Geological and petrological study of the middle Miocene regional basalts from the Japan Sea side in the NE Japan arc

对日本东北部12个上新世火山中心的111个样品进行了X射线荧光分析，测定了主量元素和Rb、Sr、Nb、Y、Zr、Ni、Cr等7种微量元素的丰度。本研究和已发表的其他上新世火山岩的岩石学和分析结果表明，上新世火山岩在横跨日本东北弧太平洋一侧至日本海一侧的岩石学特征，即镁铁斑晶组合和岩系，以及K2O、Rb和Sr含量的逐步增加，具有系统的地带性变化。此外，在日本东北弧晚中新世(8-6 Ma)火山岩中也观察到了类似于上新世至第四纪火山岩的化学成分和岩石学特征的系统跨弧变化。关于…晚期火山岩的岩石学和地球化学特征从上新世到中新世，更多的中新世至第四纪表明，与板块俯冲所决定的岩浆生成机制有关的火成岩活动至少在8 Ma以前在日本东北弧内持续存在。另一方面，中新世中期(16-14 Ma)的日本东北弧以非岛弧型火山岩的产出为特征，如富钛和其他HFS元素的E型MORB状玄武岩，在块体岩石化学上介于岛弧拉斑玄武岩和弧后盆地玄武岩之间的玄武岩，以及类似冰英安山岩和英安岩的岩石与岛弧拉斑玄武岩系列和钙碱性系列火山岩共生。这些火山岩的K2O含量从日本海一侧向太平洋一侧逐渐增加，横跨日本东北弧区，表明其横向变化不同于8 Ma前至今的火山岩，这种非岛弧型火山岩可能来源于一种不同于现今日本东北弧区的上地幔物质。这些火山岩可能是在日本海扩张期间从软流圈上升的地幔底辟产生的。日本东北弧中中新世岛弧拉斑玄武岩系列和钙碱性系列火山岩可能起源于上升地幔底辟加热的上地幔和下地壳部分熔融。较少

Abundances of major elements and seven trace elements, Rb, Sr, Nb, Y, Zr, Ni and Cr were determined by X-ray fluorescence analysis for 111 samples from twelve Pliocene volcanic centers in the NE Japan are. The petrographical and analytical results in this study and the published ones on other Pliocene volcanic rocks indicate that systematic zonal variation of petrological features, i.e., mafic phenocryst assemblages and rock series, as well as gradual increase in K_2O, Rb and Sr contents are recognized in the Pliocene volcanic rocks from the Pacific Ocean side to the Japan Sea side across the NE Japan arc. Furthermore, systematic across-arc variation of chemical compositions and petrological features, similar to those in the Pliocene to Quaternary volcanic rocks, are also observed in the late Miocene (8-6 Ma) volcanic rocks throughout the NE Japan arc.Similarity of across-arc variation. with regard to both the petrological and geochemical features among the volcanic rocks from the late … More Miocene to Quaternary through Pliocene time suggests that igneous activity which is related to a mechanism for magma generation determined by plate subduction bas been continuous in the NE Japan arc at least during 8 Ma ago to the present.On the other hand, the NE Japan arc of the middle Miocene (16-14 Ma) are characterized by occurrence of rocks of non island arc-type volcanic rocks such as E-type MORB like basalt which is enriched in Ti and other HFS elements, basaltic rocks hailing intermediate nature between the island arc tholeiite and the back arc basin basalt in bulk rock chemistry and icelandite like andesite and dacite, in association with volcanics of the island arc tholeiite series and calc-alkaline series. These volcanic rocks increase gradually in K_2O from the Japan Sea side toward the Pacific Ocean side across the NE Japan arc, indicating the lateral variations dissimilar to the case of volcanic rocks produced during 8 Ma ago to the present.It is possible that such non island arc-type volcanics were derived from a upper mantle material which is different from the present one in the NE Japan arc. These volcanics could have been generated from a mantle diapir ascending from asthenosphere during the spreading of the Japan Sea. The middle Miocene volcanics of island arc tholeiite series and calc-alkaline series in the NE Japan arc may have been originated from partial melting of upper mantle and lower crust heated by the ascending mantle diapir. Less