Talc-amphibole rocks and mafic conglomerates conformably occur in slaty rocks of the Kobotoke Group in the Katsunuma area, Yamanashi Prefecture. Their occurrence is almost same to that of previously reported serpentinite and picrite basalt bodies. The talc-amphibole rocks, now mainly composed of talc and actinolite, were derived from clastic serpentinite. Detrital chromian spinels have wide range of Cr/(Cr + Al) from 0.4 to 0.8, even in one thin section. The chemistry of spinels are almost equivalent to spinels in the Circum-Izu Massif (CIM) serpentinites. The mafic conglomerates were derived from olivine-rich basalts.
Chemical characteristics of bulk rock and relic chromian spinel indicate that they are equivalent to picrite to olivine basalts from the CIM serpentine belt. Geological and petrographical data of the talc-amphibole rocks and associated conglomerates indicate that a protrusion of a mafic-ultramafic complex had occurred at the Katsunuma area of the Kobotoke sedimentary basin in the Oligo-Miocene time.
The Katsunuma mafic-ultramafic complex composed of serpentinite and olivine-rich basalts are similar in petrological characteristics to other CIM equivalents.
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