Areas of very low heat flow/low geothermal gradient (forearc and
subducting slab). Today's colder subduction zones can be less than 10C/km
and some Mesozoic rocks record geothermal gradients as low as 5-6C/km.
Archean rocks which have been suggested equivalents:
- sanukitoid: a plutonic
rock simlar to adakite in major and trace element composition. Sanukitiod-
suite are found in many Archean greenstone belts, often post-deformational.
- According to Smithies (2004), study of
Archean rocks back to about 3.3 (Pilbara) to 3.7-3.8 (Isua) and find
2 groups of boninite-like Archean volcanic rocks. The first type (earliest is 3.1) is enriched
in Al and HREE relative to modern boninite but are consistent with low pressure
melting of a previously depleted then re-enriched mantle source - similar
conditions to the mantle wedge under a modern subduction zone. The differences
to modern boninites can be explained by a hotter mantle (deeper melting =
more garnet in the melt source).
The second type is commonly found with komatiites and carries signatures of
EVEN MORE garnet in the source (even deeper melting?). Some workers suggested
subduction-plume interaction (a geochemist obviously). Smithies et al. (2004)
suggest that a refractory mantle source which was subduction modified in the
past might be melted by a plume to produce these rocks.
Turner et al. (2014) considered the question of whether Archean volcanic
assemblages (not individual rocks) resemble the volcanic assemblages of
modern day arcs. Naturally the composition of arc volcanics evolves after
the initiation of subduction as the mantle source cools and hydrates. So,
they picked Izu-Bonin where subduction has just begun:
references
Bedard, Harris and Thurston (2013) The hunting of the snArc. Precambrian
Research v. 229 20-48.
de Wit (1998) On Archean granites, greenstones, cratons and tectonics: does
the evidence demand a verdict? Precambrian Research v. 91 181-226.
Herzberg (2013) Archaean drips. Nature Geoscience v. 7, 7-8.
Moyen, Stevens, and Kisters (2006) Record of mid-Archaean subduction from
metamorphism in the Barberton terrain, South Africa. Nature v. 442
559-562.
Smithies, Champion and Sun (2004) The case for Archaean boninites.
Contributions to Mineralogy and Petrology v. 147: 705-721
Smithies, Van Kranendonk and Champion (2007) The Mesoarchean emergence
of modern-style subduction. Gondwana Research v. 11 50-68.