kbar, ≈500–;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 Mla event is attributed to a widespread magmatic‐thermal event on the Kaapvaal Craton that accompanied the formation of the Bushveld Complex at 2.05–2.06 Ga. The Mlb event developed in response to the localized exhumation of deep parts of this terrain while the crustal geotherm was still elevated (≈30°C km‐1). 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 i
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