Applicability of some trace element discrimination diagrams to rhyolites: Tectonic interpretation of silicic volcanism during opening of the Sea of Japan

Abstract

Rhyolite, a volcanic rock with high SiO₂ content (≥69 wt%; sensu lato), is commonly found in a range of tectonic settings. Because few tectonic discrimination diagrams have been proposed for rhyolites, the major and trace element compositions of rhyolites from syn-collision zones, continental arcs, oceanic arcs, within-continental plate settings, and within-oceanic plate settings were compiled and plotted on previously proposed Y versus Nb, Y + Nb versus Rb, and Zr versus (Nb/Zr)_N discrimination diagrams. The results indicate that the Y versus Nb and Y + Nb versus Rb diagrams are highly effective for distinguishing between orogenic (collision and arc-related) and anorogenic (within-plate) tectonic settings. Especially on the Y versus Nb diagram, immature arc rhyolites are characterized by significantly lower Nb concentrations compared to mature arc rhyolites, clearly discriminating between them. However, syn-collisional rhyolites often plot within the volcanic arc granite field on the Y + Nb versus Rb diagram. Thus, it is difficult to distinguish between syn-collisional rhyolites and mature arc rhyolites. The Zr versus (Nb/Zr)_N diagram enables relatively effective discrimination among within-plate rhyolites, syn-collisional rhyolites, and immature arc rhyolites. Nevertheless, this diagram also fails to clearly separate syn-collisional from mature arc rhyolites. This overlap would be due to the similar source characteristics of syn-collision and mature arc rhyolites, derived from thick continental crust. In contrast, immature arc rhyolites are interpreted to originate predominantly from a mantle-derived magma source. Although within-continental plate rhyolites tend to exhibit slightly higher Rb concentrations than within-oceanic plate rhyolites, none of the three discrimination diagrams can reliably differentiate between within-continental and within-oceanic plate rhyolites. Despite these limitations, the power of discrimination of the three diagrams was utilized to evaluate and interpret the tectonic setting of silicic volcanism associated with the back-arc spreading in the Sea of Japan (∼ 23-15 Ma). The compiled data show that the early-stage rhyolites during opening of the Sea of Japan display geochemical signatures consistent with a within-plate setting and a crustal source, whereas the late-stage rhyolites exhibit characteristics of volcanic arc settings with a strong mantle source influence. These results are consistent with tectonic models proposed in previous studies and demonstrate the utility of trace element discrimination diagrams for evaluating silicic volcanism related to back-arc spreading.