Geochronological constraints on the magmatic and tectonic development of the Pongola Supergroup (Central Region), South Africa

Abstract

The Pongola Supergroup, located at the southeast margin of the Kaapvaal Craton, is a well-preserved Meso-Archean volcano-sedimentary sequence containing the full spectrum of basalt–andesite–dacite–rhyolite volcanic rocks, emplaced on 3.1–3.3Ga crystalline basement rocks. We have carried out a zircon U–Pb dating study using high resolution, secondary ionization mass spectrometry (SIMS) on volcanic layers at various levels in the lava flow successions, and on the post-Pongola granitoids emplaced in the immediate vicinity of the Pongola Supergroup, with the primary aim of determining volcanic eruption rates, Archean crustal dynamics, and the tectonic setting of the lavas and granitoids. The pre-Pongola Tsawela Gneiss on the northeastern side of the Pongola basin gives an age of 3428±22Ma, consistent with other determinations in this area. Crystallization ages of the granitoids and gneisses in the southwest appear to be generally younger at 3.1–3.2Ga, indicating the presence of contrasting crustal blocks on each side of the Pongola depositary. However, the greenstone fragments on the southwest side are 3.5–3.3Ga in age indicating a complex history. U–Pb zircon dates set the oldest Pongola volcanic layer in the Nsuze Group at 2980±10Ma and the uppermost layer contained within the mainly sedimentary Mozaan Group at 2954±9Ma. These dates provide the first direct estimate of the period of deposition for the Pongola sequence. Xenocrystic zircons in post-Pongola granitoid intrusions have inherited cores with ages similar to those of the Pongola volcanic rocks, and have younger emplacement ages between 2837±6Ma and 2717±11Ma. ɛNd values range from −2.55 for basalt to −4.20 for rhyolite in the Pongola volcanic suite, whereas post-Pongola granitoids have ɛNd values ranging from −2.08 to −6.14. This indicates that all rock types, including the basalts, have some contribution from crustal melts or aged enriched mantle lithospheric materials. Although trace element distributions for these rocks show characteristics similar to those of modern arcs, including negative anomalies in Th, Ta, Nb, Ce, and P, these may have been inherited from previously formed crust and may not be indicative of the Pongola tectonic environment. Evidence of rapid deposition, a preponderance of intermediate lavas, discordance of bounding crustal blocks and consistent structural trends in the area, are similar to features found in continental arc basins currently observed in the southwestern USA, and may present an alternative model to those currently accepted for Archean terranes in early-formed cratons.The age determinations of the post-Pongola granitoid intrusions indicate crustal recycling on a short time scale during the Archean. The post-Pongola granitoids are classic A-type granites with strongly elevated REE patterns, deep Eu/Eu* anomalies, high Fe/Mg ratios and highly elevated HFSE contents, and are possibly the oldest occurrence of this class of granitiod rocks representing the complete and final stage of development of the world's oldest stabilized Archean craton.