FROM CONTINENTS TO ISLAND ARCS: A GEOCHEMICAL INDEX OF TECTONIC SETTING FOR ARC-RELATED AND WITHIN-PLATE FELSIC TO INTERMEDIATE VOLCANIC ROCKS

Three distinct tectonic regimes were identified for felsic and intermediate volcanic rocks using published datasets from twenty-six different geographical locations around the world. The three well-defined tectonic regimes include oceanic arcs, active continental margins and within-plate volcanic zones. This subdivision is based on concentrations and ratios of the incompatible trace elements Ta, Th and Yb as geochemical tectonic discriminants. The separation of tectonic regimes is demonstrated on two discriminant diagrams, where the three zones are separated by ca. 45° diagonal lines on one, and by horizontal lines on the other. The ca. 45° trends of the boundaries between tectonic provinces on a Ta/Yb versus Th/Yb diagram are due to the similar incompatibility of Th and Ta relative to the somewhat lower incompatibility of Yb. On a Th/Ta versus Yb diagram, the three tectonic zones are separated by horizontal lines; datasets within individual zones have characteristic Th/Ta values, ca. 1–6 for within-plate volcanic zones, >6–20 for active continental margins, and >20–90 for oceanic arcs. These discriminant diagrams can be successfully used to identify the tectonic environments of intermediate and felsic volcanic rocks, and to evaluate the tectonic history of a region.