Emplacement ages of Jurassic-Cretaceous South African kimberlites by the Rb-Sr method on phlogopite and whole-rock samples.

Rb/Sr phlogopite age determinations, interpreted as emplacement ages, are reported for 15 southern African kimberlites. Jagersfontein and Rietfontein (85 and 95 m.y.) have ages typical of the majority of well-known Cretaceous kimberlites, whereas somewhat older ages of approx 118-125 m.y. have been obtained for localities in the Postmasburg, Barkly West and Boshoff districts (Finsch, Bellsbank, Poortjie and the New Elands-Blaaubosch-Roberts Victor group). Previous zircon ages of 90 m.y. for Finsch and Roberts Victor are believed to be incorrect. Two other localities in the Barkly West area, Frank Smith and Newlands, have significantly younger emplacement ages of approx 114 m.y. relative to most Barkly West occurrences. Two off-craton kimberlites, Uintjiesberg and Mzongwana, are 100 and 150 m.y. in age respectively. Swartruggens (western Transvaal) and Elandskloof (eastern Transvaal) have ages of 150-160 and 165 m.y. respectively. A Barkly West occurrence, Klipfontein, also has an apparent age of 160 m.y., but this result cannot be considered reliable. The emplacement ages and initial ⁸⁷Sr/⁸⁶Sr ratios of southern African Jurassic and Cretaceous kimberlites that have been dated define two distinctive populations, in accord with previous whole-rock isotopic studies. With the exception of the East Griqualand occurrences (150 and 190 m.y.), Group I kimberlites are generally < approx 114 m.y. in age, with initial ⁸⁷Sr/⁸⁶Sr ratios of about 0.704, whereas Group II kimberlites are generally older than approx 114 m.y., with initial ⁸⁷Sr/⁸⁶Sr ratios of approx 0.708. The two varieties of kimberlite must be derived from distinct types of sources, possibly by distinct processes. Old crustal biotite grains present as contaminants in the New Elands kimberlite are only partially reset during emplacement, resulting in spurious ages if not detected. Since very fine-grained crustal biotite is not isotopically re-equilibrated with kimberlite, it is premature to conclude that all phlogopite xenocrysts of mantle derivation are isotopically re-equilibrated at the time of pipe formation. Important factors in obtaining precise Rb/Sr mica ages include the type of mica analysed, the use of whole-rock data to constrain initial ratios, and the use of a leaching procedure developed for mica. (Authors' abstract) -C.N.