Initiation of arc magmatism in an embryonic continental rifting zone of the southernmost part of Okinawa Trough

The Okinawa Trough is a young, intracontinental backarc basin that has form ed behind the Ryukyu arc-trench system since late Miocene time. In the Sout hernmost Part of the Okinawa Trough (SPOT), a cluster of active submarine v olcanoes delineates a volcanic belt, which is located only similar to 100 k m above the Wadati-Benioff zone. We report herein new major and trace eleme nt data for the SPOT volcanic rocks. These rocks show a compositional range from medium-K andesite to rhyolite. Their geochemical characteristics are similar to those of pre-backarc rifting volcanic rocks from the central Ryu kyu arc, and different from those of backarc basin lavas from the Middle Ok inawa Trough and the post-backarc rifting Ryukyu arc volcanics. Therefore, despite being topographically contiguous with the rest of the Trough, the S POT that developed in the Quaternary is not a simple backarc basin but inst ead an embryonic rift zone in which early arc volcanism occurs as a result of the Ryukyu subduction.