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Mantle plume injection during the spreading of the back-arc basin-Evidences from the volcanic fields in north Hokkaido, Japan-

弧后盆地扩张过程中的地幔柱注入——来自日本北海道北部火山田的证据——

基本信息

批准号：
09640541
负责人：
SHUTO Kenji
金额：
$ 2.3万
依托单位：
NIIGATA UNIVERSITY
依托单位国家：
日本
项目类别：
Grant-in-Aid for Scientific Research (C)
财政年份：
1997
资助国家：
日本
起止时间：
1997 至 1999
项目状态：
已结题

项目摘要

The results obtained from this research project are summarized as follows.1. The late Miocene Engaru volcanic field in northeastern Hokkaido contains basalts (Tomeoka basalt (TM) and Chiyoda-Kakurezawa basalt (CK)) and rhyolite (Wakamatsu rhyolite (WK)) with subordinate amounts of andesite (Sakaeno andesite (SK)) that erupted during 7-9 Ma. Both the TM and CK basalts have geochemical characteristics similar to those of back-arc basin basalt (BABB). Based on differences in major and trace element abundances and the initial values of Sr and Nd isotopic ratios (SrI and NdI), the CK basalt, SK andesite and WK rhyolite can be divided into two types (I and II), respectively. The SK type I andesite and WK type I rhyolite are of calc-alkaline series, whereas the SK type II andesite (including icelandite-like andesite) and WK type II rhyolite are of tholeiitic series. The WK type I rhyolite has remarkably high SrI and low NdI values compared with other volcanic rocks from the study area. The si … More milarity in SrI and NdI of the rhyolite to S-type granitoids and pelitic-psammitic rocks of the Hidaka belt suggests the lower crustal origin of the rhyolite. The SK type I andesite is plotted on or near mixing lines between the CK type I basalt and WK type I rhyolite in major and trace element variation diagrams and SrI vs. NdI diagram, implying its mixing origin. On the other hand, major- and trace- element and Sr- and Nd- isotopic characteristics indicate that the SK type II andesite and WK type II rhyolite were generated from the CK type II basaltic magma by fractional crystallization accompanied by a minor degree of assimilation of crustal materials. The genetic relationship of these coeval basalts, andesite and rhyolite can be attributed to spreading of the Kurile basin. BABB magma which is a partial melt in a hot asthenosphere uprising below the island arc during the basin spreading, could have heated the lower crust to generate calc-alkaline rhyolitic magma. Andesitic rocks were derived both by mixing of BABB magma with the lower crust-derived rhyolitic magma and by fractional ctystallization of BABB magma.2. Basalts and andesites from the Iwao Formation, occurring in the Hamamasu area along the Japan Sea coast in northwest Hokkaido, were derived from a volcano which has been erupted during a relatively short period of 3.7 to 3.8 Ma. Similar disequilibrium phenocryst assemblages (Mg-rich olivine + Mg-rich clinopyroxene + Mg-poor orthopyroxene + hornblende + An-rich plagioclase + An-poor plagioclase + quartz + Fe-Ti oxide) are commonly found in the basalts and andesites. Detailed microprobe analyses as well as petrographic observations of coexisting phenocrysts reveal that the disequilibrium phenocryst assemblages in these volcanic rocks may be attributable to mixing of a basaltic magma in which olivine, clinopyroxene and An-rich plagioclase were crystallizing, and a dacitic magma in which orthopyroxene, hornblende, An-poor plagioclase, quartz and Fe-Ti oxide were crystallizing. Not only chemical variation but also petographic variation from basalts to andesites of the Iwao Formation are accounted for by the difference in mixing ratios of basaltic and dacitic magmas. Less

本研究取得的主要成果如下：1.北海道东北部的晚中新世远轻火山岩区包含玄武岩（Tomeoka玄武岩（TM）和千代田-角烈泽玄武岩（CK））和流纹岩（Wakamatsu流纹岩（WK）），以及少量的安山岩（Sakaeno安山岩（SK）），喷发时间为7-9 Ma。TM玄武岩和CK玄武岩具有与弧后盆地玄武岩（BABB）相似的地球化学特征。根据主、微量元素丰度和Sr、Nd同位素比值（SrI和NdI）初始值的差异，将CK玄武岩、SK安山岩和WK流纹岩分别划分为I和II两种类型。SK Ⅰ型安山岩和WK Ⅰ型流纹岩属钙碱性系列，SK Ⅱ型安山岩（包括似冰长石安山岩）和WK Ⅱ型流纹岩属拉斑玄武岩系列。WK Ⅰ型流纹岩具有明显的高Sr I和低Nd I值。的si ...更多信息流纹岩与日高带S型花岗岩类和泥砂质岩石的SrI和NdI相似性表明流纹岩为下地壳成因。SK Ⅰ型安山岩在主量、微量元素变化图和SrI-NdI图中位于CK Ⅰ型玄武岩和WK Ⅰ型流纹岩的混合线上或附近，暗示其混合成因。另一方面，主量元素、微量元素和Sr、Nd同位素特征表明，SK Ⅱ型安山岩和WK Ⅱ型流纹岩是由CK Ⅱ型玄武质岩浆经分离结晶作用和少量地壳物质同化作用形成的。这些玄武岩、安山岩和流纹岩的成因关系可归因于千岛盆地的扩张。BABB岩浆是在盆地扩张过程中上升到岛弧下方的热软流圈中的部分熔融，可能加热了下地壳，产生钙碱性流纹质岩浆。安山岩是由BABB岩浆与下地壳流纹质岩浆混合和BABB岩浆分异结晶形成的.产于北海道西北部日本海沿岸沿着滨益地区的岩尾组玄武岩和安山岩，是在3.7 ~ 3.8 Ma的相对较短时期内火山喷发的产物。玄武岩和安山岩中普遍存在类似的不平衡斑晶组合（富镁橄榄石+富镁单斜辉石+贫镁斜方辉石+角闪石+富安斜长石+贫安斜长石+石英+ Fe-Ti氧化物）。详细的微探针分析以及共存斑晶的岩相学观察表明，这些火山岩中的不平衡斑晶组合可能是由于混合的玄武质岩浆中的橄榄石，单斜辉石和富An斜长石结晶，英安质岩浆中的斜方辉石，角闪石，贫An斜长石，石英和Fe-Ti氧化物结晶。岩尾组从玄武岩到安山岩的化学变化和岩石学变化不仅可以通过玄武质和英安质岩浆混合比的差异来解释。少