LA-ICP-MS dating of zircons from Meso- and Neoarchean granitoids of the Pietersburg block (South Africa): Crustal evolution at the northern margin of the Kaapvaal craton

Abstract

In order to unravel the late-Archean magmatic evolution of the northern Kaapvaal craton, we performed LA-ICP-MS U–Pb isotopic analyses on zircon grains from 15 granitoid samples of the Pietersburg block, including tonalitic and granodioritic gneisses (TTG) as well as weakly to not deformed high-K monzogranites, granodiorites and diorites.Our new age data, coupled to already published results, show that the Pietersburg block is schematically made up of two lithologically and chronologically distinct domains, namely (1) gneiss units that cover most of the surface of this terrane and comprising the formerly defined Goudplaats, Hout River, Groot Letaba gneisses as well as the so-called “Duiwelskloof batholith”, where crust formation and reworking took place between 3350 and 2840Ma; and (2) late-stage high-K plutons and batholiths that emplaced at ~2780Ma and ~2690Ma.In details, the tectono-magmatic evolution of the Pietersburg block can be divided into five successive episodes: (1) The first crust-forming event is related to the emplacement of juvenile TTG magmas in the range 3150–3350Ma. In our dataset, it is only represented by inherited zircon cores from younger granitoids; (2) Two TTG samples from the Goudplaats–Hout River gneiss unit and the Duiwelskloof area yielded emplacement ages of ~2950Ma. This magmatic event is very widespread in the whole Pietersburg block, and we propose that it represents an important period of crustal growth linked with the accretion of the Pietersburg block to the northern edge of the older nucleus (ca. 3600–3100Ma) of the Kaapvaal craton; (3) We obtained an intrusion age of ~2840Ma for two samples of high-K biotite-bearing granites that are intimately associated with, and probably formed by melting of, the ~2950Ma-old TTGs. Thus, this episode represents an important event of crust reworking that is likely associated with a long-lasting deformation event; (4) Further intracrustal melting led to the development of large batholiths of high-K biotite granites (e.g. Turfloop batholith) and their satellite intrusions, the latter being represented by one of our samples dated at ~2780Ma; (5) The intrusion of high-K calc-alkaline granitoids of possible mixed crust-mantle origin (Mashashane, Matlala, Moletsi and Matok plutons) took place at ~2690Ma. They are likely associated with collision between the Pietersburg block and the Central Zone of the Limpopo Belt, together with localized strain along shear zones (e.g. Hout River shear zone) and granulite-facies metamorphism that both occurred in the same time.