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Proposed Impact Trigger For Magmatism Of The 2.055 Ga Bushveld Complex, South Africa: Petrographic Evidence From The Distal Facies Of Basal Zones Of The Rooiberg Group, submitted to the annual meeting of the Geological Society of America

Elston, W.E.

The unique pseudovolcanic ~3.5 km Rooiberg Group records two catastrophes critical for interpretations of the ~1 million km3 Bushveld Complex: (1) Intense deformation and melting, bracketed by final pre-Bushveld sedimentation and coming-to-rest of initial Rooiberg ejecta (basal ~200 m of Dullstroom Fm), (2) later crustal melting and caldera-like collapse of the present Bushveld basins, recorded by megabreccia, tuff-breccia and ash-fall deposits of unprecedented scale (contact of Kwaggasnek and Schrikkloof Fms).

Rocks from the first event, previously termed volcanic or sedimentary, are reinterpreted as a new class of impactites in which heat dominated shock. Although emplaced as ignimbrite- or suevite-like inflated high-T flows and surges, textures and minerals are unlike any known volcanic or impactite association. For example, in a distal facies, preserved in three type-Dullstroom paleochannels radial to the eastern Bushveld basin, the principal components are: (1) LTi, previously termed low-TiO2 (<1%) basaltic andesite, composition average upper + lower crust with surficial contaminants, SiO2 54-64%, (2) reworked quartzose and argillaceous sediments, (3) quartzite and hornfels clasts.

Inflated LTi flows transported 10+ m quartzite blocks. After coming to rest, incipient amphibole-plagioclase laths and needles enlarged rapidly (≤1 mm) without flow alignment but with tendency for radial clusters (quench texture). At the lower contact, mm-size near-spherical granoblastic quartzite clasts (26 in one slide) accreted fine amphibole-plagioclase envelopes; in the interior, melted quartz crystallized in myrmekitic intergrowths with plagioclase. Associated surge and fall deposits, of mixed LTi and sedimentary provenance, metamorphosed in situ to biotite hornfels with andalusite, cordierite, and sillimanite.

Hot inflated debris flows of sand-size quartz with LTi matrix dominate the top of the section. They transported m-size brecciated quartzite blocks scoured off channel walls, cm-decimeter lithics, and fiamme-shaped autometamorphosed lenses with sulfide cores, epidote-carbonate and amphibole zones, and quartz coronas. Through progressive recrystallization and melting, these deposits grade upward into rhyolite-like flows in which high-tridymite crystallized from melt.