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Corundum Anorthosites-Kyshtymites from the South Urals, Russia: A Combined Mineralogical, Geochemical, and U-Pb Zircon Geochronological Study

1
Vernadsky Institute of Geochemistry and Analytical Chemistry Russian Academy of Sciences (GEOKHI RAS), Kosygin str. 19, 119991 Moscow, Russia
2
Institut für Geowissenschaften, Johannes Gutenberg Universität Mainz, J.-J.-Becher-Weg 21, 55128 Mainz, Germany
3
Bahrain Institute for Pearls & Gemstones (DANAT), WTC East Tower, P.O. Box 17236 Manama, Bahrain
4
Institute of Mineralogy SU FRC MiG UB RAS, 456317 Miass, Chelyabinsk Region, Russia
5
Ilmen State Reserve SU FRC MiG UB RAS, 456317 Miass, Chelyabinsk Region, Russia
6
Department of mineralogy and lithology, Institute of Geology and Petroleum Technologies, Kazan Federal University, 420008 Kazan, Russia
7
Institut für Mineralogie, Westfälische Wilhelms Universität Münster, Corrensstrasse 24, 48149 Münster, Germany
*
Author to whom correspondence should be addressed.
Minerals 2019, 9(4), 234; https://doi.org/10.3390/min9040234
Received: 25 February 2019 / Revised: 8 April 2019 / Accepted: 11 April 2019 / Published: 16 April 2019
(This article belongs to the Special Issue Mineralogy and Geochemistry of Gems)
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Abstract

Kyshtymites are the unique corundum-blue sapphire-bearing variety of anorthosites of debatable geological origin found in the Ilmenogorsky-Vishnevogorsky complex (IVC) in the South Urals, Russia. Their mineral association includes corundum-sapphire, plagioclase (An61–93), muscovite, clinochlore, and clinozoisite. Zircon, churchite-(Y), monazite-(Ce), and apatite group minerals are found as accessory phases. Besides, churchite-(Y) and zircon are also identified as syngenetic solid inclusions within the sapphires. In situ Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) U-Pb zircon geochronology showed the ages at about 290–330 Ma linked to the Hercynian orogeny in IVC. These ages are close to those of the syenitic and carbonatitic magmas of the IVC, pointing to their syngenetic origin, which is in agreement with the trace element geochemistry of the zircons demonstrating clear magmatic signature. However, the trace element composition of sapphires shows mostly metamorphic signature with metasomatic overprints in contrast to the geochemistry of zircons. The reason for this discrepancy can be the fact that the discrimination diagrams for sapphires are not as universal as assumed. Hence, they cannot provide an unambiguous determination of sapphire origin. If it is true and zircons can be used as traces of anorthosite genesis, then it can be suggested that kyshtymites are formed in a magmatic process at 440–420 Ma ago, most probably as plagioclase cumulates in a magma chamber. This cumulate rock was affected by a second magmatic event at 290–330 Ma as recorded in zircon and sapphire zoning. On the other hand, Ti-in-zircon thermometer indicates that processes operated at relatively lower temperature (<900 °C), which is not enough to re-melt the anorthosites. Hence, zircons in kyshtymites can be magmatic but inherited from another rock, which was re-worked during metamorphism. The most probable candidate for the anorthosite protolith is carbonatites assuming that metamorphic fluids could likely leave Al- and Si-rich residue, but removed Ca and CO2. Further, Si is consumed by the silicification of ultramafic host rocks. However, kyshtymites do not show clear evidence of pronounced metasomatic zonation and evidence for large volume changes due to metamorphic alteration of carbonatites. Thus, the obtained data still do not allow for univocal reconstruction of the kyshtymite origin and further investigations are required. View Full-Text
Keywords: blue sapphire; anorthosites; kyshtymites; sapphire geochemistry; Ilmenogorsky-Vishnevogorsky complex; in situ LA-ICP-MS U-Pb zircon dating blue sapphire; anorthosites; kyshtymites; sapphire geochemistry; Ilmenogorsky-Vishnevogorsky complex; in situ LA-ICP-MS U-Pb zircon dating
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Filina, M.I.; Sorokina, E.S.; Botcharnikov, R.; Karampelas, S.; Rassomakhin, M.A.; Kononkova, N.N.; Nikolaev, A.G.; Berndt, J.; Hofmeister, W. Corundum Anorthosites-Kyshtymites from the South Urals, Russia: A Combined Mineralogical, Geochemical, and U-Pb Zircon Geochronological Study. Minerals 2019, 9, 234.

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