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 geograph ical locations around the world. The three well-defined tectonic regimes in clude oceanic arcs, active continental margins and within-plate volcanic zo nes. This subdivision is based on concentrations and ratios of the incompat ible trace elements Ta, Th and Yb as geochemical tectonic discriminants. Th e separation of tectonic regimes is demonstrated on two discriminant diagra ms, where the three zones are separated by ca. 45 degrees diagonal lines on one, and by horizontal lines on the other. The ca. 45 degrees trends of th e 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 l ower incompatibility of Yb. On a Th/Ta versus Yb diagram, the three tectoni c 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 area. These d iscriminant diagrams can be successfully used to identify the tectonic envi ronments of intermediate and felsic volcanic rocks, and to evaluate the tec tonic history of a region.