High-Mg andesites and basalts in the southern Nishisonogi Peninsula, Nagasaki Prefecture

The late Cenozoic volcanic rocks in the southern Nishisonogi Peninsula, which are characterized by dominance of undifferentiated, primitive rocks, show a large variation in chemical composition ranging from high-Mg andesite (HMA) through tholeiitic basalt to alkali basalt and from high-Mg basalt to rhyolite. Most HMA is cpx-HMA crystallized clinopyroxene with the highest Mg# and Cr_2O_3 of 87.4 and 1.24%, respectively, following olivine with the highest Fo of 92.1 and NiO of 0.48%. An orthopyroxene andesite containing bronzite with Mg# and Cr_2O_3 as high as 89.5 and 1.06%, respectively, was found at Ebisu-zaki, the second opx-HMA occurrence in this area. Many of the pyroxene andesites reveal high Cr (>100ppm)