5:15 PM - 6:45 PM
[SGL18-P04] M-, I-, S-, and A-type classification of granitoids in the Ryoke belt, SW Japan:
The Tsukuba Igneous Complex revisited
Keywords:Granitoids, MISA classification, Tsukuba district, major element compositions
Granitoids have a unique rock types with respect to chemical composition and mineralogy due to their varieties of origin and evolution in various geotectonic processes. Many researchers have tried to classify the granitoids especially focused on the differences of their geochemical compositions; e.g., the genetic classification scheme of MISA (M: Mantle, I: Igneous, S: Sedimentary, A: Alkaline) type granitoids has been widely accepted. The petrogenetic signature of the granitoids, together with the MISA classification, are important in unraveling the origin and evolution of granitoids worldwide with various formation age.
Mesozoic to Early Cenozoic granitoids are widely distributed in Southwest Japan, and divided into three distinct groups, i.e., Ryoke, San-yo, and San-in belts, based on their lithologies, geochemical compositions, and formation ages. The MISA classification of these granitoids have been investigated, however the granitoids in the Tsukuba region might be insufficient to reliably classify the rock types. In the Tsukuba region, the exposed Cretaceous to Palaeogene granitoid were separated into three units, such as the Inada, Kabasan, and Tsukuba granites, mainly according to their intrusive relations. As a whole, the granitoid in these units was previously labeled I-type granite despite the presence of garnet which is relatively typical mineral assemblages of S-type granite. In order to provide complementary information with potential improvement of the MISA-classification accuracy in the Tsukuba region, we conducted the petrological study and geochemical labeling of granitoids in the Inada and Kabasan units.
According to the whole-rock chemical signatures of the analyzed granitoids, SiO2 contents cover the range from 70.67 to 78.20 wt.% and P2O5 contents vary from 0.01 to 0.05 wt.%. A negative correlation between the SiO2 and P2O5 contents suggest that the analyzed granitoids are origin of I-type magma (r2 = 0.87). In addition, the aluminum saturation index, i.e., A/CNK (= Al2O3/(CaO + Na2O + K2O)), have a range of 1.01 to 1.12, further indicating that the granitoids in the Kabasan and Inada units belong to I-type. From the above, the garnet-bearing granitoids can be generated from I-type magma probably because of the minor contribution of sedimentary rocks.
Mesozoic to Early Cenozoic granitoids are widely distributed in Southwest Japan, and divided into three distinct groups, i.e., Ryoke, San-yo, and San-in belts, based on their lithologies, geochemical compositions, and formation ages. The MISA classification of these granitoids have been investigated, however the granitoids in the Tsukuba region might be insufficient to reliably classify the rock types. In the Tsukuba region, the exposed Cretaceous to Palaeogene granitoid were separated into three units, such as the Inada, Kabasan, and Tsukuba granites, mainly according to their intrusive relations. As a whole, the granitoid in these units was previously labeled I-type granite despite the presence of garnet which is relatively typical mineral assemblages of S-type granite. In order to provide complementary information with potential improvement of the MISA-classification accuracy in the Tsukuba region, we conducted the petrological study and geochemical labeling of granitoids in the Inada and Kabasan units.
According to the whole-rock chemical signatures of the analyzed granitoids, SiO2 contents cover the range from 70.67 to 78.20 wt.% and P2O5 contents vary from 0.01 to 0.05 wt.%. A negative correlation between the SiO2 and P2O5 contents suggest that the analyzed granitoids are origin of I-type magma (r2 = 0.87). In addition, the aluminum saturation index, i.e., A/CNK (= Al2O3/(CaO + Na2O + K2O)), have a range of 1.01 to 1.12, further indicating that the granitoids in the Kabasan and Inada units belong to I-type. From the above, the garnet-bearing granitoids can be generated from I-type magma probably because of the minor contribution of sedimentary rocks.