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

通过对斑晶和全岩地球化学的大量微探针分析，研究了近畿和中部地区玄武岩的成因。所研究的岩石是碱性、橄榄拉斑玄武岩和钙碱性玄武岩，以及在日本中部喷发的安山岩，那里俯冲的太平洋板块（俯冲带）的深度范围从-150到-400公里。俯冲带深度小于-250 km的火山岩在矿物学和化学上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系统中，但俯冲带深度>-250 km的岩石显示出与弧类型不同的岩浆亲和力：斑晶的化学成分（橄榄石、斜长石和单斜辉石）是非弧环境的特征;玄武岩全岩分析显示Ta、Hf、Ti相对Th、Ce、Sm没有明显亏损。这些玄武质岩石可能是由一个 ...更多信息 在日本西部，通过较浅深度的较大程度熔融，产生了具有板内碱性玄武岩亲和力的新近纪碱性火山岩。在俯冲带深度为-250 km的地方，具有板内玄武岩性质的橄榄拉斑玄武岩的喷发时间为-2 Ma，而具有岛弧玄武岩性质的岩石的喷发时间<1 Ma。位于东日本火山带的弧后一侧。菲律宾海板块在-3Ma处运动的变化可能导致了两白山下地幔上部非弧型岩浆源的突然漂移。纪仓火山链下的那个K-Ar年龄数据表明，在-1.5Ma时，岩浆源区向西漂移回到两白山下地幔上部。东日本岛弧系统下地幔对流的稳定流动。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