The significance of U-Pb zircon dates in lower crustal xenoliths from the southwestern margin of the Kaapvaal craton, southern Africa

The combination of high precision ID-TIMS U-Pb geochronology with cathodoluminescence (CL) imaging and spatially resolved microsampling of zircon provides a high fidelity record of the temporal evolution of samples of the middle and lower crust. A suite of xenoliths exhumed in Cretaceous kimberlites at the southwestern margin of the Kaapvaal craton, including eclogite, granulite, amphibolite, and greenschist-facies plutonic rocks, are the subject of these detailed zircon studies. CL imaging demonstrates the complexity of chemical and growth structure in single zircons, and highlights the importance of sampling on an intragrain scale to extract end-member ages for zircon growth events. For these diverse metamorphic rocks, linking the zircon chronology to geologic events and processes requires an understanding of the kinetics and thermodynamics of zircon stability during metamorphism. Using an empirical approach, we attempt to relate zircon crystallization, dissolution, and regrowth to metamorphic conditions through a detailed analysis of zircon chemical zonation patterns, mineral inclusions, and petrography. These lines of evidence indicate the crystallization of zircon under P-T conditions ranging from eclogite through greenschist facies. We discuss four examples of this approach: (1) At the lowest grade of metamorphism examined, primary igneous zircon (1900 Ma) in a syenite xenolith from the Klipfontein-08 kimberlite underwent two episodes (1015 and 964 Ma) of probable fluid-mediated resorption and reprecipitation in the greenschist facies. (2) An upper amphibolite facies xenolith from the Markt kimberlite contains two distinct populations of blocky and fir-tree sector-zoned zircon which are suggested to date prograde hornblende breakdown reactions (∼ 1107 Ma) and melt infiltration under retrograde metamorphic conditions (∼ 1092 Ma), respectively. (3) At granulite-facies, metamorphic zircon crystallized at ∼ 1114 Ma during prograde reactions involving hornblende breakdown in a granulite xenolith also from the Markt kimberlite. Later (∼ 960 Ma) zircon resorption followed by minor secondary crystallization occurred in both the amphibolite and granulite xenoliths during retrograde greenschist facies reactions, possibly enhanced by fluids. (4) In a much older episode of eclogite facies metamorphism recorded in a xenolith from the Lovedale kimberlite, zircon crystallized with the peak metamorphic mineral assemblage, or possibly early on the cooling path, in the Late Archean (∼ 2630 Ma). Each of these examples provides unique information regarding the evolution of the Kaapvaal craton, while the links to metamorphic reactions and P-T conditions clarify the interpretation of zircon dates obtained from these samples of the middle to lower crust.