calc-alkaline

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calc-alkaline The name given to a suite of rocks comprising the volcanic association basalt-andesite-dacite-rhyolite, or the plutonic association gabbro-diorite-granodiorite-granite. The suite is defined chemically using a graph depicting the variation of CaO and (Na2O + K2O) with SiO2 (wt.%), and is classified as calc-alkaline if the SiO2 value at the intersection of the CaO and (Na2O + K2O) trends is between 56 and 61 SiO2 wt.%. (Less than 51 SiO2 wt. % and the rock suite is classed as belonging to the alkalic series, 51–56 SiO2 wt. % to the alkali-calcic series, and more than 61 SiO2 wt. % to the calcic series.) Calc-alkaline rocks are typically developed on the continental side of plate subduction zones, well-known examples occurring in the Andes of S. America and in Japan.