Fifteen new discriminant-function-based multi-dimensional robust diagrams for acid rocks and their application to Precambrian rocks

Abstract For the discrimination of four tectonic settings of island arc, continental arc, within-plate (continental rift and ocean island together), and collision, we present three sets of new diagrams obtained from linear discriminant analysis of natural logarithm transformed ratios of major elements, immobile major and trace elements, and immobile trace elements in acid magmas. The use of discordant outlier-free samples prior to linear discriminant analysis had improved the success rates by about 3% on the average. Success rates of these new diagrams were acceptably high (about 69% to 97% for the first set, about 69% to 99% for the second set, and about 60% to 96% for the third set). Testing of these diagrams for acid rock samples (not used for constructing them) from known tectonic settings confirmed their overall good performance. Application of these new diagrams to Precambrian case studies provided the following generally consistent results: a continental arc setting for the Caribou greenstone belt (Canada) at about 3000 Ma, São Francisco craton (Brazil) at about 3085–2983 Ma, Penakacherla greenstone terrane (Dharwar craton, India) at about 2700 Ma, and Adola (Ethiopia) at about 885–765 Ma; a transitional continental arc to collision setting for the Rio Maria terrane (Brazil) at about 2870 Ma and Eastern felsic volcanic terrain (India) at about 2700 Ma; a collision setting for the Kolar suture zone (India) at about 2610 Ma and Korpo area (Finland) at about 1852 Ma; and a within-plate (likely a continental rift) setting for Malani igneous suite (India) at about 745–700 Ma. These applications suggest utility of the new discrimination diagrams for all four tectonic settings. In fact, all three sets of diagrams were shown to be robust against post-emplacement compositional changes caused by analytical errors, element mobility related to low or high temperature alteration, or Fe-oxidation caused by weathering.