Thetholeiitic magma series (/ˌθoʊleɪˈɪtɪk/) is one of two mainmagma series insubalkaline igneous rocks, the other being thecalc-alkaline series. A magma series is a chemically distinct range of magma compositions that describes the evolution of amafic magma into a more evolved, silica rich end member. Rock types of the tholeiitic magma series includetholeiitic basalt, ferro-basalt,tholeiitic basaltic andesite,tholeiitic andesite,dacite andrhyolite. The variety of basalt in the series was originally calledtholeiite but theInternational Union of Geological Sciences recommends thattholeiitic basalt be used in preference to that term.[1]
Tholeiitic rock types tend to be more enriched in iron and less enriched in magnesium and aluminium than calc-alkaline rock types. They are thought to form in a less oxidized environment than calc-alkaline rocks. Tholeiitic basalt is formed atmid-ocean ridges and makes up much of the oceanic crust. Almost all the basalt found on the Moon is tholeiitic basalt.
Rocks in the tholeiitic magma series are classified assubalkaline (they contain less sodium than some other basalts) and are distinguished from rocks in thecalc-alkaline magma series by theredox state of the magma they crystallized from (tholeiitic magmas are reduced; calc-alkaline magmas are oxidized[2]). When the parent magmas ofbasalts crystallize, they preferentially crystallize the more magnesium-rich and iron-poor forms of the silicate mineralsolivine andpyroxene, 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 oxidemagnetite, 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, aternary diagram showing the relative proportions of the oxides Na2O + K2O (A), FeO + Fe2O3 (F), and MgO (M). As magmas cool, they precipitate out significantly more iron and magnesium than alkali, causing the magmas to move towards the alkali corner as they cool. In the tholeiitic magma, magnesium-rich crystals are produced preferentially, the magnesium content of the magma plummets, causing the magma to move away from the magnesium corner until it runs low on magnesium and simply moves towards the alkali corner as it loses iron and any remaining magnesium. 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. However, the felsic end members of the two series are nearly indistinguishable, so granitic rocks are generally assigned to the calc-alkali magma series. The mafic end members may be distinguished by the aluminium content, with tholeiitic basalts containing 12% to 16% Al2O3 versus 16% to 20% Al2O3 for calc-alkali basalts.[4]
Like all basalt, the tholeiitic type is dominated byolivine,clinopyroxene andplagioclase, with minor iron-titanium oxides.[5]Orthopyroxene orpigeonite may also be present in tholeiitic basalt, and olivine, if present, may be rimmed by either of these calcium-poor pyroxenes.Tridymite orquartz may be present in the fine-grained groundmass of tholeiitic basalt, andfeldspathoids are absent. Tholeiitic rocks may have a fine, glassygroundmass, as may other types of basalt.
Tholeiitic rocks are the most commonigneous rocks inEarth's crust, produced bysubmarine volcanism atmid-ocean ridges and make up much of the ocean crust. Tholeiitic basaltic magmas are initially generated as partial melts ofperidotite (olivine andpyroxene) produced by decompression melting of theEarth's mantle. Tholeiitic basalt constituting the oceanic crust is termedMORB:mid-ocean-ridgebasalt. Throughout the process ofigneous differentiation, the oceanic crust acts to reduce the magma, producing the tholeiitic trend.[2] In contrast,alkali basalts are not typical of ocean ridges, but are erupted on some oceanic islands and on continents, as also is tholeiitic basalt.[5] Because theMoon is extremelyreduced, all of its basalts are tholeiitic.
Tholeiite is named for itstype locality near the municipality ofTholey,Saarland,Germany.[6]
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| Basalts by tectonic setting | |
| Basalts by form and flow | |
| Basalts by chemistry | |
| Important minerals | |
