Nd and Sr Isotopic Study of Leg 127 Basalts: Implications for the Evolution of the Japan Sea Backarc Basin

Abstract

Sr and Nd isotopic compositions are reported for basaltic rocks collected during ODP Leg 127 from the Yamato Basin, a rifted backarc basin in the Japan Sea. The basalts are classified into two groups in terms of Nd isotopic composition: the upper sills at Site 797 are characterized by higher 143 Nd/ 144 Nd ratios (0.513083-0.513158, eNd = 8.68-10.14) and the basalts from Site 794 and the lower sills at Site 797 have lower 143 Nd/ 144 Nd ratios (0.512684-0.512862, e Nd = 0.90-4.37). All of the basalts show higher Sr isotopic compositions than those of the mantle array, which is attributed to seawater alteration. The basalts with lower Nd isotopic values ranging in age from 20.6 to 17.3 Ma have tapped an enriched subcontinental upper mantle (SCUM) with the minor involvement of a depleted asthenospheric mantle (AM). Subsequent change in composition through the physical replacement of SCUM by AM yielded the basalts of the upper sills of higher Nd isotopic compositions. This event within the upper mantle was associated with the breakup of the overlying lithosphere during the rifting of the Japan Sea backarc basin. This report presents the Nd and Sr isotopic compositions of basalts collected from the Japan Sea floor during Leg 127 of the Ocean Drilling Program (ODP) (Fig. 1). Based on the isotopic data, we examine the geochemical evolution of the upper mantle beneath the terrain during the rifting/opening process of the Japan Sea backarc basin. The opening of the Japan Sea had been, hitherto, inferred from magnetic anomalies in the Japan Sea (Isezaki, 1986; Kono, 1986) and a sharp change in marine faunas from warm to cold water in the early Miocene deposits dated at about 15 Ma along the Japan Sea coast (Chinzei, 1986). Rapid clockwise rotation of the southwest Japan arc sliver and counterclockwise rotation of the northeast Japan sliver were suggested from paleomagnetic studies of Tertiary volcanic rocks of the Japan arc (Otofuji and Matsuda, 1983; Otofuji et al., 1985b). The rotational movement of the arc slivers resulted in the fan-shaped opening of the Japan Sea at about 15 Ma (Otofuji et al., 1985a). Alternatively, Lallemand and Jolivet (1985) proposed a pull-apart basin model to explain the opening of the Japan Sea. Tamaki (1985) pointed out the existence of multirifts during the opening of the Japan Sea Basin. As for the commencement of rifting in the Japan Sea, an age of about 30 Ma, which is older than the age of rotation of the arc slivers, was estimated on the basis of stratigraphy, basement depth, and heat-flow data by Tamaki (1986). The 40Ar-39Ar dates of three dredged samples from the slope of the Japanese Islands in the basin yielded ages of 23-24 Ma and an age of 26 Ma for an andesite from the Yamato Bank (Kaneoka and Yuasa, 1988). Nohda and Wasserburg (1986) and Nohda et al. (1988) reported Nd and Sr isotopic compositions for Tertiary volcanic rocks from the northeast Japan arc and documented secular variation in the isotopic composition of the enriched to depleted signature of the magma source region through time. This secular variation was considered in the framework of backarc opening in the Japan Sea and attributed to the growth of depleted asthenospheric materials into the preexisting enriched upper mantle. Nohda et al. (1988) and Tatsumi et al. (1989) proposed a hypothetical model of asthenospheric injection to explain