2015 年 44 巻 2 号 p. 91-111
We report new Sr-Nd-Hf isotope data from 48 Cretaceous-Palaeogene granitoids from the Northeastern (NE) Japan. The spatial variation of Sr-Nd isotopic compositions in those granitoids is known to be characterised by increasing isotopic source enrichment from the northeast to the southwest. The NE Japan has therefore been divided into five geochemical zones with distinct initial isotopic compositions: the Kitakami, North, Sado, South, and Transitional Zones. There are two main models for the origin of these spatial variations: (1) the granitoids were derived from the lithospheric mantle or lower crust, therefore the spatial variation in their Sr-Nd initial isotopic composition reflects the spatial heterogeneity of the source materials; and (2) differences supply rate of subducted sediments to the source mantle beneath the subduction zone. New Sr-Nd isotope data are consistent with previously reported values, and new Hf isotopic data indicate a coupling with Nd isotopic compositions. We suggest a new diagram for discriminating isotopic zones based on the Nd-Hf isotopic characteristics of the granitoids. The Nd-Hf isotopic discrimination diagram divides the region into the Kitakami, North, South, and Transitional zones, in line with previous works based on the Sr-Nd isotopic characteristics, but the Sado zone could not be identified. The mixing model of subducted sediments and source mantle cannot explain the variation of Hf isotopic composition of the granitoids found here, and therefore the results lend support to the model of an isotopically heterogeneous mantle-lower crust source for the Cretaceous-Paleogene granitoids; these heterogeneities result directly in the spatial isotopic variation of the granitoids.