Transcript Did PT begin in Early Archean time?

Did plate tectonic begin in Early Archean
times?
Geological Survey
of Western Australia
Hugh Smithies
Martin Van Kranendonk
Dave Champion
Paleoarchean
Neoproterozoic
> 3 billion years
Back then
Earth time scale
Now

Blue schists
Accretionary wedge deposits
Classic complete ophiolite sections
Horizontal tectonics

Major strike-slip movements
Arc-like geochemistry
arc, arc….
arc, arc,
backarc
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
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
2002: Nutman et al. Interpreted early thrust imbrication of greenstones
and gneisses at 3600-3650 Ma
Courtesy A. Nutman
Old (3.53-3.2 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
Whundo
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.
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
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)
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 steepstyle subduction was certainly active,
at least locally, by ~ 3.2 Ga