Abstract
The early to middle Cretaceous melange complex crops out along the eastern part of the Nakanogawa group, central Hokkaido. We found lamprophyre (camptonite) from the Tachiiwa melange complex in the southern most part of the Nakanogawa group. This camptonite occurs as dyke form in the pelagic sediment, which is surrounded by the basic tuff matrix. The camptonite is composed of alkali feldspar, kaersutite, salite, biotite, opaque minerals, apatite and aegirine-augite. The K-Ar dating for the camptonite indicates an age of 95 Ma. The mineralogical and bulk chemical features of the camptonite shows the alkali-lamprophyre, e. g. the MORB-normalized pattern of the camptonite does not show negative Nb-anomaly. The camptonite would be emplaced in the environment of oceanic islands or sea mount during the early to middle Cretaceous age (Cenomanian) based on mode of occurrences and chemical characteristics.
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