Petrological and geochemical study lower crustal materials beneath the lnner Zone of Southwest Japan Arc.

为了阐明西南日本弧下地壳的组成和下地壳、上地幔的地球化学演化，对广泛分布于西南日本弧内带的白垩纪-第三纪火成岩进行了岩石化学和地球化学研究。日本西南弧在中新世中期漂移离开欧亚大陆，导致日本海开放之前，被认为是欧亚大陆东缘的一部分。白垩纪的L型钛铁矿系列花岗岩类和长英质火山岩中，良屋带和山阳带的Sr同位素比值比山阴带高，Nd同位素比值比山阴带低。这些花岗岩类岩石不可能是由构成日本西南部地壳上部的侏罗纪增生杂岩熔融而形成的，而可能是由下地壳部分熔融而形成的，因为侏罗纪增生杂岩具有高得多的Sr和低得多的Nd同位素比值， 关于我们 这些花岗岩类岩石。白垩纪变质闪长岩和辉长岩在Ryoke带，从幔源拉斑玄武岩岩浆结晶，也有高Sr和低Nd同位素比值的花岗岩类岩石，这表明上地幔和下地壳下Ryoke带富集在晚白垩世。从另一方面来说。山阴带古近纪花岗岩类岩石具有较低的Sr和较高的Nd同位素比值。这些数据表明，日本西南弧的下地壳是由两种或两种以上的地球化学不同的材料。这两种下地壳物质应该在舞鹤构造带之下相互接触。中新世岩浆活动对下地壳物质的影响不大，但由于软流圈岩浆的注入，使辽岩带白垩纪上地幔的地球化学特征发生了很大的变化，山阴带下地幔和下地壳在古近纪逐渐亏损，中中新世亏损最大。这可能表明山阴带下的上地幔和下地壳由于软流圈源的亏损岩浆的注入而发生了地球化学变化，西南日本弧下的上地幔和下地壳的地球化学演化得到了西南日本弧新生代火山岩中地幔和下地壳源捕虏体地球化学数据的支持。少

Cretaceous-Tertiary igneous rocks, which are extensively distributed in the Inner Zone of Southwest Japan Arc, were petrochemically and geochemically examined in order to clarify the constitution of the lower crust and geochemical evolution of the lower crust and upper mantle beneath the Southwest Japan Arc. The Southwest Japan Arc is considered to have constituted a part of the eastern margin of the Eurasian Continent before the arc drifted away from the continent in the Middle Miocene, resulting in the opening of the Japan Sea.Cretaceous l-type ilmenite-series granitoid and felsic volcanic rocks in the Ryoke and Sanyo Belts have higher Sr and lower Nd isotope ratios than those in the San'in Belt. These granitoid rocks could not be formed by melting of a Jurrasic accretinary complex, which comprise the upper part of the crust in SW Japan, but could be formed by partial melting of the lower crust, since the Jurrasic accretionary complex has much higher Sr and lower Nd isotope ratios th … More an these granitoid rocks. Cretaceous metadiabase and gabbroic rocks in the Ryoke Belt, crystallized from mantle-derived tholeiitic magmas, have also high Sr and low Nd isotope ratios comparable to the granitoid rocks, suggesting that the upper mantle asa well as the lower crust beneath the Ryoke Belt were enriched during the Late Cretaceous. On the other hand. Paleogene granitoid rocks in the San'in Belt have low Sr and high Nd isotope ratios compared with those in the Ryoke and Sanyo Belts. These data suggest that the lower crust of the southwest Japan Arc iscomposed of two or more geochemically different materials. These two lower crustal materials should have been in contact with each other beneath the Maizuru Tectonic Belt. Geochemical characteristics of the Cretaceous upper mantle beneath the Ryoke Belt had been largely changed by injection of magmas from asthenosphere during the Miocene magmatism, although the lower crus-tal materials had not been effectively affected by the magmatism.Upper mantle and lower crust beneath the San'in Belt became more and more depleated during the Paleogene and were most depleated at the Middle Miocene. This may suggest that the upper mantle and lower crust benarth the San'in Belt had been geochemically changed by the injection of the Asthenosphere-derived more depleated magmas.These geochemical evolution of the upper mantle and lower crust beneath the Southwest Japan Arc is supported by geochemical data of mantle- and lower crust-derived xenoliths in Cenozoic volcanic rocks in the Southwest Japan Arc. Less