Abstract
Late Cretaceous-Paleogene granitoids of the Inner Zone of Southwest Japan (SWIN) batholith were studied for oxygen isotopic ratios (18O/16O ratios) by measuring 130 whole-rock samples. The whole-rock δ18O values (relative to SMOW) higher than ca. 6 permil for magnetite series and ca. 9 permil for ilmenite series are considered unaltered values for the studied granitoids. The magnetite series granitoids are generally lower than the ilmenite-series granitoids in the δ18O values. The δ18O values of the magnetite series granitoids are 5.9 to 8.1 permil in the Shirakawa area of the Chubu District; 6.9 to 10.6 permil in the Okutango area to eastern Tottori Prefecture, and 6.0 to 8.2 permil in the Misasa-Kamisaihara area of the eastern Chugoku District. On the other hand, those of the il menite-series granitoids of the Sanyo-Naegi Belt are 9.2 to 9.8 permil for the unmineralized Toki granite and 7.4 to 8.1 permil for the W-mineralized Naegi granite in the Chubu District. The W mineralized Otani mine stock of the Kinki District has much higher δ18 O values of 11.7 to 12.0 permil. The ilmenite-series granitoids of the Sanyo-Naegi Belt of the Kinki and Chugoku Districts generally have δ18O values of 7.3 to 10.8 permil, except for 11.6 to 12.0 permil in eastern Yamaguchi Prefecture,where the granitoids may be related to W mineralization. The ilmenite-series granitoids of the Ryoke Belt in the Chubu District are generally high in δ18O values as follows: 9.9 to 10.9 permil for I-type granitoids in Zone I, 9.1 to 12.1 permil for I-type granitoids of the Zones II and III. The S-type granitoids of the Zone III have the highest δ18O values of 10.5 to 12.5 permil. Regional variation of the δ18O values was drawn using δ18O values normalized at 70 percent SiO2. The δ18O contours show high-value centers in the Ryoke Belt and Sanyo-Naegi Belt, and a low-value trough along the Japan Sea coast (Fig. 7). The magnetite-series/ilmenite-series granitoids are sepa rated at 8 permil δ18 O. In the δ18 O values vs. initial 87 Sr/ 86 Sr ratios diagram, the magnetite-series granitoids are plotted generally in igneous source rocks of the lower continental crust. Most of the ilmenite-series granitoids, on the other hand, are high in both the isotopic ratios and plotted along a zone between primitive basaltic rocks and the Ryoke metamorphic rocks, indicating that, together with the common occurrence of mafic enclaves, the ilmenite-series granitoids were formed by mag mas having these two components in the source region. Cretaceous gabbroids appear to have a high δ18 O value of 8.6 permil, implying the upper mantle at that time was enriched in 18O, compared with Cenozoic basalts with different tectonic settings. If this gabbroic magma mingled with sedimentary protolith similar to the Ryoke metamorphic rocks having a δ18O of 15.6 permil, the maximum contribution of the sedimentary component is calculated to be 18 to 32 percent for the majority of I-type granitoids in the Ryoke Belt of the Chubu District. The two-mica granites contain no mafic enclaves, thus generated from both sedimentary and felsic igneous materials isotopically once homogenized in the lower continental crust. The 18O-enrichment on the I-type granitoids appears to have been enhanced by regional shearing. High initial 87Sr/86Sr ratios in the Shirakawa, Toki and Okutango granitoids suggest the existence of an old basement at depths in these regions.
