Geochronology of the deep profile through archean basement at Vredefort, with implications for early crustal evolution

The presence of Archean granite forming the core of an updomed and overturned sequence of strata at Vredefort, South Africa, has been known for over 70 years. Recent geophysical, geochemical, and geological evidence has given rise to the proposal that the basement granite core has also been overturned, presenting a section of the earth's granitic crust to view and that radial traverses in the Vredefort basement penetrate through a ∼15 km thick section of the Archean crust, when moving from the contact of collar strata to the center of the dome. The upper levels of the Vredefort crust are dominated by a relatively homogeneous outer granite gneiss (OGG) displaying marked gradients in large ion lithophile elements. These rocks grade inward into the deeper levels which occur in granulite facies metamorphism. The deeper levels of the Vredefort crust exposed in the central parts of the dome consist predominantly of felsic Inlandsee Leucogranofels (ILG) which occurs interleaved with inclusions of mafic metavolcanic and metasedimentary rocks termed Steynskraal Metamorphic Zone (SMZ). A comprehensive isotopic study using four decay schemes (87Rb, 232Th, 238U, and 235U) on this crustal section is now reported. The results permit unique and direct tests on the theories of Archean crust forming processes. In the upper Vredefort crust, whole rock isochron ages of ∼3050 m.y. are obtained from the four decay schemes, and almost all evidence of an extended crustal history prior to ∼3050 m.y. has been obliterated. Establishment of strong vertical gradients in Rb/Sr ratio, shortly before ‘setting’ of Rb-Sr whole rock systems, has wide implications for the interpretation of Sr initial ratios. Although the Vredefort upper levels of the crust has a low 87Sr/86Sr initial ratio of 0.7019 ± 0.0002, it probably participated in a crustal prehistory for at least 500 m.y. In the deeper levels of the Vredefort basement the geochronology is much more complex. Rb-Sr and Th-Pb isochrons of ∼3500 m.y. are recorded in the mafic granulite relicts. A companion paper (Welke and Nicolaysen, this issue) provides evidence for an early crust-forming event in this sector ∼3800 m.y. ago. From ∼3500 m.y. onward, these deeper crustal levels did not undergo addition of new Archean crust-forming material on a major scale. Nevertheless, a most important younger Archean event affected these levels ∼2800 m.y. ago: cessation of high-grade metamorphism is recorded by setting of the Rb-Sr isotopic system in the felsic rocks and zircon U-Pb ages; an episode of strong uranium depletion affected both the felsic and mafic rocks of this sector ∼2800 m.y. ago. The Inlandsee Leucogranofels is evidently a rock unit with a long and complex history, beginning with the emplacement as a crustal unit at least 3800 m.y. ago. Since then it has been subject to reworking and resetting of isotopic systems which terminated ∼2800 m.y. ago. In the deeper crust there is scant evidence of a 3050 m.y. event. Similarly, there is no record of a ∼2800 m.y. old imprint in the upper Vredefort crust. Thus, the last major events that affected the geochemical constitution of the Vredefort upper and deeper crustal sectors differed in time and nature; nevertheless, they brought about isotopic setting in a downward moving sequence, while the overlying crust remained cooler, rigid, and unaffected. These events probably reflect significant discontinuities in the cooling of the Archean crust.