The tholeiitic magma series
is one of two main magma series in igneous rocks
, the other magma series being the calc–alkaline
. A magma series is a series of compositions that describes the evolution of a mafic
magma, which is high in magnesium
and produces basalt
, as it fractionally crystallizes
to become a felsic
magma, which is low in magnesium and iron and produces rhyolite
. The International Union of Geological Sciences recommends that tholeiitic basalt
be used in preference to the term "tholeiite" (Le Maitre and others, 2002).
Rocks in the tholeiitic magma series are classified as subalkaline (they contain less sodium than some other basalts) and are distinguished from rocks in the calc-alkaline
magma series by the redox
state of the magma they crystallized from (tholeiitic magmas are reduced; calc-alkaline magmas are oxidized). When the parent magmas of basalts crystallize, they preferentially crystallize the more magnesium-rich and iron-poor forms of the silicate minerals olivine
, causing the iron content of tholeiitic magmas to increase as the melt is depleted of iron-poor crystals. However, a calc-alkaline magma is oxidized enough to precipitate significant amounts of the iron oxide magnetite
, causing the iron content of the magma to remain more steady as it cools than with a tholeiitic magma.
The difference between these two magma series can be seen on an AFM diagram, a ternary......