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The cratons (from the Greek "power" or "power") are the oldest parts of the continental crust on Earth, and are only preserved in a few places around the world. According to scientists, the Kaapvaal craton in South Africa and the Pilbara craton in Australia (the oldest of these structures) were parts of Vaalbara, an Archean supercontinent.
The transformation of the lower parts of the cratons under the influence of the heat emitted by the Earth's mantle can lead to the formation of rocks called granulites that frame the cratons as belts. However, processes that trace granulites from the lower crust to the surface along the craton boundaries are still largely debatable. The oldest granulite belts were formed in the Archean (3 years ago), which is only several hundred million years younger than life on Earth. The youngest granulites are about 0.5 billion years old. An old granulite belt (2.7 Ga) is located in the Kaapvaal craton on the borders of South Africa, Zimbabwe and Botswana, not far from the famous Limpopo River. The Limpopo Complex is considered a natural laboratory for studying the relationships between the oldest tectonic structures of the continental crust and is therefore of great interest to geologists
"For the first time, we have strong reasons to badume that the granitic magmas the neo-archaelite granulite complex of Limpopo (South Africa) formed during the tectonic interaction of this complex with the Kaapvaal craton rocks as the complex emerged from the lower part of the continental crust, "says Oleg Safonov, a co-author of the book, Doctor of Geology and Mineralogy, Professor of the Department of Petrology of the Faculty of Geology, MSU, and Director of the Department of Geology. Korzhinski Institute of Experimental Mineralogy of the Russian Academy of Sciences
Granulite is a metamorphic rock, which means that it is formed during the transformation of Other rocks under the influence of high temperatures. In the case of granulites, these temperatures are 750 to 1000 ° C. Feldspars, quartz, garnet, pyroxenes, cordierite and other minerals are formed at these temperatures giving the rock its granular texture.
According to one of the models, CO plays an important role in the formation of aggregates 2 Fluids rich in heated at too critical temperatures. The graphite present in the metamorphic rocks can help establish if this model is true. Usually, graphite is formed during the modification of organic matter or the decomposition of carbonates (salts of carbonic acid with CO anion 3 2- ) . However, granulites form at deep levels where no organic matter is present, so the mechanism of graphite formation is different – graphite is the result of the interaction of granulites with rich flows in CO 2 . Therefore, the presence of graphite in granulites is often considered as obvious for this model. Its formation depends on pressure, temperature and other parameters, and the study of graphite can say a lot.
Geologists found graphite samples and fluid inclusions in quartz (volatile components trapped in small mineral cavities during
The researchers found that granitic rocks penetrated the Limpopo granulite belt and began to crystallize at a temperature of 900 to 940 ° C and a pressure of 7 to 9 kbar.The badysis of fluid inclusions in quartz confirmed that fluids rich in CO 2 have The deviation of the C-13 isotope content from the standard values is 6.52 to 8.65 per cent (tenth of a percent) for graphite and 2.5 to 5.58 per This isotopic composition of carbon is usually prescribed to the deep liquid flows of the mantle, confirming their outer origin once again, which in turn coincides with the fluid model. CO 2 deep external bodies involved in the formation of granulitic rocks and accompanying granites. However, after comparing these data with the carbon isotopic composition of ancient craton rocks, the scientists concluded that fluids migrated through the Limpopo complex from cratonic rocks during the collision with Kaapvaal Craton [19659009]. rocks in the Limpopo granulite complex was of fundamental nature, the knowledge of the processes of their formation can be used for the prospecting of ore. "The rocks of the ancient cratons are rich sources of various components of the ore and are transported by magmas and fluids from the transformation of these rocks." Oleg Safonov
Data on the formation of the southern granulite complex African are also relevant to Russia. Scientists plan to compare their conditions with data on granulite formation in the Lapland belt located between the Karelia craton and the Inari craton on the border between Russia and Finland.
More information:
Oleg G. Safonov et al., Composition and source of fluids in high temperature graphite granitoids badociated with granulites: Examples of the southern southern zone, Limpopo Complex, South Africa South, Gondwana Research (2018). DOI: 10.1016 / j.gr.2018.04.009
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