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Incoming links: Mafic mineral
Today the usual first melt is basaltic melt from the mantle wedge; in a hotter Archean subduction zone, melting of hydrated basaltic crust itself could generate sodic felsic melt of tonalite-trondhjemite-granodiorite composition. Distinguish a primary melt from the evolved melt ultimately added to the upper continental crust.
On the modern Phanerozoic Earth:
- Hydrothermal alteration introduces water into the incoming oceanic crust and locally into serpentinized mantle. Sediments also carry water in pore space and hydrous minerals.
- As the slab descends, heating and metamorphism break down hydrous minerals. Released fluids enter the hotter peridotite of the mantle wedge and lower its solidus, causing flux melting. Most ordinary modern slabs release fluids before extensive melting of their basaltic crust.
- The primary melt is predominantly hydrous basaltic magma. During ascent, storage and fractional crystallization, mafic minerals are removed and the remaining liquid evolves toward andesite, dacite and rhyolite compositions. Partial melting of recently emplaced mafic lower continental crust, magma mixing and some crustal contamination can contribute to felsic magma. Intrusive equivalents include diorite, tonalite and granodiorite.
This adds juvenile material derived from the mantle; simply remelting old continental crust is reworking, not net juvenile addition. The amount and depth of slab water release depend on its thermal history, as quantified in Subduction factory: 4. Depth-dependent flux of H2O from subducting slabs worldwide by Peter E. van Keken and others.
For the hotter Archean endmember posited in the question, a hotter mantle and a warm descending plate make it easier for hydrated basaltic crust to reach its solidus. Its partial melting can directly make silica-rich, sodium-rich tonalite-trondhjemite-granodiorite liquids, leaving a more mafic residue. Residual garnet and amphibole, and the presence or absence of residual plagioclase, help determine the rare-earth element and strontium signatures. These are not ultramafic liquids merely because the surrounding mantle was hotter.
The contrast is therefore commonly fluid-fluxed mantle melting followed by differentiation today, versus a greater potential contribution from direct melting of hydrated mafic crust in the hotter case. Tonalite-trondhjemite-granodiorite is not unique evidence for subduction: melting of thickened mafic crust can produce similar liquids. Nor are all modern arcs devoid of slab melting. These qualifications are central to The composite Archaean grey gneisses: Petrological significance, and evidence for a non-unique tectonic setting for Archaean crustal growth by Jean-François Moyen.