Island arc rocks in the Dzhida zone (Fig. 2) are boninitebasalt, rhyolite-andesite, and tuff, as well as mafic-ultramafic and plagiogranite-diorite assemblages, of which the mafic-ultramafic one (dismembered ophiolite) presumably composes the base of an island arc (Gordienko et al., 2007). The typically island arc plagiogranite-diorite assemblage was dated at 504–506 Ma (Gordienko et al., 2006). Note that numerous dismembered ophiolites spatially associated with thrusts are found elsewhere in the Dzhida zone north of the island arc belt (Belichenko et al., 2003) and have suprasubduction chemical features, i.e., are of a back-arc basin rather than oceanic origin (Khain et al., 2002). The dismembered ophiolites (tholeiites) are locally overlain by subalkaline