Volcanism in the earliest stage of back-arc rifting in the Izu-Bonin arc revealed by laser-heating 40Ar/39Ar dating

The back-arc region of the Izu-Bonin arc has complex bathymetric and structural features, which, due to repeated back-arc rifting and resumption of arc volcanism, have prevented us from understanding the volcano-tectonic history of the arc after 15 Ma. The laser-heating 40Ar/39Ar dating technique combined with high density sampling of volcanic rocks from the back-arc region of this arc successfully revealed the detailed temporal variation of volcanism related to the back-arc rifting. Based on the new 40Ar/39Ar dating results: (1) Back-are rifting initiated at around 2.8 Ma in the middle part of the Izu-Bonin arc (30degrees30'N-32degrees30'N). Volcanism at the earliest stage of rifting is characterized by the basaltic volcanism from north-south-trending fissures and/or lines of vents. (2) Following this earliest stage of volcanism, at ca. 2.5 Ma, compositionally bimodal volcanism occurred and formed small cones in the wide area. This volcanism and rifting continued until about I Ma in the region west of the currently active rift zone. (3) After I Ma, active volcanism ceased in the area west of the currently active rift zone, and volcanism, and rifting were confined to the currently active rift zone. The volcano-tectonic history of the back-arc region of the Izu-Bonin arc is an example of the earliest stage of back-arc rifting in the oceanic island arc. Age data on volcanics clearly indicate that volcanism changed its mode of activity, composition and locus along with a progress of rifting. (C) 2002 Elsevier Science B.V. All rights reserved.