Abstract

The ultramafic-mafic layered igneous Molopo Farms Complex straddles the border between South Africa and Botswana. Younger cover obscures this igneous complex and its country rocks, which are generally assigned to the Paleoproterozoic Transvaal Supergroup. The area intruded by the complex is characterized by abutting and contrasting successions of the upper Transvaal Supergroup (i.e., the Pretoria and Postmasburg groups), the correlation of which is critical to understanding the first significant build-up in atmospheric oxygen, also known as the Great Oxidation Event. Recent dating of the Postmasburg Group necessitates a reinterpretation of Transvaal Supergroup stratigraphy involving a 200 million year downward revision of the Postmasburg Group relative to the Pretoria Group. The geology of the area intruded by the Molopo Farms Complex may provide important insights into this correlation model. Here we report a 207Pb/206Pb ID-TIMS baddeleyite date of 2054 ± 5 Ma from a gabbroic sample from the Molopo Farms Complex in South Africa, and an U-Pb zircon date of 2056 ± 10 Ma from a highly altered and metamorphosed quartzite in direct contact with ultramafic rocks of the complex in Botswana. We interpret these as crystallization ages of the Complex, which are within error of the 2056 to 2055 Ma age of the Bushveld Complex. Also reported from drill core intersections are U-Pb LA-ICP-MS detrital zircon age data from quartzite samples of both the floor and roof country rock. The roof rock detrital zircon age populations are comparable to those of the Paleoproterozoic Waterberg Group. Age populations in floor rocks are generally similar to those of the Pretoria Group, with the addition of ~2050 Ma populations, which likely reflect the metamorphic aureole of the complex on distinctly recrystallized country rock. A revised pre-Kalahari regional geology of the Molopo Farms Complex in South Africa, that incorporates the Pretoria Group in the area, implies an unconformable relationship with the Potsmasburg Group. Future recognition of such an unconformable relationship in drill core will ultimately resolve the problem of Transvaal strata correlation.

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