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

• Metamorphic provinces such as the ~1 Ga Grenvillian, ~400 Ma Caledonide and Triassic Qinling Provinces oftencontain rocks with high-pressure assemblages such as eclogites, which formed at mantle depths in subductionzones. These are evidence of the accretion of terranes by subduction of oceans and collision to form large tec-tonostratigraphic provinces. The Mesoproterozoic Namaqua-Natal Province comprises a number of terranesthought to have been assembled by plate-tectonic processes, but they have generally yielded metamorphicpressures below 5 kbar, corresponding to<20 km, crustal depths, lacking evidence for subduction processes. TheKaaien Terrane in the Namaqua Front contains two large garbenschiefer units with the unusual paragenesisgarnet-hornblende-epidote-white mica-plagioclase-ilmenite-quartz. Their protoliths are graywackes influenced byandesitic volcanism during their deposition at ~1870 Ma, in a passive margin of the Rehoboth Province orKaapvaal Craton. Prograde garnet growth dated at 11655 Ma culminated in peak metamorphic conditions of64530C and 10.40.7 kbar, corresponding to 40 km depth. This is attributed to subduction of these rocksbefore 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 followedby rapid cooling, as the 11435 Ma argon dates of hornblende and white mica, with closure temperatures ~540C and ~440C respectively, are the same within error. This was probably due to tectonic juxtaposition of thegarbenschiefer 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 distinctlydifferent P-T-t paths, exemplified by greenschist metamorphism in the 1300 Ma Wilgenhoutsdrift Group, andmedium-pressure metamorphism in the Areachap Terrane. They were juxtaposed by late-tectonic uplift andtranspressional movements. The ~40 km depth of garbenschiefer peak metamorphism is the deepest yet found inthe Namaqua-Natal Province and strengthens the plate tectonic model of accretion by collision of terranes at theend of a Wilson cycle. The high pressure paragenesis of the garbenschiefer was preserved due to its location in theNamaqua Front, whereas most other parts of the Namaqua-Natal Province were overprinted by 1100–1020 Mathermal events after the collision events.

Ämnesord

NATURVETENSKAP  -- Geovetenskap och miljövetenskap (hsv//swe)

NATURAL SCIENCES  -- Earth and Related Environmental Sciences (hsv//eng)

NATURVETENSKAP  -- Geovetenskap och miljövetenskap -- Geologi (hsv//swe)

NATURAL SCIENCES  -- Earth and Related Environmental Sciences -- Geology (hsv//eng)

Nyckelord

Collision orogeny

P-T-t path

Garbenschiefer

Lu–Hf garnet

U–Pb zircon

40Ar/39Ar dating

Ar/Ar dating

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