<研究報告>長崎県西彼杵半島南部の高Mg安山岩と玄武岩

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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) ; they are likely to have derived from HMA magmas through crystal fractionation. Shikimi basalt with SiO_2 49.0～49.9% and MgO 13.3～12.6% has olivine showing a modal Fo value of 74-71,suggesting that its high MgO results from accumulation of olivine. Makishima alkali basalt is distinguishable from the dominant tholeiitic and calc-alkali rocks by high incompatible element abundances, especially Nb (40.9～25.4ppm), implying derivation from intra-plate magmas. Coexistence of alkali basalt and tholeiitic basalt to HMA suggests that their magmas have generated by reaction between rising magmas and mantle materials variously metasomatized previously by slab-derived hydrous fluids and/or siliceous melts. The least modified Makishima alkali basalt may represent the least reacted magmas, and reaction with intensively metasomatized hydrous, siliceous mantle has produced the most SiO_2-rich opx-HMA magmas.

<研究報告>長崎県西彼杵半島南部の高Mg安山岩と玄武岩

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