OKANO Osamu OKAYAMA UNIVERSITY,FACULTY OF SCIENCE,ASSISTANT, 理学部, 助手 (10233355)
HONMA Hiroji OKAYAMA UNIVERSITY,INSTITUTE FOR STUDY OF THE EARTH 'S INTERIOR,PROFESSOR, 固体地球研究センター, 教授 (70033131)
|Budget Amount *help
¥4,300,000 (Direct Cost : ¥4,300,000)
Fiscal Year 1994 : ¥1,100,000 (Direct Cost : ¥1,100,000)
Fiscal Year 1993 : ¥1,700,000 (Direct Cost : ¥1,700,000)
Fiscal Year 1992 : ¥1,500,000 (Direct Cost : ¥1,500,000)
The voluminous intermediate to felsic magmatism took place in the Southwest Japan Arc during late Cretaceous to Paleogene. The granitoid rocks, which are the most predominant rocks formed by this magmatism, have regional variations in their radiometric ages, petrography and geochemistry. Kagami et.al. (1992 ; 1993) divided the granitoid rocks into four zones (South, Transitional, North and northern Kyushu) based on the Sr and Nd isotopic data. The granitoid rocks of the South Zone have high initial Sr isotope ratios (0.7070 to 0.7088) and low initial epsilonNd values (-3.0 to -8.0) , whereas granitoid rocks from the North Zone have low initial Sr isotope ratios (0.7048 to 0.7068) and high initial epsilonNd values (+3 to -2.2) . Granitoid rocks from the northern Kyushu have low Sr isotope ratios comparable to those from the North Zone, but all have highter positive initial epsilonNd values. Most samples from the Transitional Zone have middle initial epsilon-values of the North Zone and
South Zone. Kagami et.al. (1992) emphasized that the geographical variations in Sr and Nd isotopic date for the granitoid rocks cannot be attributed to contamination of magmas by upper crustal materials. They suggested that the Sr and Nd isotopic differences of the granitoid rocks among each zone are caused by those of the uppermantle and/or lower crust being sources of the granitoid rocks, and proposed an idea that lithosphere of each zone has independent history one another.
Initial Nd isotopic ratios of the igneous rocks occurring in the North Zone gradually increase from late Cretaceous (ca.85Ma) to middle Paleogene (ca.39Ma) . On the contrary, their initial Sr isotopic ratios gradually decrease. These isotopic change can be explained by the change of the source materials of the igneous rocks.
Late Gretaceous igneous activity for the South Zone commenced with intermediate magmatism (ca.110Ma) , followed by voluminous granitic plutonism which can be divided into two periods at 100-92Ma and 85-75Ma. Distributions of diabasic and gabbroic masses are conspicuous in the southern most part of the South Zone, and most of them are captured in the late Cretaceous granitoid rocks. Diabase shows metamorphic granoblastic textures, and is tentatively called metadiabase (Kagami et.al., 1985) . This rock has aluminous tholeiitic chemical compositions. The gabbro has been considered to be cumulate crystallized from tholeiitic magmas under lower crustal conditions of 6 to 8kb. Norite and hornblende gabbro, which are most predominant rock type in gabbroic masses, and metadiabase plot on a same Sm-Nd isochron which gives an age of 192? B119 Ma with an initial ^<143>Nd/^<144>Nd ratio of 0.512169? B10.000021 (Kagami et.al., 1995a) .Anorthosite and anorthositic gabbro, which occur as blocks of irregular form in the norite and hornblende gabbro, have a Sm-Nd whole rock age of 169? B129 Ma with an initial ^<143>Nd/^<144>Nd ratio of 0.512148? B10.000027 (Kagami et.al., 1995a) . These age data imply that the mafic igneous activity took place in the late Cretaceous granitic terrane during early to middle Jurassic. After late Cretaceous, field of igneous activity gradually moved to the Japan Sea Side. Igneous activity in the South Zone interrupted for 60 Ma to 70 Ma and started again at middle Miocene (ca.15Ma) .The igneous activity during middle Miocene is probably related with the opening of the Japan Sea. The middle Miocene volcanic rocks including basalt, high-magnesian andesite, andesite and dacite are sporadically distributed in the South Zone. These vokcanic rocks occasionally contain lower crustal materials as xenoliths.