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Thecalc-alkaline magma series is one of two main subdivisions of thesubalkalinemagma series, the other subalkaline magma series being thetholeiitic series. A magma series is a series of compositions that describes the evolution of amafic magma, which is high inmagnesium andiron and producesbasalt orgabbro, as itfractionally crystallizes to become afelsic magma, which is low in magnesium and iron and producesrhyolite orgranite. Calc-alkalinerocks are rich inalkaline earths (magnesia andcalcium oxide) andalkali metals and make up a major part of thecrust of the continents.
The diverse rock types in the calc-alkaline series include volcanic types such asbasalt,andesite,dacite,rhyolite, and also their coarser-grained intrusive equivalents (gabbro,diorite,granodiorite, andgranite). They do not includesilica-undersaturated, alkalic, orperalkaline rocks.
Rocks from the calc-alkaline magma series are distinguished from rocks from thetholeiitic magma series by theredox state of the magma they crystallized from. Tholeiitic magmas are reduced, and calc-alkaline magmas are oxidized, with higheroxygen fugacities. Whenmafic (basalt-producing) magmas 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. (Magnesium-rich olivine solidifies at much higher temperatures than iron-rich olivine.) However, a calc-alkaline magma is oxidized enough to (simultaneously) 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 of 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. In tholeiitic magma, as it cools and preferentially produces magnesium-rich crystals, the magnesium content of the magma plummets, causing the magma to move away from the magnesium corner until it runs low on magnesium and begins to move towards the alkali corner as it loses iron and 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.
Calc-alkaline magmas are typicallyhydrous.
Calc-alkaline rocks typically are found in thevolcanic arcs abovesubduction zones, commonly inisland arcs and particularly incontinental arcs.
Rocks in the series are thought to be genetically related byfractional crystallization and to be at least partly derived from magmas of basalt composition formed in theEarth's mantle. Trends in composition can be explained by a variety of processes. Many explanations focus on water content andoxidation states of themagmas.
Proposed mechanisms of formation begin withpartial melting of subducted material and of mantleperidotite (olivine and pyroxene) altered bywater and melts derived from subducted material. Mechanisms by which the calc-alkaline magmas then evolve may include fractional crystallization, assimilation ofcontinental crust, and mixing with partial melts of continental crust.
{{cite book}}: CS1 maint: postscript (link)Two of these widely used diagrams are the total alkalies-silica (TAS) diagram and the (Na2O + K2O)-FeO*-MgO (AFM) triangular diagram, neither of which has calcium as one of the plotting parameters... Not all orogenic andesites are calc-alkaline, and not all calc-alkaline andesites are orogenic
The terms calcalkaline and calcalkalic are currently defined and used in multiple and non-equivalent ways. Generally, the variation of total Fe as FeO/MgO within evolving subalkaline rock suites is regarded as the most distinctive character of calcalkaline compared with tholeiitic suites, reflecting the relative timing of phase saturation with Fe–Ti oxides, plagioclase, and ferromagnesian silicates. Other classification schemes are widely used, including a minority of adherents to the original formalized definition of 'calcalkalic' by Peacock (1931, Journal of Geology 39, 54–67). Given the prevailing contradictory and confusing usage of these terms, which leads to miscommunication, it is proposed that the spectrum of subalkaline rocks be divided into high-, medium-, and low-Fe suites, complementing divisions made on the basis of K contents. The terms calcalkaline and calcalkalic should be restricted to rock suites that conform to Peacock's definition.
Types ofbasalts | |
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| Basalts by tectonic setting | |
| Basalts by form and flow | |
| Basalts by chemistry | |
| Important minerals | |
