Abstract
The "calc-alkaline" series of rocks was originally defined in the early 1930s on the basis of the "alkali-lime index" on a combined SiO2 versus (Na2O + K2O) and SiO2 versus CaO plot. The usage of the term has evolved considerably since, and today it is used variably for the subalkalic basaltandesite-dacite-rhyolite suite, or any rock suite containing andesite, or island-arc rocks, or rocks with high ratios of large-ion-lithophile elements (LILE) to high-field-strength elements (HFSE), or simply rocks with negative HFSE anomalies (e.g., Nb-Ta) in primitive mantle-normalized multielement diagrams. Although such variable usage is normal in science, the use of certain geochemical variation diagrams to define and depict the calc-alkaline series is not strictly appropriate. 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. The TAS diagram can be used to depict "alkaline," "subalkaline," and probably "transitional" rocks, but not "calc-alkaline" or "high-alumina" rocks. Care must be taken while using such diagrams for rock classification, inasmuch as some of them are inherently unsuitable and several together may classify even a single rock into widely different associations. "Association" or "suite" are probably better terms than "series," because they imply neither comagmatic relationships nor linear trends. The calc-alkaline suite of rocks is abundant along destructive plate margins, but calc-alkaline geochemistry is not a 100% foolproof indicator of subduction processes, inasmuch as calc-alkaline rocks are also known from regions undergoing extension, such as the Basin and Range province and the Gulf of California. Therefore, caution must be exercised in interpreting ancient terrains with complicated geology and calc-alkaline rocks as former subduction zones. Not all orogenic andesites are calc-alkaline, and not all calc-alkaline andesites are orogenic.
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