Barberton GSB Kaapvaal Craton M. J. deWit and colleagues 1982: Interpreted large recumbent folds with overturned stratigraphy, and early thrusts and glide planes 1983: Interpreted a zone of thrust-stacked
continental basement slices beneath the Komati Fm, and a major thrust separating the Komati Fm from underlying rocks 1987: Interpreted the Komati Fm as an ophiolite (Jamestown ophiolite complex)
Isua (Greenland): North Atlantic Craton 1974: Bridgwater et al. Proposed a large-scale horizontal tectonic regime with nappe-like folding accompanied by voluminous granite magmatism 1989: Nutman et al. Recognised that "homogeneous grey gneisses had variable geochronologic and isotopic histories, and
could be divided into distinct terranes separated by thin mylonite zones 1997: Nutman et al. Recognised that Isua was comprised at least two chronologically and lithologically different supracrustal belts, juxtaposed in the early Archaean
Courtesy A. Nutman (3.53-3.2 Ga) east Pilbara nucleus contains no r evidence for modern-style plate tectonics. This t tal block most likely began as some form of oceanic eau type crust. st Pilbara Superterrane ger (3.3 3.05 Ga) amalgamation of terranes cont
ive set of features that collectively present a ling case for modern steep-style subduction at ~3.2 epresents accretion peripheral to the East Pilbara ~20 km thick (10 km min) basalt dominated sequence. Uniformly upwards
Younging packages bound by unconformities. No clear evidence for tectonic boundaries NOT a collection of accreted terrains
Just a simple pile asalts show no evidence for a subduction enriched source o boninites elsic volcanics in lower part are ot TTG or calc-alkaline rocks
hey are fractionated tholeiites East Pilbara we can not unequivocally rule out a modern-style subduction involvement for the Pilbara Supergroup, but there is no evidence supporting it and there are alternative scenarios that
better explain the data. If subduction was involved here, it must have differed significantly from modern steep-style subduction. contrast with the East Pilbara ar, structurally bound packages hronolgically and isotopically distinct terrains wi
ependent stratigraphic histories exotic l Shear Zone accommodates 100s km of sinistral mov eved this configuration before 3.0 Ga Whundo 3.12 Ga Whundo Group essentially forms a ~10 km th onologically and lithologically exotic terrain comp c to felsic volcanic sequence with very juvenile is
ositions and with little physical, chemical or isot evidence for older felsic basement i.e. it was not deposited on continental crust. Calc-alkaline basalt and andesite ce element enrichments cannot be ounted for through contamination by
locally or regionally available crustal ponent. Boninites Tholeiitic basalt Adakite high-Nb basalt
high-Nb basalt Adakite Calc-alkaline basalt and andesite Boninites
Increasing melting Increasing slab component Flux melting differs from normal melting of the mantle (decompression melting) and is confined to subduction zones because it needs an external volatile input
Ba/ La Enriched in melt Ba/La Enriched in fluid
Ba (ppm) tive stratigraphic height (up ) Siliceous high-Mg basalts Distinctive LREE enrichments that cant be accounted for via contamination of any locally or regionally available crust.
Unusually consistent isotopic and trace element ratios over a very wide region. Derived from a mantle source metasomatized by a homogeneous West Pilbara Superterrane THE COMPLETE PACKAGE We have several lines of evidence that combine to present a
compelling case that modern-style subduction occurred at least by 3.2 Ga. an overall linear architecture with distinct terrains and boundaries that include strike-slip shears that account for 100s km movement Exotic terrains. isotopically juvenile crust a 3.12 Ga volcanic sequence free of any exotic continental material
Whundo calc-alkaline basalts which require an enriched mantle source Whundo calc-alkaline basalts which reflect flux-melting Whundo boninites Whundo adakite/NEB association Whundo volcanic/geochemical association/architecture Later basalts independently derived from a modified mantle source, flanking the East Pilbara
Did plate tectonic begin in Early Archean times? Thats not clear but modern steep-style subduction was certainly active, at least
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