2010 Volume 61 Issue 1-2 Pages 17-38
Chemical compositions were studied by polarized XRF method on Miocene ilmenite-series granitoids and porphyries that belong to the Outer Zone of SW Japan from the following localities: 3 porphyries from the ring dike of the Okueyama area, 21 granitoids from the Okueyama pluton, 8 granitoids in the Hoei mine area, and 7 granites from the Takakumayama stock, all occurring as isolated bodies in the North Belt of the Shimanto Supergroup. They appear to be high-level granitoids and are compared with the ilmenite-series granitoids of Late Cretaceous Ryoke granitoids, which could be a deep level batholith in the Chubu district.
The Outer Zone granitoids are less than the Ryoke granitoids on the following components. Al2O3, Ba, CaO, Sr, A/CNK, P2O5, Zn and Cr; whereas the Outer Zone granitoids are richer than the Ryoke granitoids on the following components: Gax10000/A, K2O, Rb, Pb, Na2O, MgO, TiO2, V, Y, La, Ce, Th, U, Nb, Ta, Zr and Sn. These differences are considered to reflect different source rocks for these granitoids. The most distinct difference between high and low level granitoids is a sharp increasing with silica of Rb, Pb, Y, Th and U in the latest stage of the high level granitoids, which should be due to magmatic differentiation.
Zircon saturation temperatures are the highest in the ring dike, averaged as 859oC (70.3 % SiO2). The main granodiorite of the Okueyama pluton shows an average of 774oC (68.4 % SiO2), while the leucogranitic cap reveals 750oC (74.8 % SiO2). The Takakumayama granites give similar values as 738oC for the main phase (73.8% SiO2) and 695oC (76.3% SiO2) for the leucogranite. Traditional understanding of the S-type Takakumayama granites failed to be so, because of their A/CNK less than 1.1, low K2O contents and lack of restitic alumina-silicates.
