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.