Evidence for Mesoproterozoic collision, deep burial and rapid exhumation of garbenschiefer in the Namaqua Front, South Africa

Abstract

Metamorphic provinces such as the ∼1 ​Ga Grenvillian, ∼400 ​Ma Caledonide and Triassic Qinling Provinces often contain rocks with high-pressure assemblages such as eclogites, which formed at mantle depths in subduction zones. These are evidence of the accretion of terranes by subduction of oceans and collision to form large tectonostratigraphic provinces. The Mesoproterozoic Namaqua-Natal Province comprises a number of terranes thought to have been assembled by plate-tectonic processes, but they have generally yielded metamorphic pressures below 5 ​kbar, corresponding to <20 ​km, crustal depths, lacking evidence for subduction processes. The Kaaien Terrane in the Namaqua Front contains two large garbenschiefer units with the unusual paragenesis garnet-hornblende-epidote-white mica-plagioclase-ilmenite-quartz. Their protoliths are graywackes influenced by andesitic volcanism during their deposition at ∼1870 ​Ma, in a passive margin of the Rehoboth Province or Kaapvaal Craton. Prograde garnet growth dated at 1165 ​± ​5 ​Ma culminated in peak metamorphic conditions of 645 ​± ​30 ​°C and 10.4 ​± ​0.7 ​kbar, corresponding to 40 ​km depth. This is attributed to subduction of these rocks before collision between the overriding arc-related Areachap Terrane, the Kaaien Terrane and the Kaapvaal-Rehoboth cratonic block during the Namaqua orogeny. Exhumation of the garbenschiefer slabs was followed by rapid cooling, as the 1143 ​± ​5 ​Ma argon dates of hornblende and white mica, with closure temperatures ∼540 ​°C and ∼440 ​°C respectively, are the same within error. This was probably due to tectonic juxtaposition of the garbenschiefer slab with much cooler rock units. The exhumation was accommodated along the Trooilapspan-Brakbosch Shear Zone due to ongoing transpression. Other components of the Namaqua Front have distinctly different P-T-t paths, exemplified by greenschist metamorphism in the 1300 ​Ma Wilgenhoutsdrift Group, and medium-pressure metamorphism in the Areachap Terrane. They were juxtaposed by late-tectonic uplift and transpressional movements. The ∼40 ​km depth of garbenschiefer peak metamorphism is the deepest yet found in the Namaqua-Natal Province and strengthens the plate tectonic model of accretion by collision of terranes at the end of a Wilson cycle. The high pressure paragenesis of the garbenschiefer was preserved due to its location in the Namaqua Front, whereas most other parts of the Namaqua-Natal Province were overprinted by 1100–1020 ​Ma thermal events after the collision events.