Abstract
Niijima is one of the rhyolite dominant Quaternary volcano located in northern part of Izu volcanic arc. Rhyolites in Niijima are classified into four types based on mafic phenocrystic minerals: hypersthene-(hornblende)-bearing (Hyp-Hb-type), cummingtonite-bearing (Cum-type), cummingtonite-biotite bearing (Cum-Bt-type), and biotite-bearing (Bt-type) rhyolites. Petrographical observation, mineral chemistries and whole rock chemical compositions show that Cum-type and Bt-type rhyolites were generated from different parental magmas and fractionated, each other, and that Cum-Bt rhyolites were produced by magma mixing between Cum- type and Bt-type rhyolites. Sr isotope ratios of basalts (and basaltic inclusions) are slightly higher than those of rhyolites, proving different origin of magmas. Also, newly examined xenoliths of granite and gabbro and tonalite will be discussed with host rhyolites and basalt.
