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 Th–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 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.