U-Pb Geochronologic Constraints on Deposition of the Campbellrand Subgroup, Transvaal Supergroup, South Africa

Dawn Y. Sumner and Samuel A. Bowring

Precambrian Research, v. 79, p. 25-35

(The links "*" connect to definitions of the preceeding word.)
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 25213 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 255749 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.

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Dawn Y. Sumner
Department of Geology
University of California
Davis, CA 95616