Melt segregation by localized shear deformation and fracturing during crystallization of magma in shallow intrusions of the Otoge volcanic complex, central Japan

Basaltic dikes and sheets in the Miocene Otoge volcanic complex in central Japan have thin trachyte veins, which exhibit systematic variation regarding spacing with distance from the dike wall. Compositional relationship between the trachyte veins and surrounding basalt shows that the trachytic melt filling the veins was formed by in situ fractionation and segregated from the surrounding basalt. Mass balance calculation suggests that a residual liquid of trachyte composition formed when about two-thirds of the parent basaltic magma was crystallized. Preferred orientation of groundmass crystals around the trachyte veins and echelon alignment of veinlets show that localized shear caused the deformation and fracturing in the incompletely crystallized magma and the interstitial residual melt was drawn into the fractures to form trachyte veins. The parallel development of trachyte veins and the increase of spacing toward the inner portion of the dike were controlled by the balance of cooling from the dike wall and segregation of interstitial melt. In a mobile magma body, shear will be concentrated in a crystal-mush layer in marginal parts, and the shear deformation could be an important mechanism to separate strongly-evolved residual melt from a mushy crystallizing layer.