Tholeiitic magma series

With the calc-alkaline series, however, the precipitation of magnetite causes the iron-magnesium ratio to remain relatively constant, so the magma moves in a straight line towards the alkali corner on the AFM diagram.

[3] The AFM plot distinguishes the intermediate members of the tholeiitic and calc-alkali magma series quite well.

[4] Like all basalt, the rock type is dominated by olivine, clinopyroxene and plagioclase, with minor iron-titanium oxides.

Tridymite or quartz may be present in the fine-grained groundmass of tholeiitic basalt, and feldspathoids are absent.

Throughout the process of igneous differentiation, the oceanic crust acts to reduce the magma, producing the tholeiitic trend.

Tholeiite is named for its type locality near the municipality of Tholey, Saarland, Germany.

AFM diagram showing the difference between tholeiitic and calc-alkaline magma series
AFM diagram showing the relative proportions of the oxides of alkalis (A), iron (F), and magnesium (M), with arrows showing the compositional change path of the magmas in the tholeiitic and the calc-alkaline magma series (BT=tholeiitic basalt, FB=ferro-basalt, ABT=tholeiitic basaltic andesite, AT=tholeiitic andesite, D=dacite, R=rhyolite, B=basalt, AB=basaltic andesite, A=andesite; dashed line=boundary between tholeiitic and calc-alkaline compositions)
Photomicrograph of thin section of tholeiitic basalt (in plane polarized light)
Photomicrograph of thin section of tholeiitic basalt (in cross polarized light)