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Effects of ridge subduction on Early Cretaceous magmatism in Kitakami Mountains, Japan

山脊俯冲对日本北上市山早白垩世岩浆作用的影响

基本信息

批准号：
06640574
负责人：
TSUCHIYA Nobutaka
金额：
$ 1.15万
依托单位：
Iwate University
依托单位国家：
日本
项目类别：
Grant-in-Aid for Scientific Research (C)
财政年份：
1994
资助国家：
日本
起止时间：
1994 至 1996
项目状态：
已结题

项目摘要

The Cretaceous igneous rocks in the Kitakami Mountains consist mainly of plutonic rocks with minor amounts of volcanic rocks and dike rocks. The radiometric ages of plutonic rocks are concentrated around 120 Ma, and these plutonic rocks give contact metamorphisms on some volcanic and dike rocks. The petrochemical signatures of plutonic rocks are similar to those of volcanic rocks, but are different from those of dike rocks. From this, Cretaceous igneous rocks in the Kitakami Mountains are divided into main series (plutonic rocks and volcanic rocks) and early dyke series. The main series rocks are composed mainly of calcalkine (rarely tholeiitic) rocks, adakitic pluton distributed along eastern margin of the Kitakami Mountains, and sporadic shoshonitic rocks. Petrochemical evidence indicates that the adakitic plutonic rocks can be derived by direct partial melting of subducted slab. The presence of adakitic magmas of slab melting origin indicates the subduction of young and hot oceanic crust. Detailed investigation of incompatible trace elements suggests that calcalkaline to shoshonitic rocks of the main series derived from various degrees of partial melting of a mantle wedge. The early dyke series contain high-Nb basalts with petrochemical characteristics of intraplate alkali basalts and Sr-rich andesitic rocks similar to "bajaites". Alkaline magmatism would be produced by small degrees of decompressive partial melting of mantle materials rising adiabatically through the slab window upon ridge subduction. Bajaite has the common nature of petrochemistry to adakite except high content of Mg, Cr, and Ni. This suggests that bajaitic magmas were formed by reactions between ascending adakitic melt and a mantle peridotite.The occurrence of these rocks suggests ridge subduction, which would promote passive upwelling of asthenosphere (slab window) and produce alkali basalt, and subsequent subduction of young and hot oceanic slab appropriate for slab melting.

北上山地的白垩纪火成岩主要由深成岩组成，也有少量火山岩和岩脉岩。深成岩的辐射年龄集中在120 Ma左右，这些深成岩对一些火山岩和岩脉岩产生了接触变质作用。深成岩的石油化学特征与火山岩相似，但与岩脉岩不同。由此，北上山白垩纪火成岩分为主系（深成岩和火山岩）和早期岩脉系。主要系列岩石主要由钙碱性岩（很少有拉斑岩）、分布在北上山脉东缘的埃达克岩体和零星的钾玄岩组成。石油化学证据表明，埃达克质深成岩可以通过俯冲板片的直接部分熔融而形成。板片熔融起源的埃达克岩浆的存在表明年轻而炎热的洋壳的俯冲。对不相容微量元素的详细研究表明，主要系列的钙碱性至钾玄岩源自地幔楔不同程度的部分熔融。早期岩脉系含有高铌玄武岩，具有板内碱性玄武岩和类似“贝贾岩”的富锶安山岩的石油化学特征。碱性岩浆作用是由地幔物质在山脊俯冲时通过板片窗口绝热上升的小程度减压部分熔融而产生的。除镁、铬、镍含量较高外，贝贾石具有与埃达克石相同的石油化学性质。这表明巴贾岩岩浆是由上升的埃达克岩熔体与地幔橄榄岩之间的反应形成的。这些岩石的出现表明洋脊俯冲，这将促进软流圈（板片窗口）的被动上涌并产生碱性玄武岩，以及随后适合板片熔化的年轻而热的海洋板片的俯冲。