Saturday, April 23, 2016

Evidence From South Africa Supporting PaleoProterozoic Continent Vaalbara

Source and age of upper Transvaal Supergroup, South Africa: Age-Hf isotope record of zircons in Magaliesberg quartzite and Dullstroom lava, and implications for Paleoproterozoic (2.5–2.0 Ga) continent reconstruction


Zeh et al


The Magaliesberg Formation quartzite of the upper Transvaal Supergroup in north-central South Africa represents the biggest Paleoproterozoic arenite deposit on Earth, with a present-day lateral extent of more than 600 km. It therefore has a great potential to provide detailed information about Archean to early Paleoproterozoic crust–mantle evolution in the hinterland of the Kaapvaal Craton and for global plate tectonic reconstruction. Detrital zircons in Magaliesberg Formation quartzites provide evidence for magmatic activities in the source region at 2080–2130 Ma (11%), 2220 Ma (35%), 2350–2450 Ma (8%) and, following an age gap of ca. 200 million year, at 2650–2780 Ma (11%), 2820–2910 Ma (25%), and at 3060–3100 Ma (6%). Minor zircon formation took place during Bushveld Complex contact metamorphism at 2055 Ma, and less than 3% underwent a post-Bushveld age reset. Detrital zircons in a sediment-rich Dullstroom lava yield similar ages between 2120 ± 12 Ma and 2865.2 ± 1.7 Ma, indicating that Magaliesberg and post-Magaliesberg/Dullstroom sediments were derived from the same sources, and deposited between 2080 and 2055 Ma. Distinct age spectra among the investigated quartzites indicate that the sandstone detritus was derived from different sources and rapidly deposited without complete homogenization, in a regressive shore line, braid-delta environment.

Combined Hf isotope-age data reveal two distinct groups of zircon. Zircons of group I have Archean ages (greater than 2.65 Ga), and define a crustal array that points to protracted crust reworking in the hinterland between 3.10 and 2.65 Ga. Likely sources for these zircons are granitoids of the Pietersburg Block, which is located at the northern edge of the Transvaal basin. Zircons of group II have Paleoproterozoic ages between 2.45 and 2.08 Ga. They are mostly subchondritic, and show highly variable εHft between +4 and −19. These large variations hint to their formation in a magmatic arc and/or collisional setting. As granitoids with ages between 2.65 and 2.06 Ga are absent on the Kaapvaal Craton, detrital zircons of such ages must have been derived from sources outside of the present-day craton. One potential source could be the Ophthalmia orogenic belt, which became amalgamated to the joined Kaapvaal–Pilbara and perhaps Zimbabwe cratons (Zim)vaalbara at 2.22–2.14 Ga, whereas coeval magmatic rocks in southern America (e.g., Minero Belt of the Sao Francisco Craton) or West Africa can be excluded, as they were mostly derived from depleted mantle sources in oceanic environments. Abundant occurrence of detrital zircons with ages of 2.5 Ga in the lower Pretoria Group (Duitschland Formation), but their complete absence in the upper Pretoria Group (Magaliesberg Formation), hint that (Zim)vaalbara was attached to an early Paleoproterozoic orogenic belt, which disappeared completely between 2.3 and 2.08 Ga, most likely by cratonic break up.

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