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

Abstract

Rare-element granitic pegmatites enriched in lithium are 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 in spodumene 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 postdate the 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 the primary 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 show reaction 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/39Ar muscovite age of 2027 ± 8 Ma coincides with the timing of the meteorite impact and documents its effect on the spodumene pegmatites. We suggest that the formation of petalite after spodumene was related to heating, which was generated in the target rocks by the post-shock thermal event. Thus, the unique conditions associated with the meteorite impact can not only result in the formation of rare minerals in the target rocks, but can also account for rare scenarios resulting in phases that would not otherwise form in nature.