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Origin of magnetite- and ilmenite-series granitic rocks in the Japan Arc

Research Center for Deep Geological Environments, Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology (AIST), Central-7, 1-1-1 Higashi, Tsukuba, Ibaraki, 305-8567 Japan; takagi-t{at}aist.go.jp

The exposure areas, radiometric ages, redox states, initial ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd isotope ratios, and ¹⁸O values of granitic plutons in the Japan Arc have been compiled based on the results of previous studies of the Japanese granitic rocks. The exposure areas of the granitic rocks in a given period ( activity of granitic rocks) and initial ⁸⁷Sr/⁸⁶Sr isotope ratios show positive correlations with the convergence rate of oceanic plates along the arc. In contrast, the redox states of the granitic rocks are negatively correlated with the oceanic plate convergence rate. The initial ¹⁴³Nd/¹⁴⁴Nd isotope ratios and ¹⁸O values combined with the initial ⁸⁷Sr/⁸⁶Sr isotope ratios suggest that the temporal variation of initial ⁸⁷Sr/⁸⁶Sr isotope ratios was mainly due to source contamination resulting from melting of subducted materials in the mantle source region. The granitic magmas contaminated with more than about 15 percent subducted materials became ilmenite-series granitic rocks.

These observations suggest that temporal variations in redox states in the Japanese granitic rocks are not due to the properties of the lower crust but rather to subducted materials. Sediment subduction was also increased with increasing convergence rate. Consequently, the oxidation states of granitic rocks declined and the initial ⁸⁷Sr/⁸⁶Sr isotope ratios rose, probably due to the increase in the assimilation rates of subducted sediments into granitic magmas. The increase of sediment subduction may be attributable to the acceleration of subduction-related erosion and/or the changes in the Earth’s surface conditions (the high CO₂ fugacity and temperature of the Cretaceous atmosphere, which promoted increased weathering and erosion of continental crust resulted in greater terrigenous sediment supply to subduction zones). The active deposition and subduction of carbon- and sulfide-rich sediments, caused by oceanic anoxic events during the Cretaceous, would also promote the extensive reduction of magma source regions to form ilmenite-series granitic rocks.

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