Classification of Altered Volcanic Island Arc Rocks using Immobile Trace Elements: Development of the Th–Co Discrimination Diagram

TLDR: The K2O^SiO2 diagram, used to divide volcanic arc rocks into rock types (basalts, basaltic andesites, dacites and rhyolites) and volcanic series (tholeiitic, calc-alkaline and shoshonitic), is particularly susceptible to the effects of alteration as discussed by the authors.

Abstract: Many diagrams conventionally used to classify igneous rocks utilize mobile elements, which commonly renders them unreliable for classifying rocks from the geological record.The K2O^SiO2 diagram, used to subdivide volcanic arc rocks into rock type (basalts, basaltic andesites, andesites, dacites and rhyolites) and volcanic series (tholeiitic, calc-alkaline, high-K calc-alkaline and shoshonitic), is particularly susceptible to the effects of alteration.However, by usingTh as a proxy for K2O and Co as a proxy for SiO2 it is possible to construct a topologically similar diagram that performs the same task but is more robust for weathered and metamorphosed rocks. This study uses 41000 carefully filtered Tertiary^Recent island arc samples to construct aTh^Co classification diagram. A‘testing set’comprising data not used in constructing the diagram indicates a classification success rate of c. 80%.When applied to some hydrothermally altered, then tropically weathered Cretaceous volcanic arc lavas from Jamaica, the diagramdemonstrates the presence ofa tholeiitic volcanic arc series dominated by intermediate^acid lavas overlain by a calc-alkaline series dominated by basic lavas.