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

北上山白垩系火成岩以深成岩为主，少量火山岩和脉岩。深成岩的辐射年龄集中在120 Ma左右，这些深成岩在一些火山岩和脉岩上发生接触变质作用。深成岩的石油化学特征与火山岩相似，但与脉岩不同。据此，将北上山白垩系火成岩划分为主要岩系（深部岩系和火山岩系）和早期岩脉系。主要系列岩石主要由钙钙质岩石（很少有拉斑岩）、沿北上山东缘分布的埃达克质岩体和零星的松玄质岩石组成。石油化学证据表明，阿达克质深成岩可由俯冲板块直接部分熔融形成。陆块熔融成因的埃达克岩岩浆的存在表明了年轻热洋壳的俯冲作用。对不相容微量元素的详细研究表明，主要系列的钙碱性和玄玄质岩石来源于地幔楔体不同程度的部分熔融。早期岩脉系含高铌玄武岩，具有板内碱性玄武岩的石化特征，以及类似“巴宰岩”的富sr安山岩。在脊状俯冲作用下，地幔物质通过板块窗绝热上升，发生小程度的减压部分熔融，从而产生碱性岩浆活动。巴哈尔特除镁、铬、镍含量高外，具有与埃达克岩相同的岩石化学性质。这表明，巴jaitic岩浆是由上升的埃达克质熔体和地幔橄榄岩之间的反应形成的。这些岩石的产状表明，洋脊俯冲作用促进软流圈（板块窗）被动上涌，形成碱性玄武岩，随后洋脊俯冲形成适合板块熔融的年轻热洋板块。

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.