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

北上山白垩纪火成岩以深成岩为主，少量火山岩和脉岩。深成岩的放射性年龄集中在120 Ma左右，与部分火山岩和脉岩发生接触变质作用。深成岩的岩石化学特征与火山岩相似，但与脉岩不同。据此，将北上山白垩纪火成岩划分为主要系列(深成岩和火山岩)和早期岩墙系列。主岩系岩石主要为方解石(极少拉斑玄武岩)、分布于北上山东缘的埃达克质岩体和零星的玄武岩。岩石化学证据表明，埃达克质深成岩可由俯冲板片直接部分熔融而成。板片熔融成因的埃达克岩浆的存在，表明年轻而炎热的洋壳俯冲。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武岩来自不同程度的地幔楔体部分熔融。早期岩墙系列含高Nb玄武岩，岩石化学特征为板内碱性玄武岩，富锶安山安山岩类“黄辉岩”。碱性岩浆作用可能是由于地幔物质在山脊俯冲时通过板条窗口绝热上升的程度较小的减压部分熔融而产生的。Bajaite具有与埃达克岩相同的岩石化学性质，但镁、铬、镍含量较高。这些岩石的产出表明，山脊俯冲作用促进了软流圈(板片窗口)的被动上涌，产生了碱性玄武岩，随后又发生了适合于板片熔融的年轻热洋板的俯冲作用。

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.