[SMP40-P03] Role of Basaltic Magma in the Evolution of Continental Crust: Formation process of Nikanbetsu Gabbro Complex in the Hidaka Metamorphic Belt
The An # = Ca / (Ca + Na + K) of plagioclase, and Mg # = Mg / (Mg + Fe) of clinopyroxene and orthopyroxene, decrease continuously from olivine gabbro to quartz diorite. By investigating the whole rock chemical composition focusing on fine grained rocks, which better reflect the melt composition, it was found that the less fractionated magmas, such as olivine gabbro, are classified as the tholeiite series, whereas more fractionated ones, such as quartz diorite and some gabbronorites, are classified as calc-alkaline series. This contrast is also evident in their rare earth element (REE) concentration patterns.The former shows a REE pattern depleted in LREE, whereas the latter shows a pattern enriched in LREE. Any crystal fractionation models cannot reproduce the latter from the former as a parental magma. It is concluded that there were at least two contrasting parent magmas: one belonging to the tholeiite series and the other to the calc-alkaline series. It can be inferred from its trace element pattern that the parent magma of olivine gabbro formed by adiabatic decompressional melting of the Middle Ocean Ridge Basalt (MORB) source mantle. We propose a scenario that the heat released by the crystallization of this magma elevating the temperature of the lower part of the Hidaka crust to form the parent magma of quartz diorite belonging to the calc-alkaline series by partial melting. The Nikanbetsu gabbro complex gives us an excellent chance to scrutinize processes of heat and material transportation, which takes place deep in the crust and is usually not accessible.