IMSG Post-conference Field Guide

Transcription

1 IMSG Post-conference Field Guide Jérémie Lehmann, Marlina Elburg and Trishya Owen-Smith The purpose of this short field excursion on Wednesday 18 January is to show a variety of rocks that make up the Johannesburg Dome. The Johannesburg Dome is made of Archean greenstones and granitoids which are rimmed by younger, outward-dipping supracrustal rocks of the West Rand Group (Witwatersrand Supergroup) strata to the south, and of the Transvaal Supergroup rocks to the north, east and west. The northern contact of the Dome is also marked by discontinuous units of the Ventersdorp Supergroup that are intercalated between the Transvaal rocks and the Johannesburg Dome basement rocks (Anhaeusser, 1973). Stops 1a and 1b are located at Glenburn Lodge itself, and Stop 2 is the famous Nooitgedacht platform, situated ~7 km east of the lodge, on the way to Lanseria Airport. Figure 1: Geological map of the Kaapvaal Craton (modified from Eglington and Armstrong, 2004). Thick dashed line delineates the geophysical boundary of the Kaapvaal Craton, inferred from the aeromagnetic map of the Southern African Development Community.

2 Figure 2: Regional map of the Johannesburg Dome, modified from Ormond (2016) after Alexandre et al. (2006). Figure 3: Simplified geological map of the Johannesburg Dome (from Poujol and Anhaeusser, 2001). The field trip stops are shown.

3 Figure 4: Anhaeusser s seminal geological map of the Johannesburg Dome (Anhaeusser, 1973). The field trip stops are shown.

4 Stop 1 - Archaean basement rocks of the Zwartkops Hills Stop 1a - at the river weir below the lapa. GPS coordinates: S, E Stop 1b - ~ 150 m further north along the Blaauwbank river path. GPS coordinates: S, E At these two localities, the main rock type is a (meta)diorite that shows amphibolite facies gneissic banding and local migmatisation features. Post-tectonic felsic pegmatites can be also observed along the river in between these two outcrops. Figure 5: Modified from Ormond (2016). (A) Cross-section through the east-facing cliff in the southern-most section of the Zwartkops Hills, showing the rock types comprising the basement and cover sequences. Nature of contacts between the different rock types is indicated. (B) Annotated photograph of the east-facing cliff. The box denotes the area represented in the cross-section. Five quartzite lenses are exposed in the cliff-face and are indicated on the photograph.

5 Stop 2 - Nooitgedacht Platform GPS coordinates: S, E This exceptional river platform has been investigated by Anhaeusser (1999). Crosscutting relationships between different granite-greenstone components allowed the author to infer the following succession of events: (1) mafic and ultramafic volcanic rocks and their plutonic equivalents are intruded by (2) a suite of TTG dated at 3340 Ma (Poujol and Anhaeusser, 2001). (3) Two mafic dykes cross-cut the trondhjemitic gneisses, and are in turn (4) cut by granitoids dated at 3121 Ma and pegmatites dated at ca Ma (Poujol and Anhaeusser, 2001). Anhaeusser (1999) presented REE data that suggest that the amphibolites are similar to modern day island arc volcanic rocks. In addition, trace element data suggest a genetic link between the TTG and the mafic/ultramafic rocks. Metamorphism, metasomatism and assimilation are inferred to have produced the mixed appearance of the rock assemblages that make up the platform. Overall, the geochemical data suggest that the Archaean rocks at Nooitgedacht were formed in tectonic settings similar to those of modern-day, volcanic arc environments. Figure 6. Chondrite-normalized rare earth element plots for rocks of the Nooitgedacht platform (Anhaeusser, 1999). (A) Amphibolite (metakomatiite, high Mg basalt/metatholeiite; (B) Mafic dykes; (C) and (D) Gneissic and homogeneous trondhjemites; (E) Dioritic and tonalitic gneisses; (F) Leucogranodiorite.

6 Figure 7: Geological map of the Nooitgedacht migmatite-gneiss platform (modified from Anhaeusser, 1999).

7 Figure 8: Field photograph in Anhaeusser (2011). Nooitgedacht migmatite platform displaying and summarising stages of development of the Archaean crust in the northern half of the Johannesburg Dome. The oldest rock is an amphibolite xenolith (top centre), which is intruded, in turn, by ca Ma foliated, grey, leuco-trondhjemitic gneiss (the dominant rock type), the latter displaying in situ mobilised leuco-veins of quartz and feldspar. The gneiss and the amphibolite is intruded by a lamprophyre dyke (beneath the hammer) and the entire sequence is crosscut by ca Ma homogeneous granodiorite/adamellite (above hammer) and ca Ma pegmatite (bottom left).

8 Figure 9: Field photograph of the Nooitgedacht platform from Anhaeusser (2011). Altered (feldspathised) amphibolite (beneath hammer) intruded by contaminated (assimilated and hybridised) granitoid rocks with compositions ranging from hornblende tonalite to diorite (right of hammer). Later crosscutting veins and dykes comprise K-feldspar-rich granodiorite. The tonalite-diorite component (which may have compositions similar to sanukitoid rocks) is not present in large volumes in the exposure, and is restricted to the contact zones adjacent to the amphibolites. Details of the exposure were provided by Anhaeusser (1992) who showed that the TTG components were produced by trondhjemitic magma interacting with amphibolite xenoliths resulting in very local, tonalitic to dioritic hybridised derivatives. References Alexandre, P., Andreoli, M.A.G., Jamison, A., Gibson, R.L. (2006). 40 Ar/ 39 Ar age constraints on lowgrade metamorphism and cleavage development in the Transvaal Supergroup (central Kaapvaal Craton, South Africa): implications for the tectonic setting of the Bushveld Igneous Complex. South African Journal of Geology, 109, Anhaeusser, C.R. (1973). The geology and geochemistry of the Archaean granites and gneisses of the Johannesburg-Pretoria Dome. In: L.A. Lister (Editor), Symposium on Granites, Gneisses and Related Rocks. Special Publication of the Geological Society of South Africa, 3, Anhaeusser, C.R. (1992). Archaean granite-greenstone relationships on the farm Zandspruit 191 IQ, North Riding area, Johannesburg Dome. South African Journal of Geology, 94, Anhaeusser, C.R. (1999). Archaean crustal evolution of the central Kaapvaal Craton, South Africa: Evidence from the Johannesburg Dome. South African Journal of Geology, 102, Eglington, B.M. and Armstrong, R.A. (2004). The Kaapvaal Craton and adjacent orogens, southern Africa: a geochronological database and overview of the geological development of the craton. South African Journal of Geology, 107, Ormond, R. (2016). Structural Geology and Geochronology of the Zwartkops Hills, Honours thesis, 44 pp, University of Johannesburg. Poujol, M. and Anhaeusser, C.R. (2001). The Johannesburg Dome, South Africa: New single zircon U- Pb isotopic evidence for early Archaean granite-greenstone development within the central Kaapvaal Craton. Precambrian Research, 108,