EAST-WEST CONTRAST IN THE METALLOGENY OF GRANITOID AFFINITY AROUND THE PACIFIC: A PERSPECTIVE FROM THE CIRCUM-JAPAN SEA REGION

Granitoid magmatism and related mineralization show a remarkable contrast between the eastern and the western sides of the Pacific Rim. In the Asian side, granitoids are dominated by the reduced type which is characterized by Sn mineralization. In the western Cordillera, however, granitoids are dominated by the oxidized type which is characterized by Mo and Cu mineralization and practically devoid of Sn.

For understanding the genesis of this E-W contrast, granitoid and metallogenic provinces in the circum-Japan Sea region have been studied in detail from the view point of the evolution of the crust. The results indicate that the redox state of granitoid magmas, linked strongly with metallogeny, is influenced by sedimentary materials. The reduced-type granitoids were generated in carbon-bearing sedimentary crust which was composed mainly of accretionary complexes. In contrast, the oxidized-type granitoids were generated in igneous crust which was depleted in reducing agent due to older magmatism. Therefore, the granitoid magmatism in a given area tends to become more oxidized with time (Sato et al., 2005, GSA Spec. Paper, 389, 319-337).

The western Cordillera has been situated in a relatively monotonous convergent margin since the Late Paleozoic. It is suggested that igneous crust formed by repeated magmatism may have favored for the oxidized-type magmatism which occasionally generated porphyry copper mineralization. On the other hand, East Asia has complicated geodynamic histories due to the amalgamation of continents. Sedimentary terrains around the continents may have been changed to the site of granitoid magmatism in various tectonic regimes including the retreat of trench, collapse of young back-arc basin and collision of continental fragments.