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[SCG44-02] Prograde and progressive metamorphism of low-pressure and high-temperature terrane in the Tenzan area, Saga prefecture, north Kyusyu, Southwest Japan
Keywords:Tenzan area, metamorphosed mafic rocks, MORB
Low-pressure and high-temperature type metamorphic rocks are exposed as mega blocks in the Cretaceous north Kyushu batholiths. Adachi et al. (2012) reported 120—100 Ma monazite Th—U—Pb chemical age and 105 Ma LA—ICP—MS zircon U—Pb ages from pelitic granulites in the Ukidake area, Fukuoka prefecture. This report is addressed with the petrological characters of the metamorphosed mafic rocks from the Tenzan area, Saga prefecture, and discussed in their temporal and spatial changes on the metamorphic grade.
The Tenzan area is underlain by the metamorphic rocks including ultramafic rocks and the Cretaceous granites and gabbros. The metamorphic rocks are exposed in the area covering 20 km in the east to west and 8 km in the north to south. The main structural features in the study area generally show east-west in strikes and dipping 50 to 80 degrees to the south or north. In the eastern part of the Tenzan area, the metamorphic rocks occur apart in the two areas with distance about 3 km, the Hiko-dake area in the north and the Ogi area in the south. The Cretaceous granitic rocks intruding the metamorphosed mafic rocks are situated between two areas.
The metamorphosed rocks consist mainly of amphibolite that are classified into Amphibolite-I and Amphibolite-II in terms of lithofacies. Amphibolite-I shows fine-grained and well foliated structures, and locally intercalates thin calc-silicate layers. On the other hand, Amphibolite-II has coarse-grained without any thin calc-silicate layers. Both Amphibolites-I and -II geochemically resemble N-MORB based on some discrimination diagrams. Amphibolite-I has a nematoblastic structure and is composed mainly of hornblende and plagioclase with a small amount of quartz. The hornblende shows a zonal structure with light green in cores to greenish brown in rims. Opaque minerals and titanite are included as accessory minerals. In addition to these minerals, clinopyroxene and epidote appear in the calc-siliceous thin layers. Amphibolite-II consists mainly of hornblende and plagioclase with a small amount of biotite, potassium feldspar and quartz. Hornblende possesses greenish brown in cores with light-green thin rims, and locally includes plagioclase and quartz with poikilitic textures indicating originally intrusive rocks. Potassium feldspar is produced as antiperthite. Opaque minerals, apatite, zircon and titanite are included as accessory minerals. In addition, Amphibolites-I and -II show granoblastic textures near the contact area of the Cretaceous granites.
Hornblende in Amphibolite-I compositionally shows actinolite in core and magnesio-hornblende or edenite in mantle to rim, and locally has thin actinolite rim, whereas that of Amphibolite-II has wide ferro-edenite in core to rim with locally associated with thin actinolite rim. Plagioclase grains are of Anorthite contents (An) 35 to 43 for both Amphibolites-I and -II with few chemical zoning. Zonal structures of hornblende grains in Amphibolite-I would reflect the prograde metamorphism from greenschist to amphibolite facies metamorphism, then finally greenschist facies again at the retrograde stage. The prograde change in Amphibolite-I can be explained by the reaction of tremolite and albite to form edenite and quartz. Since plagioclase in Amphibolite-I is of andesine, it is inferred that the low An plagioclase stabled in the greenschist facies has been consumed by the above reaction. The estimated peak metamorphic temperatures of Amphibolite-I in the Ogi and Hiko-dake areas were 550—600℃ and 640—650℃, respectively. The results indicate that the metamorphic rocks in this area underwent the progressive metamorphism with increasing temperatures from south to north regardless of the distribution of granites. Overall, it is inferred that the metamorphism occurred at 100—105 Ma because of resembling its mode of occurrence and metamorphic features metamorphic style to those of the Ukidake area.
The Tenzan area is underlain by the metamorphic rocks including ultramafic rocks and the Cretaceous granites and gabbros. The metamorphic rocks are exposed in the area covering 20 km in the east to west and 8 km in the north to south. The main structural features in the study area generally show east-west in strikes and dipping 50 to 80 degrees to the south or north. In the eastern part of the Tenzan area, the metamorphic rocks occur apart in the two areas with distance about 3 km, the Hiko-dake area in the north and the Ogi area in the south. The Cretaceous granitic rocks intruding the metamorphosed mafic rocks are situated between two areas.
The metamorphosed rocks consist mainly of amphibolite that are classified into Amphibolite-I and Amphibolite-II in terms of lithofacies. Amphibolite-I shows fine-grained and well foliated structures, and locally intercalates thin calc-silicate layers. On the other hand, Amphibolite-II has coarse-grained without any thin calc-silicate layers. Both Amphibolites-I and -II geochemically resemble N-MORB based on some discrimination diagrams. Amphibolite-I has a nematoblastic structure and is composed mainly of hornblende and plagioclase with a small amount of quartz. The hornblende shows a zonal structure with light green in cores to greenish brown in rims. Opaque minerals and titanite are included as accessory minerals. In addition to these minerals, clinopyroxene and epidote appear in the calc-siliceous thin layers. Amphibolite-II consists mainly of hornblende and plagioclase with a small amount of biotite, potassium feldspar and quartz. Hornblende possesses greenish brown in cores with light-green thin rims, and locally includes plagioclase and quartz with poikilitic textures indicating originally intrusive rocks. Potassium feldspar is produced as antiperthite. Opaque minerals, apatite, zircon and titanite are included as accessory minerals. In addition, Amphibolites-I and -II show granoblastic textures near the contact area of the Cretaceous granites.
Hornblende in Amphibolite-I compositionally shows actinolite in core and magnesio-hornblende or edenite in mantle to rim, and locally has thin actinolite rim, whereas that of Amphibolite-II has wide ferro-edenite in core to rim with locally associated with thin actinolite rim. Plagioclase grains are of Anorthite contents (An) 35 to 43 for both Amphibolites-I and -II with few chemical zoning. Zonal structures of hornblende grains in Amphibolite-I would reflect the prograde metamorphism from greenschist to amphibolite facies metamorphism, then finally greenschist facies again at the retrograde stage. The prograde change in Amphibolite-I can be explained by the reaction of tremolite and albite to form edenite and quartz. Since plagioclase in Amphibolite-I is of andesine, it is inferred that the low An plagioclase stabled in the greenschist facies has been consumed by the above reaction. The estimated peak metamorphic temperatures of Amphibolite-I in the Ogi and Hiko-dake areas were 550—600℃ and 640—650℃, respectively. The results indicate that the metamorphic rocks in this area underwent the progressive metamorphism with increasing temperatures from south to north regardless of the distribution of granites. Overall, it is inferred that the metamorphism occurred at 100—105 Ma because of resembling its mode of occurrence and metamorphic features metamorphic style to those of the Ukidake area.