K-Ar ages from seamount chains in the back-arc region of the Izu-Ogasawara arc

The Izu-Ogasawara are contains, hom east to west, a volcanic front, a backarc extensional zone (back-are knolls zone), and a series of across-ape seamount chains that cross the extensional zone in an east-northeast and west-southwest direction and extend into the Shikoku Basin. K-Ar ages of dredged volcanic rocks from these across-arc seamount chains and extension-related edifices in the back-arc region of the Izu-Ogasawara are were measured to constrain the volcanic and tectonic history of the are since the termination of spreading in the Shikoku Basin. K-At ages range between 12.5 and 1 Ma. Andesitic to dacitic rocks of 12.5-2.9 Ma occur mainly on the western part of the chains. The western part of the chains are the locus of volcanism behind the front which erupted mainly calc-alkaline andesitic lavas. The youngest rocks (<2.8 Ma), characterized by cpx-ol basalt, occur along the western margin of the back-are knells zone. Basaltic rocks of 12.5-2.9 Ma have relatively high concentrations of Na2O (>2.0 wt%), Zr (> 50 p.p.m.) and Y (> 20 p.p.m.) and low CaO (< 12 wt%). On the other hand, basalts of 2.8-1 Ma have lower Na2O (< 1.8 wt%); Zr (< 50 p.p.m.) and Y (< 20 p.p.m.), but significantly higher CaO (> 12 wt%). The age inferred for the initiation of back-arc rifting (similar to 2.35-2.9 Ma: Taylor 1992) behind the current volcanic are coincides with the time that basalt chemistry changed drastically (eruption of the low-Na2O and high-CaO basalt). This implies that post-2.8 Ma volcanism in the back-are knells zone is associated with rifting. Similarly, the change in chemical composition might be explained by a different type of source mantle following rift initiation. Volcanism in the western seamounts ceased after the onset of rifting at similar to 2.8 Ma.