東北日本弧,後期中新世〜鮮新世火山岩の島弧横断方向における化学組成および同位体組成変化  [in Japanese] Across-arc chemical and isotopic variation of Late Miocene to Pliocene volcanic rocks from the Northeast Japan arc  [in Japanese]

東北日本弧の12 Ma以降の火山活動の時期を4時代に区分し, 各時代の火山岩のLIL元素 (K_2O, Na_2O+K_2O, Rb, Sr) 量の島弧横断方向の変化を検討した。12-10 Maと10-8 Ma火山岩では, 現在の海溝軸からの距離が310-320 km付近まで距離の増加に伴って, LIL元素量は漸次増加するが, これより背弧側に位置する火山岩は海溝側火山岩と同程度の低い元素量を有する。8-5 Maと5-2 Maの火山岩では, 海溝軸からの距離が離れるほど, LIL元素量は増加する傾向を示す。背弧側玄武岩に注目すると, 16-8 Maの玄武岩と比較して, これより若い玄武岩の方がこれらの元素に富んでいる。これらの12 Ma以降の火山岩のSri値とNdi値の時空的変化を検討した。海溝側火山岩は高いSri値と低いNdi値で特徴づけられるのに対して, 背弧側火山岩は低いSri値と高いNdi値で特徴づけられる。脊梁地域の火山岩は海溝側と背弧側の火山岩の中間的な同位体組成を有している。このような12 Ma以降の火山岩の同位体組成の島弧横断方向の漸移的変化は, 同位体的に枯渇したソース(日本海拡大末期 (約15 Ma)に背弧側の島弧マントル中へ上昇してきたアセスフェア)と非枯渇的ソース(島弧マントル)の混合によって説明されることを議論した。また, 背弧側玄武岩のLIL元素量の時間的変化は, ソースの部分溶融程度が時間とともに低下したことに起因していることを述べた。

Twenty-six Late Miocene to Pliocene volcanic rocks from the NE Japan arc were dated by the K-Ar method. On the basis of new and published K-Ar age data, volcanic activity after 12 Ma in the NE Japan arc is subdivided into four stages at 12-10 Ma, 10-8 Ma, 8-5 Ma and 5-2 Ma. Across-arc variations of K_2O, Na_2O+K_2O, Rb and Sr at 53% SiO_2 for the respective volcanic rocks of each time span have been examined. The LILE (large ion lithophile elements) contents of 12-10 Ma and 10-8 Ma volcanic rocks increase gradually with increasing distance from the present trench axis to a distance of 320-340 km. Beyond this, volcanics of the back-arc side have LILE contents aslo was those of the trench-side volcanics. On the other hand, the LILE contents of 8-5 Ma and 5-2 Ma volcanic rocks increase gradually from the trench side toward the Japan Sea side, which is similar to across-arc variations of Quaternary volcanics. Thus, volcanic rocks on the back-arc side show a gradual increase in LILE contents through time from 12 Ma to 0 Ma. The initial ^<87>Sr/^<86>Sr and ^<143>Nd/^<144>Nd ratios (Sri and Ndi ratios) were determined for forty-four specimens from twenty-six locations in the NE Japan arc, and the space-time variations of these ratios were examined using new and published isotope data. Most Sri and Ndi ratios of volcanic rocks from 12-0 Ma show a linear trend across the NE Japan arc from an undepleted isotopic signature on the trench side to a depleted signature on the back-arc side. This can be ascribed to across-arc isotopic heterogeneity in the source mantle, due to mixing of isotopically undepleted island arc mantle with depleted asthenospheric mantle which was injected beneath the back-arc side during the opening of the Japan Sea. The time variations of LILE contents of the back-arc basaltic rocks may be explained by decreasing degree of partial melting with increasing age from about 12 to 0 Ma. At emporal decrease in the degree of paltial melting may have been caused by a decreasing geothermal gradient in the source region due to gradual cooling of hot asthenosphere which up-welled into the island arc mantle.