ELSEVIER Precambrian Research 79 (1996) 25-35 Pretnmbrinn Resenrth U-Pb geochronologic constraints on deposition of the Campbellrand Subgroup, Transvaal Supergroup, South Africa Dawn Y. Sumner, Samuel A. Bowring Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA Received 1 December 1995; revised version accepted 1 December 1995 Abstract A volcanic ash bed that crops out 40-50 m below the contact between the Campbellrand Subgroup and the Kuruman Iron Formation, Transvaal Supergroup, South Africa, yields an U-Pb zircon age of 2521 _ 3 Ma. This new age demonstrates that the Campbellrand-Malmani carbonate platform was deposited during late Archean time, and is the oldest extensively preserved carbonate platform known. In addition, the age constrains deposition of the Kuruman, Griquatown, and Penge iron-formations to latest Archean to earliest Paleoproterozoic time. Although a previously reported Pb-Pb age of 2557 _+ 49 Ma for dolomites at the base of the Campbellrand Subgroup is broadly consistent with this U-Pb zircon age, new Pb-isotopic data for carbonate cements and massive sulfides lie along the same Pb-Pb array as the dolomites. This relationship implies that the 'isochron' is the result of post-depositional resetting of the U-Pb system in dolomite by fluids responsible for sulfide mineralization, and that the age which corresponds to its slope has no geological significance. 1. Introduction The Kaapvaal Craton, South Africa, contains an extensive record of Archean to Paleoproterozoic platform sedimentation. Deposition of cratonic cover sequences including siliciclastic, chemical, and vol- canic sediments began at about 3.0 Ga (Tankard et al., 1982). These sediments include the auriferous Witwatersrand Supergroup, the oldest extensively preserved carbonate platform known, large volumes of iron-formation, and red bed deposits. The chemi- cal and physical characteristics of these and other deposits on the Kaapvaal Craton can provide impor- tant constraints on late Archean and Proterozoic sedimentary environments. For example, establishing the timing of carbonate deposits and the textures within them can help constrain variations in Archean and Paleoproterozoic carbonate facies and possible long-term changes in ocean chemistry (Grotzinger, 1989; Grotzinger and Kasting, 1993). Iron-forma- tions and red bed deposits contain important infor- mation on oxygen concentrations in both the oceans and atmosphere. Temporal comparisons of oxidation states in various sedimentary environments will lead to a better understanding of early ocean stratification (Simonson, 1985; Beukes et al., 1990) and global changes in oxygen concentration (Holland, 1984; Sumner and Grotzinger, 1996). Kaapvaal cratonic stratigraphy is poorly dated in absolute time, however, hindering the improvement of Archean to Paleoproterozoic sedimentation mod- els as well as development of inter-regional lithos- tratigraphic correlations. In order to better constrain the depositional ages of Kaapvaal cratonic sedi- 0301-9268/96/$15.00 © 1996 Elsevier Science B.V. All fights reserved SSDI 0301-9268(95)00086-0