Tectonic Discrimination of Basic and Ultrabasic Volcanic Rocks through Log-Transformed Ratios of Immobile Trace Elements

Abstract

Most of the existing major- and trace element-based discrimination diagrams are characterized by major defects such as subjective field boundaries, the constant-sum problem, and inadequacy of samples in creating them. Although major advances toward the solution of all these problems have been recently achieved through major element-based discriminant function diagrams, their applicability to old, altered rocks may be questionable. We present new discriminant function diagrams based on immobile trace elements and log-ratio transformation of the data of basic and ultrabasic rocks. These new diagrams are extremely successful in distinguishing the four tectonic settings (island-arc, continental rift, ocean island, and mid-oceanic ridge) and especially the plate margin (island-arc and mid-ocean ridge grouped together) and intra-plate or plate interior (continental rift and ocean island combined together) settings. The overall success rate for these natural log-transformed ratio-based diagrams using five trace elements (La, Sm, Yb, Nb, and Th), i.e., using four ratios ln(La/Th), ln(Sm/Th), ln(Yb/Th), and ln(Nb/Th), varies from 78.8% to 96.4%. Moreover, transitional tectonic settings such as interaction of a mid-ocean ridge with an ocean island or subduction of a ridge can be identified because basalts formed in these settings display linear trends in the present diagrams. Finally, the application of our new discrimination diagrams to altered, metamorphosed rocks from four widely separated areas lends further support regarding the usefulness of our proposal.