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

Artikel i vetenskaplig tidskrift
Författare Valby van Schijndel
David H. Cornell
Robert Anczkiewicz
Anders Schersten
Publicerad i Geoscience Frontiers
ISSN 1674-9871
Publiceringsår 2019
Publicerad vid Institutionen för geovetenskaper
Språk en
Länkar https://doi.org/10.1016/j.gsf.2019....
Ämnesord Collision orogeny, P-T-t path, Garbenschiefer, Lu–Hf garnet, U–Pb zircon, 40Ar/39Ar dating
Ämneskategorier Geovetenskap och miljövetenskap

Sammanfattning

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 1165#1;5 Ma culminated in peak metamorphic conditions of645#1;30#14;C and 10.4#1;0.7 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 1143#1;5 Ma argon dates of hornblende and white mica, with closure temperatures ~540#14;C and ~440#14;C 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.

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