Abstract
Takatumayama, amakazari, hiuchiyama, Hokogatake and Nannbayama are distributed on the Nishikubiki uprift zone, northern Fossa Magna (Hayashi, 1997; Oba and Hayashi, 1998; Yahata and Oba, 2000; Sato and Oba, 2002; and Endo and Oba, 2002) The intrusive activities of these hypabyssal rocks are Pliocene-late Pleistocene. The Nishikubiki hypabyssal rocks are divided with three types as follows:Clinopyroxene-orthopyroxene-hornblende porphyrite, orthopyroxene-hornblende porphyrite or porphyry, and hornblende porphyrite or porphyry. In the Nishikubiki hypabyssal rocks, orthopyroxene-hornblende porphyrite or porphyry, and hornblende porphyrite or porphyry are common, Clinopyroxene- orthopyroxene-hornblende porphyrite are found in Nanbayama. Oba and Hayashi(1998, 2000) reported that the K-Ar age is 4.9Ma-3.08Ma. The K-Ar ages of Hokogatake and Takatumaya are 1.5Ma-1.0Ma, These from Amakazari and Hiuchi are 1.6Ma and 1.0Ma-2.0Ma, respectively. SiO2 of volcanic rocks from Nanbayama is 54-62%, on the other hand SiO2 of other hypabyssal rocks is 60-70%. K2O content of Nannbayama porphyrite is high, while K2O content of Hokogatake porphyry is low. The most rock of the Nishikubiki are calc-alkali rocks except for the part of Nanbayama.
