Classification of altered volcanic island arc rocks using immobile trace elements: Development of the Th-Co discrimination diagram

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 using Th 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 >1000 carefully filtered Tertiary-Recent island arc samples to construct a ThCo 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 diagram demonstrates the presence of a tholeiitic volcanic arc series dominated by intermediate-acid lavas overlain by a calc-alkaline series dominated by basic lavas.