Abstract
Abundant serpentinite cobbles have been found in the Kashima River in the Omachi area, central Japan. Their abundance decreases sharply downstream to the Takase River. Small serpentinite bodies must be exposed in the Kashima River catchment, where Paleogene granitic rocks (i.e., the Ariake Granite) are widely exposed. These small serpentinite bodies have been interpreted as blocks surrounded by granitic magmas and are expected to have experienced intense contact metamorphism. They are part of the serpentinite and metaperidotitic rocks commonly found in the northwestern part of Fossa Magna. The mode of occurrence of the serpentinite bodies has not been investigated in detail because of the steep mountainous terrain of the Kashima River area, and the petrography and petrology of the bodies are only easily examined in river cobbles from the area. The serpentinite cobbles are characterized by olivine–tremolite–antigorite or olivine–diopside–antigorite assemblages with or without a small amount of talc, and most have been serpentinized further at low temperatures. Their mineral assemblages and chemical compositions are similar to those of the metaperidotites exposed at Happo-O’ne and the surrounding area, which record weak contact metamorphism (up to talc zone), although the metaperidotite protoliths had slightly but distinctly different in serpentine and diopside contents. The small serpentinite bodies exposed upstream of the Kashima River record only weak contact metamorphism, even though they occur in a granitic complex. This suggests that they were not trapped by the granitic magmas during intrusion but are tectonic blocks that were possibly emplaced into the granitic rocks by faults. This work provides an example of the value in studying the petrology of river cobbles.
