Comparative Petrologic Study of Basaltic Rocks from the Kinki and Chubu Districts

通过广泛的斑晶显微分析和全岩地球化学分析，研究了近基和中部地区玄武岩的成因。所研究的岩石为产于日本中部俯冲太平洋板块（俯冲带）深度为-150 ~ -400 km的碱性、橄榄石、拉玄武岩、钙碱性玄武岩和安山岩。从矿物学和化学上看，俯冲带深度<-250 km的火山岩是典型的岛弧玄武岩及其衍生物。另一方面，尽管它们分布在岛弧体系中，但来自俯冲带下方>-250 km深度的岩石显示出与弧型不同的岩浆亲和性：斑晶（橄榄石、斜长石和斜辉石）的化学成分具有非弧环境的特征；玄武岩的全岩分析没有显示出Ta、Hf和Ti相对于Th、Ce和Sm的明显衰竭。这些玄武岩可能来自于一个更大的来源，这个来源在日本西部产生了新近纪的碱性火山岩，具有板内碱性玄武岩的亲和力，在较浅的深度有较大程度的融化。在俯冲带深度为-250 km的地方，具有板内玄武岩亲和性的橄榄橄榄拉斑岩在-2 Ma喷发，而具有岛弧玄武岩亲和性的岩石在<1 Ma喷发。一批地幔物质可能位于东日本火山区弧后侧的良白山之下，是日本西部新近纪碱性火山岩的潜在来源。很可能是菲律宾海板块在-3℃的运动变化导致了非弧型岩浆源从良白山下的最上层地幔向Norikura火山链下的最上层地幔的突然漂移。K-Ar年龄数据表明，在-1.5 Ma时，岩浆源在东日本弧系地幔对流的稳定流动下向西漂移回良八山下的上地幔。少

The origins of basaltic rocks from the Kinki and Chubu districts were studied through extensive microprobe analysis of the phenocrysts and the whole-rock geochemistry. The studied rocks are alkalic, olivine tholeiitic and calc-alcalic basalts to andesites erupted in central Japan where the depth of the subducting Pacific plate (subduction zone) ranges from -150 to -400 km. Rocks from volcanoes beneath which the subduction zone is <-250 km deep are typical island-arc basalts and their derivatives in mineralogy and chemistry. On the other hand, despite their distribution in an island arc system, rocks from places beneath which the subduction zone is >-250 km deep show a magmatic affinity unlike the arc type: chemical compositions of the phenocrysts (olivine, plagioclase and clinopyroxene) are characteristic to non-arc settings; and the whole-rock analyses of the basalts do not show marked depletion of Ta, Hf and Ti relative to Th, Ce and Sm. These basaltic rocks formed probably from a so … More urce which has generated the Neogene alkalic volcanic rocks bearing a within-plate alkalic basalt affinity in west Japan, by greater degrees of melting at shallower depths. At places where the depth of the subduction zone is -250 km, olivine tholeiitic rocks with a within-plate basalt affinity erupted at -2 Ma, while rocks with an island-arc basalt affinity have been erupted at <1 Ma.A batch of mantle material, potentially a source of the Neogene alkalic volcanic rocks in west Japan, is likely to have emplaced beneath the Ryohaku Mts. at the back-arc side of the East Japan Volcanic Zone. It is probable that a change in motion of the Philippine Sea plate at -3 Macaused the sudden drift of the non-arc type magma source from the uppermost mantle beneath the Ryohaku Mts. to that beneath the Norikura Volcanic Chain. The K-Ar age data suggest that at -1.5 Ma, the magma source was drifted westward back to the uppermost mantle beneath the Ryohaku Mts. by a steady flow of mantle convection beneath the East Japan Arc System. Less