A geochemical classification for granitic rocks

This geochemical classification of granitic rocks is based upon three varia bles. These are FeO/(FeO + MgO) = Fe-number [or FeOtot/(FeOtot + MgO) = Fe- *], the modified alkali-lime index (MALI) (Na2O + K2O - CaO) and the alumin um 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 granito ids can further be classified into alkalic, alkali-calcic, calc-alkalic, an d calcic on the basis of the MALI and subdivided on the basis of the ASI in to 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 gran itic rocks. In this classification most Cordilleran granitoids are magnesia n and calc-alkalic or calcic; both metaluminous and peraluminous types are present. A-type granitoids are ferroan alkali-calcic, although some are fer roan alkalic. Most are metaluminous although some are peraluminous. Caledon ian post-orogenic granites are predominantly magnesian alkali-calcic. Those with <70 wt % SiO2 are dominantly metaluminous, whereas more silica-rich v arieties are commonly peraluminous. Peraluminous leucogranites may be eithe r magnesian or ferroan and have a MALI that ranges from calcic to alkalic.