Petalite postdating spodumene in pegmatite as a consequence of the ~2.02 Ga meteorite impact in the Vredefort structure, southern Africa

Rajesh, H.M.; Knoper, M.W.; Belyanin, G.A.; Safonov, O.G.; Schmidt, C.

Авторы: Rajesh H.M., Knoper M.W., Belyanin G.A., Safonov O.G., Schmidt C.
Журнал: Lithos
Том: 376
Год издания: 2020
Издательство: Elsevier BV
Местоположение издательства: Netherlands
Первая страница: 105760
DOI: 10.1016/j.lithos.2020.105760
Аннотация: Rare-element granitic pegmatites enriched in lithiumare economically important hosts for the lithium aluminosilicates,spodumene and petalite. Most spodumene and petalite crystallize from the pegmatite melt, depending on pressure and temperature. This paper reports the rare occurrence of petalite postdating the subsolidus stage inspodumene pegmatites within the ~2.02 Ga Vredefort impact structure, the largest, oldest and most deeply eroded meteorite impact structure currently known on Earth. The studied spodumene-bearing pegmatites postdatethe deformation fabric in the surrounding amphibolites of the granitoid-greenstone terrane. Impact-related veins of fragment-rich pseudotachylites cut across the pegmatites. Spodumene, albite and quartz constitute theprimary mineral assemblage of the pegmatites. Muscovite is late magmatic and contains inclusions of spodumene and albite. During the subsolidus stage, K-feldspar replaced spodumene and albite during pervasive metasomatic alteration. Samples collected closer to the pseudotachylites showreaction textures between spodumene and quartz without the involvement of muscovite. Raman spectroscopy and microprobe analyses show that the reaction textures consist of quartz and petalite. Unlike spodumene, petalite does not show a replacement by K-feldspar.The 40Ar/39Armuscovite age of 2027±8 Ma coincideswith the timing of the meteorite impact and documents its effect on the spodumene pegmatites. We suggest that the formation of petalite after spodumene wasrelated to heating, which was generated in the target rocks by the post-shock thermal event. Thus, the unique conditions associated with themeteorite impact can not only result in the formation of rare minerals in the targetrocks, but can also account for rare scenarios resulting in phases that would not otherwise form in nature.
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Petalite postdating spodumene in pegmatite as a consequence of the ~2.02 Ga meteorite impact in the Vredefort structure, southern Africaстатья

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