Magmatic processes

A quick review of the steps needed to make a magmatic rock…

I. Melting

  • Is always partial (5-50 %), there is therefore a solid residuum (95-50 %).
  • Melting of the mantle generates basalt or andsites; melting of the crust generates granites.
Melt needs to be extracted from the partially molten system.

II.  Magma evolution: fractional crystallization

As the system cools down, the magma evolves in the “solid+liquid” field, between solidus and liquidus.

  • Crystals are progressively formed, and can settle down at the bottom of the “magma chamber”.
  • The remaining magma chemically evolves. Its evolution is a “mirror” image of the crystals removed: typically towards SiO2-rich, mafic poor composition.
  • Studying the exact shape of the liquid evolution trends allows to discuss which minerals where removed from the liquid.
Three main trends of evolution:
  • Basalts evolve towards high-SiO2.
    • Undersaturated basalts remain so, and move in the undersaturated field, towards olivine- or nepheline-normative rocks (i.e., alkali series).
    • Saturated basalts remain so and evolve towards quartz-normative rocks (basalts-andesite-dacite-rhyolite, aka BADR series) (sub-alkaline series).
  • Granites are already SiO2 rich… and remain so (little or no evolution)

III. Magma transfert and emplacement

Magma moves upwards through the crust; mafic magmas can be seen to move through dyke swarms. The equivalent for granitic magma is not seen.

A.   Plutons and batholiths

If the magmas emplace at depth (typically, crustal magmas), they fill small pockets = plutons, with rocks slowly crystallizing at depth. Plutons are typically found as clusters of several to many intrusions (batholiths), corresponding to successive magma inputs, typically during a small time period.

B.   Volcanoes

If the magma reaches surface (more common for mantle-derived rocks), they erupt as volcanoes. The roots of the volcanoes are also dykes (but localized below the actual volcanic vent).

IV. Cooling and sub-solidus evolution

As the magma finally cools, it evolves from its liquidus at 900° (granites) to 1200° (basalts) down temperature towards cooler conditions: the solid magmatic rocks starts its life in the amphibolite facies (typically) and cools down to greenschist, therefore making (retrograde) metamorphic evolution possible.
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