Zircon U-Pb Ages of Tuffs and Volcaniclastic Sandstone of the Core Sample of IODP Exp. 322 at the Northern Part of the Shikoku Basin.

Abstract:

We determined U-Pb ages of zircons from core samples of IODP Exp. 322 using the laser abrasion ICP-MS (VG Plasma Quad 3 with New Wave Research UP-213). Zircon crystals were separated from four felsic tuffs from the Unit V of Site C0011, and a volcaniclastic turbidite sandstone of the lowermost horizon of the Unit V of Site C0012. Both of the drilling sites are located off the Nankai trough on the Shikoku Basin of the Philippine Sea plate, southwest Japan. Zircons from two felsic tuffs from Site C0011 are euhedral crystals, and most of their ²³⁸U-²⁰⁶Pb ages range 13 – 16 Ma. Weighted means of the ²³⁸U-²⁰⁶Pb ages of these samples are ca. 14.3 Ma. The other two felsic tuffs include zircon grains with older ages (80 – 260 Ma), however, weighted means of the ²³⁸U-²⁰⁶Pb ages of population with young ages ranges 14.5 – 14.7 Ma. These ages are coincide with those of the intense felsic magmatism occurred in the forearc region of southwest Japan (14 – 15 Ma) just after the opening of the Japan Sea and consequent clockwise rotation of the southwest Japan. Some of the felsic igneous bodies of the middle Miocene southwest Japan ejected large amount of felsic materials resulting caldera formation. So the provenance of felsic tuffs from the core of the Site C0011 are presumed to be one of the felsic igneous bodies of the forearc region of southwest Japan.

Turbidite sandstone from Site C0012 also includes Miocene zircon grains of which their weighted mean of the ²³⁸U-²⁰⁶Pb ages is ca. 14.2 Ma. Moreover turbidite sandstone contains zircons with various ages (19 – 2500 Ma). One of the possible origin of such old zircon grains is reworking from sediments of the accretionary complex in the forearc of southwest Japan. If we assume the present rate of convergence of the Philippine sea plate (ca. 4 cm/y) is invariant, the turbidite including both clastic sediment and coeval felsic igneous materials traveled ca. 600 km across the trench.