A Geochemical Classification for Granitic Rocks

Abstract

This geochemical classification of granitic rocks is based upon three variables. These are FeO / (FeO + MgO) = Fe-number [or FeOtot / (FeOtot + MgO) = Fe*], the modified alkali-lime index (MALI) (Na2O + K2O - CaO) and the aluminum saturation index (ASI) [Al/(Ca - 1.67P + Na + K)]. The Fe-number (or Fe*) distinguishes ferroan granitoids, which manifest strong iron enrichment, from magnesian granitoids, which do not. The ferroan and magnesian granitoids can further be classified into alkalic, alkali-calcic, calc-alkalic, and calcic on the basis of the MALI and subdivided on the basis of the ASI into peraluminous, metaluminous or peralkaline. Because alkalic rocks are not likely to be peraluminous and calcic and calc-alkalic rocks are not likely to be peralkaline, this classification leads to 16 possible groups of granitic rocks. In this classification most Cordilleran granitoids are magnesian and calc-alkalic or calcic; both metaluminous and peraluminous types are present. A-type granitoids are ferroan alkali-calcic; although some are ferroan alkalic. Most are metaluminous although some are peraluminous. Caledonian post-orogenic granites are predominantly magnesian alkali-calcic. Those with <70 wt % SiO2 are dominantly metaluminous, whereas more silica-rich varieties are commonly peraluminous. Peraluminous leucogranites may be either magnesian or ferroan and have a MALI that ranges from calcic to alkalic.