Correlations on the southern Kaapvaal Craton Margin, 1: Ventersdorp lavas transgressed the Doornberg lineament

Abstract

Two small exposures of quartz-porphyritic rocks occur on the farm Zoutpekel 98 in the Marydale Terrane between the Doornberg Fault and Brakbos Shear Zone, apparently overlying Kaapvaal basement granite but lacking clear field relationships due to sand and Dwyka tillite cover. They are lavas and tuffs, metamorphosed in lower amphibolite facies. They contain quartz phenocrysts with a distinctive blue colour, due to metamorphic exsolution of rutile. Microbeam U-Pb zircon dating gives a combined 207Pb/206Pb age of 2 722 ± 3 Ma (seven determinations on four samples), interpreted as the age of extrusion. Three of these samples give the same discordia upper intercept age, but one sample gives discordia intercepts of 2 688 ± 15 and 1 223 ± 120 Ma, thought to reflect metamorphic lead loss related to the ~1 210 Ma Namaqua terrane assembly collisions. The Zoutpekel exposures are coeval with the 2 720 ± 2 Ma Makwassie Formation of the Platberg Group, Ventersdorp Supergroup. They also correspond geochemically to the Makwassie Formation and no other unit of the supergroup. A sample from the T’kuip Formation of the nearest Ventersdorp Supergroup inlier on the Kaapvaal Craton (east of the Doornberg Fault), gives an age of 2 716 ± 8 Ma, also confirming its lithostratigraphic and geochemical correlation with the Makwassie Formation. The Zoutpekel exposures show that not only the Kaapvaal basement granites, but also the supracrustal cover rocks of the Ventersdorp Supergroup, extend southwards across the Doornberg Fault, The Marydale Terrane is thus not an exotic terrane, but probably represents a passive continental margin developed at the beginning of the 1 300 to 1 000 Ma Namaqua-Natal Wilson cycle. The age range of the Ventersdorp Supergroup and the age and stratigraphic correlation of the Marydale Group thrust complex, which straddles the Zoutpekel exposures, will be investigated in two companion papers.