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Early Proterozoic Magmatism and Geodynamics — Evidence of a Fundamental Change in the Earth’s Evolution
Published in O.A. Bogatikov, R.F. Fursenko, G.V. Lazareva, E.A. Miloradovskaya, A. Ya, R.E. Sorkina, Magmatism and Geodynamics Terrestrial Magmatism Throughout the Earth’s History, 2020
The Lebowa Granite Suite comprises the second main component of the Bushveld Complex. According to Gruenewaldt and Harmer (1992), two main granite types predominate; the Nebo Granite, a major unit of coarse-grained, hypersolvus, mildly alkaline granite; and a more evolved, sometimes aplitic variety, the Klikloof Granite. The Nebo granites exhibit a well-developed and fairly systematic chemical and mineralogical zonation, characterized (from base to top) by a decreasing modal plagioclase concomitant with increasing albite component in the plagioclase; decreasing hornblende and increasing biotite; and increasing quartz. These variations are also reflected in the geochemical trends, i.e. Si, K, Rb increase, and Re, Ti, Ca, P, Ba, Sr and Zr decrease upwards through the sheet. The entire granite mass was probably emplaced as an unusually fluid, very hot (perhaps >900°C), relatively anhydrous (initial content ~2.2%) restite-free magma. The granites exhibit all the classical features of mildly alkalic A-type magmas.
Granite suites: a problematic concept?
Published in Australian Journal of Earth Sciences, 2020
As seen above, the restite-unmixing model lies at the heart of the concept of granitic rock suites, at least in eastern Australia. As used locally, the term ‘restite’ encompasses all the crystalline solids that would coexist with a granitic partial melt, in its source, including unreacted or partially reacted grains of both major and accessory minerals from the original source rock as well as the peritectic crystals that were formed as products of the melting reactions. In the restite-unmixing model, restite is regarded as an integral part of the magma and therefore of the final rock. If we were to allow sampling to be guided by this model then restites, potentially even large, polycrystalline aggregates, should logically form part of a given bulk-rock analysis, since one ultimate aim of analytical work is to characterise the composition of the magma source terrane. Such a characterisation could be done, in a simple manner, if it were assumed that the restite-unmixing process had operated. If such an analytical strategy were followed, it could be considered as self-confirmatory bias in favour of a guiding model.