Original paper

Using the Rittmann Serial Index to defi ne the alkalinity of igneous rocks

Yang, Xue-Ming

Abstract

The Rittmann Serial Index [σ = (Na2O + K2O)2/(SiO2 - 43), units in wt.%] ( Rittmann 1957, 1962) is used as a convenient petrochemical parameter to discriminate igneous rock series on the basis of alkalinity: calcic- (σ 8.8). For silicic (SiO2 > 70%) rocks with Na2O > K2O, (2 × Na2O) is used to replace the total alkalis in the above equation for σ calculation. Based on a simple calculation of the σ value, each whole-rock sample corresponds to a specific igneous series. The discriminant results are compatible to the assignment for igneous rock series originally defined by the Peacock Index (PEACOCK 1931), and the method presented here is numerical and relatively easy to handle. Although this method is applicable only to igneous rocks with SiO2 higher than 43 wt.%, it can cover most igneous rocks ranging from basic through intermediate to acid rocks. The alkalinity of an igneous rock series is not only related to tectonic setting, but also it may be used as an indicator for mineralization. Therefore, the Rittmann Serial Index may be utilized as a supplementary tool in aiding tectonic setting identification and mineral exploration, and in particular, applied to exploration for mineral deposits associated with granitoid intrusions and (or) complexes.

Keywords

alkalinitythe rittmann serial index (sigma)tectonic settingmineralizationexplorationgranite