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Impact Cratering Research Group, School of Geosciences, University of the Witwatersrand, Private Bag 3, PO Wits 2050, Johannesburg, South Africa
Impact Cratering Research Group, School of Geosciences, University of the Witwatersrand, Private Bag 3, PO Wits 2050, Johannesburg, South Africa Corresponding author, Fax: +27 (11) 339 1697; e-mail: reimoldw{at}geosciences.wits.ac.za
Geophysical Institute, University of Alaska, PO Box 757320, Fairbanks, Alaska 99775-7320, USA
Tectonites from ~2.7-Ga-old Platberg (mid-Ventersdorp) rift-related fault zones in H.J. Joel and Harmony gold mines (southern Free State Goldfield) were dated by the 40Ar-39Ar step-heating technique. A mylonite from a bedding-parallel thrust zone at H.J. Joel Gold Mine yielded an age plateau with ages between ~2.08 Ga and ~2.07 Ga (interpreted age 2076 ± 5 Ma). A cataclasite from a north-striking shear zone yielded a partial (60%) age plateau with ~2.095 to ~2.065 Ga ages, resulting in an interpreted age of 2088 ± 7 Ma. Both partially recrystallised tectonites exhibited 39Ar-loss at, or more recent than, ~1.83 to ~1.75 Ga. A completely recrystallised ultracataclasite from a major northeast-striking fault zone yielded only ages <~1.75 Ga. Seemingly two phases of low-temperature resetting at ~1.83 to ~1.75 Ga and ~1.35 to ~1.20 Ga affected this sample.
The argon age systematics of mylonitic fault rocks from Harmony Gold Mine that are crosscut by younger ultracataclastic-pseudotachylitic breccias and that are developed along individual shear planes of the Merriespruit Thrust and De Bron Fault zones are disturbed. Only one mylonite yielded a plateau age of 2019 ± 8 Ma, which is within errors of the age of breccia development due to the Vredefort impact event. Most of these mylonites record ~1.83 to ~1.75 Ga ages. Several other ultracataclastic-pseudotachylitic breccias could not be precisely dated, as the samples do not contain sufficient potassium. The spectra of these samples are saddle-shaped with strong indications of significant excess argon. Saddle ages of 2841 ± 15 Ma and 2020 ± 20 Ma were obtained for one sample each from the De Bron and Merriespruit fault zones.
Ages in the interval ~2.095 to ~2.065 Ga document a thermal overprinting event of pre-Bushveld-age, which led to complete resetting of the isotope system of older cataclasites and mylonites. This thermo-metamorphic event is believed to represent the initial phase of the Palaeoproterozoic Magondi (Ubendian) Orogeny at ~2.1 to 2.06 Ga along the western margin of the Zimbabwe Craton, which resulted in intense magmatic activity throughout the northern and central Kaapvaal Craton. Pre- to early orogenic magmatism in the Kaapvaal Craton culminated in the structurally controlled emplacement of the Bushveld Igneous Complex, which caused regionally persistent greenschist facies metamorphism (~2 to 3 kbar; 350°C) and the formation of peak-metamorphic mineral assemblages in West and Central Rand Group lithologies in the northern and central Witwatersrand Basin at ~2.06 to ~2.05 Ga, but appears not to have thermo-metamorphically effected the southern portion of the basin.
Besides geological evidence for Vredefort-related generation of ultracataclastic-pseudotachylitic breccias along some of the major fault zones in the Free State Goldfield, ages that could be related to the Vredefort impact event at ~2.02 Ga were obtained from ultracataclastic-pseudotachylitic breccias cutting across earlier developed cataclasites and mylonites along the De Bron Fault and Merriespruit Thrust Zone. Ages <~2 Ga represent post-Vredefort tectono-thermal or hydrothermal events that have previously also been documented from breccias in the north-central Witwatersrand Basin. Expected Platberg formation ages of ~2.71 Ga were not obtained from any of the analysed cataclastic and mylonitic fault rock specimens, due to the intense and almost complete resetting of their isotopic systems during later (pre-Bushveld, Bushveld, Vredefort, post-Vredefort) tectono-thermal events. These results clearly indicate that post-2 Ga tectono-hydrothermal activity affected the central part of the Kaapvaal Craton. This activity is interpreted as a manifestation of orogenic activity in the Proterozoic mobile belts around the craton margins during the Eburnean (Kheis) and Kibaran/Grenville (Namaqua-Natal) orogenies.
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