THE ALUMINIUM SATURATION INDEX AND THE MgO/Ti02 RATIO: TWO PARAMETERS INFLUENCED BY PH20 AND THEIR USE TO DISCRIMINATE MAGMA SERIES

Abstract

The Aluminum Saturation Index is related to the MgO/TiO2 ratio and both may be added to other typological parameters commonly used in the granitic series discrimination. Some well-known examples are listed and they lead to propose new geochemical limits characterizing different geodynamic settings. Four main groups may be recognized, 1: calc-alkaline granitoids (high Ca and low K) together with the island arc tholeiitic granitoids (ASI=1 for MgO=1.8% and MgO/TiO2 =2); 2: crustal peraluminous granitoids (ASI>1 and MgO/TiO2 = 3) produced in the collision zones; 3: tardi- and post-collision (K calc-alkaline) granitoids (ASI=1 for MgO=3.7% and MgO/TiO2 =4) and 4: the alkaline and peralkaline granitoids and the mid-ocean ridge tholeiitic granitoids emplaced in rifting or doming zones (ASI=1 for MgO=0.8% and MgO/TiO2=l).