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

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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.