Did plate tectonic begin in Early Archean times? Hugh Smithies Martin Van Kranendonk Dave Champion Geological Survey of Western Australia

Back then Now > 3 billion years PaleoarcheanNeoproterozoic Earth time scale

Blue schists Accretionary wedge deposits Classic complete ophiolite sections  Horizontal tectonics Major strike-slip movements Arc-like geochemistry

Uniformitarianism arc, arc…. arc, arc, backarc Archaean Branch tectonic fence

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) 1990: Age dating of older over younger rock packages

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 dividedinto 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 2002: Nutman et al. Interpreted early thrust imbrication of greenstones and gneisses at Ma Courtesy A. Nutman

Old ( Ga) east Pilbara nucleus – contains no clear evidence for modern-style plate tectonics. This thick crustal block most likely began as some form of oceanic plateau type crust.

The West Pilbara Superterrane A younger (3.3 – 3.05 Ga) amalgamation of terranes – contains extensive set of features that collectively present a compelling case for modern steep-style subduction at ~3.2 Ga. This represents accretion peripheral to the East Pilbara nucleus.

~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 of autochthonous groups.

Basalts show no evidence for a subduction enriched source No boninites Felsic volcanics in lower part are not TTG or calc-alkaline rocks – they 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.

Vivid contrast with the East Pilbara Linear, structurally bound packages Geochronolgically and isotopically distinct terrains with independent stratigraphic histories – exotic Sholl Shear Zone accommodates 100’s km of sinistral movement Achieved this configuration before 3.0 Ga

The 3.12 Ga Whundo Group essentially forms a ~10 km thick geochronologically and lithologically exotic terrain comprising a mafic to felsic volcanic sequence with very juvenile isotopic compositions and with little physical, chemical or isotopic evidence for older felsic basement i.e. it was not deposited on continental crust. Whundo

Calc-alkaline basalt and andesite Trace element enrichments cannot be accounted for through contamination by any locally or regionally available crustal component.

Boninites

Tholeiitic basalt

Adakite high-Nb basalt

Adakite Calc-alkaline basalt and andesite Boninites

Increasing melting Flux melting – differs from normal melting of the mantle (decompression melting) and is confined to subduction zones because it needs an external volatile input Increasing slab component

Ba/La Enriched in fluid Enriched in melt Ba/La Ba (ppm) Relative stratigraphic height (up )

Siliceous high-Mg basalts Distinctive LREE enrichments that can’t 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 mix of ‘old’ Pilbara crust and Whundo crust

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 100’s 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? That’s not clear – but modern steep- style subduction was certainly active, at least locally, by ~ 3.2 Ga