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Incoming links: Amphibole
Amphibolite 2026-10-25
Diorite 2026-10-25
A coarse-grained intrusive rock with intermediate composition, commonly containing plagioclase and amphibole or pyroxene.
Mafic mineral 2026-10-25
A useful idealization is a thin sedimentary cover over a felsic upper continental crust, an intermediate middle continental crust, and a more mafic lower continental crust, ending at the Mohorovičić discontinuity at about 35–40 km. The boundaries within the continental crust are approximate compositional divisions, not three universal sharp interfaces.
An idealized modern continental crustal column
. An original schematic for a 38 km thick continental crust. Depth ranges illustrate a compositional tendency; individual provinces have different lithologies and boundaries. Details.
- At the surface, a variable, commonly kilometre-scale sedimentary cover contains sediments and sedimentary rocks. Exposed crystalline basement can instead occur at the surface.
- In the upper roughly 15 km, representative rocks are granite, granodiorite and compositionally related gneiss, with quartz and feldspar. Sedimentary rocks, metamorphic rocks and volcanic rocks make this layer heterogeneous.
- At roughly 15–25 km, intermediate gneiss and amphibolite-facies assemblages become important. Diorite to granodiorite compositions and amphibole- and plagioclase-bearing rocks are representative.
- At roughly 25–38 km, mafic granulite and gabbro-derived metamorphic rocks are a useful average, with plagioclase, pyroxene and sometimes garnet. Some provinces instead have substantially intermediate or felsic lower continental crust.
- Below the Mohorovičić discontinuity is peridotite of the mantle, dominated by olivine and pyroxene. This is not a fourth crustal layer.
The important trend is increasing average mafic character with depth, rather than a change from granite directly to peridotite. Granulite describes a metamorphic facies, not a unique chemical composition. The synthesis in Nature and composition of the continental crust: A lower crustal perspective by Roberta L. Rudnick and David M. Fountain combines seismic and petrological evidence for these distinctions.
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.