ORIGIN OF PSEUDOTACHYLITE IN THE LOWER WITWATERSRAND SUPERGROUP, VREDEFORT DOME (SOUTH-AFRICA) - CONSTRAINTS FROM METAMORPHIC STUDIES

Small, sub-millimetre to centimetre thick, pseudotachylite veins in me tapelitic rocks of the lower Witwatersrand Supergroup in the collar of the Vredefort Dome are recrystallized to a biotite+/-cordierite parag enesis, indicating the attainment of lower amphibolite facies metamorp hic conditions (T greater than or equal to 500 degrees C) following th eir formation. The veins occur along ubiquitous anastomosing fracture networks displaying millimetre-to centimetre-scale displacements and t runcate, and incorporate fragments of, an older, ca. 2.05 Ga, mid-amph ibolite facies, peak metamorphic assemblage. The post-pseudotachylite metamorphism is attributed to the same event that caused highly variab le annealing of ca. 2.02 Ga shock microdeformation features in rocks a cross the dome. Together with the disproportionately large volume of p seudotachylite in the rocks relative to that found in tectonic provinc es, and the absence of evidence of genetically related large-scale she ar or fault zones, this metamorphic evidence suggests an origin for th e pseudotachylite related to the 2.02 Ga Vredefort bolide impact event that produced the dome. Possible origins for the pseudotachylite incl ude shock brecciation and melting, post-shock decompression melting, a nd friction melting associated with the post-impact high-velocity exca vation and modification phases of crater formation. The post-pseudotac hylite metamorphism is attributed to the combined effects of an elevat ed pre-impact crustal geotherm (similar to 25 degrees C/km) related to the waning stages of the 2.05 Ga peak metamorphism, release of elasti c strain energy from the impact shock event and contact effects beneat h the impact melt body.