Serpentine minerals and their textural changes on solid intrusion tectonics of the Kurosegawa Belt in the northwestern Kanto Mountains, central Japan

Serpentinite bodies, which belong to a component of the Kurosegawa Belt, are distributed along fault boundaries between the Lower Cretaceous Sanchu Group (forearc basin-fill sediments) and the beds of the Southern Chichibu Belt (Jurassic to Early Cretaceous accretionary prism) in the Jikkoku Pass area, northwestern Kanto Mountains, central Japan. The serpentinites were divided into three types (massive, antigorite, and chrysotile serpentinites) based on meso- and microstructures and combinations of serpentine minerals. The pseudomorphic textures of massive serpentinite are initially formed in all the textures of serpentinites and such textures indicate non-deformation after serpentinization. Antigorite serpentinite shows a shape-preferred orientation of antigorites along with fine-grained recrystallization and porphyroclastic grains. It appears that antigorite serpentinite is characterized by a ductile deformation under higher pressure-temperature conditions. Antigorites are developed by replacing the original minerals (lizardite and/or chrysotile) to form pseudomorphs. Chrysotile serpentinite is eventually formed by overprinting the textures of pre-existing serpentinites. Chrysotile fibers are penetratively developed and form a foliation. The foliations of antigorite and chrysotile serpentinites strike parallel to the elongate directions of the serpentinite bodies, although there are differences in temperature stability between their major constituent minerals; this fact may indicate a continuous deformation during solid intrusion. It is inferred that the serpentinite bodies were emplaced through diapiric intrusion along the fault zone, after receiving complete serpentinization at deeper levels (lower crust and upper mantle).