ANCIENT OCEANIC CRUST IN ISLAND ARC LOWER CRUST: EVIDENCE FROM OXYGEN ISOTOPES IN ZIRCONS FROM THE TANZAWA TONALITIC PLUTON

Island arc crust is a fundamental building block of the continental crust and is important for understanding continental growth. Although the initial steps of island arc evolution have been intensively investigated in the Izu-Bonin-Mariana (IBM) arc, formation processes and constituent rocks for middle- and lower-crust of island arcs still remain uncertain owing to their limited exposure.

Oxygen isotopic ratio of igneous zircon directly reflects that of parental magma, in contrast to whole rock d¹⁸O, which is readily altered and increases with increasing WR SiO₂. To better understand island arc lower crust, in-situ analysis of d¹⁸O was carried out for 202 zircons separated from 4 plutons (Ishiwariyama, Otakizawa, Yusin, Azegamaru) in Tanzawa Tonalite, which are regarded as the exposed middle crust of the former IBM arc, using an IMS-1280 Secondary Ion Mass Spectrometer. These zircons have δ¹⁸O of 4.6-5.2 ‰ corresponding to the lower part of the mantle range (δ¹⁸O(Zc) = 5.3 ± 0.6‰). The lower zircon δ¹⁸O values from the Tanzawa Tonalite suggest that parent rocks had slightly lower δ¹⁸O than the mantle. Because the Tanzawa Tonalite is suggested to be generated by partial melting of island arc lower crust and to intrude basaltic-andesitic pyroclastic rocks (Tanzawa Group), candidates for the low δ¹⁸O values are limited to these rocks. Generally, rocks reacted with surface water in high temperature have low δ¹⁸O values, and hydrothermally altered gabbros at lower oceanic crustal level often have δ¹⁸O ranging from 4 to 5‰. We interpret the low zircon δ¹⁸O values in the Tanzawa Tonalite to reflect melting of island arc lower crust including former lower oceanic crust. Thus, it is suggested that ancient oceanic crustal material such as hydrothermally altered gabbros remained as island arc lower crust when the Tanzawa Tonalite was formed (4-9 Ma).

Combined with the formation age of arc lower crust beneath the Tanzawa Tonalite (>43 Ma), it is implied that ancient oceanic crust is preserved in the island arc lower crust at least 40 million years after the initial arc generation, especially in the IBM arc.